SMITHSONIAN CONTRIBUTIONS TO BOTANY NUMBER 13 

Swollen-Thorn Acacias of 

Central America 

Daniel H. Janzen 

SMITHSONIAN INSTITUTION PRESS 

City of Washington 

1974 
ABSTRACT 

Janzen, Daniel H. Swollen-Thorn Acacias of Central America. Smithsonian 

Contributions to Botany, number 13, 131 pages, 119 figures, 1974.-This nomenclatural, taxonomic, and ecological treatment of 1 1 Central American obligate 

ant-acacias (Acacia allenii, A. chiapensis, A. collinsii, A. cookii, A. cornigera, 

A. gentlei, A. globulifera, A hindsii, A nzayana, A. melanoceras, and A. sphaerocephala) and one quasi-obligate ant-acacia (Acacia ruddiae) is based on extensive field study from 1963 to 1972 and on herbarium specimens where of use. 

7 he population boundaries of all species are mapped and described with respect 

to ecological parameters. Morphological variation, details of the interaction with 

the ants, and acacia reproductive biology are presented for most species. 

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded 

in the Institution‘s annual report, Srnithsonian Year. SI PRESS NUMBER 4817. SERIES COVER DESIGN: 

Leaf clearing from the Katsura tree Cercidiphyllurn japonicum Siebold and Zuccarini. 

Library of Congress Cataloging in Publication Data 

Janzen, Daniel H: 

Swollen-thorn acacias of Central America. (Smithsonian contributions to botany, no. 13) 

Bibliography: p. 

1. Acacia. 2. Ants-Central America. 3. Botany-Central America-Ecology. I. Title. 11. 

QKl.S27 no. 13. [QK495.L52] 581’.08s [583’.321] 73-4401 

Series: Smithsonian Institution. Smithsonian contributions to botany, no. 13. 

For sale by the Superintendent of Documents, US. Government Printing O5ce 

Washington, D.C. 20402 . Price $2.35 (paper cover) 
Contents 

Page 

Introduction ................................................... 1 

General Biology ................................................. 2 

General Comments on Swollen-Thorn Acacia Taxonomy .............. 6 

Materials, Methods, and Terminology ............................... 14 

General Morphological Traits .................................. 15 

Taxonomic Information .............. ....................... 25 

A . Key to Mature Central American Swollen-Thorn Acacias, 

Based on All Traits ........................................... 28 

B . Key to Mature Central American Swollen-Thorn Acacias, 

Based on Vegetative Traits .................................. 29 

Acacia cookii Safford ................. ................... 29 

Acacia ruddiae, new species ..................................... 34 

Acacia melanoceras Buerling ..................................... 42 

Acacia allenii, new species .......................................... 53 

Acacia chiapensis Safford ....................................... 61 

Acacia globulifera Safford ........................................ 67 

Acacia sphaerocephala Schlectendal and Chamisso ..................... 73 

Acacia cornigera (L.) Willdenow .................................. 80 

Acacia mayana Lundell ........................................... 93 

Acacia gentlei Standley ....................................... 95 

Acacia collinsii Safford ......................................... 99 

Acacia hindsii Bentham ........................................ 113 

Literature Cited ................................................. 130 

Swollen-Thorn Acacias of 

Central America 

Daniel H. Janzen 

Introduction 

The swollen-thorn acacia group (Acacia species) 

is one of the two most diverse and widespread of 

the neotropical myrmecophytes. These acacias are 

the characteristic ant-plants in the drier lowlands 

(“tierra caliente”) of Central America, just as the 

genus Cecropia (Aloraceae) is the most conspicuous 

and diverse myrmecophyte of wetter habitats in 

Central and South America. 

This monograph on the taxonomy and natural 

history of neotropical swollen-thorn acacias is offered at this time for several reasons. (1) The 

neotropical swollen-thorn acacias do not appear to 

constitute a tight phjletic unit, yet they share many 

adaptive ecological and morphological traits. They 

are thus outstanding examples of evolutionary convergence. The most noteworthy of the traits they 

have in common is that they are protected from 

herbivores by colonies of ants in the genus Pseudomyrmex, rather than by chemical traits as are most 

plants (Janzen, 1966a, 1967b; Rehr et al., 1973). (2) 

The ants can be readily removed, just as though 

one were able to easily remove the alkaloids and 

cyanogenic glycosides from the leaves of ordinary 

acacias such as Acacia farnesiana. This ease of experimentally disarming the plant means that the 

protective role of the obligate acacia-ants has been 

easy to study and provides an excellent system for 

understanding the coevolution of complex interactions between higher organisms. For further and 

more detailed comparative studies among the swolDaniel H. Janzen, Department of Zoology, University of Michigan, Ann Arbor, Michigan 48105. 

len-thorn acacias, a stable nomenclature is essential 

at this time. (3) Detailed data on populations of native wild Central American plants are noticeably 

absent from the literature, A better concept is 

needed of the population structures of a wide variety of tropical plants of different life forms and in 

different habitats in order to generate specific hypotheses that can lead to an understanding of complex tropical communities. (4) The lowland 

deciduous forests of Central America have nearly 

disappeared. The population structures of the species remaining in these habitats are already greatly 

altered. The swollen-thorn acacias are clearly in this 

category. I would like to summarize the ecological 

behavior of swollen-thorn acacias before their patterns are so far altered that we can no longer recog 

nize the conditions that produced the genotypes we 

find in today’s cattle pastures, roadsides, and 

abandoned fields. Most wild plants in high-yield 

agricultural areas must be studied under conditions 

not unlike studying elephants and deer in a zoo. 

It is hoped that the incompleteness of the data 

and hypotheses offered here will encourage field 

biologists to look harder at why plants do what they 

do in natural habitats as well as in those badly 

altered by agriculture. 

ACKNOWLEDGMENTS.-~ am greatly indebted to 

Dr. Velva E. Rudd of the National Museum of 

Natural History for her very competent assistance 

and advice throughout this study. Her patience 

with the bumblings of an entomologist in the world 

of taxonomic botany immensely facilitated the unraveling of the nomenclatorial spaghetti surrounding the swollen-thorn acacias. Personnel in various 

1 
SMITHSONIAX CONTRIBUTIOXS TO BOTANY 

herbaria have kindly allowed me to examine types 

and at times to borrow specimens. 

A special acknowledgment is due Karen Janzen, 

who participated in the laborious task of recording 

field data and voucher specimens. 

This study was supported by National Science 

Foundation Grant GB-5206, and is in part a byproduct of NSF Grants GB-91, 1428, 7805, and 

7819. Substantial portions of this study were completed while I was serving in Costa Rica as a faculty 

member for the Organization for Tropical Studies 

in Costa Rica (1965-1971). The paper is Contribution No. 1360 from the Department of Entomology, 

The University of Kansas, Lawrence, Kansas. 

General Biology 

The 12 species of swollen-thorn acacias in aggregate range from the Tropic of Cancer on both 

coasts of Mexico only as far south as northwestern 

Colombia. They are generally found at elevations 

below 1200 m (Figure 1). One Argentinian acacia, 

Acacia cauen (Molina) Molina, has been suggested 

as a swollen-thorn acacia (Fiebrig, 1909; Chodat 

and Vischer, 1920); however, Wheeler (1 942), 

Havrylenko (in litt.), and Sliillink (in litt.) have 

convinced me that the ants living in the thorns 

were only fortuitous invaders. My own unpublished 

observations of A. cazlen in Chile give no hint of 

myrmecophytism with this plant. Greenhouse A. 

cazlen (from Argentinian seeds) shows thorn and 

leaf development like that of normal acacias and 

therefore will not be discussed further. The African 

swollen-thorn acacias, or “whistle-thorn” acacias 

(e.g., A. seyal, A. drepanolobium, A. formicarum, 

A. bussei), appear to have a mutualistic interaction 

with ants of the pantropical myrmicine genus 

Crematogaster (Hocking, 1970; Monod and 

Schmitt, 1968; Bequaert, 1922) . This is a clear case 

of mild convergence on the Central American system and is excluded from generalizations made in 

this paper. This restricted view should not, however, be taken to suggest that hypotheses based on 

the much better understood Central American 

system are irrelevant to the African system. 

The pantropical and subtropical mimosoid legume genus Acacia contains at least 750 species, 

though the number may be reduced substantially if 

the synonymy found in this study is representative 

of the remainder of the genus. Plants of the genus 

Acacia are primarily in areas with less than 3 meters of annual rainfall and with a well-developed 

dry season of 3-11 months duration. Throughout 

Africa and the neotropics, those species with foliar 

nectaries (probably all species) have a loose association with ants. Many genera of arboricolous ants 

(e.g., Crematogaster, Pseudomyrmex, Camponotus) 

visit these nectaries and in the process probably increase the intensity of their foraging activity on 

these plants compared with adjacent plants lacking 

nectaries. This, accompanied by similar benefits 

from parasitic Hymenoptera attracted to the nectaries, has probably been the selective force leading 

to the evolution of these nectaries (cf. Janzen, 

1966a; Bequaert, 1922). 

In a few cases, however, the ant and acacia interaction has evolved to a much higher level of complexity, that of obligatory mutualism (Janzen, 

1966a). For background information, a brief resume

of the interaction in Central America is given in 

the following paragraphs. More details can be obtained from Janzen’s (1967b) lengthy description of 

Acacia cornigera and Pseudomyrmex ferruginea in 

lowland eastern Mexico and in peripheral papers 

on ants and acacias (Janzen, 1967a, 1969a, 1969b, 

1973; Beulig and Janzen, 1969). 

A colony of obligate acacia-ants lives in the swollen, stipular thorns of one to several individual 

shrubs or small trees that have been given the subgeneric common name of “swollen-thorn acacia” 

(Janzen, 1967a). New World obligate acacia-ants 

are a behaviorally distinct group (at least 13 species: 

P. belti, P. ferruginea, P. nigrocincta, P. venefica, 

P. satanica, etc.-Janzen, MS) within the exclusively 

New World, and mostly tropical, genus Pseudomyrmex (150-200 species). Most species of Pseudomyrmex have small colonies in dead hollow stems 

above the ground in habitats ranging from brushy 

pastures to well-developed forest. The exceptions, 

besides obligate acacia-ants, are those with an obligate mutualistic interaction with the neotropical 

trees Trip 1 ar is (Pol ygonaceae) and Tach iga lia 

(Leguminosae). I have never found any of the many 

species of obligate acacia-ants as a viable colony 

living elsewhere than in a swollen-thorn acacia. 

Those rare cases where swollen-thorn acacia poplations are unoccupied by obligate acacia-ants are 

discussed in later parts of this paper. 

New World swollen-thorn acacias differ morphologically and behaviorally from other acacias in 
XUMBER 13 3 

FIGURE 1 .-General distribution of swollen-thorn acacias in Central America, based on herbarium 

records, field observations, and inferences from general vegetation. Contemporary distributions 

are usually continuous throughout the areas in black, owing to human destruction of the primary forest that previously isolated the swollen-thorn acacia subpopulations along river edges 

and in other kinds of disturbed vegetation. 

that they have: (1) large and partly hollow stipular vested by the ants-these are the protein, lipid, and 

thorns that the ants clean out through an entrance major carbohydrate sources of the ant larvae; (3) 

hole cut in one thorn tip while the thorn is still greatly enlarged foliar (petiolar) nectaries that are 

green and relatively soft; (2) modified leaflet tips continually secreting the nectar that constitutes the 

called Beltian bodies (see Rickson, 1968, 1969, 1971, primary carbohydrate and moisture source of the 

for micromorphological details), which are har- adult ants; (4) year round leaf production and 
4 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

retention by most individuals, yielding a relatively 

continuous food source for the ants; and (5) lost 

most of the chemical and structural traits that protect other acacias from most herbivores (cf. Rehr 

et al., 1973). Swollen-thorn acacias differ ecologically 

from other acacias in that in natural habitats they 

cannot survive to reproductive maturity without 

the patrolling activities of a large colony of obligate 

acacia-ants. The ants remove herbivores (primary 

insects) and intrusive vegetation by biting and 

stinging the invading pait of the plant (as do their 

relatives, Pachysinza, living in Barteria trees in 

Africa-Janzen, 1972). By analogy, the ants substitute for the chemical and physical abilities of 

most acacias to deal with their predators and competitors. The one exception to the above generalizations, A. rziddiae, will be discussed in its own species 

account. 

The species of obligate acacia-ants are not specific 

to a swollen-thorn acacia species, but rather to its 

life form. For example, A. collinsii has at least eight 

species of obligate acacia-ants living in it over its 

range from Mexico to Colombia, and all of these 

ants also live in other swollen-thorn acacias. In 

nature, one ant colony may be found occupying 

two adjacent individuals of two different swollenthorn acacia species. The only species of swollenthorn acacia that are occupied by only one species 

of ant are A. melanoceras (occupied by Pseudoinyrmex sntanica) and A. allenii (occupied by 

Pseudonzy~mex ferruginea) . They appear to be this 

way because the climate or resources will not support other ant species, rather than because the 

plants are unsuitable for other obligate acacia-ants. 

Both of these species of acacias will support other 

obligate acacia-ants in the greenhouse. 

Swollen-thorn acacias grow in natural and manmade disturbed vegetation-both primary and 

secondary succession. A. cornigera, A. hindsii, and 

A. collinsii have very successfully invaded brushy 

pastures, roadsides, and fallow fields, and may constitute a major part of the standing biomass in 

some l-5-year-old fallow regeneration. On the other 

hand, A. allenii, A. melanoceras, and A. ruddiae 

are apparently in danger of extinction because they 

do not maintain a breeding population except in 

very lightly disturbed, or undisturbed, primary rain 

forest. 

In contrast to most other acacias, swollen-thorn 

acacias originally inhabited naturally disturbed 

sites in forests-deciduous to evergreen-with 2-5 

m annual rainfall. Other acacia species usually live 

in drier sites. The ability to compete with rapidly 

growing vegetation in wetter habitats is the direct 

result of the behavior of the occupant acacia-ants. 

In pruning vines out of the swollen-thorn acacia 

crown and in killing intruding branches from 

neighboring trees, they place the acacia in a highly 

insolated microhabitat (this is, incidentally, a form 

of allelopathy by the acacia-Janzen, 1969a). Second, since the growing points of the acacia are very 

heavily patrolled (protected) by ants, they lack the 

usual structural toughness associated with protection from herbivorous insects that is found in other 

acacia shoot tips. This tenderness is associated with 

a swollen-thorn acacia rate of vertical height increment high enough to make the acacia an effective 

competitor with other plants in wet sites. Ironically, 

this high growth rate would probably not be possible in drier sites, and in drier sites the acacia would 

not be able to support the ant colony owing to loss 

of leaves during the dry season. From the wettest to 

the driest sites the species are: melanoceras, allenii, 

ruddine, cookii, gentlei, chiapensis, globulifera, 

hindsii, mayana, sphaeyocephala, cornigera, and 

collinsii. This progression also ranges from those 

with the smallest geographic ranges to the largest, 

except for A. mayana and A. sphaerocephala, which 

have minute ranges. 

It is noteworthy that some swollen-thorn acacias 

have the largest leaves of any acacia species. This 

may be useful in the cloudy weather which is more 

probable in the wetter sites than drier habitats; it 

is also “allowed” by the generally reduced problem 

of foliar water loss in wetter habitats. In the driest 

swollen-thorn acacia habitats, many of the larger 

leaves are dropped during the dry season, and then 

axillary clusters of very small leaves provide nectar 

and Beltian bodies for the ant colony. Very few 

swollen-thorns and their associated large leaves are 

produced during the dry season. 

Even though they occupy wetter habitats than 

other acacias in general, swollen-thorn acacias disappear when annual rainfall exceeds about 4 m 

(e.g., the Puerto Viejo region of northeastern Costa 

Rica) and the dry seaeon is less than two months’ 

duration. The reason for this probably centers on 

the decreasing number of sunny hours per day as 

rainfall increases. When days are overcast in rainforest areas, the amount of light reaching small 
NUMBER 13 5 

shrubs on the forest floor is very low, and it may 

be that the swollen-thorn acacia has been unable to 

convert its primitively heliophile physiology to that 

of total shade tolerance. 

Their upper elevational limit is about 800 to 

1200 m, depending on the moisture and temperature conditions at upper levels. The wetter and 

colder an elevational transect, the quicker the 

swollen-thorn acacias disappear from the vegetation 

with increasing altitude. Cold weather does not 

kill the ants on the acacias but restricts patrolling 

activities to the point where the acacia is either 

covered over by the rising vegetation, defoliated 

severely, or both. All species are found at sea level 

in at least some part of their range; their upper 

elevation limits vary widely, with A. globulifera, 

A. sphaerocephala, and A. cornigera attaining the 

greatest heights. 

In contemporary communities (disturbed by 

man), swollen-thorn acacias reproduce very commonly by stump and root suckers and only rarely 

by seedlings. In naturally disturbed sites (where 

clearing of the vegetation is not periodically repeated), however, the rare seedlings are the usual 

means of population recruitment. Though a large 

seed crop is produced annually, most seeds are 

killed by bruchid beetles, against which the ants 

offer no defense (Janzen, 196713, 1969~). There is a 

very high mortality rate of seedlings until the trees 

become occupied by an ant colony. Sucker shoots 

(often from lateral roots) are often near a large 

obligate acacia-ant colony, which simply incorporates the sucker into the colony’s territory; this is 

especially noticeable with the obligate acacia-ant 

species that have multiple queens and thus unlimited colony size (e.g., Pseudomyrmex venefica in 

A. hindsii in western Mexico north of the Rio 

Balsas basin; Janzen, 1973). An unoccupied seedling 

must wait 9 months or longer for a founding queen 

ant to grow a young colony and by then the seed- 

!ing is often well below the surrounding vegetation 

and therefore at a severe competitive disadvantage. 

Sucker shoots produce their first seed crop about 2 

years earlier than do seedlings. Adults of either 

seedling or sucker shoot origin generally live 15-35 

years. 

Swollen-thorn acacias flower during the dry season, as do so many other deciduous forest woody 

plants (Janzen, 1967c), and are pollinated by relatively unspecific pollen-collecting bees. In the 

greenhouse, swollen-thorn acacias never set fruit 

though they flower copiously. I inEer from this that 

they are obligatory outcrossers. The bees are 

occasionally attacked by a worker ant, but these 

bees are persistent in getting at the flowers, and 

the ant’s aggressiveness probably does little more 

than maximize cross-pollination by forcing bees 

to visit several trees. The small pods remain on 

the receptacle until the following dry season, 

growing large and maturing several months before the next crop of flowers. There may be 

mature pods on the tree with the flowers, but these 

are from the previous year except in the species in 

very wet sites, such as A. allenii, A. ruddiae, and 

A. melanoceras. The seeds are distributed by birds 

that either eat the pod entirely (only A. cornigera 

and A. sphaerocephala) or eat the cluster of seeds 

imbedded in a sweet moist pulp that is yellow or 

white pulp. Swollen-thorn acacias appear to be the 

only neotropical acacias with bird dispersed seeds 

(Janzen, 1969b). If it were not for bird dispersal, 

there would be virtually no survival of seeds because the bruchid beetles will kill virtually every 

seed that remains on the tree (Janzen, 1969c, 1971). 

Swollen-thorn acacias have a representative 

flowering phenology. At any given site, nearly all 

mature plants bear open inflorescences for 4-8 

weeks at about the same time each year. This 

flowering period may start anytime during the first 

2 months of the dry season, depending on the species and the area. For a given species and area the 

timing is the same from year to year. A single 

species may show quite extreme variation from site 

to site in both the absolute dates of flowering and 

the dates relative to onset of wet and dry season. In 

general, however, at least 80 percent of the subpopulations of swollen-thorn acacias flower during 

the dry season. 

In addition to the major peak of flowering shown 

by each species at each site, there are almost invariably some individual acacias that are out of phase 

with the remainder of their population. These are 

usually damaged, growing on locally peculiar drainage types (creek bottom or hill crest) or senescent. 

They are often helpful in taxonomically identifying 

the population, but are very misleading in understanding the morphology, ecology, and behavior of 

the population. Botanists have a tendency to collect 

these flowering or fruiting plants in preference to 

sterile individuals. This leads to erroneous records 
6 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

of flowering times. These individuals often set little 

or no seed, which gives a false impression of seedcrop size. Those seeds that are produced are sometimes only mildly attacked by bruchids, giving the 

false impression that seed mortality is trivial for the 

population (see Janzen, 1968, for a discussion of 

this point among plants in general). These plants 

also may bear a reduced number of inflorescences 

per axil and inflorescence-bearing branch, and these 

branches are often sparser and smaller than on 

healthy acacias. 

Swollen-thorn acacias have very thoroughly occupied most of the Central American habitat that 

looks as though it should support them; however, 

they have not yet spread into the semiarid areas of 

South America that are presently occupied by the 

species of plants that co-inhabit their Central 

American habitats. Their absence from the deciduous and semideciduous forests of Venezuela, Brazil, 

and Colombia is particularly striking. It seems reasonable to expect that these areas are not inimical 

to swollen-thorn acacias, but rather that they simply 

have not yet gotten there. A. collinsii in Colombia, 

however, must surely be a seed source for the remainder of the country. If swollen-thorn acacias 

were not such an economic pest in brushy Central 

American pastures, the obvious transplantation experiment would be well worthwhile. 

Swollen-thorn acacias have not been generally 

successful at colonizing islands. They occur on offshore islands such as those in the Gulfo de Nicoya, 

Costa Rica, in the Gulfo de Fonseca, Honduras, and 

off the eastern coast of Yucatan, Mexico. The only 

far island they have reached is Isla Providencia, 

Colombia, in the Caribbean (about 125 miles off 

the coast of Nicaragua). A. collinsii is here occupied 

by one or two species of obligate acacia-ants. In 

colonizing islands, the acacia must already be there 

when the founding ant queen arrives; however, the 

acacia probably has little chance of surviving to 

reproduction unless occupied by an ant colony. It 

will take a high rate of simultaneous colonization 

by both ants and acacias for establishment. A. 

cornigera, introduced as an ornamental into Cuba 

(and Florida) without ants, has not escaped to form 

a noticeable population. While this may be in part 

due to a lack of pollinators, the absence of obligate 

acacia-ants may also contribute. 

Since agricultural man came on the scene (at 

least since 1200-900 B.c.-Coe et al., 1967), the 

population structure of most species of swollenthorn acacia has probably been grossly altered. The 

majority of species have greatly increased their local 

density, absolute population sizes, and thoroughness 

of geographic coverage by invading fields, pastures, 

and roadsides which now connect the natural disturbance sites that occur along rivers, streams, and 

lakes. This means that the populations now observed are operating with a genotype evolved in 

slightly to grossly different habitats, and the local 

barriers to gene flow (climax forest on hills and 

ridges) have in great part been removed. 

Present-day populations of swollen-thorn acacias 

show distinct clinal variation in traits such as leaflet 

size, thorn size and shape, life form, seed-pod 

morphology, and Beltian body traits. Rather than 

being solely the result of extreme sensitivity to 

small linear environmental changes, these may be 

the consequence of the recently established genetic 

continuity between previously isolated populations 

on different drainage systems. 

Humans have probably not severely altered the 

gross geographic range of swollen-thorn acacias by 

their activities. They have, however, served as 

efficient long-range-dispersal agents. Fruits of A. 

cornigera are eaten by Central American indians 

and, with modern bus transport being commonplace, seedlings are sometimes found near bus stops 

several hundred miles from a known acacia population. Further, cattle are often trucked long distances 

in Central America, and cattle eat the pods of A. 

cornigera and others. There are pockets of A. 

cornigera around cattle corrals scattered through 

the northern part of the Yucatan Peninsula; the 

natural vegetation at these sites is normally occupied by A. collinsii (though A. cornigera appears 

native to local wet sites in Yucatan). 

General Comments on Swollen-Thorn Acacia 

Taxonomy 

A close examination of the taxonomic status of 

swollen-thorn acacias is warranted by the fact that 

(1) as many as 25 percent of the swollen-thorn 

acacias in some herbaria are incorrectly determined, 

(2) there are at least 32 synonyms for the 10 previously described species, (3) swollen-thorn acacias 

have never been treated with monographic thoroughness (though there are short discussions in 

Safford, 1910, 1914, 1915, 1922; Rudd, 1964; 
NUMBER 13 7 

Schenck, 1913; Janzen, 1967b), and (4) their species-level taxonomy has become so confused that it 

is currently nearly impossible to generate a detailed 

discussion of the mutualistic interaction in a comparative and evolutionary framework. 

Examination of the distributions of the type 10- 

calities on the range maps of each species of swollenthorn acacia leads one to suspect what is shown by 

examination of the types, namely that most synonyms are the result of the taxonomist possessing a 

herbarium sheet from two widely separated portions 

of a species population but having no information 

on the clinal changes across the connecting part of 

the population. Further taxonomic difficulties were 

generated by the greatly differing thorns that may 

be found on a single plant. There is no evidence 

that we are dealing with many sibling species but 

rather that there is extreme geographic variation of 

several conspicuous vegetative traits. Finally, the 

more conservative (reliable) vegetative traits of 

swollen-thorn acacias have not generally been used 

as taxonomic characters. 

The early parts of this study emphasized the 

location and delimitation of populations of swollenthorn acacias in Central America. Once these populations were understood geographically and 

morphologically, then the type specimens of swollen-thorn acacias were associated with them in the 

herbaria. Since virtually all the variation encountered in this study was clinal, and since the clines 

showed little or no concordance with the subpopulations of a given species (being contiguous almost 

without exception), there is no basis for the designation of infraspecific Latin names. Fortunately, the 

breaks between species are clearly delimited by 

many morphological, behavioral, and ecological 

traits in the numerous areas of sympatry. 

The species discussed here are clearly morphospecies, but all circumstantial and ecological evidence indicates that they are independent 

evolutionary units that do not normally exchange 

genetic information. The various species grow side 

by side over much of their ranges with almost no 

obvious hybridization, though ant-acacias may introgress with nonant-acacias on occasion (e.g., A. 

hindsii X A. pennatula (Schlect. and Cham.) Benth. 

and A. hindsii X A. cochliacantha Humb. and 

Bonpl. in western Mexico). Those species that have 

no range overlap with other swollen-thorn acacias 

are presumed to be distinct species because of the 

P 

BRACTEOLE 

PULVINUS 

FIGURE 2.-Swollen-thorn acacia terminology. 

many morphological and ecological differences between them and other acacia populations. 

The total morphological variation within the 

populations of some of the widely distributed species (e.g., A. cornigera, A. collinsii, A. hindsii) is 

much greater than that within pairs of closely 

related species (e.g., A. allenii and A. melnnoceras, 

A. nzayana and A. cornigera). This, plus the impossibility of designating what might be called a 

“species-level” trait, prevents a uniform criterion 

for the designation of “species.” 

Swollen-thorn acacias are readily determined to 

species in the field, either in vegetative or sexual 

phases. In fact, it is easier in the field than from 

herbarium specimens. Because of this, only occasional voucher specimens were collected in this 

study. Since swollen-thorn acacias are generally 

recognizable even from a fast-moving car, continuous distribution records along roads were often 

obtained in this study and are plotted on the 

distribution maps as continuous data rather than as 

“spot localities” as are usually taken from herbarium specimens. In most parts of Central 

America, the total absence of swollen-thorn acacias 

for more than 5 km along a road or watercourse 

means that they are absent for reasons other than 

persistent attempts by humans to remove them. 

Their absence within areas of secondary vegetation 

of a few hectares in extent may be due to nonhuman 
FIGURE 3.-Representative leaves subtended by type A swollen thorns: A, B, Acacia cookii; c, A. 

hindsii; D, A. gentlei; E, A. ruddiae; F, A. globulifera; G, A. rnelanoceras; H, A. collinsii; I, A. 

cornigera. (All photographs taken to same scale; the leaf of A. cookii in upper left-hand corner 

is 40 cm long.) 
FIGURE 4.-Representative leaves subtended by type A swollen thorns: A, Acacia allenzi; B, A. 

maynna; c, A. cliiapensis; D, A. sphaevocephala; E, A. chiapetzsis. (Scale as in Figure 3.) 
10 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

TABLE 1.-Meristic traits of two representative fully developed leaves subtended by type A thorns on 

lateral branches of occupied adult swollen-thorn acacias (mean for large samples are not presented since 

this would be a study in itself; these two leaf exemplars are judged to be representative by my own observations) 

- - - Total Total Total Length X X X Total Number of Number of 

Species and thorn length 

(mm) 

allenii (55+56) . . . . . . . , 

allenii (50 + 49) , , . , , . , , 

hindsii (41 +42) . . . . . . . . 

hindsii (36+38) . , . . , , . . 

cookii (51 + 48) . . . . . . , , 

cookii (67 + 71) , , , , , , , . 

mayana (51 +48) . . . , , . . 

collinsii (46 + 48) . . . . . . . 

collinsii (42+43) , . . . . , . . 

gentlei (48 + 48) . . . . . . , 

gentlei (38+39) . . . . . . . . . 

globulifera (43+44) . . , . . 

globulifera (39+33) . . . . . 

cornigera (45 +47) , , , . . , . 

cornigera (43 + 42) . . . . . . . 

ruddiae (no thorn) . . . . . . 

ruddiae (no thorn) . . . . . . 

chiapensis (48+43) , . . . , . 

chiapensis (79+75) , , . , . , 

sphaerocephala (63+63) 

sphaerocephala (54+52) , 

melanoceras (36+34) . . . . 

melanoceras (32 + 45) , , . . 

- leaf number number longest length length width pinnules nectaries nectaries 

ength pinnae pinnules pinna pinnae pinnules pinnules on longest on rachis on petiole 

(mm) (mm) (mm) (mm) (mm) pinna 

383 84 1170 98 70.7 10.53 3.08 47 19 4 

40 1 

178 

150 

404 

367 

225 

101 

101 

100 

106 

252 

228 

110 

81 

205 

258 

288 

190 

155 

149 

255 

237 

38 

42 

42 

96 

58 

20 

20 

22 

14 

16 

48 

52 

20 

12 

38 

74 

74 

42 

26 

24 

48 

46 

1‘630 

1924 

1912 

12242 

7013 

1049 

588 

632 

784 

988 

4007 

4080 

848 

536 

3200 

8180 

7644 

2955 

952 

1072 

2134 

1592 

121 

47 

42 

69 

87 

100 

42 

47 

65 

58 

62 

62 

54 

55 

67 

62 

63 

48 

54 

53 

49 

35 

81.1 

36.3 

35.6 

61.5 

77.5 

72.5 

33.6 

36.6 

55.0 

52.1 

49.2 

46.9 

43.2 

50.5 

61.4 

47.6 

46.7 

39.2 

36.7 

46.1 

38.9 

28.8 

10.30 2.94 63 

4.46 1.58 56 

4.17 1.26 52 

4.04 0.60 140 

6.82 1 .oo 136 

10.90 1.93 66 

6.39 1.89 34 

8.18 2.22 38 

8.45 1.98 62 

8.53 1.93 64 

5.51 1 .oo 89 

5.14 1.00 96 

7.09 1.61 52 

7.26 1.82 48 

5.77 1.18 88 

2.87 0.80 144 

2.83 0.77 130 

4.46 1 .oo 88 

6.34 1 .go 46 

6.28 1.76 52 

4.90 1.12 54 

5.46 1.10 42 

28 

21 

21 

54 

28 

9 

1 

0 

4 

8 

14 

1 

4 

20 

37 

35 

0 

0 

0 

25 

24 

a 

‘3 

0 

0 

0 

1 

1 

4 

1 

2 

4 

2 

1 

1 

2 

0 

4 

14 

1 

I 

18 

21 

8 

factors, but more likely is due to repeated cutting 

immediately followed by burning. Relative changes 

in density are especially susceptible to human activity. It is therefore very difficult to determine 

what climates and/or habitats are “best” for 

swollen-thorn acacias. Cutting, burning, and grazing patterns up to at least 20 years previous may 

grossly influence both density and presence of 

swollen-thorn acacias. There are even cases of heavy 

insecticide use, where the ants, and apparently most 

other insects, have been killed and the unoccupied 

acacias are relatively healthy (e.g., A. hindsii in the 

Leon area of western Nicaragua, along cotton field 

edges). 

Some of the herbarium specimens collected during 

this study have been deposited in the National 

Museum of Natural History of the Smithsonian 

Institution (US), Gray Herbarium (Harvard) (GH), 

New York Botanical Garden (NY), University of 

California at Berkeley (UC), Missouri Botanical 

Garden (Mo), Field Museum of Natural History (F), 

University of Michigan (MICH), and University of 

Mexico (MEXA). The majority have been deposited 

as intact series at the Field Museum. 

Despite their spectacular association with ants, 

swollen-thorn acacias have been poorly collected 

and generally receive little attention in regional 

floras. They are not famous for medicinal uses, 

which is not surprising since the ants are their 

“chemical” defense system. Collectors consistently 

avoid them (or more properly, avoid their ants). 

Those parts of the plants that are collected tend to 

be old foliage and branches, which are poorly 

patrolled by the ants and often unrepresentative of 

the foliage of the plant as a whole. The branches 

with well-developed swollen thorns, especially those 

with very large thorns on short laterals, are almost 

entirely missing from herbaria, presumably because 

they do not press easily and usually add a large 

number of ants to the plant press. This produces 

the illusion, when examining herbarium material, 

that the thorns of swollen-thorn acacias are gen- 
NUMBER 13 11 

FIGURE 5.--Shoot tip from end of an elongating healthy 

branch: A, Acacia coo& frorn the Cardenas, Tabasco, Mexico, 

region; B, A. allenii from the Rincon, Osa Peninsula, Puntarenas Province, Costa Rica, region. 

erally small, monomorphic, and without much 

geographic or intraplant variation. 

Wheeler (1942) reviewed the generally undistinguished presence of swollen-thorn acacias in the 

literature prior to 1930. Acacia cornigera was the 

first to be described (Linnaeus, 1753) as Mimosa 

cornigera, but since it was based on a European 

greenhouse specimen, the ant interaction was not 

discussed at that time. The last species to be described in the current literature, A. mayana in 

1937, is the rarest and most restricted of the swollenthorn acacias. The two species described in the 

present paper (A. allenii, A. ruddiae) had been 

collected earlier, but the herbarium specimens were 

misidentified. 

Herbarium specimens are often misleading in respect to the ant-acacia interaction. There is generally no way of knowing if the specimen is from a 

heavily occupied acacia, one that has recently lost 

its ant colony, or one that was never occupied. The 

presence of ant-entrance holes in the thorns are not 

reliable indicators of occupation because founding 

queens may have made them and because, once 

made, they last as long as the thorn (2 or more 

years). Occupied acacias (those with an ant colony 

large enough to patrol the tree) have fully developed lateral branches, short lateral branches with 

very large thorns, inflorescence-bearing branches, 

etc. Unoccupied ones have stunted branches with 

short internodes, smaller leaves, fewer inflorescences 

at times only partial damage and shading; directly, 

they are not a product of the ants. 

From 60 to 95 percent of the mature swollenthorn acacias of each species examined in this study 

were occupied by obligate acacia-ants. The exceptions are A. ruddiae, which is never occupied by 

obligate acacia-ants, and A. sphaerocephala. Where 

the latter species grows on the dunes south of Veracruz, Veracruz, Mexico, it is unoccupied within 

range of heavy wind and salt spray on leading 

dunes, but tenanted by Crernatogaster. Those plants 

tenanted by other ants (they often use the hollow 

thorns for nest sites as they would use hollow 

branches on other trees) are unoccupied from the 

plant’s viewpoint. The statements about vegetative 

characters in this paper are drawn from healthy 

and occupied acacias. Numerically, the majority of 

any swollen-thorn acacia population in repeatedly 

disturbed sites is made up of unoccupied seedlings 

and sucker shoots, but these are almost never noticed or collected because they are less than 50 cm 

tall and below the general weed canopy (mature 

acacias are usually emergent). These small plants 

are often much more difficult to identify to species 

than are the adults, owing to their convergence in 

leaf and thorn characteristics. 

The swollen-thorn acacias did not escape from 

Britton and Rose’s (1928) attempts to split the genus 

Acacia into more manageable subunits, The genera 

Tauroceras and Myrmecodendron were proposed 

but are not retained in the present study because 

they represent only one of the many ways the 12 

species could be arbitrarily grouped, and thus add 

little to the understanding or ease of manipulation 

of the morphological affinities within the swollenthorn acacias. 

Perhaps of more importance in subgeneric taxonomy, the evolutionary affinities of the 12 species 

are not clear in most cases, and their speciation 

may have involved some very interesting complications. Despite their possession in common of many 

unique vegetative traits-Beltian bodies, swollen 

thorns, etc.-the swollen-thorn acacias appear to be 

a polyphyletic group within the genus Acacia. Pairs 

or triplets of species appear to be more similar in 

the majority of their traits to acacia species not 
12 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE &-Variation in nectary morphology on mature A. globulifera plants (and see Figure 

7c): A, maximal number of petiolar nectaries, as occasionally found on old trees in tufts of 

leaves not subtended by swollen thorns (Honduras, site 26); B, maximal condensation of the 

petiolar nectary area, as characterizes leaves from type A thorns at high (cold) and dry areas 

(Honduras, site 28) ; c, maximal attenuation of the petiolar nectary area, as characterizes leaves 

from type A thorns on rapidly growing 2-4-year-old trees in riparian (wet) sites (Honduras, 

site 26). 

Acacia collinsii sometimes has nectaries similar to Figure B, and A. gentlei sometimes has 

nectaries similar to Figure C (and see Figure 7). 

involved in mutualism with ants than they are to 

other swollen-thorn acacias. Even the designation 

of subgenera based on these small groups seems to 

have little usefulness (e.g., Tauroceras would contain A. cornigera, A. sphaerocephala, and A. 

may an a). 

The possibility of a polyphyletic origin for 

swollen-thorn acacias is especially noticeable if only 

the reproductive parts are compared. Three explanations are available. The first, and least likely in 

view of dissimilarity of sexual reproductive parts, 

is that a single swollen-thorn acacia evolved, and 

by classical speciation patterns split into the 12 

modern populations (species) discussed in this paper. This carries the implausible implication that 

floral and fruit morphology are as evolutionarily 

unconservative as the vegetative parts of the plant, 

and that the similarity of the flowers and fruits of 

swollen-thorn acacias to those of nonant-acacias is 

simple convergence. The second possibility is that 

each cluster of similar swollen-thorn acacias had an 

independently evolved, swollen-thorn acacia parental species. Here, the vegetative similarity between 

the clusters would be convergence selected for by 

the requirements of the mutualistic interaction. 

This also seems unlikely since it requires that a 

complex interaction must have arisen several times 

independently. The third hypothesis, and the one 

that I favor, is a combination of the two above by 

the mechanism of introgression. This would yield 

a reticulate phylogeny. Starting with one species of 

swollen-thorn acacia, new acacia species become 

part of the system by introgression of the set of antacacia traits into their genotype. The outcome is 

two species of swollen-thorn acacia, one of which 

(the donor) is probably not much changed while 

the other may change its ecology considerably since 

it now possesses a very different, and in some ways 

more effective, set of defenses against herbivores 

and competitors. 
NUMBER I3 I3 

FIGURE 7.-A, Representative petiolar nectary on a leaf subtended by a type A thorn of A. collinsii from the moister end of the species’ range; the number of nectaries may range from 1-4 

and there never are rachis nectaries (Mexico, site 15). B, Representative petiolar and rachis 

nectary of A. gentlei on a leaf subtended by a type A thorn (Guatemala, site 21). c, Representative petiolar and rachis nectary of A. globulifera on a leaf subtended by a type A thorn (Guatemala, site 13). 
14 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE %-A, Representative petiolar and rachis nectaries on leaf from type A thorn of A. hindsii; the number and size of rachis nectaries is variable (Mexico, site 17). B, Representative 

petiolar nectary of A. cornigera; on rare occasions there are two petiolar nectaries and a rachis 

nectary (Mexico, site 6). c, Representative petiolar and rachis nectaries on leaf from type A 

thorn of A. ruddiae; nectaries vary greatly in height (Guatemala, site 8). 

This type of “speciation” (note that there is no 

net increase in the number of species of Acacia) 

will almost certainly result in a range extension for 

the ant-acacia interaction, and perhaps lead to contact with, and subsequent incorporation of, other 

nonant-acacia species. It appears to be occurring 

now between A. hindsii and A. pennatula (see later 

discussion of hybrids). This process, coupled with 

the usual kind of speciation, could easily lead to 

the present 12 species. 

Materials, Methods, and Terminology 

A bit of jargon has evolved in the discussion of 

swollen-thorn acacias and needs brief review here 

(also see Janzen, 196713). An “occupied” swollenthorn acacia is one that has an ant colony large 

enough to patrol its surface with sufficient intensity 

such that the acacia will gain or remain in moderate 

to excellent health. An unoccupied acacia may 

therefore have founding queen ants in its thorns or 

have a large colony of an unprotective ant species 

tenanting its thorns. The “basal circle” is that 

circular patch of bare ground directly under a 

swollen-thorn acacia that results from the ants 

attacking seedlings and sucker shoots beneath the 

acacia. A “multiple-queen” ant colony is of one of 

the species that has egg-producing queens in many 

of the tree’s thorns, in contrast to having only one 
NUMBER 13 15 

queen per colony. Multiple-queen ant colonies tend 

to be associated with swollen-thorn acacias that 

naturally occur as dense thickets, such as A. hindsii 

in western Mexico. 

The stipular thorns of Neotropical swollen-thorn 

acacias are not galls in any sense of the word. Incidentally, despite the caution displayed by Hocking 

(1970), the enlarged thorns of African acacias are 

not galls either, as shown by their full development 

in North American greenhouses. ‘‘Type A” thorns 

are those more ordinary-appearing thorns to be 

found on long, lateral branches and on the central 

axis; “type B” thorns are the large and often convoluted thorns found on greatly shortened lateral 

branches. See Figure 2 for other morphological 

characteristics. 

For precise localities, the lists under each species 

should be referred to, rather than to the distribution maps. Localities are listed under the state or 

province containing a reference town. The projected ranges on the distribution maps are just 

that; they are based on my intuition and the general ranges of other plants from the same habitats. 

They are probably overall correct in general, but 

cannot be expected to be accurate in detail. The 

key collection localities are given in the distribution 

figures as sites or as continuous distributions along 

roads (solid broad lines). Where my own distribution records overlap those obtained from herbarium 

specimens, to avoid redundancy I usually have not 

given the herbarium data. The site numbers are 

unique only within a species; for example, site 16 

for A. cornigera is not the same locality as site 16 for 

A. hindsii. While some data are given as to the 

reproductive condition of the plants on the dates of 

observations or collection (fl = inflorescences, gp = 

immature fruits, mp = mature fruits), little reliance should be placed on these data in determining 

sexual timing owing to the collectors’ habit of 

seeking out plants in flower and fruit but out of 

phase with the remainder of the population. 

All scales shown in photographs are in millimeters unless the caption indicates otherwise. 

In the terminology of this paper, “isotypes” are 

collections made from the same plant at the same 

time, while the “holotype” is only that herbarium 

sheet with the type on it. “NC” is used to signify a 

plant examined in the field but not collected. While 

Safford’s descriptions sometimes were not in Latin, 

they were published before 1 January 1935 and are 

still recognized as valid in accordance with Article 

36 of the International Code of Botanical Nomenclature (1972). 

General Morphological Traits 

The leaves of swollen-thorn acacias differ from 

those of other Central American acacias primarily 

in being larger overall and having larger leaflets; 

in short, the leaves extend far past the tips of the 

swollen type A thorns and can be easily removed 

from the tree without contacting the thorns. On 

any given tree the leaves vary strongly in size as a 

function of season (smaller leaves being produced 

during the dry season) and part of the tree (smaller 

leaves being subtended by type B thorns). However, 

if only the leaves produced during the rainy season 

and subtended by fully developed type A thorns 

on lateral branches from healthy trees are considered (Figures 3 and 4, Table 1) , there are diagnostic interspecific differences in leaf and leaflet size 

and shape. Not surprisingly, those species from the 

driest habitats have the smallest leaves (e.g., A. 

collinsii, A. cornigera, A. hindsii). There may be 

considerable variation within a species over its 

range on the parameter of leaf size. In Figure 3, a 

leaf from A. cookii at the northern and dry end of 

its range in the Cardenas, Tabasco, Mexico, area 

(Figure 3~) may be compared with a leaf from the 

wet end of the range in the Puerto Barrios, Guatemala region (Figure 3~). One species, A. chiupensis, 

shows strong variation at a given site as well, as 

earlier discussed for the Temascal, Oaxaca, Mexico, 

area (Janzen, 1967b); this variation (Figures 4c 

and 4E) is probably an expression of introgression 

of Acacia macracantha and A. cornigera genes into 

the A. chiupensis population in this area (and see 

Rehr et al., 1973). 

The shoot tips on elongating branches of swollenthorn acacias are unprotected physically (as well 

as chemically) when compared with other plants. 

The shoot tip of A. allenii in Figure 5~ is representative of swollen-thorn acacias in being free of 

hairs or defensively sticky compounds and in having 

the newly expanding leaves quite separate and exposed. They are extremely brittle and easily broken, 

reminding one of the inner shoot of a celery plant. 

The shoot tips of A. cookii (Figure 5A) and A. 

ruddiae are more like those of nonant-acacias, with 

the new leaves clasped tightly around each other 
16 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 9.-Ontogenetic development of nectaries of A. collinsii: A, characteristic seedling leaf 

produced at about the 20th node and often not subtended by swollen thorns; B, leaf subtended 

by type A thorn on juvenile but well occupied plant; c, leaf subtended by type A thorn on 

relatively short lateral branch in crown of mature acacia. 

and covered with a dense pile of short stiff hairs. 

In the northern part of its range, A. cookii is sometimes unoccupied, and in this area the difference is 

more pronounced than in the Puerto Barrios, 

Guatemala, region where the shoot tips appear 

more like those of a regular swollen-thorn acacia. 

The extrafloral nectaries on the petiole 

and rachis of swollen-thorn acacia leaves are 

one of the most useful and reliable characters for 

species-level determinations. While extrafloral nectary morphology may vary strongly in detail from 

area to area for a few species (the most extreme 

case being A. globulzfeia, Figure 6), for the majority of species there is little geographic variation 

and at any given site the morphology is quite constant; it may be the only way to distinguish between 

young plants of species that are easily separated as 

adults on the basis of thorn shape. Since the entire 

function of the extrafloral nectaries appears to be 

physiological-excreting sugar for the ant colonythey are not under strong pressure for adaptive external morphologies; much as with the thorns, I 

suspect that numerous different kinds of external 

morphology may be equally functional and that the 

interspecific variation is primarily due to interspecific differences in the adaptive peaks from which 

the different swollen-thorn acacia genotypes were 

derived. The extrafloral nectaries in Figures 7 and 

8 and in Table 1 are representative of the gamut 

of interspecific variation. As a seedling grows, there 

is a progression of extrafloral nectary development 

(young sucker shoots from cut stumps tend to have 
NUMBER 13 17 

FIGURE ~O.-A, Cross section of a 5- 

year-old main trunk of A. hindsii 

(Mexico, site 17). 8, Conspicuous 

lenticels on 6-month-old branch of 

A. ruddiae (Costa Rica, site 5). 

FIGURE 1 1.-Representative bark topography of 2-5-year-old main trunks of swollen-thorn acacias: A, Acacia cornigera (Mexico, site 26) ; B, A. gentlei (Guatemala, site 11); c, A. chiupensis 

(Mexico, site 9) ; D, A. hindsii (Mexico, site 15) ; E, A. ruddiae (Costa Rica, site 5) ; F, A. allenii (Costa Rica, site 5). 
18 

Species 

mayana , , . . , . . , . . . 

SMITHSONIAN CONTRIBUTIONS TO BOTANY 

Knobby Thorn 

projection shape in 

Thorn on inner cross 

surface External thorn wall section General thorn 

contour pubescence at base near base configuration 

smooth c+ none present elliptical flat V that is bent downward 

slight 

ridges 

smooth 

smooth 

with 2 sharp 

elongate 

projections 

absent round to 

slightly 

elliptical 

slightly in larger thorns; 

never recurved around stem 

gentlei . . . . . . . . none flat V, slightly distorted and bent 

irregularily in the plane of the 

V; thorn may be closely appressed 

to stem longitudinally and partly 

bent around it 

,... 

,... 

..., 

.... 

globulifera . . , . 

melanoceras . . . 

cookii . . . . . , . . 

absent round flat V, slightly to strongly curved 

downward; never recurved 

around stem 

flat V, slightly curved downward 

in larger thorns 

heavily warped U, asymmetrical 

with one side or both twisted 

around the stem partly or fully; 

asymmetry highly regular in 

pattern 

flat V, tendency for one thorn half 

to be larger than the other; never 

recurved about stem 

none 

smoothed 

longitudinal 

ridges 

smooth 

smooth c

slight ridges 

sometimes 

smooth 

smooth 

smooth 

none 

heavy 

present round to 

elliptical 

absent round 

ruddiae very light absent round to 

slight 

ellipse 

chiapensis .... round 

.... 

,... 

none absent flat V that may be bent slightly 

downward in very large type B 

thorns 

flat V; never recurved around the 

stem 

flat V or U, bent upward or downward to various degrees; type B 

thorns often heavily recurved 

around stem, at right angles to it 

sphaerocephala 

cornigera . . . . . 

none 

none 

absent 

absent 

round 

round to 

slightly 

elliptical 

collinsii none usually 

present 

smooth 

smoothed 

longitudinal 

ridges 

smooth to 

faint 

smoothed 

longitudinal 

ridges 

round to 

elliptical 

to very 

elongate 

and thin 

elliptical 

round to 

flat V that may be very open and 

curve around the stem at various 

angles 

allenii . . . . , 

hindsii . . . . 

flat V; never recurved around the 

stem 

none present 

none absent elliptical to 

very elongate 

and thin 

flat V; never recurved around the 

stem 
NUMBER 13 

on adult 

plant over 

entire range 

brown, nearly 

black; trace 

of red on 

green thorns 

dark brown, 

nearly black 

to ivory 

white; bicolor 

beige + ivory, 

brown+ ivory 

ivory; darker 

thorns with 

reddish cast 

ivory to beige 

to black 

very dark 

brown to 

very dark 

brown, nearly 

black; no 

trace of red 

on green 

thorns 

red to brown 

to very dark 

brown 

19 

foliar nectaries and other leaf traits essentially 

identical at the qualitative level to those of the 

adult crown). A characteristic progression is displayed in Figure 9; in general, with increasing age 

of the plant, the number of nectaries increases, their 

size increases, the distance between the first pair of 

pinnae and the thorn decreases, and nectar flow 

increases. 

Swollen-thorn acacias are more “treelike” than 

other Central American acacias in that they tend 

to have more of the growth concentrated in the 

central axis (see figures in Janzen, 1967b). The 

wood is harder than that of most fast-growing 

tropical trees and generally displays obvious annual 

growth rings (Figure 10~); the slower wood production coincides with the dry season and, in areas 

with almost no dry season, the rings are much more 

difficult to see if they are present at all. The tree 

bark (Figure 11; Table 5) is thin and often greenish; presumably considerable photosynthesis occurs 

just under the bark. There are well-developed 

lenticels on young branches (Figure ~OB), and many 

of these persist on trunks 2-6 years old (e.g., Figures 

The swollen thorns on adult acacias are very 

distinctive at the species (and sometimes localpopulation) level, but the differences are difficult 

to quantify and incorporate in a dichotomous key. 

Tables 2 and 3 attempt to organize some of these 

differences, but for a good feeling for the differences 

the reader must also compare the thorn photographs 

associated with each species account. Thorn color 

is perhaps one of the most useful diagnostic specieslevel traits at a given site, but varies strongly with 

location and the plant’s history. Damage to green 

thorns may produce premature darkening of the 

llA, C, E, F). 

TABLE 3.- Color of swollen thorns for seasoned and undamaged types A and B thorns from living 

branches on occupied adult swollen-thorn acacias 

1 Thorn color Usual color Color at any Color among 

Species 

allenii . , , . . . . . 

cornigera 

collinsii 

hindsii 

rnelanoceras . . 

gentlei 

.. 

change from one site 

50 cm to among 

adult plant adult plants 

none no variation 

observed 

beige to dark beige+brown; 

brown; white brown+very 

to ivory dark brown; 

ivory+ brown 

beige to beige + brown; 

brown beige + ivory 

beige to beige + black; 

black; white ivory+ beige; 

to ivory; brown + black 

beige to 

brown 

none no variation 

observed 

brown to red + dark 

reddish; brown 

light brown 

to dark brown 

seedlings Color clines 

over entire where obvious 

range 

as adult plants in full sun have slightly 

browner thorns than those in full 

shade 

white to 

brown, usually 

beige 

ivory in dry and hot lowlands to very 

dark brown in wet upper elevations; 

ivory in dry lowlands to brown in 

wet lowlands 

white to none obvious 

beige to 

brown 

ivory to beige, lightest thorns in driest sites but black 

thorns there also; black thorns most 

common at low elevations, light at 

high elevations 

usually brown 

as adult none obvious 

reddish brown none obvious 

to brown 
20 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

TABLE 3.-Color of swollen thorns for seasoned and undamaged types A and B thorns from living 

branches on occupied adult swollen-thorn acacias-continued 

Species 

globulifera . . . . 

chiapensis 

ruddiae 

mayana , . . . . . . 

sphaerocephala . 

cookii 

Thorn color Usual color Color at any Color among 

on adult change from one site seedlings Color clines 

plant over 50 cm to among over entire where obvious 

entire range adult plant adult plants range 

brown to bicolor brown + brown to bicolor brown+ none obvious except that wet sites 

olive to ivory; brown black to bi- ivory; brown 

bicolor brown to white; color brown+ with white 

ivory and white to black ivory thorn tips 

ivory+ black 

very dark gray 

reddish cast 

to green thorns 

black; reddish 

cast to green 

thorns 

brown (seedling 

unknown) 

lark brown to none 

dark brown to none 

very dark none 

lark brown to none 

ivory: bicolor 

brown + beige, 

black or very 

dark brown + 

ivory; yellow 

dark red 

lark brown to none 

no variation dark brown to 

observed very dark 

gray 

no variation dark brown to 

observed black 

no variation as adult? 

observed (only (seedling 

one plant unknown) 

known from 

each site) 

brown to usually cream 

ivory; bicolor to beige 

dark brown + 

ivory to yellow 

dark brown to as adult 

dark red 

appear to have the darkest thorns 

none obvious 

none obvious 

none obvious 

darkest in wettest sites, most strongly 

bicolor inland and solid color near 

coast 

none obvious 

thorn wall. The death and drying (seasoning) 

of the thorn wall proceeds from tip to base, often 

resulting in temporarily bicolor or tricolor thorns; 

in some areas, however, all thorns are bicolor irrespective of age. The thorns of all species turn lighter 

with age and bleaching by the sun. There is likely to 

be some adaptive significance to thorn color in the 

context of optimizing the internal thorn microclimate for the ants. In addition to detailed swollenthorn morphology, the placement of the thorns on 

the branches varies from species to species; when 

looking down the long axis of a branch, the thorns 

either radiate out in all directions or lie in two 

parallel planes, one on each side of the branch. This 

appears to be a by-product of leaf placement, rather 

than being adaptive in the context of the thorns 

themselves. In general, the species of dry regions 

tend to have small leaves-and thorns-radiating 

outward from all sides of branches that are oiten 

more vertical than horizontal (e.g., A. collinsii), 

while those of wetter areas often have the leaves in 

a more or less horizontal plane from more horizontal branches. There are conspicuous differences 

among the species as to the proportion of leaves 

lacking swollen thorns (as in Figure 10~) but quantitative data are not available on this parameter. This 

parameter is also greatly confused by strong variation in swollen-thorn production as a function of 

tree age and health; in general, seedlings and senescent trees produce the fewest swollen thorns, and 

leaves produced during the dry season often lack 

swollen thorns. Though not discussed here, there is 

moderate interspecific and intraspecific variation in 

thickness of the thorn wall, thorn-wall hardness, 

longitudinal fiber orientation in the thorn wall, 

insulating quality of the thorn wall, point of attachment of the leaf to the thorn, ease of thorn removal, 

and rates of natural thorn shedding. 

Healthy swollen-thorn acacias produce large numbers of bright yellow inflorescences during a short 

part of the dry season. They are either elongate 

cylindrical (Figure 12) or spherical (Figure 13) and 
NUMBER 19 21 

FIGURE 12.--Swollen-thorn acacias with elongate inflorescences: A, Acacia cornigera (Honduras, 

site 63) : B, A. collinsii (Mexico, site 15) : C, A hindsii (Mexico, site 16). 

composed of many individual florets (Table 4). 

Pollinators-pollen-gathering bees-generally move 

around over the surface, scraping off pollen with 

their legs; there is no evidence of nectar production. 

Those inflorescences that are pollinated are retained with their very small and immature pods 

through the following rainy season, but only a small 

fraction of the inflorescences are retained. Presumably the plant produces a great number of inflorescences to attract pollinators, and then sheds what 

it does not have sufficient energy to mature. A. 

cornigera and A. collinsii have produced large 

quantities of flowers in greenhouses at the University of Kansas and the University of Chicago. 

Swollen-thorn acacias differ from all other Central American acacias (and probably from the remainder of the genus) in having bird-dispersed seeds 

(Janzen, 1969~). The seeds are enclosed in a sweet 

white to yellow pulp or aril, and in all species except the A. cornigera, A. sphaerocephala, and A. 

mayana group the pod opens to expose the pulp 

when ripe (Figure 14; Table 5). Presumably this 
22 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 13.-Swollen-thom acacias with spherical inflorescences: A, Acaciu globulifera, florets 

unopened; two immature pods on receptacle on far right (Honduras, site 28); B, A. melanoceras 

(Canal Zone, site 6); c, A. allenii (Costa Rica, site 2). 
NUMBER 13 23 

Species 

collinsii . . . , , . . . 

Distance Range Infl. In$ 

Znfl. Znfl. Range Range Number from sexual peraxil bearing 

General max. max. inp. Peduncle peduncle florets bracteole branchlet of axils per 

shapeof diam. length length length length this toinfl. length sexual sexual 

inflorescence (mm) (mm) (mm) (mm) (mm) infl. base (mm) (cm) branchlet branchlet 

elongate 4 17 14-20 12 11-13 227 10 5-14 3-6 5-8 

cylinder 

mayana , . , . . . . . 

gentlei . . . . , . . 

globulifera . . . . . 

melanocerac . , . . 

cookii . . . , . . . . . 

terminally 

tapered 

cylinder 

cylinder 

elongate 

globular 

globular 

globular 

ruddiae . . , . . . . . 

chiapensis . . . . . 

sphaerocephala . 

hindsii 

cornigera , , . . . . 

. . . . . . . . 

allenii . . . . . . . . . 

3.5 

4.5 

3.5 

5.0 

4.5 

4.0 

3.8 

4.0 

4.0 

5.5 

5.5 

? 

5 

5 

9 

6.5 

6 

6 

6.5 

6 

7 

7 

6 

6 

8 

9 

5 

5 

globular 

globular 

nearly 

globular 

cylinder 

cylinder 

tapered 

tylinder 

elongate 

slightly 

globular 

17.5 I, 

26 22-26 

23 20-25 

30 25-34 

46 40-50 

23 20-24 

21 

28 26-28 

29 

5 4-5 

5 11 

? ? 

5 45 

5 

9 9-10 

6.5 6-7 

6 5.5-6.5 

6 

6 

11 10-14 

10 

27 25-36 

22 15-23 

n 

w 

I, 

I1 

6.5 5.5-7.0 I, 

I, 

26 

28 

I1 

I1 

5 4-5 

5 It 

10 

16 

8 

3 

10 

14 

8 

14 

15 

6 

12 

12 

3 

6 

37 

8 

9 

9 

18 

20 

12 

12 

20 

20 

5 

6 

3 

7 

I, 

14-18 

13-17 

1-4 

5-12 

8-12 

10-15 

6-12 

10-12 

3-6 

26-38 

8-18 

6-10 

16-20 

8-12 

A 

,, 

I1 

I, 

I1 

I1 

,I 

1422 

1-10 

11 

I1 

3-6 

,I 

222 

301 

283 

1003 

1408 

533 

534 

494 

519 

69 

124 

? 

54 

117 

537 

105 

198 

182 

124 

120 

340 

319 

153 

142 

1112 

992 

65 

91 

8 

12 

5.5 

.2 

2 

8 

5 

8 

10 

6 

12 

12 

1 

1 

18 

4 

3 

3 

5 

3 

10 

10 

13 

13 

4 

5 

2 

3 

I, 

2-3 

4-13 

13 

5-15 

2-14 

6-9 

1-9 

8-22 

10-20 

85-1 80 

10-23 

4-12 

15-30 

II 

I, 

,I 

I, 

I1 

11 

2-18 

,I 

10-23 I, 

5-20 

r, 

3-15 

I1 

I, 

2-3 

3-5 

1-2 

1 

1 

1 

2-5 

It 

w 

n 

2-5 

3-5 

37-86 

25-46 

n 

8-8 

,I 

35-60 

34 

,I 

n 

5-7 

1-4 

I, 

I1 

20-40 I, 

I, 

5-6 

4-8 

20 

21 

4-15 

4-10 

41 1 

10-20 

29-36 

16-20 

5-18 

2-4 

10-15 

2-12 

I1 

I, 

,, 

II 

0 

I1 

II 

8-1 3 11 

4-1 3 I1 

2-10 

,I 

difference reflects the ease with which birds can 

enter the crown and area beneath an occupied swollen-thorn acacia, as contrasted to mammals which 

are much more bothered by the ants. The pods of 

swollen-thorn acacias are very diagnostic at the 

species level except within the A. cornigera group. 

A. collinsii displays the most extreme geographic 

variation in pod morphology of all the species, ranging from very short and linear pods with thick 

walls in the drier parts of Panama to much longer, 

curved, and thin walls in Mexico and Guatemala. 

All swollen-thorn acacias have very hard seeds (except A. allenii and A. melanoceras) which pass 

through the intestinal tract unharmed and are scarified in the process. Swollen-thorn acacias are subject to heavy seed predation by bruchid beetles 

(e.g., Janzen, 1969c) except for those species in 

very wet habitats-A. melanoceras and A. allenii 

and perhaps A. ruddiae. The general pattern is that 

the eggs are laid on the full sized and nearly mature 

pod, the larvae bore in through the pod wall and 

into a seed, and one larva matures inside each seed. 

Upon emerging from the pod, the new adult beetle 

will oviposit on pods that remain, resulting in destruction of that portion of the seed crop that has 

not been removed by dispersal agents (Janzen, 

1969c.) Undamaged and scarified seeds yield almost 

100 percent germination if taken from truly mature 

pods. 

With considerable experience in studying the 

populations in the field, all species of swollen-thorn 

acacias can be correctly identified from specimens 
24 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

TABLE 5.-Reproductive and other traits of swollen-thorn acacias recorded from occupied adult trees 

Species 

mayana , . . . . . . 

gentlei . . . . . 

glo bulifera 

melanoceras 

cookii , . . . . . . . . 

ruddiae . . , . . , . 

chiapensis . , . . , 

sphaerocephala . 

hindsii . . 

cornigera , . . . . 

allenii . . . . . . . . . 

collinsii . . . . . . . . 

Estimated Maximum Maximum Life Bark 

age at first Pod valve Pod Color of height basal form color 

seed repro- color when dehiscion Aril mature of adult diameter of and 

iuction (yrs) mature behavior color seed (m) (4 adult texture 

4-6 maroon indehiscent yellow dark 45 10 single smooth 

6 

5 

8 

a 

10+ 

6-7 

4 

6 

4 

7 

4 

brown 

black dehiscent pale olive 

along both yellow green 

margins 

but centrally 

together 

along one 

margin 

and 

strongly 

curled 

black dehiscent yellow 

greenish dehiscent white 

black along one 

margin 

olive 

green 

black 

black ? 

greenish ? 

black 

trunk gray - tree green 

trunk rust red 

tree 

14 12-25 single rough 

8 12 

dark dehiscent white 

brown along both 

margins 

maroon indehiscent yellow 

yellow 

red 

brown 

with 

waxy 

bloom 

maroon 

yellow 

red 

greenish 

black 

dark 

brown 

12 15 

? ? 20 30 

? 7 18 50 

dehiscent yellow 

along one 

margin 

yellow 

dehiscent white 

along both 

margins 

along both 

margins 

dehiscent yellow 

greenish 

beige 

black 

dark 

brown 

black 

black 

olive 

drab to 

black 

single olive drab 

trunk to reddish 

tree 

single or green to 

multiple light 

trunk tree brown 

single dark 

trunk brown 

tree 

single smooth graytrunk green with 

tree longitudinal 

whitish 

cracks 

5 20 single rough 

trunk rust 

tree red 

1-4 15 single trunk smooth 

tree to graymultiple green 

trunk shrub smooth 

la 35 single trunk graytree to green 

multiple 

trunk tree 

6-18 20 single trunk smooth graytree green to 

rough 

25 25 single trunk brown 

tree green to 

light brown, 

6 15 single trunk rough 

tree silvery gray 

to brown 
NUMBER 13 25 

species accounts. Table 6 presents the degrees of 

sympatry and gives some feeling for the amount of 

caution required to identify the species on collection locality only; A. melanoceras and A. allenii are 

the only species that are never sympatric with other 

swollen-thorn acacias. 

While hybrids of swollen-thorn acacias with other 

acacias (A. globulifera, A. hindsii, A. collinsii, and 

A. chiapensis are the offenders in this case) may be 

difficult to place to species, swollen-thorn acacias 

apparently hybridize only very rarely (though understandably they might be difficult to notice); I 

have seen only one, and that was a single plant of 

A. cornigera x A. sphaerocephala on the dunes 

south of the city of Veracruz, Mexico. 

FIGURE 14.-~, Newly dehisced fruits of A. collinsii, with the 

yellow pulp enclosing the seeds and hanging down beneath 

the valves on thin fibers; birds pick these off in flight (Costa 

Rica, site 71), B, Opened pod valves of A. hindsii; the pulp 

and seeds have been removed by birds (Mexico, site 17). 

from adult trees even if the localities are unknown. 

Individual variation owing to local ecological factors, however, can make this process confusing, and 

knowledge of the collection site often aids tremendously in quick and accurate identification; the 

field work in this study was extensive and intensive 

enough so that the reader should be extremely 

suspicious if his identification leads him to decide 

that he has located a population of a given species 

far from the ranges listed under the individual 

Taxonomic Information 

While working with a population of swollenthorn acacias, one should have no difficulty in 

identifying them with key A. Key B is provided 

for use in conjunction with key A, when flowers and 

fruit are not available. On occasion there may be 

some difficulty in identifying old and very incomplete herbarium material with these keys, but that 

is not likely to be a significant problem inasmuch 

as the value of species-level determinations for such 

material is highly questionable anyway. When a 

population of swollen-thorn acacias is encountered 

during the rainy season, sexual material may be 

difficult to locate. If one searches for damaged trees, 

however, often an occasional inflorescence-bearing 

branch may be located. Since the immature infructescences are retained on the tree through the rainy 

season, careful search for these usually succeeds in 

locating sexual material. It is worth noting that the 

inflorescence receptacle generally retains its shape 

while bearing immature or mature fruits (e.g., 

Figure 112), allowing easy use of the first couplet of 

key A even when fresh inflorescences are not available. The physical locations of the type specimens 

of swollen-thorn acacias are given in Table 7, but 

these are generally of little use in identification. 

Vernacular names are of little use in correct 

identification of swollen-thorn acacias, inasmuch as 

they tend to be lumped under “cornizuelo” or some 

similar word at a given site (Table S), in contrast 

to the other acacias which generally carry names 

such as “guisache.” 
26 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

Name 

collinsii . . . . . . . 

sphaerocephala 

TABLE 6.-Sympatry of swollen-thorn acacias (+ indicates that they may be 

found growing within 200 m of each other in some part of the range and that 

the populations overlap) 

Species 1 COT. sph. may. glo. coo. hin. col. mel. all. rud. gen. chi. 

Type location 

and sheet number Synonyms Location of synonym types 

US 692159* yucatanensis B (presumed destroyed) 

NY (cotype)* GH (“type collection”)* 

US 571618 (“type COIL”)* 

F 417619 (paratype)” 

F 417618 (paratype)” 

US 943274 (potential 

lectotype) * 

GH (cotype)* 

K (same as US 943274)t 

US 677927 (isotype)* 

US 764673 (photo of type)* 

nelsonii US 399366 (type)” 

costaricensis B (presumed destroyed) 

penonomensis NY (type)* 

panamensis B (presumed destroyed) 

(presumed destroyed) ueracruzensis (presumed destroyed) 

US 764680 (frag. and photo)* US 764683 (frag. and photo)* 

US 764684 (frag.)” 

US 692164 (isotype). 

NY (isotype)* 

dolichocephala F 189552 (type)* 

cornigera 

sphaerocephala 

mayana 

globulifera 

cookii 

hindsii 

collinsii 

melanoceras 

allenii 

ruddiae 

gentlei 

chiapensis 

+ 

++ 

+o+ 

+oo+ 

+ooo+ 

+oo+o+ 

+oo+o++ 

ooooooo+ 

oooooooo+ 

oooo+oooo+ 

+ooo++ooo++ 

+oo++ooooo++ 

mayana . . . . . . . 

cornigera . , . . . . 

US 1688106 (isotype)* 

GH (isotype)* 

MICH (type)* 

NY (isotype). 

BM (no number) 

US 2396019 (frag. and 

photQ)* 

spadicigera US 764687 (frag. and photo)* 

cu bensis GH (type)* 

US 764633 (frag. and photo)* 

MO 16066, 46866 (type)’ 

US 577752 (“type toll.")* 

GH (“type collection”)* 

NY (isotype)* 

nicoyensis B (presumed destroyed) 

K (cotyfie)t 

hernandexii US 692170 (type)* 
NUMBER 13 

Acacia species 

mayana . . . . . . . . . . . . . 

globulifera . . . . . . . . . . 

cornigera , . . . . . . . . . , 

27 

Vernacular Geographic region 

cruzetillo Campeche, Yucatan, Mexico 

cachetoro 

ishcanal 

cornezuelo blanco Yucatan, Mexico 

cachetoro 

ishcanal 

cornizuelo lowland eastern and southern 

cornezuelo Mexico, generally throughout 

Coban, Alta Vera Paz, Guatemala 

Coban, Alta Vera Paz, Guatemala 

Central America 

TABLE 7.-Location of type specimens of swollen-thorn acacias-continued 

Name 

hindsii . . , . . . , , . 

cookii 

globulifera . . . . . 

chiapensis 

gentlei . . . . . . . . 

melanocerm . . . 

allenii . . . . . , . . . 

ruddii . . . . . , , . . . 

Name 

gladiata . . . . . . . , 

hirtipes 

standleyi 

oaxacanum 

Examined in 

.- - Type location 

and sheet number Synonyms Location of synonym types 

GH (isotype)* 

K (no number)? 

US 574387 (type)” 

US 764658 (paratype)* 

F 58707 (type)” 

F 36458 (isotype?)* 

US 1267507 (isotype?)* 

GH (istotype?). 

NY (isotype). 

US 692157 (type)‘ 

F 464247 (photo)* 

F 1000801 (type)* 

furcella 

interjecta 

rossiana 

campecheana 

turgida 

bursaria 

tepicana 

sinaloensis 

bucerophora 

donnelliana 

NY (isotyie)* 

US 692166 (type)* 

unknown 

MO (no number)* 

US (photo). 

B (presumed destroyed) 

(photo only, in Wheeler, 

B (presumed destroyed) 

GH (cotype)* 

US 637234 (type)* 

US 636818 (type)* 

NY (isotype)* 

US 764696 (frag. & photo)* 

F 659304 (“type”)* 

GH (“type”)* 

US 943282 (type)” 

1942) 

K (cotype)t 

locality unknown multiglandulosa B (presumed destroyed) 

US (photo, no. 1281)* 

us (type)* 

us (type)* 

Type location 

US 764660 (frag. & photo)* 

MO 46838 (type)* Bonpl. ex Willd. 

US 764659 (frag. & photo)* 

US 716563 (“paratype”). 

US 694036 (type)* A. hindsii x A. pennatula Benth. 

US 637251 (type)* 

US 1208317 (type)* 

iis study; numbers refer to herbarium sheet numbers. 

and sheet numbers Hybrids 

A. hindsii x A. cochliacantha Humb. and 

A. hindsii x A. pennatula or A. macracantha 

A. hindsii x A. pennatula 

t Information provided by Sir George Taylor (in litt.). 
28 

Acacia species 

....... 

gentlei .............. 

collinsii ............ 

SMITHSONIAN CONTRIBUTIONS TO BOTANY 

Vernacular Geographic region 

cuernitos Veracruz, Oaxaca, Mexico 

ishcanal blanco El Salvador 

cuernos de tor0 

ishcanal Pacific lowlands of Guatemala 

ixcanal 

carnizuelo Central Honduras 

torito Salina Cruz, Oaxaca, Mexico 

cornezuelo Michoacan, Guerrero, Mexico 

gara terrera Sinaloa, Nayarit, Mexico 

jarretadera 

cockspur British Honduras 

red cockspur 

cockspur Isla Providencia, Colombia 

crowsfoot British Honduras 

San Luis Potosi, Veracruz, Chiapas, 

Tabasco, Mexico 

TABLE 8.-Vernacular names of swollen-thorn acacias in Central America as 

hindsii 

I cachitos Pacific coast of Panama 

melanoceras ........ 

sphaerocephala ..... 

cookii ............. 

1 cockspur Guatemala 

subin 

cornezuelo 

cuerni tos 

cachitos 

cornezuelo 

guascanal 

Yucatan Peninsula, Mexico 

Chiapas, Yucatan, Mexico, 

Porto Bello, Panama 

Panama 

Veracruz, Mexico 

Guatemala 

Colombia 

A. Key to Mature Central American Swollen-Thorn Acacias, Based on 

All Traits 

1. Inflorescence distinctly globular ................................................... 2 

Inflorescence very elongate or at least distinctly longer than broad ................... 3 

2. Leaves from type A thorns (on lateral branches) over 30 cm long ..... 

Leaves from type A thorns (on lateral branches) less than 25 cm long . 

3. Pinnules less than 1 mm wide ...................................................... 6 

4. Petiole deeply grooved, young thorns pubescent, nectaries b ised above surface of 

Petiole slightly grooved, young thorns glabrous, nectaries str outstanding conical, 

Pinnules 2 or more mm wide .............................. ................. 7 

petiole and elongate, occupied by obligate acacia-ants ..................... .I. A. cookii 

Petiole with many nectaries (8-30) .................................. 3. A. melanoceras 

Petiole with few nectaries (0-6) ........................................... .4. A. allenii 

very soft .............. ......................................... .5. A. chiupensis 

shaped .................................................. 8 

not occupied by obligate acacia-ants ...................... . . , . . , . .2. A. ruddiae 

5. 

6. Petiole with many nectaries (6-25) closely packed and often not in a straight line, wood 

Petiole with few and widel aced nectaries (1-5) in a straight line ..... .6. A. globulifera 

7. Inflorescence peduncle very thick, at least half as wide as inflorescence, nectaries trough 

Inflorescence peduncle very less than one-fourth as wide as the inflorescence, nec8. Inflorescence 34 times as long as broad; secondary venation clearly visible on leaflets. ... 9 

Inflorescence one and one-half times as long as broad, approaching a globular form; 

.................. 

taries round .......................................... 

secondary venation on leaflets not visible ........................ 7. A. sphaerocefhzla 

9. Swollen-thorns round in cross section ................................... .8. A. cm'gma 

Swollen-thorns triangular in cross section ................................. .9. A. mayam 

10. Inflorescence approaching globular form, but nearly twice as long as wide ................ ................................. ..Hybrid of A. hi&$ with other Acacia species 

Inflorescence very elongate, 3-8 times as long as broad ................................ 11 
NUMBER 13 29 

11. Seed pods less than 8 cm long, valves curling heavily upon opening ................... 12 

Seed pods more than 12 cm long, curling only slightly upon opening ....... .lo. A. gentlei 

12. Petiolar nectaries dome shaped in lateral view with 1 mm or greater diameter, usually 

3-4 very close together in widened part of petiole and none on rachis .... .11. A. collinn'i 

Petiolar nectaries cylindrical in lateral view with less than 1 mm diameter, usually 1 or 

more on the petiole and one at junction of each pinna with rachis ....... .12. A. hiMi 

B. Key to Mature Central American Swollen-Thorn Acacias, Based on 

Vegetative Traits 

1. Petiolar nectary trough shaped, much longer than broad (Figure 8~) .................. 

2. 

Type A thorns circular or elliptical in cross section .................................. 3 

3. Secondary venation on leaflets clearly visible .......................... 8. A. codgera 

Secondary venation on leaflets not visible ........................ .7. A. sphaerocephala 

4. Type A thorns very heavily flattened, nectaries scattered on petiole and rachis, and small 

enough so as not to cause heavy swelling of petiole or rachis (Figure 8A) ,12. A. hindn'i 

Type A thorns circular or elliptical in cross section, nectaries various ................. 5 

5. Nectaries restricted to petiole and broad dome shaped (Figures 7A, 9) ... .11. A. collinsii 

Nectaries other than above ......................................................... 6 

6. From type A thorns on lateral branches, leaves 30 cm or more long . . , . 

From type A thorns on lateral branches, leaves less than 25 cm long . . 

7. Pinnules less than 1 mm wide ...................................... 

Pinnules more than 2 mm wide , , . . .................................... 9 

8. Petiole deeply grooved, young thorns (and sometimes old thorns) , nectaries 

elongate and low (Figures 18~, 18~)~ obligate acaciarant entrance holes in thorns near 

Petiole only slightly grooved, young thorns glabrous, nectaries conical and well raised 

above petiole and rachis (Figure 8c), never with obligate acacia-ant entrance holes in 

thorns near apex .............................................. , 2. A. ruddiae 

Petiole with few nectaries (0-6) (from Costa Rica) ......................... 4. A. allmii 

10. Petiole with many nectaries (6-25) closely packed and often not in a straight line 

Petiole with 1 to few nectaries; if more than one, then linearly arranged . . , , . , , , , , , . 11 

11. Extrafloral nectaries as in Figures 6 and 7c, a1 always more than one on the petiole 

........................................... ................... 6. A. globulifera 

Extrafloral nectaries as in Figure 78, almost never more than one on the petiole, bark and 

thorns deep red or mahogany in color, type A thorns very slender (as little as 3 mm di2 

Petiolar nectaries circular (domes, cups, conical) . . , , . . ....................... 4 

Type A thorns triangular in cross section ................................. .9. A. mayana 

apex .................................................. ........... 1. A, cookii 

9. Petiole with many nectaries (8-30) (from Panama and Canal Zone) . , 

......................... ............ ................. 5. A. chiapensis 

amrter at base) 

1. Acacia cookii Safford 

Acacia cookii Safford, 1910:677. 

Acacia bucerophora B. L. Robinson, 1913:502. 

Mymecodendron cookii (Safford) Britton and Rose, 1928:93. 

Acacia cookii is easily distinguished from all other 

swollen-thorn acacias by its tomentose, long and 

thin thorns that bend back around the stem and are 

often bilaterally asymmetrical (Figures 15, 16). A. 

collinsii type A thorns may also wrap around the 

branch that bears them, but they are very short and 

thick. Only the type B thorns of A. cornigera wrap 

around the branch that bears them. A. ruddiae 

thorns may be tomentose when green, but they are 

glabrous upon maturing. 

10. A. gentlei 

The type specimens for both A. cookii and A. 

bucerophora are representative of specimens from 

the southern part of the population (Figure 17). 

The branch designated as the type of A. cookii 

(from site 1) lacks swollen thorns and was probably 

taken from the end of a long and old reproductive 

branch such as that in Figure 15~. Since the inflorescences on the type are still in bud, the peduncles are much shorter than they are when in 

flower (e.g., Figures I~B, 16~). The leaves of the type 

specimen are not from the inflorescence-bearing 

branch; they are of the form normally subtended 

by large type A thorns (cf. ends of branch in Figure 

18) and were probably broken from the branches in 
30 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 15.-Lateral branches from mature A. cookii (Guatemala, site 4): A, sterile branch with 

fully developed thorns characteristic of the southern part of the acacia’s range; B, reproductive 

branch with axillary inflorescences characteristic of the southern part of the acacia’s range. 

order to avoid pressing the ant-filled thorns. The 

type specimen of A. bucerophora (site 3) differs in 

no significant way from that of A. cookii. The single 

thorn of the specimen is old and rotted and therefore not the dark brown that is characteristic of 

most thorns on a tree of A. cookii. The single small 

seed pod is immature and shrunken, giving the 

illusion of being thin, while in fact A. cookii pods 

are over 1 cm wide and very long (Figure 19). 

The northern and southern parts of the A. cookii 

population differ on a number of morphological 

traits; however, these differences appear to be the 

result of comparing the two ends of a cline, since 

the intermediate elevation wet-forest habitat of A. 

cookii is continuous from the northern to southern 

part of the range. This hypothesis can only be verified by collections from the area between the broken 

distribution in Figure 17. Some of the more outstanding changes in A. cookii in progressing from 

the northern to the southern part of the range are: 

(1) shortening of the type A thorns (cf. Figures 15~ 

vs. 16~); (2) shortening of the sexual branchlets (cf. 

Figures 15~ vs. 16~); (3) increased number of inflorescences per inflorescence-bearing axil; (4) increased number of Beltian bodies per mature leaf; 

and (5) increased percentage of the acacia plants 

being occupied. 

KEY COLLECTION LOCALITIES 

1. 

2. 

3. 

4. 

5. 

6. 

Type of A. cookii: Finca Trece Aguas near Secanquim, 

Depto. Aka Verapaz, Guatemala (300 m) . 111-8-1907, 

G.P. Goll, no. 102 (US 692154), fl. 

Paratype of A. cookii: Secanquim, Aka Verapaz, Guatemala (300 m) . Vl-4-1905, O.F. Cook, no. 4. 

Type of A. bucerophora: about Toledo, 4 mi from the 

coast and 5 mi from Punta Gorda, Belize Prov., British 

Honduras (sea level), 111-29-1907, ME. Peck, no 632. 

2.8 mi W Puerto Matias de Galvez (on road to Escobas) , 

Depto. Izabal, Guatemala (10 m). VIII-10-1967, DHJ, 

no. 1524, gp. 

9.8 mi W Cardenas on Mexico 180, Tabasco, Mexico 

(10 m). VI-17-1966, DHJ, no 515. 

From Km 56 sign post, 9.8 mi N turn off Tapilula to 10.8 
NUMBER 13 31 

FIGURE 16.-Lateral branches from mature A. cookii: A, sterile branch with fully developed 

thorns characteristic of the northern part of the acacia’s range (Mexico, site 7); B, reproductive 

branch with long sexual branchlets characteristic of the northern part of the acacia’s range 

(Mexico, site 6). 

mi S Pichucalco on Villa Hermosa-Tuxtla Gutierrez 

highway, Chiapas, Mexico (240 to 380 m) . VII-10-1966, 

DHJ, nos. 530, 537, fl. 

7. N side of Chontalpa, 26 mi S Cardenas, Tabasco, Mexico 

(80 m). VI-17-1966, DHJ, nos. 548, 550. 

8. Along river, Lancetilla Valley, near Tela, Atlantida, Honduras (5 m) . XII-19-1927 to 111-27-1928, P.C. Standley, 

no. 54627. 

9. Rd. from Secanquim to Sepacuite, Depto. Alta Verapaz. 

Guatemala. IV-1-1902, O.F. Cook and R.F. Griggs, no. 

374 (US 408103). 

10. Sierra del Mico, between Los Amates and Lake Izabal, 

Depto. Izabal, Guatemala (650 m) . 11-27-1908, W.A. KelZerman, no. 7169 (US 2442365, US 2442347), fl. 

11. Between Virginia and Lago Izabal, Montana del Mico, 

Depto. Izabal, Guatemala (50-100 m) . IV41940, J.A. 

Steyermark, no. 38758 (F 1056559). 

12. British Honduras. 1944 (F 1403119). 

13. Big Rock, British Honduras (65 m) . XII-16-1932, W.A. 

Schipp, no. 1072 (F 713932). 

14. Maskall, British Honduras? 1-1934, P.H. Gentle, no. 1121 

(F 739552). 

NATURAL HISTORY 

Acacia cookii is a narrowly distributed species 

that has its highest densities at intermediate elevation sites (200 to 400 m) with very mild dry seasons 

(e.g., sites 1,2,6). At site 6, this habitat was slightly 

higher than that of A. cornigera and A. chiapensis, 

where all three occur along the east-facing foothills 

of the Sierras in western Tabasco, Mexico. Occasional adults of A. cookii were found at lower and 

drier sites (sites 5,7) in the northern part of the 

range, but their sympatry with A. cornigera and 

A. chiapensis may have been the result of seed dispersal by road-building machinery. In the southern 

part of the range, isolated plants (e.g., site 4) generally grow in naturally disturbed sites along rivers 

in very wet forests. They are sympatric with A. 

gentlei at these sites (as at other Guatemalan sites 

such as sites 10 and ll), and there is no evidence 
32 SMITHSONlAN CONTRIBUTIONS TO BOTANY 

FIGURE 17.-General postulated distribution (stippled area; type localities indicated by arrows). 

collection site of A. cookii A collection site of A. rnayana 

that they are unnatural populations. A. cookii disappears when the wet forest is cut in areas like site 

4; this local extinction is probably due to the same 

causes as cited for A. allenii and A. melanoceras. In 

the northern part of the range, where the dry season 

is more severe, A. cookii appears to be able to persist 

in the face of repeated wilting and occasional burning. It was very common as a roadside and pasture 

tree shrub at site 6. 

The hard seeds of A. cookii are attacked by unidentified bruchids to the same degree as those of 

swollen-thorn acacias of drier areas. The seeds are 

dispersed by birds; the pods dehisce incompletely 

and the dark seeds are buried in a white, sweet 

pulp. The seed crops are not unusually large, but 

the large number of inflorescences per axil is 

matched only by the axils of A. chiapensis (Figure 

20~). In the southern part of the range, where the 

inflorescence-bearing branch is reduced to a bump 

of inflorescences per axil may be as high as 64 for a 

FIGURE 18.-Crown of mature 5-m-tall A. cookii growing in the thorn (Figure 15B)9 an average number 

along creek in rain forest (Guatemala, site 4). 
NUMBER 13 

FIGURE 19.-Acuciu cookii (Guatemala): A, rachis nectaries on leaf siibtended by type A thorn 

(site 4); B, petiolar nectaries of leaf; c, mature seed pods (site 1, US 692155). 

large sample (Figures ~OB, 22). In the northern part 

of the range, where the sexual branchlets are long, 

a sample of 10 axils on a representative branch had 

a mean of 32 inflorescences per axil (16, 29, 29, 29, 

31, 31, 35, 37, 38, 42). 

The pattern of development of A. cookii from 

seedling to adult is unknown, but I have no reason 

to suspect it differs in any substantial way from that 

of other wet-forest, swollen-thorn acacias such as A. 

allenii. At site 4, the mature trees had a relatively 

dense crown (Figure IS) perched on top of a single 

and straight thin trunk. Based on growth rings, in 

wet forest the trees die of senescence when about 

15 to 25 years old. In slightly drier sites (definitely 

site 6 and probably sites 1 and 2) the old trees may 

FIGURE 20.-Axillary inflorescences: A, immature pods of A. attain the largest size of any swollen-thorn acacia, chiupensis (Mexico, site 9); 8, bud scars (with two infrutescence peduncles projecting to 12:OO and 2:OO o'clock) except perhaps '* in Mexico and 

of A. cookii (Guatemala, site 4). A. ruddiae in the Costa Rican mountains. Growing 
34 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

in old landslide scars at site 6, there were trees of 

A. cookii up to 30 m tall, up to 20 m across the 

crown, and with a DBH as much as 40 to 50 cm. 

The ant colonies were not examined in these trees, 

but I suspect that they were either a multiple-queen 

species or they restricted their patrolling to the 

newer foliage. While the crowns of these acacias 

were very large, the actual amount of green foliage 

was small compared to what it would be in another 

species of legume of equal size during the rainy 

season. 

It is possible that the herbivore pressure is somewhat reduced at this site and/or that A. cookii in 

this area has better chemical defenses than do most 

swollen-thorn acacias. I suspect this because roadside clumps of poorly occupied A. cookii at site 6 

had foliage with very little herbivore damage. 1 

favor this hypothesis because the A. cookii growing 

at site 5 was in a lowland roadside pasture where 

unoccupied A. cornigera was being heavily defoliated by insects that normally feed on other species 

of plants. This A. cookii was, however, completely 

unoccupied and almost in perfect condition. At 

least 5 years old, the plant was 4 m tall and had a 

crown similar in foliage density to the one in Figure 

18. About one out of every four shoot tips had been 

- 

160 Y 

I X 

t-1 

u 

2 

W 

cl 

eaten off by insects, which is about the frequency 

usually observed during the rainy season on nonant-acacias and occupied swollen-thorn acacias. 

These comments should not be construed to indicate that A. cookii does not have a mutualistic association with its ants. At site 6 there was ample 

defoliation of unoccupied sucker shoots, in contrast 

to occupied ones. This indicates that the results of 

removing the ants would have had an effect on A. 

cookii much like that with A. allenii, but not as 

dramatic as with ,4. cornigera or A. collinsii. 

It is possible that A. cookii is still in the process 

of coevolving a tighter mutualism with its occupant 

ants-Pseudomyrmex ferruginea in the northern 

part of the range, and P. ferruginea plus an unidentified species in the southern part. Its petiolar 

nectaries (Figure 19~) are morphologically poorly 

developed-their sugar production rate may be 

high, however-though the nectaries on the rachis 

may compensate for this (Figure 19~). On some 

individuals at site 6 there were Beltian bodies on 

only the basal 10 to 30 pinnules, and even at the 

sites where the healthiest ant colonies occur (e.g., 

site 4), Beltian bodies are produced on only about 

half the pinnules. The swollen thorns on A. cookii 

in the northern part of the range are very thinwalled-birds could probably break into them. with 

ease-and they harden very soon after attaining 

full size, which allows the ant only a short time to 

cut an entrance hole. The very narrow thorns have 

a volume (Figure 21) representative of that of other 

swollen-thorn acacias because of their extreme 

length; however, these narrow thorns probably 

make very cramped living quarters for the ants. The 

largest thorns to be found (Figure 21) had a mean 

volume of only 1.64 cc, which is representative only 

of A. chiupensis, A. globulifera, A. gentlei, and A. 

ruddiae. 

2. Acacia ruddiae, new species 

t-1 100 11 

lo0 200 3 00 

TOTAL THORN VOLUME ( cc ) 

FIGURE !?I.-Distribution of 131.3 cc of thorn volume among 

19 very large type B thorns from one adult A. cookii (Guatemala, site 4). While the largest to be found, these thorns had 

a maximum diameter in cross section of 5-7 mm. 

Arbol usque ad 20 m alta; ramuli novelli 

puberuli, glabrescentes; stipulae spinescentes interdurn magnae, teretisculae, usque ad 10 cm longae, 

9 mm latae; folia circiter 15-40 cm longa; nectaria 

sessilia cupulata ad juncturas pinnarum omnium et 

0-2 in petiolis; pinnae (12-) 25-40-jugae, 4-5 cm 

longae; pinnulae 65-75-jugae, lineariae, 2-4 mm 

longae, 0.5-1.2 mm latae, acutae vel obtusae, 
NUMBER 13 

v) 

W 

W 

V 

v) 

30 

el 

IL 

w 

I 

3 

z 

m 

123456 

A NUMBER OF PODS PER INFLORESCENCE 

u) 

cl 

x 

< 

IL 

0 

U 

I 

1 35 7 9 11 

B NUMBER OF INFLORESCENCES WITH 

PODS PER AXIL 

FIGURE 22.-Acacia cookii (Guatemala, site 4): A, distribution of 113 seed pods among 63 infructescences; B, distribution of the 63 infructescences among their 10 axils. There were 642 

inflorescences produced by these 10 axils. 

puberulae; flores aurei in capitulo globoso; involucellum annulare in medio pedunculo; legumina 

(immatura) atrobrunnea, puberula, granulata glabrescentes, oblonga, 5 cm longa, 7 mm lata, complanata, obtusa, stipite 5 mm longo; legumina 

matura probabiliter usque ad 6-8 cm longa. 

TYPE.-”. H. Janzen 828, Costa Rica, Limon 

Province, ferry landing, Moin River, Moin, 13 February 1965 (holotype US 2639554; isotypes CAS, F, 

GH, MEXU, MICH, MO, NY, UC). 

Type tree 20 m tall with 35 cm diameter trunk 

1 m above ground; gray-green bipinnate leaves 

mostly 15 to 40 cm long with 50 to 80 pinnae per 

leaf, pinnae 40 to 50 mm long with 130 to 150 pinnules per pinnae, pinnules 2.5 to 3.0 mm long and 

0.75 to 1.0 mm wide (up to 9,000 pinnules per leaf); 

occasional swollen-thorns on less than 10 percent of 

the nodes and very frequently only one side of the 

thorn swollen, thorns not occupied by obligate 

acacia-ants but by a miscellany of other ant species; 

no food bodies (Be1 tian bodies) on pinnules; single 

nectary at junction of each pinna with rachis, 0 to 

2 nectaries on petiole, nectaries cylindrical up to 1 

mm tall; yellow inflorescences spherical and up to 

6 mm diameter, peduncle up to 10 mm long and 

0.50 to 0.75 mm thick; immature fruits flat and 

slightly curved, probably 6 to 8 cm long at maturity. 

Acacia ruddiae is distinctive by never being occupied by obligate acacia-ants, lacking swollen thorns 

on most branches except the bases of some lateral 

branches, having better than 5,000 very small leaflets 

per leaf, growing in rain forest, becoming a very 

large tree, and by its range (Figure 23). It also 

differs from other Central American members of 

the genus Acacia by the same traits; it is undoubtedly distinguishable from the remainder of the 

genus by fine details of flower, fruit, and leaf structure, but it is impossible to know these differences 

at the present’state of Acacia taxonomy. 

The tree from which the type specimens (13 

sheets) were collected was at least 30 years old, and 

perhaps much older, and was still standing in 1967. 

It was about 80 m south (west side of the road) of 

the south beach of the ferry landing on the Moin 

River and 200 m from the ocean beach, at Moin, 

Costa Rica (site 1). The branches were cut from 

high in the tree and were representative of the 1-2- 

year-old branches in the canopy. Although the specimens were collected during the dry season, the area 

was still subjected to frequent rains and the tree was 

only partly deciduous. It was at the end of its flowering season, and the full sized but immature green 

pods were from the same flower crop, rather than 

from the previous year’s flowers as is usual with 

acacias of drier habitats. As is usual with the canopy 

branches from A. ruddiae this size, there are few 

swollen thorns on the branches. The tree from 

which the type specimens were collected was exceptional in two respects: First, most of the A. 

ruddiae population in this area has long since been 

destroyed by housing development. Second, the tree 

grew in a relatively open site and thus had strong 
36 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE Pt.-General postulated distribution (heavily stippled area) and specific collection sites 

of A. ruddiae (all on the Caribbean side of Ccntral America). General postulated distribution 

(lightly stippled area) and specific collection sites of A. allenii (all on the Pacific side of Central 

America). 
NUMBER 13 87 

development of lateral branches; most A. ruddiae 

are involved in intense intertree competition and 

thus are tall and slender. 

This acacia is named in honor of Dr. Velva E. 

Rudd whose unfailing assistance and encouragement has greatly aided in bringing this study to 

completion, and whose taxonomic work with Leguminosae will always be a tremendous assistance to 

field biologists. 

Acacia ruddiae is included with the swollen-thorn 

FIGURE 24.-Acacia ruddiae: A, 

very young seedling; note nodules on roots (Costa Rica, site 

5); B, young sapling (Costa Rica, 

site 5). 

acacias, despite the fact that it is not occupied by 

obligate acacia-ants, because it appears to represent 

an intermediate step in one of several possible evolutionary pathways to the evolution of swollenthorn acacias. Its ecology is therefore of special 

significance in this study. A. ruddiae is the only 

Central American acacia, aside from hybrids of 

swollen-thorn acacias with nonant-acacias, that appears to represent an intermediate step in the evolution of a New World swollen-thorn acacias. 
38 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

KEY COLLECTION LOCALITIES 

1. TYPE.-Ferry landing, Moin River, Moin, Limon Prov., 

Costa Rica. 11-13-1965, DHJ, no. 828, fl. gp. 

2. Pandora, Limon Prov., Costa Rica. VIII-26-1963, C.D. 

Michner, nc. 

3. 16 mi N Vara Blanca on road from Heredia to Puerto 

Viejo (Sarapiqui Dist.), Heredia Prov., Costa Rica (320 

m) . V-1-1965, DHJ, no. 819, fl. 

4. 1 km W La Virgin, Sarapiqui Dist., Heredia Prov., Costa 

Rica (80 m) , IX-5-1971, DHJ, nc. 

5. 1 km W La Lola (on Limon-Siquirres railroad), Limon 

Prov., Costa Rica. VIII-19-1963, DHJ, nos. 1868, 1872, 

1873, 1874, 1875, 1876, 1877, 1878, 1881. 

6. 11 mi N Florencia, San Carlo Dist., Alajuela Prov., Costa 

Rica. VII-14-1963, DHJ, no. 1883, 1885, 1866. 

7. Along Changuinola River, Changuinola Valley, Bocas del 

Tor0 Prov., Panama. 11-29-1924, V. C. Dunlap, no. 462 

(F 709739, US 1405662) (listed incorrectly as “A. cookii” 

in Woodson and Schery), fl. 

8. Lancetilla valley near Tela, Depto. Atlantida, Honduras. 

VI-6-1927 to 111-20-1928, P. C. Standley, no. 54627 (US 

1408382). 

9. 1.9 mi W Puerto Matias de Galvez (on road to Escobas), 

Izabal, Guatemala. VIII-10-1967, DHJ, no. 1582. 

10. 27.4 mi S Puerto Barrios, Izabal, Guatemala. VIII-8-1967, 

DHJ, nc. 

NATURAL HISTORY 

Acacia ruddiae is one of the most peculiar Central 

American members of the genus. It lives in very 

wet forest habitats, which is distinctly unlike other 

acacias, yet does not have the obligate acacia-ants 

which generally allow survival of swollen-thorn 

acacias in wet habitats (Janzen, 1966a). 

In Costa Rica, where the only large populations 

of A. ruddiae have been encountered, the natural 

habitat of this tree is in primary succession along 

lowland rivers through rain forest (sites 4, 5), swamp 

edges (site 5), and in large, old landslide scars on 

the sides of very steep gorges (site 3). These areas 

receive 4 to 5 m of rain a year and have a poorly 

developed dry season of 1 to 2 months with occasional rains. Some of the sites occupied by A. melanoceras and A. allenii receive as much rainfall, but 

have a more noticeable dry season. Judging from the 

paucity of collections, A. ruddiae apparently also 

exists as a very rare tree in the rain forest that extends in a narrow strip up along the Caribbean 

coast to the Puerto Barrios area of Guatemala (sites 

8 and 9), but there is no reason to believe that it 

does not become locally common in these forests 

just as it does in Costa Rica. 

Acacia ruddiae is by no means generally distributed throughout the rain forest of the Costa 

Rican Caribbean lowlands. At site 4 it was common 

in 1- to 30-year-old primary succession on gravel 

river banks and grew to a 25-m-tall tree with 30 

cm DBH; it had not, however, extended its population out into the surrounding old cacao groves and 

pastures. At site 5 there was a large population 

along about a mile of old river bank, and occasional 

large saplings in the immediately adjacent parts of 

a banana plantation and on swamp edges in primary forest. Many of the swamp trees were old and 

scattered widely, in contrast with the many adjacent 

and even-aged A. ruddiae along the river. Extensive 

search throughout many other swamps and disturbed forest habitats within ten miles located no 

other A. ruddiae. At site 3, A. ruddiae occurs only 

FIGURE 25.-Acacia ruddiae (Costa Rica): A, representative flowering branch (site 1); B, naturally 

split type A thorn (site 6). 
NUMBER 13 39 

FIGURE 26.-Representative branches of A. ruddiae: A, without swollen thorns from the crown 

of a large adult (Guatemala, site 9); B, from main trunk of large tree (Costa Rica, site 5). 

in a small portion of the total landslide area in this 

strongly dissected topography. It has not spread 

along the road into other disturbed habitats from 

this site. While detailed weather data are not available for site 3, it may be slightly drier-hence better 

insolated-than other disturbed sites slightly above 

or below it in elevation. This is indicated by the 

high success of grasses in early stages of succession 

here, in contrast with dicot regeneration at casually 

cleared sites a few miles farther up or down the 

road. 

Seedlings growing in deep shade resemble those 

of A. melanoceras and A. allenii in that the leaves 

are arranged in a whorl so that in vertical view, 

they do not overlap. When growing on bare gravel 

bars, the seedlings are more bushy and asymmetrical 

in appearance (Figure 24~). The nodules on the 

rootlets of A. ruddiae seedlings are more noticeable 

than on any other swollen-thorn acacia (Figure 

24~). 

The seedlings differ from those of all other 

swollen-thorn acacias in lacking swollen thorns. The 

first swollen thorns do not appear until the plant 

is 1 to 3 m tall and has several hundred leaves. The 

nectaries (Figure 8c), however, are produced as 

soon as on any swollen-thorn acacia. The first ones 

appear on the rachis just below the point where the 

pinnae join the rachis, and petiolar nectaries begin 

to appear on the 25th to about the 50th leaf. There 

are no Beltian bodies on A. ruddiae, and thus the 

only contact that the seedlings may have with ants 

is when occasional worker ants visit the nectaries 

and forage on the leaf surfaces. In this sense the 

young A. ruddiae are no different from nonantacacias, except that the nectaries are larger than on 

nonant-acacias and may therefore produce more 

nectar. 

On saplings, the first swollen thorns are scattered 

along the main trunk and lateral branches. They 

are very thin-walled, hollow, easily crushed, and 
40 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 27.-Acacin ruddiae (Guatemala, site 9): A, type B thorns on short lateral branches from 

main trunk; B, representative crown section of 4-m-tall sapling; c, well developed type A thorns 

from the crown in Figure B. 

occasionally only half of the stipule is enlarged. 

They often split at the base upon drying (Figure 

Z~B), and ants, especially Crematogaster, often use 

this as an entrance hole. As the tree grows larger, 

some of the lateral branches have no thorns at all 

(Figure %A), some branches have large swollen 

thorns at the base and tufts of leaves at the ends 

(Figures 27~, 27~), and some branches are very short 

and bear thorns best classified as type B (Figures 

27~, c). The A. ruddiae in the Puerto Barrios area 

(sites 8 and 9) had most of its swollen thorns on 

branches such as those in Figures 26~ and 27, while 

in Costa Rica the branches were usually very short 

with many thorns (as in Figure 27~) or very long 

with just a few scattered thorns (Figure 28~). The 

lower trunk of a large, old A. ruddiae is covered 

with short branches with many thorns, making the 

tree very difficult to climb. 

The crowns of A. ruddiae are more diffuse than 

any other swollen-thorn acacia and therefore look 

more like most mimosaceous legumes than do 

swollen-thorn acacias. Looking up through a crown 

(e.g., Figure 28~), one can often see large amounts 

of sky; only the most densely leafed branches (e.g., 

Figure 5 1~) look like typical swollen-thorn acacia 

branches. This is the case despite the fact that the 

crowns of A. ruddiae are usually fully insolated; 

while the young seedlings appear to grow fairly 

well in heavy shade, the mature trees almost always 

have canopy level or emergent crowns, suggesting 

that A. ruddiae probably cannot mature in heavy or 

broken shade. 
NUMBER 13 41 

FIGURE 28.-Acacia ruddiae 

(Costa Rica, site 6): A, lateral 

branches in crown of adult; 

B, looking up through crown 

of young adult. 

The trees of A. ruddiae do not bear flowers until 

they are in the 8 to 15 m height and 15 to 30 cm 

DBH class. Small clusters of yellow and bee-pollinated inflorescences (Figure 25~) are scattered 

through the crown and not concentrated on main 

sexual branches as is common for swollen-thorn 

acacias. The seed pods appear to mature during the 

2 to 4 months immediately following flowering, as 

with A. melanoceras and A. allenii, rather than 

waiting until the following dry season as with dryland, swollen-thorn acacias. 

Adults of A. ruddiae show no more obvious insect 

damage than do other acacias not occupied by 

obligate acacia-ants. Since some have no ants living 

on them at all-though some worker ants forage 

in the foliage for nectar and probably insects-this 

immunity is likely due to the usual kinds of chemical defenses encountered in ordinary acacias. Shoot 

tips of A. ruddiae are slow growing in comparison 

with more usual swollen-thorn acacias. Associated 

with this they are very tough, compact, bitter in 

taste, and covered with dense hair (Figure 5A). 

They strongly resemble those of A. macracantha 

and A. farnesiana. 

By virtue of the very well developed foliar nectaries and enlarged thorns, A. ruddiae has far more 

ants foraging on its leaf surfaces than ordinary 

nonant-acacias. None of these ants are obligate 

acacia-ants, but rather those species-Pseudomyrmex, Crematogaster, Camponotus-that live 

throughout the general forest canopy and that 

forage for dead, moribund, or live insects on foliage 

in general. While queens of obligate acacia-ants 

may occasionally be dispersed into habitats with A. 

ruddiae (definitely the case at sites 9 and 10 in 

Guatemala), I have never seen one of the characteristic type of entrance holes made by these queens 

in about 2,500 swollen-thorns examined carefully 

on 16 individual trees of A. ruddiae. 

To examine the use made of A. ruddiae thorns 

by ants, a representative 3 m tall, fully insolated 

sapling growing in river-edge vegetation at site 4 

was completely dissected. The acacia had 798 leaves 

on 51 branches; far less than 20 percent of the large 

leaves produced by the acacia were subtended by 

swollen thorns. Of the 371 thorns, 302 were, or had 

been, tenanted by some insect (on a “typical” 

swollen-thorn acacia, all thorns would have been). 
42 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

Nineteen percent of the thorns had split naturally 

at the base, and all of these thorns had been 

entered by insects. Of all the thorns that had been 

entered by insects, by any means, 20 percent had 

parts of ant colonies in them or queens attempting 

to start a colony. Of the remaining thorns that had 

been entered, 95 percent contained microlepidopterous larvae, weevil larvae, and Orthoptera eggs. 

When these insects emerge, the acacia thorns are 

very likely to be occupied by ants. Spiders and 

other insects tenanted the remaining 5 percent of 

the thorns lacking ants. The ants represented at 

least 7 species in 5 genera-Solenopsis, Crematogaster, Camponotus, Pseudomyrmex, Paracryptocerus. While a detailed census is not available, 

cursory examination of A. ruddiae thorns in large 

old trees shows almost every thorn to have a part 

of an ant colony in it. It is extremely likely that 

the continual foraging of these ant species on the 

foliage occasionally results in less herbivore damage 

to the tree than if the ants were not present. 

If a mutant strain of A. ruddiae provided any 

type of solid food to the ants, the probability is 

very high that it would quickly lead to the evolution of a strongly mutualistic interaction. It is impossible to know if A. ruddiae represents the 

gradual degeneration of a once intense mutualism 

or is on its way to more mutualistic interaction. 

The latter is more likely, since the former would 

require a strong relaxation of selection pressure by 

herbivores, such as might occur if a swollen-thorn 

acacia were to immigrate to a small oceanic island. 

The evolution of a type of Beltian body may be 

starting to occur with A. ruddiae; some of the 

plants at site 4 have clear tips on the pinnules, 

extending into the leaf about 0.25 to 0.5 mm. If the 

ants were to start to chew on this for moisture or 

nutrients during the time of year when insect prey 

is scarcest, and therefore maintain larger colony 

sizes than on plants lacking this modified pinnule 

tip, the evolution of a food body might occur. This 

is likely to be the way that typical Beltian bodies 

first appeared on acacias. Plants with a larger yearround population of ants would probably be less 

damaged by the periodic small outbreaks of hostspecific insects that probably occur on many tropical 

trees, and certainly occur with nonant-acacias. 

Interestingly, the species of ant that began this 

behavior would be likely to concentrate more of its 

foraging on the acacia and less off, and be likely to 

tenant an increasingly greater proportion of the 

thorns on the acacia. This could occur either 

through faster colony growth than could occur with 

ant species totally dependent on insect prey for 

growth, or through the ants moving into thorns left 

untenanted during periodic population lows in the 

other species of ants. Once the entire ant colony 

becomes concentrated in the acacia, it has a vested 

interest in the health of the acacia and coevolution 

of the interaction should occur at an ever increasing 

rate (Janzen, 1966a). As will be discussed further 

in respect to hybrids, this acacia appears to be a 

plant that would have greatly increased fitness by 

the gaining of the genetic information necessary 

for the production of Beltian bodies. This could 

well happen through introgression with a wet-forest 

acacia such as A. melanoceras at the southern end 

of the range of A. ruddiae, or A. gentlei or A. 

cookii, and the northern end of the range of A. 

ruddine. 

3. Acacia melanoceras Buerling 

Acacia melanoceras Buerling, 1854:362. 

Acacia multiglandulosa Schenck, 1913:362. 

Myrmecodendron melanoceras (Buerling) Britton and Rose, 

1928:93. 

Acacia melanoceras is easily recognized since it is 

the only swollen-thorn acacia in the wet lowlands 

of central Panama and the Canal Zone (Figure 29). 

Where its distribution is tangent with that of A. 

collinsii, about halfway through the Canal Zone, 

A. melanoceras is easily distinguished from that 

dry-forest species since A. melanoceras has a very 

large number of nectaries on the leaf petiole, 

longitudinally ridged swollen thorns, spherical inflorescences, and huge leaves, 

1 have been unable to locate the type specimen 

of A. melanoceras and it is likely destroyed; however, the type specimen of A. multiglandulosa (at 

Berlin, now destroyed, but represented by Field 

Museum photo 1281 ex Berlin) is from the same 

locality as that of A. melanoceras (site 1) and is 

probably described from the same specimen or a 

duplicate specimen of the same collection. The 

description of the type of both A. melanoceras and 

A. multiglandulosa agrees completely with the 

characteristics of the swollen-thorn acacia population whose distribution is plotted in Figure 29. The 

type specimen of A. multiglandulosa is a representa- 
NUMBER 13 43 

FIGURE 29.-General postulated distribution (stippled area) and specific collections sites of 

A. melanoceras in Panama. 

tive distal end of a fertile branch from an adult 

acacia. Very long (20 cm) inflorescence-bearing 

branchlets such as the one on the type of A. multiglandulosa are commonly seen on old trees growing 

along river edges that have their crowns partly 

shaded by taller trees. The absence of swollen 

thorns on a branch bearing sexual branchlets, as is 

the case with the type specimen, is commonplace 

in old crowns of A. tnelanoceras. In 1915, Safford 

redescribed A. melanoceras, but under the name of 

A. 

1. 

2. 

3. 

4. 

3. 

multiglan dulosa. 

KEY COLLECTION LOCAL~TIES 

Types of A. melanoceras and A, multiglandulosa: Portobello (Porto Bello), Colon, Panama (sea level). 1824, 

J.G. Billberg, no. 289. 

Head of Gatun Valley, Canal Zone, Panama. 1914, H. 

Pittier, no, 6745 (US 716562, US 716560, US 716561; Safford 1915, Wheeler 1942). 

Headwaters Rio Agua Salud, Canal Zone, Panama. 

Wheeler, 111-6-1923 (Wheeler, 1942). 

Marajal, near Colon, Canal Zone, Panama. Wheeler, VII26-1924 (Wheeler, 1942). 

Cerro Jefe, several mi N Cerro Azul, Panama, Panama. 

R.D. Dressier, VIII-28-1967 (pers. comm. plus thorn 

sample). 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

400 m upriver (east bank) from the bridge at Rio Piedras, Colon, Panama. VII-3-1967, DHJ, nos. 1641, 1912, 

1913, 1915, 1916, gp, mp. 

Southwest corner of Zorro Island (in Gatun Lake), 5 km 

NW Nueva Providencia, Canal Zone, Panama. VII-7- 

1967, DHJ, no. 1894, gp. 

10 m post on Al. Ar. Trail, Barro Colorado Island, Gatun 

Lake, Canal Zone, Panama. DHJ, VII-7-1967, nc (I am 

indebted to R.D. Dressler for showing me this plant). 

7 km SW Gatun Locks on road to Escobal, Canal Zone, 

Panama. DHJ, VII-10-1967, nc (I am indebted to M. 

Nauman for showing me these plants). 

France Field, Canal Zone, Panama (sea level). VIII-1924. 

D. Fairchild (US 1405611). 

Between France Field and Catival, Colon Prov., Panama. 

1-9-1924, P.C. Standley, no. 30378 (US 1219256). 

Near Frijoles, Rio Agua Salud, Canal Zone, Panama. 

111-6-1923, C.V. Piper, no. 5853 (US 1167694, US 

1166375). 

Margarita Swamp, Canal Zone, Panama. VI-28-1923, O.F. 

Cook and R.D. Martin, no. 14 (US 1491829). 

Peiia Blanca Bay, Barro Colorado Island, Canal Zone, 

Panama. VII-3-1934, Shattuck, no. 1087 (US 2366760). 

Hills N of Frijoles, Canal Zone, Panama (50 m). XII-19- 

1923, P.C. Standley, no. 27582, (US 1217650). 

Margarita Swamp near France Field, Canal Zone, Panama 

(sea level). 11-1925, J.B. Shropshire (US 1208884, US 

1208883, US 1208885), mp. 

Barro Colorado Island, Canal Zone, Panama. VIII-1927, 

L.A. Kenoyer, no. 373 (US 1319142). 
44 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

NATURAL HISTORY 

Acacia melanoceras is one of the seven species of 

swollen-thorn acacias with small geographic ranges 

(A. allenii, A. mayana, A. ruddiae, A. sphaerocephala, A. cookii, A. gentlei, the others), and like 

these species is restricted to comparatively wet and 

undisturbed forests. In 1928 Standley reported A. 

melanoceras as common in the forests on the Atlantic slopes of the Canal Zone; in comparison with 

the more widely distributed dry-land species, it is 

nevertheless very rare. For example, in contemporary forested habitats there appear to be at least 

100 adult plants of A. collinsii on the dry Pacific 

side of Panama for every one adult A. melanoceras 

on the Atlantic side. It is of course impossible to 

know to what degree this difference is due to A. 

collinsii becoming much more common with the 

development of brushy cattle pastures and wasteland. One thing may be said with certainty, however, and that is that A. melanoceras disappears 

from any habitat subjected to disturbances that are 

any more catastrophic than infrequent logging. 

Since the Atlantic side of Panama, aside from 

banana farming, is infrequently logged, it is likely 

that the contemporary distribution of A. melanoceras is not much different from that of pre-Colombian times. 

Every reproductively mature A. melanoceras 

found in this study (sites 6 to 9) was in secondgrowth woody vegetation that had grown better 

than 15 years without further disturbance. All sites 

were either areas that had been clear-cut long before for logging or farming (e.g., Barro Colorado 

Island) or had been swept clean by a flooding 

river. Seedlings (Figure 30~) were found only in 

the full shade of canopies 10 to 15 m tall, and on 

wet to moderately well-drained soil. A. melanoceras 

generally becomes reproductively mature under, or 

in, jagged breaks in the general canopy; they are 

almost never fully insolated. Senescent canopies are 

often directly under a small hole in the general 

canopy (Figure 31). Neither mature plants nor 

seedlings are common at any site. Two adults per 

acre is a high density for A. melanoceras. Adults 

and young plants almost always occur as single 

FIGURE 30.-Acacia melanoceras: A, seedling from deep forest shade (scale in inches; Panama, 

site 6); B, normal lateral branch from 6-rn-tall plant (Canal Zone, site 9). 
NUMBER 13 45 

FIGURE 31.-Acacia melanoceras (Panama, site 6): A, looking up through crown of 15-m-tall 

mature tree that is still reproducing, but only poorly; B, looking up through senescent crown 

of 12-m-tall tree that has lost its ant colony. 

shoots. The only clump of shoots encountered in 

this study was at site 6 on the edge of an island 

at Gatun Lake. A single tree-based on tree rings, 

at least 27 years old-had produced a large number 

of root suckers and seedlings covering an area about 

20 by 30 m. The cluster was on the north side of a 

hill and situated such that the taller forest trees 

shaded it, yet the opening caused by the lake led to 

day-long indirect insolation. Wheeler (1942) illustrates a small clump at site 4; the plant has the 

appearance of a cluster of stump and root sprouts 

from a large adult that was cut when the swamp 

was cleared of forest. 

Acacia melanoceras appears to be completely 

absent in repeatedly disturbed pasture, field edges, 

roadsides, cleared swamps, etc., in the central and 

northern part of the Canal Zone and adjacent areas 

in Panama; however, it is still present in adjacent 

relatively undisturbed forest, or forest that has 

been allowed to regenerate for a long time. Extinction of A. melanoceras following human disturbance is probably based on the following factors: 

(1) A. melanoceras does not set seed until at least 

8 to 10 years old; I have found heavy seed production only on very healthy old trees that are in 

partial shade of a tall broken forest canopy. Such a 

long time in reaching reproductive maturity is 

likely due to the shady microhabitat occupied by 

this acacia. (2) When adults or subadults are exposed to direct insolation by clearing the forest, 

they become very shrubby, stunted, and die after 

1 to 3 years. They produce no seed during this time. 

Failure to grow in full sunlight appears to have 

two causes: First, the leaves are probably genetically 

programmed to operate most efficiently at low-light 

density and air temperature, and the opposite 

microclimate is probably damaging. Second, large 

numbers of phytophagous insects in young secondary succession probably find the acacia to be very 

edible. Shoot tips are repeatedly eaten off the vertically elongating shoots, resulting in a bush life form 

which is probably very inefficient. It is not obvious, 

however, why the occupant Pseudomyrmex satanica, 

which is so effective at protecting A. melanoceras 
46 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE SZ.-Acacia melanoceras: A, reproductive branch with extraordinarily heavy pod set 

(Canal Zone, site 7); B, reproductive branchlets with representative pod set (Panama, site 6); 

c, longitudinal section through immature but full-sized pod (Canal Zone, site 7). 

in the forest, is apparently so ineffective in more 

open sites. It may be that their protective abilities 

per insect attacking are the same in both habitats, 

but that the plant is less able to repair the more 

frequent damage in the open. (3) A. melanoceras 

does not sprout readily from cut stumps. The 

stumps of many young and mature A. melanoceras 

die without sucker-shoot regeneration, in strong 

contrast to the behavior of other swollen-thorn 

acacias. It may be that heavily shaded plants have 

insufficient energy reserves stored in their roots for 

reproduction by sucker shoots. (4) The seed of A. 

melanoceras (Figure 3%) has a soft seed coat and 

apparently needs to germinate immediately. Even 

if a seed is dispersed into early stages of succession, 

it would probably die of desiccation if it arrived 

during the dry season of one of the dry “spells” 

during the wet season. (5) Being soft-seeded, A. 

melanoceras cannot be dispersed by the same species 

of birds that disperse the hard seeds of species 

living in drier habitats (eg., A. cornigera, A hindsii, 

A. collinsii); these birds (e.g., jays, derby fly-catchers) pass the hard seeds through their gizzard 

unharmed, while a seed of A. melanoceras would 

surely be digested. Any dispersal of these soft seeds 

must be via birds that regurgitate seeds (e.g., trogons, cotingas). These birds characteristically live in 

older stages of succession and primary forest and are 

unlikely to disperse A. melanoceras seeds out into 

large tracts of young second growth. (6) Since the 

seedlings of A. melanoceras are normally lound 

only in heavily shaded forest understory, it would 

not be surprising to find that the queens of P. 

satanica do not search in dry or fully insolated 

sites. The young acacia would then be competing 

with many fast-growing plants with a very high 

density of herbivorous insects. It is possible that 

the searching queens of P. ferruginea, coming from 

A. collinsii growing in young second growth, could 

occupy young A. melanoceras. A. collinsii, however, 

has not yet invaded much of the disturbed land 

left when forest suitable to A. melanoceras was 

cleared. This general interaction could perhaps be 

observed if half of an island such as Barro Colorado 
NUMBER 13 47 

Island were clear-cut and turned into brushy pasture. 

Acacia melanoceras, A. allenii, and A. ruddiae 

live in the wettest sites occupied by the genus in 

Central America. The dry season in its habitats is 

indistinct and the sites generally receive 3.5 to 5.5 

m of rainfall. At sites 6 and 7, the roots of A. 

melanoceras were under water or in mud almost all 

year. The cluster of fruits in Figure 32~ hung down 

to within a few inches of Gatun Lake. The bark of 

A. melanoceras is almost always damp from rain 

and those parts of the acacia not patrolled by ants 

often become covered with lichens and other 

epiphytes. 

That A. melanoceras lives in wet forest that lacks 

other acacia, and for the most part other mimosoid 

legumes, may explain in part why quite healthy 

but unoccupied adults are occasionally encountered 

in the forest. Judging from the holes in the thorns, 

these acacias were at one time occupied by large 

ant colonies, which died for unknown reasons. In 

contrast to the subsequent death of the tree that 

would have occurred were this a dry-land acacia, 

the abandoned acacias appear to live a number of 

years, bear successive seed crops, and are at times 

tenanted by a variety of other ants that normally 

live in hollow twigs in the general canopy. These 

unoccupied acacias suffer herbivore damage, but it 

is not very severe (in contrast to individuals even 

with a strong ant colony in insolated sites a few 

hundred meters away). I suspect that in leaving 

v) 

W 

U 

U 

m 

W 

el 

!A 

z 

6 40 

30 

- 20 

z 10 

B 

3 

z 

behind the younger stages of succession and the 

large number of insects it harbors, especially those 

that normally feed on mimosoid legumes, A. 

melanoceras has moved into a habitat of reduced 

herbivore pressure. Such a strategy is likely to be 

effective until over evolutionary time other mimosoid legumes make the same move for the same 

reason. 

While A. melanoceras appears to be pollinated 

by small bees-Halictidae, Megachilidae, Ceratina 

in the Apidae-pollination is either very ineffectual or the plant has little reserves for maturing 

fruits. The seed crops on almost every tree examined have been very small, ranging from 2 to 30 

pods; the long flowering branchlets (Figure 32~) 

set only 1 or 2 pods per branchlet and often none 

at all. The 13-seed pod in Figure 32c is representative of the number of seeds per pod; A. melanocerns does not show the strong variation in number 

of seeds per pod found in the acacias of drier sites. 

The large pod crop on the acacia at site 7 provides 

several lines of evidence to suggest that the usual 

small seed crop is due to lack of energy reserves 

rather than poor pollination: (1) This tree was 

growing in an exceptionally good site, as is evidenced by the large number of healthy sucker 

shoots, had an immense ant colony, and large 

amounts of lush foliage (as in Figure 30~ and 

contrasted with Figure 31~). There were no other 

adult swollen-thorn acacias within at least 200 m 

of this plant (down the island) and at least 500 m 

123 13 57 A NUMBER OF PODS PER 6 NUMBER OF INFLORESCENCES WITH 

INFLORESCENCE PODS PER BRANCHLET 

FIGURE BB.-Acacia melanoceras (Canal Zone, site 7): A, distribution of 64 mature pods among 

the 49 infructescences on 10 sexual branchlets from one tree with an extraordinary seed crop; 

B, distribution of the 49 infructescences in Figure A among the 10 sexual branchlets. 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 

across the lake. This plant set at least 900 pods in 

large clumps such as that in Figure 32~. ‘The implication is that the tree is self pollinated, or 

else the clump of shoots originated from two seeds. 

(2) The total number of inflorescences produced by 

each sexual branchlet appeared to be no different 

from that of other acacias examined that had produced only 0 or 1 pod per sexual branchlet. The 

number of sexual branchlets per major tree branch 

was likewise no different in any obvious way; however, the number of inflorescences that produced 

pods, per branchlet, was much higher (Figure 3313) 

than is usually observed with A. melanoceras. Two 

sexual branchlets had 7 pods each on them, and 

none had only one. (3) There were even more than 

one pod set per inflorescence in many cases (Figure 

33~). That the pods were distributed among the 

inflorescences in this manner, rather than being 

uniformly distributed (i.e., retain more inflorescences, each with one fruit), suggests the possibility 

that the plant chooses out-crossed female florets as 

those to mature. This tree had an average of 31 

inflorescence-bearing axils per sexual branchlet 

(=lo). Its total crop of about 18,540 inflorescences 

produced about 960 pods on 735 inflorescences for 

a total seed crop of about 10,560 seeds. There are 

about 10 female florets on each inflorescence, meaning that only 0.6 percent of the tree’s ovules 

matured. 

Associated with the small seed crop observed on 

most adult A. melanocel-as, it is particularly noteworthy that this species is free from predation by 

the bruchid beetles that take such a high toll of 

hard-seeded acacias. This means that virtually every 

FIGURE 34.-Acacia melanoceras (A-C, Canal Zone, site 7): A, type B thorns on short lateral 

branches from the main trunk of a mature tree; B, immature type B thorns on newly developing short lateral branch from same trunk; ants are Pseudomyrmex satanica; c, maximally developed cluster of type B thorns on lateral branch of same tree; D, type B thorns on lateral 

branch from crown of very old A. inelanoceras (Panama, site 6). 
NUMBER I3 49 

seed dispersed by the plant is viable, in contrast to 

the case with hard-seeded, swollen-thorn acacias 

where most seeds are killed by bruchids before the 

seeds are dispersed. In short, the average number of 

viable seeds produced per adult A. melanoceras is 

likely to be as great as for acacias of drier sites 

that initially produce many more seeds. 

A seedling of A. melanoceras (Figure 30~) can 

probably germinate and grow slowly to adulthood 

almost anywhere in the forest, in strong contrast to 

the seedlings of hard-seeded, swollen-thorn acacias, 

which must land in a fairly young stage of succession to survive. This means that a certain large fraction of the viable seeds produced by dry-land acacias 

will not make it to the stage where they have any 

chance at all of surviving. Therefore, the apparently 

smaller seed crop of A. melanoceras may in fact not 

mean that any fewer saplings develop per adult 

than is the case with dry-land, swollen-thorn 

acacias. 

Throughout its range, I found A. melanoceras 

to be occupied only ,by Pseudomyrmex satanica 

(Figure 3413). (The “Pseudomyrmex satan” cited by 

Standley in 1928 is a misspelling.) Wheeler (1942) 

likewise had no records of other obligate acacia-ants 

in A. melanoceTas. Since P. satanica is restricted to 

A. melanoceias, the ant will probably go extinct 

as the remaining forest in Panama is converted to 

agricultural land. P. satanica is probably a “wet 

forest derivative” of the widespread P. ferruginea, 

which occupied A. collinsii in the drier parts of 

Panama, among other places. As will be discussed 

later, the part of the P. ferruginea population occupying A. allenii on the Osa Peninsula and the 

Golfito region in Costa Rica probably represents 

an intermediate form of the same type of speciation 

event. P. satanica is easily distinguished from P. 

ferruginea in Panama by the former’s black color, 

habit of holding the gaster curled under the body 

(Figure 34~), and having multiple-queen colonies. 

Unoccupied adults of A. melanoceras are tenanted 

by the usual miscellaneous array of ants found in 

unoccupied swollen-thorn acacias: Crematogaster, 

Azteca, Camponotzis, Paracryptocerus, and Pseudomyrmex. Since, as mentioned previously, unoccupied A. melanoceras seems to survive fairly well in 

well-developed forest, these colonies may attain 

unusually large size for ants tenanting swollen-thorn 

acacias. At times an Azteca or Crematogaster colony 

will take over all the thorns on the tree, but the 

ants’ foraging on the acacia foliage is not thorough 

enough to prevent frequent damage by herbivores. 

P. satanica founding queens establish themselves in 

the thorns of A. melanoceras seedlings at the stage 

shown in Figure 30~. Until the ant colony has 

grown large enough to patrol the young shoot, 

growth is not rapid enough for the plant to have 

more than 3 to 6 intact thorns less than two years 

old) at any one time. Not only is the number of 

founding queens in the system small as compared 

with dry-land, swollen-thorn acacias, but one very 

rarely finds searching queens on the outside of the 

acacia. This indicates that few queens are produced 

by adult colonies or that the queens have a difficult 

time locating the widely spaced seedlings. Both 

possibilities are probably true, 

FIGURE 35.-Dormant shoot tip and “crown” of A. melanoceras seedling above point where latest swollen-thorns were 

produced (Panama, site 6). 
50 SMITHSONIAN CON'I'RIBUTIONS TO BOTANY 

FIGURE 36.-Acacia melanoceras (Panama, site 6): A, petiolar nectaries characteristic of seedling 

leaves near swollen thorns; B, maximal development of petiolar nectaries in crown of old plant; 

c, minimal petiolar nectary development of large leaves subtended by type A thorns on lateral branches of 5-m-tall subadult. 

Leaf production on thc seedling acacia differs 

from that of dry-land acacias. The seedling produces 1-4 leaves subtended by swollen thorns followed by a cluster of leaves without swollen thorns 

(Figure 35). By the time that swollen thorns have 

rotted off (about 1.5 years later), the leaves have 

been shed and the process is repeated if an ant 

colony has not become established. How long the 

seedling can continue this process is unknown, but 

I have found seedlings that have gone through 

this process at least five times. The seedling in 

Figure 30~ had done it at least four times, yet was 

only 53 cm tall. 

On the very young seedlings there may be no 

nectaries on the leaves (Figure 35) except those 

TABLE 9.-Distribution of leaflets among the 9 

leaves on a 40-cm-tall seedling of A. melanoceras at 

the time of production of its first swollen thorn 

(a total of 52 leaves had been produced by the 

seedling, and the thorn was on the 50th node) 

Rachis Petiole Number of Number of 

number length (mm) length (mm) pinnae pinnules 

Leaf 1 ~ 83 44 16 428 

108 

104 

117 

112 

113 

116 

108 

109 

22 22 

31 18 

30 24 

27 21 

24 28 

28 24 

30 22 

32 21 

604 

492 

660 

492 

174 

654 

598 

512 
NUMBER IS 51 

280 

240 

200 

160 

120 - 

z 

Y 

L4 

4 

UI 

cl 

0 

X 

Fu 

L 

m 

u *O 

el 40 

. 

. . . . . .* 

. . t m. 

.. .. . *. '. *.*. 

.. . *. .*. . ..*. . .. t 

0.- .* 

.* 

. . . . 

. 

. . .. . .. .* .' 

: **. . . . . t *. 

. 

. 

. 

. 

. 

I I I 1 I I 

0 5 10 15 20 25 

NUMBER OF NECTARIES PER LEAF 

FIGURE 37.-Distribution of 1256 petiolar nectaries among 24 leaves of A. melanoceras from 

sterile lateral branches in an adult crown (Panama, site 6). 

subtended by, or close to, swollen thorns (Figure 

36~). As the plant matures, the number of petiolar 

nectaries increases through the progression shown 

in Figures QA, B, c. The distribution of petiolar 

nectaries among the leaves (Figure 30~) in a mature 

acacia's crown (Figure 37) displays all the frequencies that can be observed by watching seedlings 

develop, but the majority of leaves have between 

5 and 20 nectaries. The leaves of A. rnelanoceras 

(Figure 30~) are at least three times the size of 

those found on dry-land acacias. The number of 

nectaries is higher on adults of A. melanoceras than 

on any other swollen-thorn acacia. On some A. 

melanoceras there are even nectaries on the pinnae. 

Owing to the shaded nature of its habitat, however, 

nectar production may be very low. 

Beltian body production by A. melanoceras is 

never fully developed, again probably from the 

relatively high cost to the plant of Beltian bodies 

in a habitat with low solar energy input and (perhaps) somewhat reduced herbivore loads. The first 

leaves on a seedling, as is usual with swollen-thorn 

acacias, have no Beltian bodies. By the time the 

leaves are developed to the level described in Table 

9, there are 1 to 3 Beltian bodies per new leaf; a 

dry-land acacia would have been producing 10 to 

20 per leaf by the time it had produced this many 

(52) leaves, even when growing in the shade. Even 

the very large and healthy acacia at site 6 was 

producing Beltian bodies on only about half its 

pinnules. 

On adult A. melanoceras, type B thorns appear 

on a small stubby branchlet (Figure 34~) which 

quickly elongates to 5 to 10 cm (Figure 34~). On 

large and healthy occupied acacias, such as the one 

at site 6, there may be many of these short shoots 

in relatively tight clusters (Figure 34c). On acacias 

in poorer condition, they are usually scattered along 
52 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

140 

120 

E 

E 

Y 

E 100 

W 

cl 1 .i. 

140 

,120 

s 

E - 

A 1.4 1.8 2.2 2.6 

i.101181111'' 

TOTAL THORN VOLUME (cc) 

r 

c. 

W 

cl 

g 100 

B 

0 2 80 

1.0 1.4 1.8 2.2 2.6 3 .O 

TOTAL THORN VOLUME (cc) 

FIGURE 38.-Mature A. melanoceras (Canal Zone): A, distribution of total thorn volume among 

the 25 largest type B thorns (site 9); B, distribution of thorn volume among the 31 largest type 

B thorns (site 7). 

the branches (Figure 34~). The volumes of A. 

melanoceras type B thorns (Figure 38) are not spectacular and are rarely more than twice that of type 

A thorns. The similarity of the two different sets 

of thorns in Figure 38 is probably representative 

of variation in type B thorns throughout the range 

of A. melanoceras. 

Several aspects of the biology of A. melanoceras 

reflect its occupation by a multiple-queen species of 

ant: (1) As noted, there is not a great difference in 

size or toughness between type A and type B thorns. 

While P. satanica colonies do not have the great 

number of queens characteristic of P. venefica and 

the undescribed multiple-queen Pseudomyrmex in 

Acacia hindsii in western Mexico (Janzen, 1973), 

they do have 5 to 20 or more per colony and this 

means that the loss of a queen-thorn to a bird or 

other predator is not fatal to the colony. This 

means that the primary selective pressure for very 

large and strong type B thorns has been removed. 

(2) When there is the rare opportunity for an A. 

melanoceras to reproduce vigorously with root suckers, as at site 6, the multiple-queen colony can expand indefinitely to provide patrolling for the 

increased surface area of foliage. Therefore, while 

A. melanoceras normally occurs as single plants 

that could easily be patrolled by a single-queen 

colony, when the situation arises, the plant can 

greatly increase in size (and presumably seed 

production). The acacia is effectively polymorphic 

for growth form. (3) The new searching queens 

may on occasion be absorbed by the colony 

that produced them rather than go off in 

search of seedlings. This may result in a 

lower survival of seedlings than would be the case 

with a single-queen ant species situation. On the 

other hand, the presence of several queens in a 

young seedling are all contributing workers to the 

development of the young plant. (4) When the 

opportunity occurs for the growth of a cluster of 

A. melanoceras shoots occupied by one multiplequeen colony, the shoots are free of competition 

from immediately adjacent vegetation. As is usually the case with large multiple-queen colonies, 

P. satanica is especially effective at killing foliage 

that contacts its acacia. Incidentally, as the name 

suggests, P. satanica is very aggressive toward a wide 

variety of foreign objects. These ants are perhaps 

the most difficult of the obligate acacia-ant species 

to work with, primarily because they run inside 
NUMBER 13 53 

one’s clothing rather than just on top as is the case 

with the other species. As mentioned before, this 

means that a large cluster of A. melanoceras occupied by a large colony of P. satanica tends to be 

in extremely good condition. 

4. Acacia allenii, new species 

Arbol usque ad 25 m alta; ramuli novelli minute 

velutini, glabrescentes; stipulae spinescentes interdum magnae, teretiusculae, porcatae, usque ad 4 

cm longae, 1 cm latae; folia 20-40 cm longa; nectaria sessilia cupulata ad juncturas pinnarum 

omnium et 1-10 in petiolis; pinnae (5-) 10-20- 

jugae, 7-10 cm longae; pinnulae (1%) 20-30-jugae, 

oblongae, lineare, vel ellipticae, (3-) 5-10 mm 

longae, 1-4 mm latae, obtusae, glabrae; flores aurei 

in capitulo globoso; involucellum annulare in 

medio pedunculo vel inferiore, interdum basi; 

legumina (immatura) atrobrunnea, puberula, 

granulata, glabrescentes, lineares, 8 cm longa, 8 mm 

lata, complanata, acuta sessiles vel subsessiles. 

TYPE.-”. H. Janzen 1769, Costa Rica, Puntarenas Province, Osa Peninsula, bank of tributary 

of Rio Agua Buena, 3.5 mi SW of Rincon, 9 March 

1967 (holotype US 2639555; isotypes F, GH, 

Rica: Puntarenas: Forested hills near Golfito, Golfo 

Dulce, P. H. Allen 5B7 (US). 3.4 mi E of Golfito, 

Janzen 1658 (F, MICH, MO, NY, UC, US). 6 mi S 

of Rincon on the "Road to the Pacific," Janzen

2000 (F, GH, MEXU, MICH, MO, NY, UC, US). 

Type tree 8 m tall with widely spaced branches 

and very dark green bipinnately compound leaves 

20 to 40 cm long. Pinnules 8 to 11 mm long, 2 to 

3.5 mm wide and up to 60 per pinna. Pinnae 7 to 

10 cm long and up to 40 per leaf. 2 to 10 widely 

spaced petiolar nectaries and 1 to 2 nectaries on 

rachis at bases of pinnae. Swollen stipular thorns 

dark brown and up to 4 cm long and 1 cm wide at 

base, external thorn walls with low and rounded 

longitudinal ridges. New leaves with orange-yellow 

food body on tip of each pinnule. Internode length 

5 to 10 cm on lateral branches. Sterile specimen. 

Fruits and flowers described below from other 

specimens. Janzen field collection number 1769. 

Types deposited in U. S. National Herbarium, 

Smithsonian Institution. 

This plant is named in honor of Paul H. Allen, 

who collected the first specimen (site 1) and did so 

MEXU, MICH, MO, NY, UC). PARATYPES.-cOSta 

much to aid plant biologists working in Central 

American wet forests. In his book (Allen, 1962) 

A. allenii is identified as A. melanoceras, a mistake 

easily made if one is working from herbarium specimens of either species. His specimen from site 1 

bears immature inflorescences but was not chosen as 

the type because its leaves were badly damaged and 

very small. This type of foliage is typical of flowerbearing branches but not sterile branches. Sexual 

material is of almost no use for identification or 

reference since plants bearing fruits or flowers are 

extremely rare in time and space. For these reasons, 

a sterile plant was chosen for the type (site 6). 

Specimens of mature pods are not available from 

any A. allenii, and the only good collection of 

flowers (site 5, no. 2000) is from a nearly leafless 

tree. The tree from which the type specimen was 

taken was growing approximately in the center of 

the geographic range of A. allenii (Figure 23), and 

along a creek bank in lightly disturbed “rain 

forest,” which seems to be the normal habitat for 

A. allenii. Its foliage is very representative of sterile 

and mature plants of the species; the plant was 4 m 

tall and originated as a sucker shoot from a 22 cm 

diameter downed trunk that was at least 25 years 

old. I estimate that the shoot would have flowered 

for the first time within the next 3 to 4 years. It 

was occupied by a large colony of Pseudomyrinex 

ferruginea. 

Acacia allenii is very similar (and presumably 

very closely related) to A. melanoceras. I feel justified in distinguishing it as a separate species on the 

following grounds: 

1. The population of A. allenii appears separated 

from that of A. melanoceras by at least 100 miles 

(and probably more) and by the Continental Divide. Despite careful searching, neither plant has 

been found in the wet and relatively undisturbed 

forests along the Pacific side of the mountains 

running through the western half of Panama. It 

seems that by the time one moves high enough in 

the mountains to find sufficient rainfall for A. 

allenii or A. melanoceras, it is too cold for the 

species. A. collinsii occupies the drier areas between 

the ranges of A. allensii and of A. melanoceras. In 

view of the long geographic distance between A. 

allenii and A. melanoceras, genetic exchange by 

either pollen or seed flow seems extremely improbable. This is not meant to imply, however, that 

there would not be extensive exchange of genetic 
54 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

10 

u) 

-1 

z 

V 

t; 

25 

b 

a 

m 

a 

w 

I 

3 

X 

m 

40 

30 

u) 20 -1 

x 

4 

l-l 

!!3 10 a 

w 

3 

X 

B 

1 

13519 15 13 5 7 9 11 1315 

A NUMBER OF INFLORESCENCE-BEARING B NUMBER OF INFLORESCENCES PER AXIL 

AXILS PER BRANCHLET 

FIGURE 39.-Acacia allenii (Costa Rica, site 5): A, distribution of 242 inflorescence-bearing axils 

among 37 sexual branchlets from the crown of a 22-m-tall tree: B, distribution of 2255 inflorescences among the 242 axils in Figure A. 

FIGURE 40.-Acacia allenii (Costa Rica): A, left, type B thorns on the base of a lateral branch 

with terminal type A thorns: right, type B thorns on a short lateral branch from the main 

trunk (site 2): B, left, type A thorns on lateral branches and main trunk of 3-m sapling; right, 

type B thorns on short lateral branches of the same tree (site 5). 
NUMBER 13 55 

material were this gap to be bridged in some manner, This seems unlikely, since both species become 

even more rare than they already are as their 

habitat comes under agricultural influence. 

2. The foliage of A. allenii is more similar to 

that of A. melanoceras than it is to any other 

acacia. However, leaves from mature A. allenii are 

easily distinguished from those of A. melanoceras, 

since a leaf of A. allenii usually has only 1 to 5 

petiolar nectaries and has pinnules with about twice 

the surface area of those of A. melanoceras. These 

two differences, along with the floral traits described 

below, are consistent throughout the population of 

A. allenii. 

3. Inflorescences of A. melanoceras are borne on 

long (up to 45 cm) sexual branchlets that tend to 

be bunched near the ends of sterile branches; infloresences of A. allenii are borne on short (3 to 15 

cm) sexual branchlets scattered along major 

branches and therefore throughout the crown. The 

number of axils with inflorescences on a sexual 

branchlet (Figure 39, 42~) is about half that of 

A. melanoceras. While each sexual branch of A. 

melanoceras has 1 to 5 inflorescences per axil, each 

inflorescence-bearing axil on A. allenii has up to 

15 or 16 with a mean at about 9 for large samples. 

4. The type B swollen thorns of A. melanoceras 

are normally found on short branches, while those 

of A. allenii are much more likely to be strung 

along the lower parts of a branch that bears large 

leaves and type A thorns on its distal portion 

(Figure 40~). A. allenii thorns regularly have an 

inward projection that partly occludes the basal 

part of the thorn cavity. This is almost always 

absent from the thorns of A. melanoceras. 

5. The differences between A. allenii and A. 

melanoceras are of the same magnitude that can 

be encountered over the ranges of one widely spread 

species such as A. collinsii, A. cornigera, or A. 

hindsii. However, these widespread species appear 

to have continuous ranges and almost always the 

variation is clinal in nature. A. allenii and A. 

melanoceras each appear to be going its own way 

evolutionarily, and I have no reason to suspect 

that their similarity will persist through time. If 

for no other reason, they are likely to diverge since 

A. melanoceras is occupied exclusively by a multiple-queen ant colony while A. allenii is occupied by 

a single-queen species. There is always the possibility that P. ferruginea will evolve multiple-queen 

behavior and thereby evolutionarily “catch up” 

with P. satanica in A. melanoceras. In the meantime, however, A. melanoceras will likely have 

further diverged from A. allenii. 

6. When describing the ecology of these two 

populations, lumping them under one Latin name 

destroys resolution. To call them mutual “subspecies” is to infer a common evolutionary origin, 

when in fact A. melanoceras may be a wet-forest 

derivative of the Panamanian portion of the A. 

collinsii population, while A. allenii may be a 

wet-forest derivative of the Costa Rican portion of 

the A. collinsii population. Even if one is in fact 

the result of a long-distance, overland-seed dispersal 

from the other’s population (the most likely origin), 

we still have no way of knowing which is the 

“parent” species and which the “subspecies.” 

KEY COLLECTION LOCALITIES 

All specimens are from Puntarenas Province, 

Costa Rica. 

1. 

2. 

3. 

4. 

5. 

6. 

Forested hills near Golfito. 111-3-1951, P. H. Allen, no. 

5997 (USNH 2085794). Allen (1952) reports A. allenii as 

occasional in pastures around Golfito. 

2.5 mi E (inland) of Dominical. DHJ, VI-25-1967, no. 

1669, mp. 

3.4 mi E Golfito. DHJ, VI-28-1967, no. 1658, fl. 

11.0 mi E Golfito. DHJ, VI-28-1967, 11-28-1965, nc. 

Within 8 miles of Rincon along road to the Pacific and 

along highland road, Osa Peninsula. DHJ, 111-1-3-1965, 

no. 791, 803, 811, 812, 813; 111-10-1968, no. 2000, fl. 

TwE.-bank of tributary of Rio Agua Buena, 3.5 mi SW 

Rincon, Osa Peninsula. DHJ, 111-9-1967, no 1769. 

NATUML HISTORY 

Like A. melanoceras, A. allenii has a very narrow geographic range (Figure 23) which probably 

reflects rather narrow tolerances of moisture and 

temperature. Its uppermost elevation record (160 

m at site 2) is probably representative for the entire 

range, and, incidentally, is probably about the same 

as that for A. melanoceras on the Atlantic side of 

Panama. A. allenii is occasionally found in somewhat drier sites than A. melanoceras (such as landslide succession, roadsides), but otherwise it appears 

to have the same ecological difficulty with invading 

disturbed habitats as detailed for A. melanoceras. 

Acacia allenii provides a somewhat better test 

than A. melanoceras of the prediction that both 

species are likely to become extinct with the intro- 
56 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 41 .-Sedling of A. allenii; note single swollen thorn 

in crown and old rotted thorn near bottom of photo (Costa 

Rica, site 5). 

duction of European-type agriculture to a particular site. Allen (1952), based on experience around 

Golfito in the 1940s, when much primary forest was 

being cleared, reported that A. allenii was an occasional plant in brushy pastures. Today (1965- 

1970) it is totally absent from the pastures in the 

Golfito region but may still be encountered on 

the forest edge where fields are being pushed back 

into the forest, especially along the road between 

Golfito and the Pan-American highway. When the 

forest is cleared, the stumps of A. allenii produce 

small shrubby bushes. Like those of A. melanoceras 

under the same circumstances, these pasture shrubs 

may live for 1 to 4 years and attain a height of 2 

to 3 m-in the forest they live as long as 25 years 

and attain a height of 22 m. Since the ant colonies 

in these shrubs are quite ineffective at clearing the 

vegetation from around the acacia, pasture fires 

normally kill the acacia and its ant colony. In late 

stages of woody succession such as on landslides or 

undisturbed regeneration following logging, light 

ground fires kill neither the acacia nor ant colony 

(Janzen, 1967a). 

In the general area of Rincon on the Osa Peninsula, healthy seedlings of A. allenii in the 0.5 to 2 

m size class occur at a density of at least 1 per 

hectare in old natural secondary succession (such as 

along creeks and rivers) ranging from 20 to 40 years 

of age. These young plants are often occupied by 

colonies of P. ferruginea with 25 to 300 workers. A 

high percentage of these seedlings appear to attain 

reproductive maturity since the same habitats have 

densities of adults as high as one per 5 hectares. 

When an area is cleared by logging and then allowed to return to forest, the density may be much 

higher, indicating that in uniform woody succession, seedling survival is higher than in the fragmented successional stages along rain-forest waterways and landslides. For example, the lower part 

of the hillside immediately north of the airstrip 

near Rincon was almost clear-cut in the late 1940s 

and then allowed to regenerate undisturbed. The 

density of adult or subadult A. allenii in this site 

is about 3 per hectare. Along logging roads, A. 

allenii adults or subadults may attain a density as 

high as 4 per linear mile of road. 

The seedlings (e.g., Figure 41) of A. allenii are 

morphologically and physiologically very similar to 

those of A. melanoceras. For purposes of later comparison with dry-land, swollen-thorn acacia seedlings, I offer the following description of a 90 cm 

tall A. allenii seedling (site 5). The plant was 

growing in deep shade in undisturbed forest on 

level ground and had a tap root 22 cm long and 

8 mm in diameter at ground level. As do other 

swollen-thorn acacias, the rootlets had nodules on 

them. The taproot went straight down and had 8 

lateral roots of 1 to 2 mm diameter. The stem 

had narrowed to 5 mm diameter at 10 cm above 

the ground. Based on leaf scars, between 93 and 

110 leaves had been produced, 8 of which were on 

the plant. Between 10 and 14 swollen thorns had 

been produced, 2 of which were still intact and 

contained founding queens of P. ferruginea. The 

swollen thorns had the longitudinal shallow grooves 

characteristic of adult plants of both A. allenii and 

A. melanoceras. Their volumes were 0.8 and 0.9 cc. 
NUMBER 13 

FIGURE 42.-Acacia allenii (Costa Rica): A, sexual branchlet (site 2); B, mature seeds: seed in 

lower left had split its thin seed coat and was starting to germinate before it left the pod (site 

2); C, rachis nectaries on leaf subtended by a type A thorn (site 5). 

FIGURE 43.-Acacia allenii (Costa Rica, site 4): A, sapling that was growing in full shade; B, type 

A thorns in crown center of the sapling. 

57 
58 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

As with A. melanoceras, the seedling produced a 

whorl of leaves without swollen thorns which were 

either followed or preceded by several leaves with 

swollen thorns. The leaves were nearly as large as 

those of an adult, and radiated outward such that 

they did not overlap on a horizontal plane. Freedom from overlap was aided by the leaf having a 

3 cm petiole about twice the length of the petiole 

on a mature leaf. The leaves ranged from 15 to 18 

cm in length, had pinnae 22 to 29 mm long, and 

had pinnules 6 to 7 mm long and 1.5 to 2.0 mm 

wide. Beltian bodies were on only the basal 3 to 6 

pinnules of each pinna, and were 2 to 4 mm long. 

These large Beltian bodies are about twice the 

length and 4 times the volume of those on a mature 

plant. There were no petiolar nectaries, but there 

was a single small nectary at the base of each pair 

of pinnae (as in Figure 42c). 

Once such a seedling is occupied by an active 

colony of ants, it will grow into a sapling in 2 to 3 

years such as the one in Figure 43~. As the sapling 

grows, it maintains a single-stem life form and 

produces virtually no root sucker shoots. In very 

young second growth, the plant becomes very bushy 

owing to insects eating off the growing shoot tips 

(again as with A. melanoceras). 

Seed-bearing adults have 12 to 20 growth rings 

and are probably 15 to 23 years old. They may 

attain a DBH of 35 cm and are generally 12 to 25 

m tall. The adults are almost never emergent to 

the general canopy (in strong contrast to dry-land 

species). Senescent canopies look like those figured 

for A. melanoceras. After the top has rotted out 

and the trunk fallen, the root crown sometimes 

produces a single sucker which may become occupied and grow to adult status. The same happens 

if a healthy adult is cut, but if the sucker shoots 

are then removed, the stump usually fails to produce more. It appears to have very limited food 

reserves (again, in contrast with dry-land acacias). 

As does A. melanoceras, A. allenii lives in a 

habitat (heavily shaded understory of wet forest) 

of less intense defoliation pressure as contrasted 

with the dry-land, swollen-thorn acacias. In an attempt to quantify this difference, an experiment of 

the type conducted extensively with A. cornigera 

in lowland eastern Mexico (Janzen, 1967b) was 

performed on the A. allenii population at site 5 

(cf. similar experiment for A. collinsii for direct 

comparative data for a dry-land species). During 

the two weeks preceding March 1, 1968, I searched 

intensively for adult or subadult A. allenii in site 

5. The 21 healthy plants located were in various 

kinds of vegetation and ranged from 5 to 20 m in 

height. None was fully insolated. On March 1, all 

were cut, leaving 60-cm-tall stumps in heavy to 

light shade. The control plot of 12 acacias was 

established by placing the acacia crown, with its ant 

colony, over or adjacent to the cut stump. The 

experimental plot of 9 acacias was established by 

dragging the crown at least 5 m from the stump 

and usually farther. Experimental and control 

plants were mixed in sequence, with the treatment 

determined by a flip of a coin. As anticipated, the 

ants-or the colony-abandoned 6 of the control 

acacias and reoccupied one of the experimental 

stumps by moving the colony 6.5 m from the old 

crown to the new sucker shoots from the stump. 

This left a final tally of 7 occupied stumps and 

14 unoccupied; these adjustments occurred within 

the first month after the stumps were cut. On 

September 17, 1968, six and a half months later, a 

variety of parameters were recorded (Table 10). 

The effect of an ant colony on sucker regeneration 

is obvious, but it is not nearly as dramatic as were 

this experiment to have been conducted with a 

dry-land acacia during six and a half months of the 

rainy season. These data, and some of the more 

obvious reasons for this difference are discussed in 

the following paragraphs. 

The diameter of the stump of a swollen-thorn 

acacia is weakly correlated with the amount of 

reserves the plant has for the production of sucker 

shoots. While the stumps of the occupied acacias 

appear to have smaller diameters, thus biasing the 

experiment in the favor of unoccupied shoots, in 

fact they are not statistically different. Further, 

differences in growth between occupied and unoccupied shoots for other reasons are usually far 

greater than those resulting from difference in 

stump size. Without much larger samples, it is impossible to segregate out the effect of stump size. 

The tallest sucker shoot may originate at any 

point on the side of the 60 cm stump. For this 

reason, small samples tend to have a very high 

variance in the total height of the plant in the 

early stages of sucker regeneration. That the occupied and unoccupied acacias have significantly 

different total heights only at the 0.025 (t-test) 

level is a reflection of this high variance. In this 
NUMBER 13 59 

TABLE 10.-General condition of occupied and unoccupied A. allenii after 6.5 months of sucker shoot 

growth from cut stumps (see text for details), Costa 

Rica, site 5 

Measurements 

Stump diameter (mm) 

average ........... 

s.d. ............... 

median ........... 

range ............. 

Maximum height of 

shoots (a) 

average ........... 

s.d. ............. 

median ........... 

range ............ 

Length of longest 

shoot (cm) 

average ........ 

s.d. ............... 

median .......... 

range ............ 

Number of suckers 

per stump 

average ........... 

s.d. ............... 

median ........... 

range ............. 

Number of intact 

shoot tips per stump 

average .......... 

s.d. ............... 

median ........... 

range ............. 

Number of damaged 

shoot tips per stump 

average .......... 

s.d. ............... 

median ........... 

range ............. 

Number of swollen 

thorns per shoot 

average ........... 

s.d. ............... 

median ........... 

range ............. 

Number of leaves 

per stump 

average ........... 

s.d. ............... 

mediah .......... 

range ............. 

Total live weight 

of leaves per stump 

(gm) ............... 

Occupied Unoccupied 

(n=7) (n i= 14) 

40 

17 

44 

18-67 

157 

73 

172 

41-245 

112 

105 

117 

18-202 

6.6 

1.8 

6 

4-9 

5.0 

2.0 

5 

3-7 

1.9 

1.6 

2 

0-5 

22 

22 

15 

0-57 

48 

20 

44 

16-74 

76 

56 

28 

52 

11-1 15 

81 

50 

85 

0-182 

34 

29 

37 

0-83 

10.6 

6.5 

13 

0-1 8 

1.8 

1.6 

1 

0-6 

12.0 

9.1 

11 

0-29 

20 

23 

5 

0-69 

32 

26 

27 

0-73 

10 

respect, the results are not outstandingly different 

from those expected in a drier habitat, except that 

in the dry land experiment this high variance 

would have disappeared within about 3 months of 

sucker regeneration. 

Much less so than with dry-land acacias, A. 

allenii suckers are not “racing” to maintain a spot 

in the general canopy. They are already far below 

it and a difference of a few centimeters in height, 

between occupied and unoccupied, is not so important as it is with an acacia whose survival to 

reproductive maturity depends on remaining insolated. With A. allenii, success in height increment 

is measured in the size of eventual seed crops, 

rather than in living or dying during a short 

period. 

Of course it is the length of the “leading” sucker 

shoot that usually determines the height of the 

regenerating acacia, though sometimes the longest 

sucker originated low on the stump and this is in 

fact shorter than the tallest sucker. The significant 

difference at the 0.005 level (t-test) between the 

length of occupied and unoccupied suckers is the 

result of feeding damage by insects. If no shoot tips 

are damaged on the growing sucker, one or two 

quickly become the primary suckers and vertical 

elongation of the others nearly ceases. There is an 

occasional loss of a shoot tip on an occupied acacia 

(usually the moth larva, Coxina hadenoides, which 

appears to be host-specific on swollen-thorn acacias, 

cf. Janzen, 1967b). If the main sucker shoot is 

generally undamaged, however, a new vertically 

elongating branch is produced from a leaf axil high 

upon the main sucker shoot when the shoot tip is 

eaten and the “single trunk” life form is not lost. 

If there is repeated loss of sucker shoot tips to insects, however, as is the case of unoccupied shoots, 

many short suckers are produced, none of which 

grow very tall. 

In striking contrast to sucker height, length of 

shoot, and condition of the shoot tips, the number 

of swollen thorns and leaves are not significantly 

different between the occupied and unoccupied 

acacias. However, the total live weight of leaves on 

the occupied acacias was 8 times as great as on the 

unoccupied acacias. 

All of the above data suggest that the following 

system is operating. On an unoccupied acacia, the 

insects that will feed on it encounter it at wide 

intervals. In between times, the plant has the 
60 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

chance to grow new sucker shoots to replace those 

eaten off. The long time between attacks by 

herbivores almost certainly reflects a low density of 

these insects. This in turn is probably brought 

about by the low productivity of the understory 

community and the absence of other mimosaceous 

legumes. This is in strong contrast to dry-land sites, 

where the insects arrive so frequently that most new 

shoots are eaten off before they have grown more 

than a centimeter. Phytophagous insects, however, 

find the A. allenii shoots often enough so that no 

one shoot ever becomes large enough to assume 

physiological dominance over the others. The result 

is a stump crowned with a scrawny clump of as 

many as 30 short shoots. Not only do suitable 

herbivores more rarely encounter the acacia, but 

when they do, they appear to feed primarily on 

the newest leaves and shoot tips. The old leaves are 

gradually nibbled at, but are rarely eaten in 

entirety. This results in the very great difference in 

leaf weight between the occupied and unoccupied 

acacias. A swollen thorn is edible only when very 

young, and thus if herbivores find the plant only 

occasionally, swollen thorns may accumulate almost 

as fast as damaged leaves. 

Judging from natural experiments of the types 

described above, unoccupied A. allenii eventually 

die. During the six and a half months of this 

experiment, however, only two (14 percent) of the 

unoccupied stumps died. In a dry-land experiment, 

the percentage would have been between 30 and 

60 percent during one rainy season. Lowered 

herbivore pressure is partly responsible, but the 

obvious shade tolerance of A. allenii is another 

factor. Suckers of dry-land, swollen-thorn acacias in 

shade, such as the A. allenii stumps received, would 

have ceased growth almost entirely within about 3 

months. In the dry areas, a major effect of the 

herbivores was to make the acacia lose the race for a 

spot in the sun. Such an interaction does not occur 

here. Incidentally, P. ferruginea has not lost its 

propensity to kill vegetation contacting the canopy 

FIGURE 44.4eneral postulated distribution and collection localities: 

A. chiapensis (stippled area): A. gentlei (cross-hatched area). 
NUMBER 13 61 

of A. ullenii. Several of the occupied sucker shoots 

in the experiment grew through vine canopies and 

the ants literally chewed a hole through for the 

sucker shoot. 

5. Acacia chiapensis Safford 

Acacia chiapensis Safford, 1915:356. 

Acacia chiapensis is known only from Mexico 

(Figure 44) and the type specimen was collected at 

the same site as the type of A. collinsii (site 1). 

A. chiupensis is distinguishable from all other 

swollen-thorn acacias by the combination of globular inflorescences, multiple and circular petiolar 

nectaries, semi-indehiscent seed pods, and straight 

or slightly curved thorns that are circular in cross 

section. It is easily distinguished from A. collinsii, 

since the latter species has elongate inflorescences 

and few petiolar nectaries. The type specimen of A. 

chiupensis is representative of its species throughout 

the range except that the single type A thorn is 

light brown, while A. chiupensis thorns are usually 

dark brown to black. The immature seed pods on 

the type specimen are full-sized and display the 

distinct ridge bordering the margins of the valves, 

which is characteristic of A. chiupensis, and absent 

from other swollen-thorn acacias. 

KEY COLLECTION LOCALITIES 

(ALL IN MEXICO) 

1. San Fernando, Chiapas. VII-12-1907, G. N. Collins, no. 

164. 

2. Shores of lake behind Presa Miguel Aleman, Temascal, 

Oaxaca (70 m) . XI-3-1963, DHJ, nc (occasional). 

3. Temascal, Oaxaca to La Granja, Veracruz (16 m), All 

year, DHJ, nos. 118, 130, 140, 154, 158, 164, 165, 170, 185, 

190, 193, 200, 204, 209, 214, 222, 230, 235, 242, 247, 266, 

277, 281 (common in permanently wet sites such as below 

the dam at Temascal, Rio Chichicazapa, and Arroyo Enmedio, very rare otherwise). 

4. 3.4 to 8.5 mi E Papaloapan on Cd. Aleman-Loma Bonita 

Hgy., Oaxaca (25 m). XI41963, DHJ, nc (3 plants). 

5. 19.6 mi SW Tuxtepec on Tuxtepec-Oaxaca Hgy., Oaxaca. 

XI41963, DHJ, nc (1 plant). 

6. 26.4 mi SW Tuxtepec on Tuxtepec-Oaxaca Hgy., Oaxaca. 

XI-4-1963, DHJ, nc (common). 

7. 0.9 mi S Chontalpa on Cardenas-Presa Malpaso Rd., 

Tabasco (about 80 m). VI-17-1966, DHJ, nc (3 plants). 

8. 12.6 mi W Cardenas on Hgy. 180, Tabasco (30 m) . VI15-1966, DHJ, nc (1 plant), fl. 

10. 

11. 

12. 

on Hgy. 185, Oaxaca (300-160 m). VIII-18-1967, DH], 

nos. 1500, 1510 (8 plants). 

Rio Cascabel, Oaxaca. 111-15-1934, C. D. Mell, no. 2239 

(US 1642830) fl. 

Upper Rio Coatzacoalcos, Oaxaca. 111-8-1930, C. D. Mell, 

no. 600 (US 1492290), fl. 

Ubero, Oaxaca (30-90 m). IV-1937, L. Williams, no. 

9124 (US 1741272, F 897560). 

NATURAL HISTORY 

Acacia chiupensis is the only dry-land lowland, 

swollen-thorn acacia that could be called rare, unFIGURE 45.- Acacia chiafiensis (Mexico, site 3): A, emergent 

crown of an occupied 4-m-tall adult; B, two adults, 2-m tall, 

~- in an open field; c, looking up through crown of 5-m-tall I 

9. 15.1 mi S and 5.2 to 11.0 mi N turn-off to Matias Romero adult. 
62 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 46.--Acacia chiupensis (Mexico, site 3): A, stump (4 inches in diameter) with multiple 

sucker shoots; 8, branch pattern of 1.5-m-tall tree. 

less it is found that the rare, moist forest species 

A. mayana also occurs regularly in dry sites. It is 

often represented by only one or two plants where 

found. At both places where it was common (sites 

3 and 5), it was growing along or near watercourses 

in vegetation heavily disturbed at 2 to 4 year intervals by earth-moving machinery or logging; the sites 

were subjected to a severe dry season, but the 

acacias were growing in sites of locally high soil 

moisture. Based on the general condition of plants 

growing in a variety of sites, it appears that the 

pre-Colombian habitat of A. chiupensis was the 

evergreen or semideciduous forest along the edges 

of foothill streams running through deciduous 

forest. Road-building machinery appears to have 

been primarily responsible for movement out of 

this habitat into human disturbance sites. A. 

chiapensis has not invaded brushy pastures or upland regeneration following clearing for cornfields 

or logging, in striking contrast to A. collinsii and 

A. cornigera, which probably occupied the same 

pre-Colombian habitat as A. chiupensis. 

Some of the ecology of A. chiupensis at site 3 was 

discussed briefly as a contrast with A. cornigera 

(Janzen, 196713) and A. globulifera is incorrectly 

listed there as a synonym of A. chiupensis. A more 

detailed discussion of A. chiupensis is offered here 

after having seen the plant growing at a variety of 

sites. 

Acacia chiapensis has a life form much like that 

of dry-land nonant-acacias such as A. farnesiana 

and A. macracantha (Figures 45~, B and 46~) but if 

given ample water will grow into a 8-m-tall tree. 

When repeatedly cut, a stump of A. chiupensis 

tends to form a large cluster of shoots from one 

root crown (Figure 46~), rather than also sprouting 

from lateral roots as the custom with A. cornigera. 

A. chiupensis is generally found in the general 

canopy or slightly emergent to it (e.g., Figure 45~); 

seedlings in the shade die even faster than those of 

A. cornigera and thus are rarely noticed. 

Acacia chiupensis resembles nonant-acacias in another respect. The seeds are surrounded by a dry 

pulp that is at best only slightly sweet and is probably only rarely eaten by birds. Since the dehiscent 

pods (Figure 47c) do not open far enough for the 

seeds to fall out, the pod can either be wind dispersed (perhaps up to 2 or 3 m), water dispersed, or 

rodent dispersed. None of these methods even approaches being as effective as bird dispersal for 
NUMBER 13 63 

FIGURE 47.-Acacia chiupensis 

(Mexico, site 3): A, flowering 

branch from a heavily shaded 

tree; B, flowering branch from 

a fully insolated tree; c, mature 

pods on a branch like that in 

figure A. 

rapidly invading newly created patches of young 

regeneration. All other swollen-thorn acacias, with 

the possible exceptions of A. cookii and A. ruddiue, 

are regularly bird dispersed. 

The flowers are pollinated by the wide variety of 

small bees that are available in deciduous forest 

habitats and contemporary second growth during 

the dry season. The inflorescences are borne in 

very dense clumps in the leaf axils of vegetative 

branches (as on A. cookii in the southern part of 

its range) produced in the early part of the previous 

rainy season. At any one site, the number of inflorescences per axil can vary strongly (Figures 

47~, B) and the largest numbers are found on 

branches that are fully insolated and from bushes 

that are fully insolated. Inflorescences generally 

set 1 to 5 pods, but may on occasion have as many 

as 10. 

The seedling ecology of A. chiupensis has not 

been examined but the morphology of young 

bushes indicate that it is more like that of nonantacacias than like that of swollen-thorn acacias. It 

appears that the first swollen thorns often do not 

appear until the fiftieth or even later leaf, and 

even in fully insolated bushes, many leaves on 

lengthening lateral branches are not subtended by 

swollen thorns. This behavior persists even in very 

old A. chiupensis, resulting in branch segments 

with many nodes that lack swollen thorns. The 

canopy section in Figure 45c has about the maximum number of thorns in it to be found in an 

A. chiapensis canopy. 

While the large leaves of mature A. chiupensis 

have large numbers of petiolar nectaries (Figure 

48~), sometimes making the petiole resemble that 

of A. melunocerus, the leaves on young plants have 

smaller numbers (e.g., Figure 48~), and I suspect 

that very young seedlings may only have one or zero 

petiolar nectaries per leaf. While nectar production 

of A. chiupensis is apparently very high, as with 

the other swollen-thorn acacias, Beltian body production is clearly reduced as compared with other 
64 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 46.-Petiolar nectaries of A. 

chiupensis leaves (Mexico, site 9): 

A, from fully insolated old crown; 

B, from a young sapling. 

FIGURE 49.-Acacia chiupensis (Mexico, site 3): A, type A thorns from lateral branch of adult; 

B, thorns intermediate between type A and type B on lateral branch from crown of tree in 

Figure 45c; c, type B thorns from crown of same tree. 
NUMBER 13 

130 - 

I 

a 

- 110 - I * 

z W 

w cl 

90 - 0 

I 

65 

dry-land, swollen-thorn acacias such as A. cornigera, 

A. hindsii, and A. collinsii. A. chiupensis leaves on 

fully mature and well-insolated plants may have 

Beltian bodies on only half the pinnules of a large 

leaf subtended by a swollen thorn. 

There is strong variation in the degree of 

herbivore damage to occupied A. chiupensis-all 

occupied plants censused have been occupied by 

P. ferruginea or by a smaller undescribed species of 

obligate acacia-ant. Even plants with the largest ant 

colonies to be found on A. chiapensis usually have 

most shoot tips eaten, as with an unoccupied A. 

cornigera. The result is that plants of A. chiupensis 

are generally very shrubby and stunted in appearance. The cause appears to be that with the reduced 

number of Beltian bodies and reduced number of 

thorns, almost none of which have a large volume 

or could be regarded as type B thorns (Figures 49, 

50), the ant colony never grows very large. Since 

the colony may be old, its workers may be very 

aggressive, but there is too much acacia canopy to 

patrol effectively. A 2-m-tall A. chiapensis such as 

the one on the left in Figure 3a in Janzen (1967b) 

may have a maximum colony size of about 2,000 to 

4,000 worker P. ferruginea, while an adjacent A. 

cornigera of equal size and with an equal number 

of leaves will have between 5,000 and 10,000 worker 

ants. Younger plants of A. chiapensis characteristically have a colony size reduced to the same 

degree, with the result that they are slow to gain 

effective patrolling by ants, and thus have to be on 

thoroughly cleared ground in order to mature. 

When roads or dams are built, A. chiapensis becomes established on the bare soil where there is 

no competition from fast-growing suckers from old 

stumps of woody plants (as would have been the 

case in areas cleared by fire or machete). At site 6, 

there was a nearly pure stand of 3- to 6-m-tall A. 

chiapensis covering a gravel island of about 2 acres 

in the middle of a river; a newly bulldozed site 

obviously simulates such a situation for primary 

succession. 

If an A. chiupensis is growing within 1 to 2 m of 

an A. cornigera, the latter species always contains 

the queen and the A. chiupensis serves as an 

auxiliary unit to the main colony. This means that 

during a dry season sufficiently severe to cause the 

A. chiupensis to drop its leaves, the ant colony is 

very likely to abandon it and retreat into the acacia 

with the queen. All of these observations suggest 

150 1 

t 0 

b 

0 

a 

a 

i 

c 

I I I I I 

0.4 0.6 0.8 1.0 1.2 1.4 

TOTAL THORN VOLUME (cc) 

FIGURE 50.-Distribution of 89 cc of thorn volume among 100 

type A thorns from an adult occupied A. chiapensis (Mexico, 

site 3). 

that A. chiupensis is a marginal host for obligate 

acacia-ants, but that it is still dependent on the 

ants. However, it may not be as dependent as other 

dry-land, swollen-thorn acacias since its leaves contain cyanogenic glycosides that are also present in 

nonant-acacias (A. farnesiana) but absent from A. 

cornigera (Rehr et al., 1973). 

Not all A. chiupensis, however, demonstrate strong 

susceptibility to insect attack. At site 3, and only 

at this site, it appears that A. chiupensis may be 

hybridizing with nonant-acacias such as A. macracantha. Numerous plants occur there that are intermediate in thorn, leaf, and branching morphology between these two species. A gradient may be 

recognized between the “typical” A. chiupensis life 

form-dark green large leaves with large pinnules, 

swollen thorns subtending more than half of the 

large leaves on lengthening branches, new swollen 

thorns soft for several days allowing easy entry 

and with soft inner parenchyma allowing easy 

cleaning, fast-growing shoot tips, upright-cylindrical 

life form with a few major branches from a single 

trunk, and severe herbivore damage to uppermost 

shoot tips-and a life form like the shrubs in Figures 45~ and 45~ which tend on all the above characters to be much more like A. macracantha. A. 

mucracantha is common in the area and shows the 

opposite traits of those listed above for A. chiapensis. It is particularly obvious that the forms intermediate between A. chiapensis and A. macracantha 
66 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

are more damaged by herbivores than ordinary 

nonant-acacias, but they are very weakly occupied, 

if at all. The lack of damage appears to be due to 

chemical and morphological defenses, since these 

only very mildly damaged bushes are often immediately adjacent to heavily damaged more typical 

A. chiapensis. 

The large stump of one of these intermediate 

acacias in Figure 47~ is particularly instructive in 

understanding the poor development of an ant 

colony in the apparent hybrids. It has produced at 

least 50 small sucker shoots since it was cut, but 

only five swollen thorns; sucker shoots of “typical” 

A. chiapensis or A. cornigera from an equal-sized 

stump would have had a swollen thorn subtending 

almost every new leaf (e.g., Figure 53 in Janzen, 

1967b). Not only are there few swollen thorns, but 

they harden rapidly, and the queen of P. ferruginea has a very difficult time hollowing out enough 

of the inside of the thorn for its own body, to say 

nothing of enoligh space for brood and workers. 

Many of the new leaves on a stump like this have 

no Beltian bodies and the nectaries are very poorly 

developed, again in strong contrast to A. chiapensis 

or A. cornigera new sucker shoots. Not only are the 

suckers slow growing, but dominance is poorly developed with the result that many shoots develop 

simultaneously. The dense shrub resulting eventually matures in completely insolated sites-as might 

be expected of dry-land nonant-acacias-but appears incapable of competing with dense vigorous 

vegetation on land cleared by fire or machete. 

In short, there appears to be a gradient in ability 

to deal with the surrounding vegetation from A. 

macracantha to the apparent hybrid to A. chiapensis to A. cornigera. Along this gradient, the ant 

becomes progressively more important to the plant 

in protecting its faster growing shoot tips that have 

progressively fewer chemical defenses. Both A. 

macracantha and A. cornigera represent adaptive 

peaks that are successful in vegetation types of 

great areal extent. The hybrid and A. chiapensis 

Contour lines at 1000, 5000, and 10,000 feet 

FIGURE 5l.-Collection localities of A. globulifera; its detailed collection records are too patchy 

to allow an estimation of the general distribution at this time. 
NUMBER I3 67 

represent adaptive peaks that do well under much 

rarer combinations of vegetative competition and 

rainfall. It is not meaningful to say that one is any 

more or less successful than the other, except in 

terms of the number of acres of vegetation occupied. 

6. Acacia globulifera Safford 

Acacia globulifera Safford, 1915:360. 

Acacia donnelliana Safford, 1915:361. 

Myrmecodendrum globuliferum (Safford) Britton and Rose, 

Myrmecodendron donnellianum (Safford) Britton and Rose, 

1928:98. 

1928:93. 

Acacia globulifera is easily distinguished from 

other swollen-thorn acacias by the possession of 

spherical inflorescences, completely dehiscent pods 

with valves that curl back on themselves (as with 

A. hindsii), and 2 to 6 circular or oval petiolar 

nectaries (Figures 6, 7c). Its range (Figure 51) 

overlaps slightly with that of A. collinsii, A. 

cornigera, A. gentlei, A. cookii, and A. chiapensis, 

but it is very rarely encountered growing side by 

side with these plants. Herbarium specimens of 

A. globulifera are incorrectly listed as a synonym of 

A. chiupensis in Janzen (1967b), but a comparison 

of the petiolar nectaries and mature pods-barely 

indehiscent in A. chiapensis, fully indehiscent in 

A. globulifera-will easily separate them. The 

petiolar nectaries of A. globulifera may on occasion 

be confused with those of A. collinsii, but A. 

collinsii has elongate inflorescences. In field studies, 

failure to examine plants closely may result in misidentifying A. globulifera as A. collinsii or as A. 

gentlei because it has a very similar life form to 

either of these when growing in their habitats. 

Safford's type specimen of A. globulifera (site 1) 

appears to have been taken from a fully insolated 

shrub growing in a locally exceptionally dry site, 

since its thorns are unusually light in color (beige) 

and short (the largest type A thorns are only 4 cm 

long), and its leaves only attain 8 cm in length. In 

the coastal areas of the Yucatan Peninsula such 

plants are usually found on old beach dunes; the 

more inland plants have thorns that are nearly 

black and twice as long, and have leaves up to 20 

cm long. The inflorescences on the type specimen 

are representative of A. globulifera throughout its 

range except that they have peduncles only 10 mm 

or less in length, and in wetter areas the peduncles 

of A. globulifera may attain a length of 20 mm. 

Safford's type specimen for A. donnelliana was 

collected from that portion of the A. globulifera 

population growing between San Pedro Sula and 

Santa Rosa, Honduras (site 24), and is representative of the plants in that area growing in very wet 

habitats (swamps, creek edges); its thorns are nearly 

black, its leaves are up to 20 cm long, its inflorescence peduncles are up to 18 mm long, and its 

sexual branchlets are very long (e.g., Figure 52~). 

KEY COLLECTION LOCALITIES 

Mexico: 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

Type of A. globulifera: Silam (Dzilam), north coast of 

Yucatan. April 1895, G. F. Gaumer, no. 1909. 

48 mi NE Puerto Real (Isla Aguada) on Mexico 180, 

Campeche. VI-22-1966, DHJ, nos. 611, 615, 617, 623 (OCcasional) , mp. 

11.3 mi N intersection of Mexico 185 and 190, on Hgy. 

185, Matias Romero, Oaxaca (290 m). VIII-18-1967, 

DHJ, no. 1502 (single plant) . 11.8 mi N intersection of Mexico 185 and 190, on Hgy. 

185, Matias Romero, Oaxaca (290 m) . VIII-18-1967, 

DHJ, nc (occasional plants). 

25.8 mi N intersection of Mexico 185 and 190, on Hgy. 

185, Matias Romero, Oaxaca (250 m). VIII-18-1967, 

DHJ, nc (very abundant). 

26.3 mi N intersection of Mexico 185 and 190, on Hgy. 

185, Matias Romero, Oaxaca (240 m) . VIII-18-1967, 

DHJ, nc (3 plants). 

31.5 mi N intersection of Mexico 185 and 190, on Hgy. 

185, Matias Romero, Oaxaca (260 m). VIII-18-1967, DH], 

nc (1 plant). 

Chivelia, Oaxaca, Mexico. 111-6-1928, C. D. Me11 (US 

1406953), fl. 

Silam, Yucatan, Mexico (sea level). IV-1895, G. F. 

Gaumer, no. 655 (US 1267146), fl. 

Silam, Yucatan, Mexico (sea level). IV-1909, G. F. Gaumer, (US 1267508) fl. 

Guatemala: 

11. 

12. 

13. 

14. 

15. 

16. 

Finca San Joaquin, slopes above Rio Negro, near San 

Cristobal de Verapaz, Alta Verapaz (1080 m). VII-14- 

1967, DHJ, nc (very abundant), mp. 

20.5 mi NE Guatemala City on CA9 (980 m). VIII-8- 

1967, DHJ, no. 1565 (occasional), fl., gp. 

26.7 mi to 26.9 mi NE Guatemala City on CA9 (700 m) . VIII-9-1967, DHJ, nc (abundant), gp. 

29.6 mi NE Guatemala City on CA9 (850 m). VIII-9- 

1967, DHJ, nc (rare). 

23.4 mi SW of turn off to Coban on Hgy. 9, Progreso, 

Dept. Progreso. VIII-5-1966, DHJ, no. 444. 

16.2 mi NE Guatemala City on CA9 (870). V-14-1965, 

DHJ, no. 1741 (1 plant). 
68 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

17. Sanarate, Depto. Progreso (900 m) . XI-9-1916, Popenoe, 

no. 720 (US 2089415, US 1319702), mp. 

18. La Libertad, Chicbul, Depto. Peten. IV-29-1933, C. L. 

Lundell, no. 3088 (US 1635206, F 738059), fl. 

19. Rio Teculutan, above Teculutan, Depto. Zacapa (250- 

275 m). 1-7-1942, J. A. Steyermark (F 1191231) fl., mp. 

20. Fallabon-Yaxha Rd., Depto. Peten. 111-22-1933, C. L. 

Lundell, no. 2214 (F 740476), mp. 

21. Lake Yaxha, Depto. Peten. 111-23-1933, C. L. Lundell, 

no. 2208 (F 738041). 

British Honduras: 

22. Vaca, El Cay0 Prov. 111-2-1938, P. H. Gentle, no. 2275 

(F 945039), fl. 

23. El Cayo, El Cayo. 111-5-13-1931, H . H . Bartlett, no. 11992 

(US 1493114), fl. 

Honduras: 

24. 

25. 

26. 

27. 

28. 

29. 

30. 

Type of A. donnelliana: San Pedro Sula, Cortes (200 m) . March 1888, C. Thieme, no. 5216. 

19.4 to 19.9 mi SW San Pedro Sula on Honduras 18 (road 

to Santa Rosa) Depto. Cortes (160 m). VIII-2-1967, 

DH], no. 1682, 1688 (8 plants), fl., gp. 

24.6 to 26.0 mi SW San Pedro Sula on Honduras 18 

(banks of the Rio Chamelecon; road to Santa Rosa), 

Depto. Santa Barbara (380 to 420 m). VIII-3-1967, DHJ, 

no. 1604, 1608 (occasional). 

28.4 mi SW Sula on Honduras 18 (road from San Pedro 

Sula to Santa Rosa), Depto. Copan (780 m) , VIII-3-1967, 

DH], no. 1630 (1 plant). 

10.4 to 16.3 mi (Cucuyagua) SW Santa Rosa on Honduras 18 (road to Nueva Ocotepeque), Depto. Copan 

(980-1180 m) . VIII-3-1967, DHJ, no. 1634 (occasional), 

gP. 

Yoro, Depto. Yoro (850 m). 1-26-1934, J. B. Edwards, no. 

P-746 (US 1828182, F 759556), fl. 

Santa Rosa de Copan, between El Portillo and San Juan 

Opoa, Depto. Copan (1000 m). IX-23-1963, A Molina, 

no. 12873 (F 1622365). 

NATURAL HISTORY 

As with the other widespread swollen-thorn 

acacias, it is difficult to characterize the contemporary or pre-Colombian habitats of A. globulifera 

in one statement. It is commonly encountered at 

elevations that represent the upper elevational 

range of swollen-thorn acacias (e.g., sites 11, 28, 30); 

however, all of these higher elevation sites are 

characterized by being dry and dusty even during 

the rainy season. It is only at these sites that A. 

globulifera is sympatric with no other swollen-thorn 

acacia, though at some lowland sites A. globulifera 

may be the only riparian species, with A. cornigera 

or A. collinsii on the hillsides (e.g., site 26). At lower 

elevations in the wetter part of the range (e.g., sites 

25, 26), it is most commonly encountered in 

riparian and swamp successional stages and occasionally on the edges of pastures and in roadside 

right-of-way. At sea level in the drier parts of the 

range (e.g., sites 2, 3-7) it is found on old sand 

dunes, rocky slopes, swamp edges, and badly eroded 

creek edges. No healthy plants have been found in 

habitats where they are heavily shaded, and almost 

all reproductive individuals have crowns that are 

emergent to the general canopy or are growing on 

nearly bare ground. One is left with the distinct 

impression that A. globulifera can withstand a very 

wide range of temperature and moisture regimens 

if it is moderately to heavily insolated. Since the 

largest individuals encountered have only been 3 m 

tall, and most are much shorter, this restricts the 

plant to young stages of succession, or to habitats 

with low or badly broken vegetative cover. At the 

higher elevation sites, A. globulifera is often associated with pines and oaks, and at sites 11 and 13 

became very common-up to 20 adults per acrein old pastures and regenerating fields formed by 

clearing mixed pine-oak forest. 

Acacia globulifera is distinctly shrubby in life 

form, and like the other dry-land acacias may bear 

a large seed crop when only 1 to 2 m tall if well 

insolated. If growing in full sun continuously, A. 

globulifera generally does not grow over 2 m tall 

(Figure 53~) but if slightly shaded, it has a slightly 

more attenuated life form (Figure 53B), and its 

crown does not expand much laterally until it is 

emergent to the general canopy or otherwise insolated. 

The length of the sexual branchlets of A. globulifera appears to be more directly related to the 

health of the plant than is the case with other 

swollen-thorn acacias. For example, the very long 

branchlets in Figure 52~ arefrom a 2.5-m-tall acacia 

in its third year of sexual reproduction and fully 

insolated with a large crown and occupied by a very 

large ant colony. The same is true of the long 

sexual branchlets bearing immature pods in Figures 

54B and 54c. On the other hand, the very short 

sexual branchlets in Figure 53B are on the healthiest 

appearing branch in the senescent crown of a 4-mtall acacia in a swamp. Though fully insolated, 

this 14-year-old acacia had lost its ant colony and 

was heavily damaged by herbivores. The extreme 

differences in number of inflorescences borne by the 

two branches in Figure 52 are documented in 
NUMBER 13 69 

FIGURE 52.--Acacia globulifera (Honduras): A, elongate sexual branchlets from a very healthy 

adult (site 28): B, axillary inflorescences from the crown of a senescent tree (site 25). 

FIGURE 53.-Acacia globulifera (Honduras): A, heavily insolated subadult (site 

26); B, main trunk of subadult growing 

under intense lateral competition (site 

28). 
70 

- 

- 

SMITHSONIAN CONTRIBUTIONS TO BOTANY 

60- 

a 2 50- 

w 40 - 

30- 

U 

a 

I 

W 

u 

z 

U 

v) 

W 

ci 

U 

20- U 

0 

$10- 

z 

W 

cl 

FIGURE 34.-A, lateral branch of A. hindsii with large number of immature infructescences (Nicaragua, site 85); B, long sexual branchlets of A. globulifera with heavy crop of immature infructescences (Guatemala, site 13) : c, the lateral branch bearing the infructescences in Figure B. 

0 

0 

0. 0: 

0 

I I 

5 15 25 

A NUMBER OF INFLORESCENCES PER BRANCHLET 

- 

32 

U 

z 

4 

IT 

L9 

m 

230 a 

4 

W 

W 

U 

z 

0 

LL! 

a 

U 

4' 210 

5 190 

2 170 

E 

u, 150 0 

32 

z 

W 

ci 

130 

0 

0 0 

0 

0 

40 60 80 100 

NUMBER OF INFLORESCENCES PER BRANCHLET 

FIGURE 55.-Number of inflorescences per sexual branchlet in A. globulifera (Honduras): 

A, crown of a senescent tree that had an inflorescence crop of about 250 (site 25): B, crown of 

a healthy adult that had an inflorescence crop of about 12,000 (site 28). 
NUMBER 13 71 

Figure 55; the size of the inflorescence crop is clearly 

proportional to the condition of the acacia. 

Acacia gEobulifera with many long sexual branchlets usually bear large seed crops (Figures 54B, C, 

56). I suspect, however, this means that a large 

percent of the seeds are killed by bruchid beetles. 

As with the other dry-land acacias, the young green 

pods (Figure 112~) remain on the plant throughout 

the rainy season and do not mature until about 

the time the new flowers are produced during the 

following dry season. Mature undehisced pods of 

A. globulifera have not been collected and therefore there is no information available on seeds. 

Since the pods curl open and yet stay on the tree 

for several months after all seeds are gone, I suspect 

that they are bird-dispersed as with A. hindsii. 

Seedling biology of A. globulifera has not been 

investigated except to note that they develop their 

first swollen thorns, Beltian bodies, and petiolar 

LO v) 

W 

u 

2 

w 

u 

v) 

W 

3 s5 H 

U 

0 

CT 

W 

m 

9 

z: ~ 13 

1 4 5 7 9 11 13 

NUMBER OF PODS PER INFLORESCENCE 

FIGURE 56.-Distribution of 207 immature seed pods among 

the infructescences from a healthy adult A globulifera (Honduras, site 28). 

FIGURE 57.-Acacia globulifera: A, main trunk covered with type B thorns bleached white by the 

sun (Guatemala, site 12); B, central axis of subadult with downward curved type A thorns 

(Honduras, site 28). 
72 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

120 

110 

- 

E 

E 100 I 

X 

F 

z 

W 

4 

w 90 

2 80 a 

0 

z 

k 

4 70 

a 

t; b 

FIGURE 58.-Acacia globulifera: A, type B thorns (Honduras, site 26); B, left, type B thorns and 

on right, type A thorns (Guatemala, site 15). 

TOTAL THORN VOLUME (cc) 

FIGURE 59.-Distribution of 49 cc of thorn volume among 69 

type A thorns from lateral branches in the crown of a 

healthy adult A. globulifera (Honduras, site 28). 

nectaries very early in development, like other dryland, swollen-thorn acacias. Also as with other 

dry-land, swollen-thorn acacias, seedlings of A. 

globulifera appear to die within 1 to 2 years if they 

are in heavy shade. 

Acacia globulifera produces large numbers of 

swollen thorns early in its development, and continues to do so on all branches. The thorns are 

exceptional among swollen-thorn acacias in showing 

extreme differences in color from one habitat to 

another. Adding to the complexity is the fact that 

A. globulifera thorns bleach exceptionally easily in 

direct sunlight (e.g., Figure 57~). They are commonly bicolored as well, with lighter tips than 

bases. Type B thorns are somewhat larger than type 

A thorns (Figures 57~, 58~, 59, 60), but not spectacularly so. They also do not differ in morphology 

from type A thorns in any outstanding manner; 

even the largest type B thorns are usually round 

in cross section. Type A thorns often curve downward (Figure 57~) or project outward almost at 

right angles from the branch bearing them (Figure 

58). 

Acacia globulifera shows greater variation in 

petiolar nectary morphology than any other species 

of swollen-thorn acacia. The major factor appears 

to be in the moisture of the ground where the 

individual plant is growing; the wetter the site, 

the longer the leaves. A great deal of the variation 

in nectary morphology can be viewed as simply the 

effect of lengthening or shortening the leaf at the 
NUMBER 13 

140 - 

120 - 

100 - 

73 

6 

6 

z 

U 

0 

X 

I+ 

160 -I 0 

ooo. 

0. 

n 

0 

0 

0 

1.0 2.0 3.0 

TOTAL THORN VOLUME (cc) 

FIGURE 60.-Distribution of 48 cc of thorn volume among 31 

type B thorns from the trunk of A. globulifera in Figure 

57A. 

time of formation. The effect of this variation in 

nectary size and number on the amount of nectar 

produced per leaf is unknown. There is no evidence 

that the ants react differently to any of these 

different forms of nectary. 

The impact of herbivore on unoccupied plants of 

A. globulifera has not been examined experimentally, but plants that lose their ant colony 

through natural causes are badly damaged by insects in the lowlands. At the upper end of the 

elevational distribution of ‘4. globulifera, apparently healthy acacias in reproductive condition are 

encountered that obviously have been unoccupied 

for at least a year. They are often tenanted by other 

arboreal ants in the genus Camponotus. At these 

high elevations, where the weather is often dry, 

windy, and cool, herbivorous insect populations 

appear to be much reduced as compared with the 

lowlands and this is probably the reason why unoccupied A. globulifera is relatively free of insect 

damage at these sites. 

7. Acacia sphaerocephala Schlectendal 

and Chamisso 

Acacia sphaerocephala Schlectendal and Chamisso, 1830:594. 

Acacia ueracruzensis Schenck, 1913:362. 

Acacia dolichocephala Safford, 1915:355. 

Acacia sphaerocephala is easily distinguished 

from all other swollen-thorn acacias by having the 

elongate canoe-shaped petiolar nectaries found also 

on A. cornigera and A. mayana, but having a 

globular or thimble-shaped inflorescence. Sterile 

specimens of A. sphaerocephala are usually distinguishable from A. cornigeya because the pinnules 

of A. sphaerocephala usually lack readily visible 

secondary venation (Rudd, 1964). As can be seen 

from its distribution (Figure Sl), A. sphaerocephala 

is probably a dry-land derivative of the widespread 

A. cornigera just as A. mayana is probably a wetforest derivative of A. cornigera; the direction of 

this evolution cannot, however, be stated with 

certainty. 

Schlectendal and Chamisso’s type specimen of A. 

sphaerocephala has elongate internodes and moderate-sized thorns, indicating that it was collected 

a few meters inland from the beach dunes where 

most of the population of A. sphaerocephala now 

grows (site 1). While its spherical inflorescences are 

immature, they are representative of those found 

throughout the population of A. sphaerocephala 

except in the northern part of its range where 

some plants have more thimble-shaped inflorescences. Rudd (1 964) has placed A. ueracruzensis and 

A. dolichocephala in synonymy with A. sphaerocephala and I agree. The small maroon seed pods 

growing from a globular inflorescence, and the very 

thick thorns on the type of A. ueracruzensis identify 

it clearly as having been collected from the dense 

mat of A. sphaerocephala growing on the dunes 

immediately above the high-tide line south of the 

city of Veracruz (sites 2 and 6). At this site, the 

thorns of A. sphaerocephala may attain a diameter 

of 15 mm (e.g., Figure 65c) while equal-length 

thorns of A. cornigera are only 8 to 10 mm diameter 

at the base. Also, A. sphaerocephala thorns at this 

site (as at most sites) are bicolored with ivory to 

beige tips while those of A. cornigera are uniformly 

brown or gray. Safford’s type of A. dolichocephala, 

like that of A. sphaerocephala, was taken from the 

population just inland from the dunes and thus has 

as long internodes and thin thorns as are to be 

found on A. sphaerocephala. Its immature inflorescences are slightly elongate but those of A. cornigera 

at the same site are 3 to 4 times as long. 

Acacia globulifera has been commonly misidentified as A. sphaerocephala where the former species 

grows along the dunes sympatric with A. collinsii 

on the Yucatan Peninsula. Careful search has, how- 
74 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

1000 R W 8. - I96 7 1 I I # 

FIGURE 61.4eneral postulated distribution (stippled area) 

and collection localities of A. sphaerocephala. 

ever, located no A. sphaerocephala south of Alva- 3. Type of A. dolichocephala: Along shore north of Veracruz, Veracruz (sea level). 1-24-1906, J. M. Greenman, rado, Veracruz, Mexico. _- 

KEY COLLECTION LOCALITIES 

(ALL IN MEXICO) 

no. 87. 

XI-10-1963, DHJ, nos. 1921, 1924 (common), mp. 

4. 5.1 to 38.1 mi SE Jalapa, Veracruz, along Mexico 140. 

5. 23.5 to 42.0 mi WNW Huatusco, Vcracruz, road from 

1. 

2. 

Type of A. sphaerocephala: Actopan, Veracruz (sea level). 

March, 1820, Schiede and Deppe, no. 684. 

Type of A. veracruzensis: sand dunes south of Veracruz, 

Veracruz (sea level). X-13-1908, Schenck, no. 916. 

Orizaba to Hgy. 140. XI-10-1963, DHJ, no. 1920 (common) , mp. 

6. Within 1-2 km of the ocean beach, from the southern 

city limits of Veracruz, south to Anton Lizard0 (approxi- 
NUMBER 13 75 

mately 15 miles north of Alvarado) , Veracruz. DHJ, nos. 

1922, 1925, .l926, 1927. 

7. 26.2 mi N Ciudad Valles to Antiguo Morelos on Mexico 

85 (100-300 m). VIII-25-1967, DHJ, nc. 

8. Antiguo Morelos to where the Tropic of Cancer crosses 

Mexico 85 (e.g., 1.8 mi S Llera on Hgy. 85; 22 mi s 

Ciudad Victoria, Tamps.). VI-10-1964, DHJ, no. 505. 

9. Ciudad Mante to Tampico (Mexico 80) (primarily in 

liver beds) and Tampico to the Tamuin area (e.g., 8 mi 

E Tampico on 110). 1-2-1964, DHJ, no. 504, Tamps. and 

SLP. 

10. Antiguo Morelos to Ciudad del Maiz, Tamps. and S.L.P. 

(occasional populations on dry southeast facing slopes) . 

11. 10.3 mi NE Ciudad Valles on Mexico 110, S.L.P. I-II1964, DHJ, no. 1923. 

12. Tanquian, Partido de las Valles, Tamcanhuitz Dist., 

S.L.P. 1908, L.G. Cuevas, (US 692167), fl. 

13. Hda. Buena Vista, Tamaulipas, Mexico. VI-15-1919, E.O. 

Wooton; (US 989707). 

14. Between Tdla and Tampico. XII-1830, no. 728 (US 

40123), fl. 

15. Tampico, Tamaulipas (sea level). 1-1-31-1910, E. Palmer, 

no. 133 (US 764678), fl., mp. 

16. Between San Luis Potosi and Tampico (Huasteca Region), 1878, E. Palmer (US 764678), fl., mp. 

17. Pueblo Viejo, 2 km S Tampico, Veracruz (sea level). 

V-23-31-1910, E. Palmer, no. 448a (US 463371), fl. 

18. Between Rio de Santa Maria and Rio de 10s Pescados, 

Veracruz. X-7-1908, Schenck, no. 837 (US 764677). 

19. 1 mi N Cd. Madero, Tamaulipas (sea level). 111-1-1961, 

R.M. King, no. 3990 (US 2365430), fl., mp. 

XII-28-1963. 

~~ATURAL HISTORY 

The distribution of Acacia sphaerocephala invariably borders the dry edge of the A. cornigera 

distributions. At sites 2 and 6, it occupies the dunes 

and only extends inland to the wetter sites occupied 

by A. cornigera as scattered individuals. At sites 4 

and 5, A. sphaerocephala extends the distribution 

of swollen-thorn acacias into that part of the state 

of Veracruz with the most severe dry season. All 

along the northern part of the range of A. sphaerocephala, the northernmost individuals are found in 

semidesert vegetation while the southernmost individuals are scattered among the northernmost 

individuals of the A. cornigera population. A. 

sphaerocephala is the only species of swollen-thorn 

acacia that inhabits newly formed dunes, and it 

occupies some of the driest habitats of any swollenthorn acacia population. It is the only Central 

American acacia involved in the early stages of 

beach succession. In the Isthmus of Tehuantepec 

on the Pacific side of Mexico, A. collinsii is also 

found on occasion in extremely dry habitats, but 

it is not clear if the low density population is 

reproducing itself or maintained by seed immigration from wetter sites. 

For purposes of discussion, the A. sphaerocephala 

population can be divided into two ecotypes: the 

dune ecotype and the inland ecotype. The dune 

form occurs only on new dunes in the vicinity of 

the city of Veracruz (sites 3 to 6), while the inland 

form occurs throughout the remainder of the range 

and does not approach the ocean as closely as does 

the dune form, When seeds of the dune form are 

grown at well-watered inland sites, the resulting 

shrub closely resembles the inland ecotype except 

for having slightly shorter internodes and slightly 

thicker swollen thorns. On the dune, however, it is 

a very different plant and worthy of special discussion. 

On the dunes closest to the ocean, the dune form 

of A. sphaerocephala forms a dense pure stand from 

0.2 to 1.8 m tall that generally crowns the leading 

edge of the dune (Figure 62~; Figure 10 in Sauer, 

1967). The origin of the mat of A. sphaerocephala 

is seed, but most of the plants in the larger mats 

are from sucker shoots from roots. The branches 

are so intertwined that the entire mat appears as 

though it were one crown. The largest mats may be 

30 m wide by 200 m long and contain at least 6,000 

major stems. 

The dune ecotype of A. sphaerocephala differs 

from other swollen-thorn acacias in not being occupied by obligate acacia-ants when it grows on 

new dunes. Its success as an unoccupied swollenthorn acacia in this habitat is apparently due to the 

continuous strong onshore winds, which probably 

make life very difficult for phytophagous insects 

and prevent their flying to the acacias in the first 

place. The absence of other adjacent shrubby vegetation in this barren habitat also removes the source 

for most of the insects that defoliate unoccupied 

swollen-thorn acacias. Plants grown at inland sites 

from dune ecotype seeds are heavily defoliated if 

unoccupied, just as are adjacent unoccupied A. 

cornigera (experiments conducted 1963-1964, at 

Cotaxtla Exp. Sta. and Temascal, cf. Janzen, 1967b, 

for site details). Inland ecotypes of A. sphaerocephala interact with ants in the same manner as do 

A. cornigera. 

Failure of Pseudomyrmex ferruginea (the only 

obligate acacia-ant at the site) to occupy the dune 
76 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

ecotype is due to the following reasons: (1) Searching queens appear to rarely arrive at the dune 

plants, probably because the prevailing winds blow 

from the dunes toward the inland swollen-thorn 

acacias (A. cornigera and A. sphaerocephala), and 

because they would have to cross many feet of very 

hot sand to walk there. Their absence is indicated 

by the almost complete lack of their characteristic 

entrance holes in thorns of seedlings and adult 

plants of the dune ecotype of A. sphaerocephala. 

(2) The dune ecotype produces almost no Beltian 

bodies and the few that are produced appear to be 

harvested by colonies of Crematogaster atrata Mayr 

or C. coruina Mayr that tenant the thorns in large 

colonies. These ants cut entrance holes at the base 

of the thorns, tend mealybugs (Pseudococcidae) 

and aphids (Aphididae) inside the thorns, and 

scavenge on the acacia surface. Their foraging is 

almost entirely nocturnal and very patchy in coverage. There is no evidence that they protect the 

acacia; inland A. sphaerocephala dies as fast from 

herbivore damage when tenanted by this ant as it 

does when unoccupied. (3) The Crematogaster ants 

thoroughly harvest the petiolar nectar produced by 

the dune ecotype of A. sphaerocephala and would 

probably be very severe competitors for the young 

FIGURE 62.-~, pasture dotted with 

young 1-2-m-tall A. sphaerocephala; 

the columnar structure is typical of 

pasture plants in this area (Mexico, 

site 9); B, beach dune created by dense 

mat of A. sphaerocephala (Mexico, 

site 6). 

queens attempting to start a colony. The total 

absence of Crematogaster, however, from those inland acacias that are producing Beltian bodies and 

can be found by many searching queens of P. 

ferruginea, indicates that obligate acacia-ants can 

easily exclude Crematogaster in more ordinary 

habitats. (4) During the latter part of the dry season most of the dune ecotype individuals of A. 

sphaerocephala drop their leaves (e.g., Figure 63c), 

and this behavior makes colony establishment and 

maintenance very difficult. 

As dune succession proceeds, the dense mats of 

A. sphaerocephala are broken up, and the population consists primarily of very flat-topped scattered 

individuals that rarely exceed 1.5 m in height (Fig 

ure 63~). Once the succession has reached a height 

of 3 to 5 m, A. sphaerocephala disappears completely, though A. cornigera may still be common. 

Birds apparently disperse seeds continually into 

these inland sites, however, because if they are 

cleared, both A. cornigera and the inland form of 

A. sphaerocephala appear in the early successional 

stages. In the inland habitat, these two acacias are 

very difficult to tell apart until they bear flowers. 

Seedling development in situ of the dune ecotype 

has not been examined closely, but leaflet develop- 
NUMBER 15 77 

FIGURE 63.-Acacia sphaerocephala (Mexico): 

A, adult in cattle pasture (site 4); B, adult 1 

m tall, leafless, in low vegetation several hundred meters behind the high-tide line (site 

6); c, leafless subadult on beach dune (site 

6). 

inent of a representative seedling from an inland 

plant is depicted in Figure 64. The gradual increase 

in leaf size and leaflet number with age is representative of dry-land, swollen-thorn acacias. It is 

characteristic of all the swollen-thorn acacias to 

have this short period of growth before developing 

the characteristic traits of a swollen-thorn acacia. 

In short, the seed probably lacks sufficient reserves 

to produce both a small amount of shoot and root, 

and swollen thorns, nectaries, and Beltian bodies. 

That these traits increase gradually in quantity 

indicates that the seedling continues to divide its 

new photosynthate between conventional vegetative 

growth and production of resources for the developing ant colony. 

No matter in what habitat it is growing, a young 
78 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

525+ 

.O4?& 

28 5\ 

26 % 

16 i, 

/& 54 

# 30 

A34 

FIGURE 64.--Schematic representation of 46-day-old A. 

sphaerocephala seedling: the number of pinnules for each 

pinna are recorded on the figure and .the first petiolar nectary appeared on the 7th leaf with the first swollen thorn 

and Beltian bodies at the 11th node. A fully mature leaf 

on the 50th or later node may have as many as 20 pinnae 

with 50-70 pinnules on each (Mexico, site 6). 

FIGURE 65.-Acacia sphaerocephala (Mexico): A, sexual branchlet with type B thorns (site 9); 

B, type B thorns (two very large thorns) and type A thorns from the plants found on beach 

dunes (site 6): c, fruits and type A thorns (site 6). 
NUMBER 13 79 

100 

90 

80 

h 

I = 

L 

T 

c 

K 

w 

- 

g 70 

n 

f 60 t9 

L 

w 

-1 

cl 

4 

s 

c. 50 

. . 

5. . .... .. ... . b 

. i *. .. 

0. 

. 

0 

. 

0 

. 

. 

. . . 

0. 

. 

. 

. e. 

I I I I I I I I 1 

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

TOTAL VOLUME PER THORN (CC) 

FIGURE 66.-Distribution of 30 cc of thorn volume among the 59 thorns on a 50-cm-tall A. sphae. 

rocephala from the mat of shrubs on beach dunes (Mexico, site 6). 

A. sphaerocephala starts out with a single major 

trunk (e.g., Figure ~ZA), as with A. cornigera. These 

plants, however, are generally in dry and, therefore, 

relatively well-insolated sites and thus rapidly 

spread out laterally (Figure 63~). A. cornigera in 

the same sites tends to grow taller and more slender. 

Within 2 to 4 years after first becoming occupied 

by obligate acacia-ants, the young A. sphaerocephala bears flowers. It is generally one year ahead 

of A. cornigera growing at the same site. After 

flowering, A. sphaerocephala rarely increases in 

height much in later years. The pods (Figure 65c) 

are produced in dense clusters, again as with A. 

cornigera, but generally have very thin wallseasily crushed between one's fingers-in contrast to 

the stiff and tough walls of A. cornigera at the same 

sites. A. cornigera has a morph with thin pod walls, 

but it is not sympatric with A. sphaerocephala. 

In an effort to document the thorn volume of 

A. sphaerocephala, and its relationship to external 

thorn dimensions, the swollen-thorns of three different plants were measured. The young seedling 

(Figure 66) has the very small thorns (as compared 

with those of the adult in Figures 67 and 68) characteristic of young plants; in general, the larger the 

thorn, the higher it was up on the plant. For the 

inland ecotype (Figures 65~, 69~, B), the maximum 

thorn volume is 3.5 to 4.5 cc and probably represents the maximum possible inside thorn diameter 

for optimal ant brood care. The great increase in 

thorn volume found in the dune ecotype (Figures 

65~, 69c) would be negatively adaptive for plants 

occupied by obligate acacia-ants, since it is produced by inflating the thorn rather than increasing 

its length. It is very difficult for the ants to organize 

and care for brood in thorns where the thorn walls 

are more than $bout 4 to 6 mm apart. The ants of 

Crematogaster, however, that normally tenant these 

thorns are accustomed to living in large cavities in 

plants and build carton partitions in the thorns to 

keep the brood separate. One may envision this as 

the first possible step in the development of an 
80 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

.. 

I .. .‘ . * ..* .. .. . .: .. $;;:;:*’ *. 

. I!: 

..:i!:: 

:l:il . 

. a:!!! FIGURE 67.-Relationship between thorn length, volume, and width for the 244 

.* *. : . thorns on a 2-m-tall adult A. sphaerocephala from the beach dune mat (Mexico, 

.. .I.. site 6): A, total thorn volume as a function of the total thorn length (sum of the 

:* two halves); B, total thorn volume as a function of the sum of the two halves of 

the thorn; c, relationship of total thorn length to the sum of the maximum width 

.‘I:;. * 

... 

I 5 10 15 20 25 ,b of the two halves of the thorn. 

c SUM 0’ UAXIIJII LIDT’ OF THE TWO THORN HALVES (mmj 

interaction in the New World as has evolved between the African swollen-thorn acacias and Acacia turgida Safford in Wheeler, 142:plate 45. 

Acacia campecheana Schenck, 1913:361. 

Crematogaster. 

8. Acacia cornigera (L.) Willdenow 

Acacia cornigera (L.) Willdenow, 1806: 1080. 

Mimosa cornigera L., 1753:520. 

Acacia cornigera var. americana Dc., 1825:460. 

Acacia spadicigera Schlechtendal and Chamisso, 1830:594. 

Acacia cubensis Schenck, 1913:360. 

Acacia nicoyensis Schenck, 1913:360. 

Acacia hernandezii Safford, 1914:358. 

Acacia furcella Safford, 1914:359. 

Tauroceras spadicigerum (Schlechtendal and Chamisso) BritTauroceras cornigerum (L.) Britton and Rose, 1928:86 [exAcacia interjecta Schenck, 1913:361. 

Acacia rossiana Schenck, 1913:361. 

ton and Rose, 1928:85. 

cluding synonymy]. 

Acacia cornigera is the best known of the swollenthorn acacias. It is readily distinguished from all 

other swollen-thorn acacias by the unique combination of elongated “canoe-shaped” petiolar nectaries (Figure 8~), very elongated infloresc, ences 

(Figure 12~), thorns usually round in cross section, 

and completely indehiscent seed pods. 

Linnaeus’ type specimen of A. cornigera is a 

sterile branch representative of the eastern Mexican 

portion of the total population; it was collected 

from a cultivated plant in Holland, presumably 

grown from Mexican seed (Rudd, 1964). 

Rudd has compared pinnules of the type with 

those of A. sphaerocephala and found that the 

pinnules of the type have the readily visible 

secondary venation lacking on A. sphaerocephala 
NUMBER 13 

B 

r 20- 

E 

I i5. 

b 

: 

81 

.... -. .... ................ .......... ....... . ........ ..... . ...a ....... _..I 

..... .. a. 

.. 

15C 

12: 

p ii 

C 

.. 

.... ...... ....... 

............. 

... 

.. 

I 1234567BD1011I2 L 2 3 4 5 6 7 0 9 10 11 

TOTAL THORN VOLUME Iccl B TOTAL TLiORN YOLJME lccl 

*. -. 

-2. : 

.* *. FIGURE 68.-Relationship between thorn length, volume, and width for the 200 

thorns from half of a 1.5-m-tall adult occupied A. sphaerocephala growing like a 

representative swollen-thorn acacia 200 m inland from the beach (contrast these 

data with those in Figure 67) (Mexico, site 6): A, total thorn volume as a function 

of the total thorn length (sum of the two halves); B, total thorn volume as a function of the sum of the maximum width of the two halves of the thorn; c, relationship of total thorn length to the sum of the maximum width of the two halves of 

the thorn. 

*: .:.a. * 

*:. !i;.:* 

... 'i,ili:i. i.1 ... ::!i!.i . . . 11: .*. 

IS 20 25 10 35 

SUM OP MPXIMUM WIDTH OF THE MI0 THORN MLYES lmml 

FIGURE 69.-Acacia sphaerocephala (Mexico): A, black type A thorns on lateral branch (site 5); 

B, bicolor type A thorns from a lateral branch of an inland adult (site 6); c, bicolor type A 

slightly inflated thorns of plants growing in the dune mat (site 6). 
82 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

leaflets. Owing to the peculiar thorns of A. mayana, 

there is no doubt that the type does not belong to 

that species. 

The type specimen of A. spadicigera (site 1) is 

easily identified as A. cornigera because of its 

elongate inflorescences and thorns round in cross 

section. Its straight short thorns are representative 

of A. cornigera from lowland wet sites in Veracruz. 

Safford’s type specimen of A. hernandezii is unmistakably from the northern wet lowlands portion 

of the A. cornigera population. A. cornigera is the 

only species of swollen-thorn acacia in the Rascon 

area (site 2). Safford’s leafless type specimen of A. 

furcella is representative of the A. cornigera population in the Lake Catemaco region (site 5). The type 

has the very dark brown thorns and widely spread 

type B thorns characteristic of highslevation, eastern Mexican A. cornigera. The inflorescences of the 

types of both A. hernandezii and A. fnircella are 

representative of A. cornigera inflorescences. 

Schenck’s type of A. rossiana (site 4) is representative of that portion of the A. cornigera population 

inland from Minatitlan, Veracruz, Mexico. I have 

been unable to locate his type specimen for A. 

campecheana; possibly it was destroyed at Berlin. 

He states that its leaves are like those of A. 

spadicigera and A. rossiana, and it was therefore 

most likely collected from that part of the A. 

cornigera population that extends along the coastal 

lowlands from Veracruz to Yucatan. Its name suggests that it was collected in Campeche, and there 

appear to be no swollen-thorn acacias in that area 

that could be confused with ‘4. cornigera. Schenck 

described A. interjecta from material growing in 

the Singapore and Kew botanic gardens; the description agrees with that of A. cornigera and the 

seed probably came from Mexico, since that appears 

to have been the source of most introduced A. 

cornigera seed. Schenck described A. nicoyensis 

from material representative of the southern end 

of the geographic range of A. cornigera (site 77). 

His type specimen has the white and slightly flattened type A thorn found on Costa Rican A. 

cornigera growing in very dry and insolated sites. 

The inflorescences and seed pods of the type of A. 

nicoyensis are unmistakably A. cornigera. 

Schenck’s type specimen of A. cubensis was taken 

from a cultivated plant (site 82); the white and 

partially recurved type B thorns indicate that the 

seed came from the A. cornigera population somewhere north of Guatemala. A. cornigera apparently 

occurs only in gardens .in Cuba and its presence 

unlikely is due to natural introduction by birds or 

other means. The absence of ant entrance holes in 

the thorns of the type specimen of A. cubensis indicates that the ants have not yet been introduced 

to Cuba. 

Since A. turgida is supported only by a photograph (Wheeler, 1942), it probably represents a 

name that Safford intended to propose. I cannot 

find the locality (site 5) for the specimen, but it 

appears to have been collected from that rather 

distinctive portion of the A. cornigera population 

growing where the Pan-American highway crosses 

the Guatemala-Mexico border. 

KEY COLLECTION LOCALITIES * 

Mexico: 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

Type of A. spadicigera: La Laguna Verde, Veracruz, 

Mexico. March 1820, Schiede, no, 685. 

Type of A. hernandezii: Rascon, S.L.P., Mexico. VII-19- 

22-1905, E. Palmer, no. 669. 

Type of A. furcella: Catamaco, Veracruz, Mexico (300 

m). IV-26-1894, E.W. Nelson, no. 427. 

Type of A. rossiana: Santa Lucrezia, Isthmus of Tehuantepec, Veracruz, Mexico, X-8-1906, H. Ross, no. 918. 

“Type” of A. turgida: Herba Santa, Chiapas, Mexico 

(see plate 45 in Wheeler, Bull. Mus. Comp. Zool. Harvard, 90: 1-262). 

Guatemala-Mexico border (near Ciudad Cuahtemoc) to 

19.0 mi N on Hgy. 190, Chiapas (720-600-780 m) . VIII7-1966, DHJ, no. 499; VIII-16-1967, DHJ, nc (common 

to rare). 

2.6 mi NE to 0.9 mi W Chiapa de Corzo on Hgy. 190, 

Chiapas (580-475 m) . VIII-17-1967, DHJ, nc (common 

to rare). 

9.7 mi W Tuxtla Gutierrez to 8 mi NE Cintalapa on 

Hgy. Chiapas 190 (860-1090-570 m) . VIII-18-1967, DHJ, 

nc (common to rare to occasional). 

13.5 to 9.0 mi E of intersection of Hgy. 190 and 185 (La 

Ventosa) , Oaxaca (80 m) . VIII-18-1967, DHJ, nc (com 

mon to rare). 

9.9 to 23.4 mi N of intersection of Hgy. 190 and 185 (La 

Ventosa) , Oaxaca (210-290-250 m). VIII-18-1967, DHJ, 

nc (common to occasional). 

33.3 mi N of intersection of Hgy. 190 and 185 (La Ventosa) to Acayucan, Oaxaca-Veracruz (23040 m) . VIII18-1967, DHJ, nc (common). 

* Hundreds of collection records of A. cornigera in the 

U.S. National Herbarium and elsewhere are included within 

the inclusivc localities given here. For the sake of brevity, 

only type localities or collections made in sites not visited in 

this study are cited. 
NUMBER 13 83 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

22. 

23. 

24. 

25. 

26. 

27. 

28. 

29. 

30. 

31. 

32. 

33. 

34. 

Near Salina Cruz, Oaxaca (sea level). X-3-1965, DHJ, 

nc (occasional). 

Coatzacoalcos to Veracruz on Hgy. 180, Veracruz (0-350- 

0 m) . 111-21-1964, DHJ, no. 1935; VI-15-1966, DHJ, nc 

(occasional to common). 

8.3 mi SE bridge over Rio Coatzacoalcos, Veracruz, to 

Cardenas, Tabasco, on Hgy. 180 (0-40 m) . VI-15-1966, 

DHJ, nc (very rare, about 1-2 plants per 2 miles). 

Cardenas to 37.1 mi S, Tabasco (40-100 m) . VI-17-1966, 

DHJ, nc (rare to common). 

Cardenas to Paraiso, Tabasco (40-10 m) . VI-17-1966, 

DHJ, nc (rare). 

Cardenas to Ciudad del Carmen, Tabasco-Campeche 

(40-0 m) , VI-22-1966, DHJ, no. 494 (rare, and only on 

well drained sites), fl. 

Campeche to Ruinas Edzna (29 mi E on Hgy. 180), Campeche (0-50 m) . VI-22-1966, DHJ, nc (very rare to occasional). 

Hopelchen to Ticul on Hgy. 180, Campeche-Yucatan 

50-80). VI-28-1966, DHJ, nc (very rare to occasional). 

12.2 mi S Pet0 (Santa Rosa) on Hgy. to Cd. Chetumal, 

Yucatan-Quintana Roo (50-80 m). VI-29-1966, DHJ, nc 

(common), fl. 

Cd. Chetumal to 69.6 mi NW on Hgy. to Peto, Quintana 

Roo (0-20 m) . VI-30-1966, DHJ, nc (common), fl. 

30 mi S to Campeche on Hgy. 180, Campeche (0-80-0 

m). VI-25-1966, DHJ, no. 491; VII-8-1966, DHJ, nc 

(rare to common). 

3.2 mi E Cruz Grande to 19.1 mi NW Puerto Escondido 

on Hgy. 200, Guerrero-Oaxaca (50-250-70 m). VI-18- 

1967, DHJ, nc (rare to common to occasional) . 9.3 mi N Playa Pie de la Cuesta to 16.5 mi SE Papanoa 

on Hgy. 200, Guerrero (0-20 m). VI-17-1967, DHJ, nc 

(very common in swamps, absent on dry hills). 

Alvarado to Tlacotalpan to Cd. Aleman to Las Tinajas 

to Paso del Toro, Veracruz (0-100-0 m). Throughout 

the year, DHJ, no. 1939 (common), flower February-May. 

Tierra Blanca to La Granja to Temascal, VeracruzOaxaca (20-70 m). Throughout the year, DHJ, nos. 

1966, 1967, 1968, 1969, 1970, 1971, 1972, 1973, 1974, 1975, 

1976, 1977, 1978, 1979, 1980, 1981 (very common). 

Shores of the lake behind Presa Miguel Aleman, Temascal, Oaxaca (70 m). X-20-1963, DHJ, no. 1937; XI-3- 

1963, DHJ, nc (occasional to very common). 

Cd. Aleman to Loma Bonita to Rio Playa Vicente, Veracruz-Oaxaca-Veracruz (50-100 m) . XI-4-1963, DHJ, no. 

1938 (common to very rare on red clay hills), 

Cd. Aleman, Veracruz to Valle Nacional, Oaxaca, on Hgy. 

to Oaxaca, XI-6-1963, DHJ, nc (common to occasional). 

22.8 to 27.5 mi SW Conejos (turn off of Hgy. 140 to 

Huatusco) on Hgy. to Huatusco to Cordoba, Veracruz. 

XI-10-1963, DHJ, nc (common to rare). 

4.3 mi E Cordoba, Veracruz on Hgy. 150, Veracruz (800-0 

m) . X-16-1963, DHJ, nc (rare to very common). 

Hgy. 150 SE to San Ignacio de la Llave, Veracruz (60-10 

m) . X-16-1963, DHJ, nc (common). 

Tampico to Tuxpam on Hgy. 105, Veracruz (0-100-0 m) . 

1-2-1964, DHJ, nos. 1942, 1943; VI-13-1966, DHJ, nc 

(common to rare). 

Tuxpan to 1.2 mi S Villa A. Camacho on Hgy. 130, Vera35. 

36. 

37. 

38. 

39. 

40. 

41. 

42. 

43. 

44. 

45. 

46. 

cruz. 1-2-1964, DHJ, no. 1940; VI-13-1966, DHJ, nc (occasional to rare). 

Poza Rica to Nautla on Hgy. 180, Veracruz (50-0 m), 

1-20-1964, DHJ, nc (occasional to rare). 

6 mi SW Tamazunchale to Cd. Valles on Hgy. 85, San 

Luis Potosi (500-100 m) . 1-1-1964, DHJ, no. 1936; XII31-1963, DHJ, nc (rare to common). 

36.6 mi E Rio Verde to Cd. Valles on Hgy. 86, San Luis 

Potosi (1140-100 m). VIII-24-1967, DHJ, nc (rare to 

common). 

Cd. Valles to 6.4 mi N Antiguo Morelos on Hgy. 85, San 

Luis Potosi-Tamaulipas (100-250 m) . VIII-24-1967, 

DHJ, nc (rare). 

Antiguo Morelos to 20 mi R’W Cd. del Maiz, TamaulipasSan Luis Potosi (250-? m). XII-31-1963, DHJ, nc (common on wetter hillsides) . Cd. Valles to Tampico on Hgy. 70, San Luis PotosiVeracruz (100-10 m). VI-11-1966, DHJ, nos. 496, 497; 

VIII-24-1967, DHJ, nc (common to occasional). 

Pt. Talisman (Guatemala-Mexico border) to 3 mi N 

Arriaga on Hgy. 200, Chiapas (50-150 m). 1-11-1965, 

DHJ, no. 1047 (common). 

Presa Benito Juarez, near Jalapa de Marquez on Hgy. 

190, Oaxaca (200 m). IX-7-1968, DHJ, nc (1 plant). 

Pichucalco, Chiapas. 1-1907, GN. Collins and C.B. Doyle, 

no. 260 (US 692169) mp. 

3 mi S Pinola Las Rosas along road to Soyatitan, Venustiano Carranza (W of Comitan), Chiapas (1400 m). VII27-1965, D.E. Breedlove, no. 11367 (US 2484541), fl. 

Zacualpan, Veracruz (1000 m) . VII-1917, C.A. Purpus, 

no. 7748 (US 884536), fl., and IX-1908, H. Schenck, no. 

836 (US 764665). 

Fortuno, Rio Coatzacoalcos. 111-1937, L. Williams (F 

897476), fl. 

British Honduras: 

47. Sta. Elena to Belize on main Hgy. (5-0 m) , VI-30-1966, 

DHJ, nc (rare to occasional), fl. 

48. El Cayo, El Cay0 Prov. V-5-1931, H.H. Bartlett, no. 

13007 (US 1493273), mp. 

Guatemala: 

49. 

50. 

51. 

52. 

53. 

54. 

55. 

Escuintla to Taxisco along Hgy. CA-7, Depto. EscuintlaSanta Rosa (330-100 m). 1-13-1965, DHJ, nc (occasional) , 

Escuintla, Drpto. Escuintla, to Puente Talisman, Depto. 

San Marcos (Guatamela-Mexico border) on Hgy. CA-7 

(330-50 m). VII-16-1966, DHJ, no. 466; 1-11-1965, DHJ, 

nos. 1045, 1046 (common to occasional), fl. 

Retalhuleu SW to Champerico, Depto. Retalhuleu (200- 

0 m) . VIII-1-1966, DHJ, nc (occasional). 

Retaljuleu to 14.0 mi NE on Hgy. S, Depto. Retalhuleu 

(200-900 m). VII-28-1966, DHJ, no. 464 (common to 

rare). 

Escuintla to 7.4 mi NE on Hgy. 3, Depto. Escuintla (330- 

880 m) . VIII-13-1967, DHJ, nc (common to occasional). 

0.3 mi SW to 4.1 mi NE Los Amates on Hgy. CA-9 (150- 

110 m). VIII-9-1967, DHJ, nc (common) . Finca San Joaquin, Slopes above Rio Negro, near San 
84 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

56. 

57. 

58. 

59. 

Cristobal de Verapaz, Depto. Alta Verapaz (1420-1080 

m) . VIII-14-1967, DHJ, nc (rare to common). 

14.9 mi SE to Guatemala-Mexico border (near Ciudad 

Cuauhtemoc) , Depto. Huehuetenango (900-1000-720 m) . VIII-16-1967, DHJ, nc (common). 

Patalul, Depto. Solola. 11-14-1906, WA. Kellerman, no. 

5915 (US 578841), fl. 

Finca Sepacuite, Depto. Alto Verapaz. IV-1905, O.F. Cook 

and R.F. Griggs, no. 8 (US 590344), mp. 

1-2 mi N Ocos, Depto. San Marcos (sea level). III-15- 

1940. ].A. Steyermark, no. 37867 (F 1035247). 

Honduras: 

60. 12.5 to 19.5 mi NW Siguatepec on Tegucigalpa-Puerto 

Cortes Hgy., Depto. Comayagua (910-540 m). VII-31- 

1967, DHJ, nc (comnion to very rare). 

61. 0.7 mi N Rio Lindo to 18.6 mi S Puerto Cortes on 

Tegucigalpa-Puerto Cortes Hgy., Depto. Cortes (90-10 

m) , VIII-2-1967, DHJ, nc (common to occasional). 

62. San Pedro Sula to 8.1 mi SW on Hgy. to Santa Rosa, 

Depto. Cortes (90-130 m) . VIII-2-1967, DHJ, nc (occasional along streams). 

63. 20.6 mi NE to 28.4 mi SW Sula on San Pedro Sula-Santa 

Rosa Hgy., Depto. Santa Barbara (260-780 m). VIII-3- 

1967, DHJ, nc (common to rare), fl. 

64. Vegas del Rio Agua, 3 km from Yoro, Depto. Yoro 

(1000 m), V-8-1956, A. Molina, no. 6807 (US 2412019), 

fl. 

El Salvador: 

65. From 11.2 to 26.3 mi N (Depto. Cuscatlan) San Salvador 

on San Salvador-Nueva Ocotopeque Hgy. (420-590 m) . VIII41967, DH], nc (common to rare). 

66. 6 mi E San Salvador on Interamerican Hgy., Depto. San 

Salvador (650 m) . 1-14-1965, DHJ, nc (occasional), 8. 

67. 29.7 to 31.6 mi E La Libertad on La Libertad-La Union 

Hgy., Depto. La Paz (50-80 m). VIII-5-1967, DHJ, nc 

(rare). 

68. Intersection of Hgy. from La Herradura with La Libertad-La Union Hgy., Depto. La Paz (20 m) . VIII-5-1967, 

DHJ, nc (1 plant). 

fig. 15.2 to 10.2 mi SE Santa Ana on Interamerican Hgy., 

Depto. Santa Ana (440-700 m). VIII-6-1967, DHJ, nc 

(common). 

70. San Vicente, Depto. San Vicente (350-500 m), III-2- 

1922, P.C. Standley, no. 21687 (US 137448, US 1137096), 

fl. 

71. Ateos, Depto. La Libertad. IV-17-1922, P.C. Standley, 

no. 23360 (US 1139047), fl. 

72. San Martin to Laguna de Ilopango, Depto. San Salvador. 

IV-1-1922, P.C. Standley, no. 22580 (US 1138317), fl. 

Nicaragua: 

73. Penas Blancas (Depto. Rivas) to 7.2 mi NW on Hgy. 2 

(50 m). VII-25-1967, DHJ, nc (common along rivers). 

74. 3.4 mi SE Leon on Hgy. 12, Depto. Leon (20 m). VI-27- 

1967, DHJ, nc (30 acre patch), fl. 

75. 1.2 mi W Matagalpa to 3.6 mi N on Hgy. 3, Depto. Matagalpa (650-880 m) . VII-28-1967, DHJ, nc (common), 8. 

76. 8 mi S Esteli on Hgy. 1 (940 m) . VII-28-1967, DHJ, nc 

(1 plant). 

Costa Rica: 

77. Type of A. nicoyensis: Shores of the Gulf of Nicoya, 

Guanacaste, Costa Rica (sea level). February 1900, Tonduz, no. 13538. 

78. 20 mi NW turnoff of Interamerican Hgy. to Puntarenas, 

to Pefias Blancas (border of Costa Rica and Nicaragua) 

on Interamerican Hgy., Guanacaste Prov. (10-100 m) . January-September, periodically in 1963-1967, DH], nc 

(rare to common, patchy distribution and associated with 

wet spots), fl., February-April. 

79. Liberia to Playa COCO along Hgy. 21, Guanacaste Prov. 

(100-0 m). January-September, periodically in 1963-1967, 

DH], nc (rare to very common, primarily in wet spots), 

fl. and mp., in February. 

80. Hda. Palo Verde, COMELCO, Bagaces, Guanacaste Prov. 

(sea level). IV-2-1966, DHJ, nc (occasional), 

81. Filadelphia, Guanacaste Prov. 111-2-1946, Echeuerria, no. 

279 (F 1442853), mp. 

Cuba: 

82. Type of A. cubensis: North coast of Cuba. IV-21-1963, C. 

Wright, no. 2402. 

NATURAL HISTORY 

Acacia cornigeru (Figure 70) and A. collinsii 

have the greatest geographic and ecological range 

of all the swollen-thorn acacias. The riparian and 

swamp habitats that were occupied by A. cornigeru 

previous to human interference are in lowland areas 

that are ideal for slash-burn milpa agriculture. A. 

cornigera has probably been common in lowland 

agriculture fallow fields as long as Indians have 

lived in the Central American lowlands. Since occupied A. cornigeru may reach reproductive maturity in 3 to 4 years, and since birds, people, and 

cattle distribute its seeds widely in second growth 

vegetation, A. cornigera populations have expanded 

widely into pastures, roadsides, and short-fallow 

fields generated by contemporary agricultural practices. At many locations, A. cornigera is found in 

all types of disturbance sites, from creek bottoms to 

hilltops. Only A. sphaerocephala and A. collinsii 

are on occasion found in drier habitats than A. 

cornigera. The natural history of A. cornigeru in 

eastern Mexico has been described in detail (JanZen, 1967b), and the comments in the remainder of 

this section are designed to highlight the way it 

differs in other parts of its range from that portion 

of the population. 

Local short-distance dispersals of A. cornigera by 
NUMBER IS 85 

humans make it difficult to understand its preColombian range. For example, at site 42 in western 

Mexico, a single A. cornigera was found growing 

by a new hydroelectric dam. The closest native 

population of A. cornigera is several hundred miles 

away in the Isthmus of Tehuantepec and along the 

west coast in the Acapulco region (sites 23, 24) (also 

an introduction?). This plant probably arrived by 

either road-building machinery or through humans; 

farmers and Indians often eat the sweet pulp 

around the seeds and ingest many seeds simultaneously (Janzen, 196713). The work crews on public 

construction in Mexico often come from several 

hundred miles away. If more than one plant was 

introduced in the area of the dam, it is likely that 

viable seeds will be produced. The bruchids that 

normally kill most A. cornigera seeds will be absent, 

at least at first, and this swollen-thorn acacia will 

likely spread down the watercourse away from the 

dam. It will probably not spread around the lake 

since the general area of the lake is extremely dry 

and water levels will fluctuate greatly. The single 

plant was occupied by Pseudomyrmex ferruginea. 

The queen probably came from the population of 

A. collinsii about 25 miles to the south or the 

population of A. hindsii 50 miles to the northwest. 

As a second example, in the central Yucatan Peninsula, A. cornigera is only common around small 

settlements, cattle corrals, and old Indian ruins 

(sites 18 to 21). These small sites are also usually 

the wetter habitats in the region (the outlying areas 

are heavily occupied by A. collinsii), and it is impossible to know if the presence of A. cornigeru 

is due to recent and frequent introduction by humans and cattle, or by natural dispersal by birds. 

Deliberate long-distance introduction of A. 

cornigera may have some interesting repercussions 

in the future. Introduced to the southern tip of 

Florida as a garden plant, A. cornigera has occasionally escaped to form feral populations (e.g., 

Oneco and Chapman Field, US 1433083 and US 

764685 in the U.S. National Herbarium). These 

plants are severely damaged by the frosts that occasionally damage orange orchards, but they do 

flower and set fertile seeds if there are several 

together. Obligate acacia-ants have not been introduced, but it would be interesting to examine the 

interaction of occupied swollen-thorn acacias with 

the subtropical communities of southern Florida. 

Reproductively isolated from the Mexican parent 

genotype, A. cornigera might even develop a frostresistant population in Florida. A. cornigeru has 

also escaped on Martinique, Guadeloupe, and Cuba 

in the Caribbean (US 2229384, US 1714928, US 

764657). Here, with a climate not unlike that of 

some parts of eastern lowland Mexico, the popula- 
86 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

tions (again without their ants) have a chance to 

expand into the native vegetation just as many 

other species of introduced trees have done on 

Caribbean islands. The central question here is 

whether the herbivore pressure in the island communities is strong enough to require occupation by 

obligate acacia-ants for the survival of a large breeding population of A. cornigera. All introductions 

to the Old World tropics appear to have been to 

botanical gardens, and I have no information on 

their escape into natural habitats. 

Acacia cornigera and A. collinsii occupy adjacent 

habitats over much of their range. Always A. cornigera is in the wetter creek bottoms and swamp 

edges while A. collinsii is on the drier hillsides (e.g., 

Guanacaste Province, Costa Rica). In the wetter 

parts of the range, A. hindsii and A. cornigera often 

occur together (e.g., Pacific foothills of Guatemala) 

and here, A. cornigera is the drier hillside plant 

with A. hindsii along the watercourses. The distinctiveness of these separations by habitat was probably once very clear. Current agricultural practices 

have rendered wetter habitats drier and more insolated by clearing the vegetation, with the consequence that the formerly separated swollen-thorn 

acacias now grow side by side. 

In the southern part of its range, the A. cornigera 

population is much more broken up into subpopulations than in eastern Mexico where it is found 

in all sorts of disturbance and climatic regimes 

from sea level to 1200 m, and from areas with a 

7-month dry season to areas with almost no obvious 

dry season. In habitats where one expects to find 

A. cornigera in the northern part of the range, one 

instead encounters A. collinsii and A. hindsii. To 

determine the role of competitive exclusion in this 

system, we need much more information on the 

parasites and predators (specifically, bruchid 

beetles) that these species have in common, and 

the ability of each to grow and maintain the largest 

possible obligate acacia-ant colony in the shortest 

period of time. 

Why A. cornigera has stopped its southward 

niovement in Guanacaste Province in Costa Rica 

is very enigmatic. A. collinsii continues south to 

Colombia with a nearly continuous population that 

occupies many habitats similar to those occupied by 

A. cornigera further north. It may well be that it 

has not stopped at all, but rather that it is in the 

process of slowly moving southward. The only 

possible ecological explanation that comes readily 

to mind is that the Guanacaste plants of A. 

cornigera are specialists at surviving the severe dry 

season there and therefore cannot advance slowly 

through the wet-forest area immediately down the 

coast from Puntarenas. What is needed is a longdistance dispersal of more than one seed simultaneously from Guanacaste to the dry Pacific coast of 

Panama. 

The dependency of A. cornigera on a colony of 

obligate acacia-ants for survival to reproductive 

maturity has been well documented for lowland 

eastern Mexican habitats (Janzen, 1967a). Judging 

from the badly damaged condition of naturally unoccupied A. cornigera throughout its range, this 

dependency is the general case. It is clear, however, that the intensity of damage to unoccupied 

A. cornigera varies from one habitat to another, 

just as it did from one season to another in the 

lowlands of eastern Mexico. High-elevation sites 

subjected to wet, cool, and windy weather (e.g., site 

75) produce the healthiest appearing unoccupied 

A. cornigera, but even here it is doubtful if an 

unoccupied tree can survive until reproductive 

maturity unless growing in a cluster of acacias in 

a large and well-grazed cattle pasture. In central 

highland Guatemala (e.g., site 53) and on the highland Atlantic side of Honduras (site 60), A. 

cornigera is occasionally “occupied” by a large 

colony of Camponotus planatus which appear to 

compete directly for thorns with the obligate acaciaants colony. The C. planatus patrol the acacia quite 

effectively and the plants are healthy in appearance. 

How long an acacia can be occupied by C. planatus 

and survive is not clear. On some occasions, a 

colony of C. planatus and one of obligate acaciaants occupy the same acacia, but usually in different 

branches. C. planatus occurs throughout much of 

the range of swollen-thorn acacias but in other areas 

does not appear to offer any protection to the 

acacia. It also lives in hollow twigs in many other 

species of tree. 

Acacia cornigera has gained the same “freedom” 

from herbivorous insects gained by A. hindsii in 

areas of intense agriculture (e.g., site 74 in El 

Salvador); however, it is much rarer than A. hindsii 

in these sites and will probably become extinct in 

the near future. Seedlings are almost never encountered. Without the ants to keep the vines off, 

the stiff and upright young shrub of A. cornigera 
NUMBER IS 87 

FIGURE 7l.-Acacia cornigera (Mexico, site 26): A, unoccupied seedling from heavily shaded 

site; B, unoccupied seedling from strongly insolated site. 

makes an excellent vine standard leading to its 

death by shading. The young plants of A. hindsii 

are more shade tolerant and less rigid than A. 

cornigera of the same age. 

As with A. hindsii and A. collinsii, the seedlings 

of A. cornigera are scattered throughout the habitat 

occupied by the parents. As stressed earlier (Janzen, 

1967b), they attain the height of 10 to 50 cm and 

die after 1 to 2 years if not occupied by a colony 

of obligate acacia-ants. When in heavily shaded 

sites, the seedlings tend to produce few swollen 

thorns (Figure 71~), while in fully insolated sites 

thorn production is much greater (Figure 71~). 

Once occupied, a variety of different shrub life 

forms may develop. In dry areas, about as much 

wood goes into the central trunk as into branches 

(e.g., Figure 72~), and the shrub ends up being very 

bushy in appearance (Figures 73~, B). Only very 

rarely does A. cornigera become a large tree to FIGURE 7P.-Acacia cornigera: A, crown of healthy adult after 

losing its leaves during the last part of the dry season (Nical5 in height; such are ragua, site 75); B, crown of healthy subadult growing in 

found in natural disturbance sites along rivers and marshy pasture (Honduras, site 61). 
88 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 73.-Acacia cornigera: A, pasture with adult (Mexico, site 23); B, type B and type A 

thorns in center of crown of adult (Nicaragua, site 75); c, type B thorns on main trunk of adult 

(Mexico, site 24). 

may at one time have made up a much larger 

proportion of the reproducing population than at 

present . Over most of the distribution of A. cornigera, 

sexual branchlets and their products are rather 

uniform; however, wetness of the habitat does appear to influence this. In the Pacific foothills of 

Guatemala (site 50) the sexual branchlets may be 

20 to 50 an long (Figure 74~). In contrast to the 

situation at this wet site, most populations of A. 

cornigera produce sexual branchlets 1 to 10 cm long 

(Figure 74c). The inflorescence itself is not influenced by the amount of moisture at the site, except 

that at some of the driest sites, some of the shortest 

inflorescences were found (again suggesting that A. 

sphaerocephala is a dry-land offshoot of A. cornigem). The number of florets per inflorescence is 

poorly correlated with the length of the inflorescence (Figure 75) at any one site but over a variety 

of sites shows a weak correlation (Figure 76). 
NUMBER 13 89 

FIGURE 74.-Acacia cornigera: A, exceptionally long sexual branchlets (Guatemala, site 50); 

8, mature pods ripped open by birds (Guatemala, site 50); c, last year’s pods and this year’s 

inflorescences on short sexual branchlets in the crown of an adult (Nicaragua, site 75). 

The indehiscent pods of A. cornigera are split 

open by birds (Figure 75~) and the seeds dispersed 

by birds that frequent second-growth vegetation 

(Janzen, 1967b). Over most of the range of A. 

cornigera, the pod walls are thick and hard to 

break between ones fingers. They tend to have sharp 

and stiff long points on the end (Figures 74B, c); 

however, in riparian habitats at site 26, the pod 

walls are very thin and even split of their own 

accord when drying. The plants on the adjacent dry 

hills have the usual thick-walled pods. This difference is genetic as shown by a lack of intermediates 
90 

0 

SMITHSONIAN CONTRIBUTIONS TO BOTANY 

0 

0 

LT Lu 

1 

3 

z 

m 

I 26 28 30 35 311 3’6 38 4b 

LENGTH OF INFLORESCENCE (mm) 

FIGURE 75.-Distribution of 27,262 florets among 23 inflorescences from a single adult A. cornigem; the inflorescences averaged about 35 mm in length (excluding peduncle) and 8.9 mm in 

width (Mexico, site 26). 

and occasional plants bearing the “wrong” type of 

pods for the habitat where the plant is encountered. 

It is interesting that this site is only about 80 miles 

from that portion of the A. sphaerocephala population below the city of Veracruz that also has very 

thin pod walls. The number of seeds in A. cornigera 

pods is representative for dry-land, swollen-thorn 

acacias (Figure 77) but nearly all the seeds are 

killed by bruchid beetles. The very high predation 

on A. cornigera seeds by these beetles may be due 

to the high concentration of pod crops to be found 

in fields and roadsides; when the plants were widely 

spaced in natural disturbance sites, the bruchids 

might have a much more difficult time locating the 

pod crops. 

The thorns of A. cornigera display every color 

found in swollen-thorn acacias, but there is little 

intertree or intratree variation at any one site. In 

general the thorns in very hot and dry lowlands are 

white, ivory, or pale gray. At high elevations they 

are often very dark brown or nearly black. Though 

the correlation is not perfect, it is tempting to 

interpret these differences as being adaptive in 

respect to the ant colony. In lowland areas, temperatures in fully insolated thorns can be high enough 

to drive out the ants, and light-colored thorns may 

reflect radiation and slightly lower the temperature 

inside the thorn. It is perhaps significant in this 

connection that A. coZlinsii also has very lightcolored thorns in dry and hot areas and dark thorns 

in high and wet sites. The dark thorns of A. 

cornigera could aid in warming up the ant colony 

on cool days and thereby increase the degree of 

patrolling by ants. It is significant in this respect 

that A. sphaerocephala has very dark thorns at the 

one high (cool) and dry site where it occurs, in 

strong contrast to the light-colored thorns it bears 

in dry lowland areas. All of the wet-forest swollen- 
NUMBER 13 91 

In 

25 

20 

15 VI 

8 

p 10 

E5 

I 

z 

a 

FIGURE 76-Distribution of 125,773 florets among 122 inflorescences from 16 adult A. cornigera 

from throughout the range of A. cornigera; there was no evidence of a geographical trend in 

size of inflorescence or number of florets per inflorescence. 

""'nnn 4 13 12 11 10 n 8 6 

NUMBER OF SEEDS PER PO0 

FIGURE 77.-Distribution of 1436 seeds among 100 pods from 

an adult A. cornigeru with a normally heavy seed crop of 

about 500 pods (Mexico, site 26). 

thorn acacias have very dark brown thorns, and 

this may be important in warming the thorns by 

occasional direct insolation. Bicolored thorns (Figure 78c) are somewhat puzzling in this respect. 

They are most prominent in the small and insolated 

population of A. cornigera on the west coast of 

Mexico; here A. cornigera grows only in swamps, 

with A. collinsii on the extremely dry and hot adjacent hillsides. 

The spectacular "bull's horns" swollen-thorns of 

A. cornigera are extremely variable in shape, but 

in a consistent and often clinal manner. Many sites 

are characterized by a particular thorn type, and 

many of these thorn types resemble thorns found 

on other species of swollen-thorn acacia (though 

often not sympatrically). Type A thorns of A. 

cornigeru (Figures 78, 79) are usually round or 

nearly so in cross section and form a shallow to 

steep "V" with the branch wearing them. When 

flattened so that they superficially resemble A. 

hindsii thorns (Figure ~OD), they are still easily 

distinguished by having very thick and rough walls. 

Type B thorns (Figures 80, 81) are highly variable 

in form, but all are much larger in volume than 

type A thorns. There is no part of the range of 

A. cornigera where it is consistently occupied by 

multiple-queen species of obligate acacia-ants, so 
92 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 78.-Geographic variation in type A thorns from occupied adult A. cornigera: A, Mexico, 

site 33; B, Mexico, site 40; c, Mexico, site 24; D, Guatemala, site 56. 

FIGURE 79.-Geographic variation in type A thorns from occupied adult A. cornigera: A, Mexico, 

site 33; B, Mexico, site 36; c, Mexico, site 33. 
NUMBER 13 

FIGURE 80.-Geographic variation in type B thorns from occupied adult A. cornigera: A, Mexico, 

site 33; B, Guatemala, site 52; c, Mexico, site 84; D, Guatemala, site 56; E, Mexico, site 35. 

selection for large type B thorns is omnipresent. 

Type B thorns of A. cornigera are the hardest to 

break into of any of the swollen-thorn acacias. 

Several are often twisted together, making it difficult to even pull one off the acacia. 

9. Acacia mayana Lundell 

Acacia mayana Lundell, 1937:210. 

Acacia mayana (Figure 17) is the most easily 

recognized of the swollen-thorn acacias, owing to 

the pair of bladelike longitudinal flanges extending 

from the thorn base to tip. The type specimen is 

representative of the other specimens examined, 

though it should be noted that there are only 6 

specimens at hand of this exceptionally rare swollenthorn acacia. The leafless isotype of the U.S. National Herbarium was collected from the upper part 

of the canopy of a mature plant that has been reproducing at least 2 years, since it bears both 

flowers and fruit. The fruits and flowers of A. 

A 

1. 

2. 

3. 

4. 

3. 

mayana are similar to those of A. cornigera and A. 

sphaerocephala (and therefore very different from 

the other 9 species of swollen-thorn acacia), but the 

fruits differ from those of these two species by 

being longer, thinner, and having a long thin base 

or stalk before swelling out around the seeds. The 

inflorescences differ from those of A. cornigera and 

A. sphaerocephala in that they taper to a point, 

rather than having a blunt end. More specimens of 

mayana are needed for study. 

KEY COLLECTION LOCALITIES 

Type: Rio Pasion, San Diego, Peten, Guatemala. VI-10- 

1935, M. Aguilar, no. 495 (US 1688106), mp. 

Forest S of Finca Yalpemech, near Alta Verapaz-Peten 

boundary, Depto. Alta Verapaz, Guatemala (100-150 m) . 111-25-1942, J. A. Steyermark, no. 45184 (F 1188721), mp. 

3 mi W Cardenas, Tabasco, Mexico (20 m). VI-18-1966, 

UHJ, no. 458. 

Fortuno, Coatzacoalcos River, Veracruz, Mexico (30-50 

m) . 111-1937, L. Williams, no 8425 (F 896540). 

Retiro, Tenosique, Tabasco, Mexico. VI-19-25-1935, E. 

Matuda, no. 3410 (F 1027159) (“shrub in virgin forest”). 
94 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 81.4eographic variation in type B thorns from occupied adult A. cornigera: A, Mexico, 

site 26; B, Mexico, site 28; c, Mexico, site 26; D, Mexico, site 41. 

6. Rio Icuolay, NNW Finca Cubilquitz to Quebrada Diablo, 

Depto. Alta Verapaz, Guatemala (300-350 m) ; 111-6-1942, 

1. A. Steyermark, no. 44726 (F 1148694). 

NAT~JRAL HISTORY 

Acacia mayana is by far the rarest of the swollenthorn acacias. If the small number of collections 

represents the true density and spatial distribution 

of the species, it may well be self-pollinated in contrast to the other swollen-thorn acacias. The only 

specimen that I have been able to find (site 3) was 

growing in the same type of old second-growth 

cornfield regeneration that commonly harbors A. 

cornigera, but with one exception. The forest was 

about 15 m tall and the single A. mayana was 

heavily shaded, yet growing very well; an A. 

cornigera in the same shade regime would have 

been spindly with undersized thorns. It is possible 

that A. mayana is a species of primary forest, such 

as A. melanoceras and A. allenii; Matuda recorded 

it at site 5 as a “shrub in virgin forest.” If this is 

the case, then its rarity in areas under heavy agricultural pressure probably means that it will become extinct in the lowlands. 

If the above hypothesis is correct, then A. mayana 

is a mature and wet-forest edition of A. cornigera. 

It is interesting in this context that A. cornigera has 

perhaps produced both a beach-edge species (A. 

sphaerocephala) and a wet mature forest species. 

It is tempting to view the oddly shaped thorns of 

A. mayana as the result of a long past hybridization 

with a sword-thorn acacia such as A. macracantha. 

When A. macracantha hybridizes with A. chiapensis, the result is often a “roundish” thorn in cross 

section, with longtitudinal shallow-sided ridges 

down each side of the thorn. On the other hand, 

A. mayana displays no other traits suggesting a 

history of introgression with other acacias. 

Whether the ridges are the result of introgression, or a novel mutation, they have a possible 

adaptive value. A. cornigera thorns on heavily 

shaded shoots have weak walls and are easily opened 

by birds for the ant brood inside; apparently the 

starving shoot does not have the energy to make 

the thick-walled thorns that effectively protect the 

ant colony in more insolated microhabitats. The 

longitudinal ridges on A. mayana thorns may be a 

way to solve the strengthening problem without 
SUMBER I3 95 

FIGURE 8P.-Acacia mayana: A, type B thorns (the two largest), the largest thorn contained the 

ant colony queen; B, type A thorns split by birds in search of ant brood (Mexico, site 5). 

putting a lot of bulk into the thorn wall (this bulk 

is also adaptive as insulation in heavy insolated 

sites) in a heavily shaded habitat. First, it is important to note that the ant colony grows slowly 

at a comparatively large cost to the acacia in the 

shaded habitat, and the loss of thorns with brood 

will be proportionately larger to the acacia than in 

an insolated site. Second, the slow replacement rate 

of thorns in shaded sites, observed on all forest 

species of swollen-thorn acacias, means that the 

birds have a longer time per thorn to locate the 

tree and go after its ants. Figure 82 shows that 

birds can be a threat to the ant colony in A. mayana, 

and it is important to note that they opened all 

the thorns on the shoot except the few largest ones. 

All specimens of A. mayana examined have been 

occupied by mature colonies of P. ferruginea. In 

view of the small sample size, however, this should 

not be taken to indicate that it is a suitable host 

for only this species of obligate acacia-ant. The 

colony in the specimen at site 3 was as normal as 

can be expected of a colony that has just lost about 

three-fourths of its brood and workers to a bird. 

While the petiolar nectaries of A. mayana are 

nearly twice as long as those of A. cornigera and 

A. sphaerocephala, it should not be assumed that 

the ant colony gets twice as much nectar per leaf 

from the shaded plant. There are no data to show 

a correlation between nectary size and nectar production. 

A total of 12 pods from herbarium sheets was 

examined and there was no attack by bruchids. 

Owing to the small sample size, and the small sizes 

of the crops from which the pods were taken, this 

should not be taken as an indication that A. 

mayana is free from predispersal seed predation. 

On the other hand, if in fact it does occur as widely 

scattered individuals in mature forest, it is possible 

that the bruchids that commonly infest A. cornigera 

and A. sphaerocephala cannot cope with the very 

low prey density that such a rare population of 

swollen-thorn acacias presents (as with A. melanoceras and A. allenii). 

10. Acacia gentlei Standley 

Acacia gentlei Standley, 1940:77. 

Acacia gentlei is the common acacia of wetter 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 

sites in British Honduras and lowland northern 

Guatemala (Figure 44). It is easily distinguished 

from other swollen-thorn acacias by the combination of elongate inflorescences, one or at the most 

two elongate petiolar nectaries, and a large number 

of very thin thorns. Where it is sympatric with A. 

collinsii and A. cornigera in northern British 

Honduras, it is easily distinguished from both by 

attaining heights of 3 to 8 m, while they are generally 1.5 to 3 m in height when reproductive, and 

by having much darker green foliage that strikingly 

contrasts with the rust-red bark. Both are generally 

found in the drier sites, while A. gentlei is generally in the wetter sites. Where the distribution of 

A. gentlei overlaps with that of A. ruddiae and A. 

cookii in northern Guatemalan wet forest, A. 

gentlei is again easily distinguished from the former 

by being occupied by obligate acacia-ants and by 

having Beltian bodies on the new foliage. It is 

distinguished from the latter by having oval petiolar 

nectaries and elongate inflorescences. Some specimens of A. globulifera have oval petiolar nectaries 

as well, but the spherical inflorescence of A. globulifera is very easily distinguished from the elongate 

one of A. gentlei. 

Standley’s type specimen of A. gentlei was obviously collected from the upper part of the crown 

of a large acacia such as the one in Figure 83~, and 

it is representative of the population whose distribution is plotted in Figure 44. The thorns on the 

type are slightly redder than most thorns in nature 

from British Honduras; they look more like those 

common on the north side of Lake Izabal. The 

seed pods on the type specimen are not mature but 

are nearly full length (19 to 20 cm). 

KEY COLLECTION LOCALITIES 

British Honduras: 

1. 

2. 

3. 

4. 

5. 

Type of A. gentlei: British Honduras. P. Gentle, no. 185. 

13.5 mi S Sta. Elena, Corozal Prov., to 43.8 mi SE Orange 

Walk (on road to Belice), Belice Prov., British Honduras 

(10-30 m). VI-30-1966, DHJ, nos. 582, 575 (occasional to 

common) fl., mp. 

17.9 to 36.1 mi SW Belice (on road to Cayo), Cay0 Prov., 

British Honduras (50-100 m). VII-3-1966, DHJ, nc 

(common). 

2 mi S Sta. Elena, Corozal Prov., British Honduras (sea 

level), VII-4-1966, DHJ, nc (3 plants). 

Gallon Jug, British Honduras. 111-5-1938, D. A. Lancaster, no. 31 (US 2219935) fl., mp. 

6. Maskall, British Honduras. 1-31-1934, P. H. Gentle, no. 

1121 (US 1635002), mp. 

7. Little Cocquericot, Belize River. 111-27-1933, C. L. 

Lundell, no. 4406 (F 738238). 

8. Valentin, El Cay0 Prov. VI-1936, C. L. Lundell, no. 6328 

(F 894493). 

9. Camp 6, El Cay0 Prov. 111-9-1938, P. H. Gentle, no. 2311 

(F 945128). 

10. Rio Grande (southern B. H.) (80 m). 111-20-1933, W. A. 

Schipp, no. 1142 (F 669139) ft. 

Guatemala: 

11. El Zapotillo, 1 km E El Estor, Depto. Izabal, Guatemala 

(10 m). V-16-1965, DHJ, nos. 657, 659, 665, 667, 671, 

690 (common), fl., mp. 

12. 16.1 mi SW (Puente El Lobo) Los Amates to 29.4 mi 

NE (Santa Rosa) Los Amates on CA-9, Depto. Izabal, 

Guatemala (200 to 50 m) . VIII-9-1967, DHJ, nc (common but scattered). 

13. 1.0 to 9.4 mi SE Mariscos (S side Lake Izabal), Depto. 

Izabal, Guatemala (100 m). V-13-1965, DHJ, nc (common). 

14. Uaxactun, Depto. Peten, Guatemala, IV-21-1931, H. H. 

Bartlett, no. 12676 (US 1493227, US 1493228), fl., mp. 

15. Aguacaliente, Depto. Santa Rosa. 1-25-1908, W. A. Kellerman, no. 7710 (F 224606) (locality doubtful). 

16. La Libertad, Depto. Peten. IV-1-1933, C. L. Lundell, no. 

2379 (F 738067) fl. 

17. N of Sebol, between Sebol and Carrizal, Depto. Alta 

Verapaz (200-300 m) . IV-18-1942, J. A. Steyermark, no. 

45762 (P 1180368, F 1180419). 

18. S of Sayaxche, Laguna Petexbatum. (Laguna Mexico), 

Depto. Peten (50 m). V-3-1942, J. A. Steyermark, no. 

46221 (F 1180409). 

19. 2-4 mi W Livingston, Depto. Izabal. IV-16-1940, J. A. 

Steyermark, no. 39546 (F 1038624). 

20. Fallabon-Yaxha Road, Depto. Peten. 111-1933, C. L. Lundell, no. 2195 (F 739642). 

21. 2.8 mi W Puerto Matias de Galvez (on road to Escobas) , 

Depto. Izabal (10 m) , VIII-10-1967, DHJ, nc. 

NATURAL HISTORY 

While A. gentlei is clearly a plant that was 

originally associated with wetter forest than extreme 

dry-land species such as A. cornigera, A. hindsii, and 

A. collinsii, it has survived very well in the face 

of human disturbance, in contrast to A. allenii, 

A. melanoceras, and A. ruddiae. Wherever the forest is cut with at least a 15 to 20 year fallow rotation 

period, A. gentlei becomes very common, and has 

even become a roadside plant in the wetter parts of 

British Honduras. It is very common in fallow 

Indian milpas in the Izabal region of Guatemala. 

In Guatemala, it is commonly encountered in 

swamps and river-edge vegetation in forest undisturbed by man for a long time. 
NUMBER 13 97 

FIGURE BS.-Acacia gentlei (Guatemala, site 12): A, crown of 5.m-tall emergent adult; B, type B 

thorns on main trunk of adult. 

The obvious success of A. gentlei in moving from 

natural disturbance sites is probably due to two 

factors. First, the seeds are surrounded by a white 

and sweet pulp that is readily picked from the 

semidehiscent pods by birds such as kiskadees 

(Ptangus sulpheratus) and saltators (Saltator spp.). 

These, and other birds of second-growth vegetation, 

are probably responsible for frequent dispersal of 

seeds into newly cleared sites. Second, as will be 

discussed later, A. gentlei is quickly occupied and 

heavily patrolled by obligate acacia-ants. 

While A. gerttlei adults in reproductive condition 

are only occasionally encountered in primary forest 

(but slightly more often than are A. allenii or A. 

melanoceras), their seeds are probably moderately 

common. A. gentlei seeds are very hard, like those 

of other dry-land acacias, and remain viable at 

least 5 years and probably much longer. The presence of A. gentlei seeds in the forest litter is suggested as the effect of the hurricane that leveled 

large expanses of wet forest to the southwest of 

Belice, British Honduras (site 3), in the late 1950s. 

In the woody regeneration that followed, by 1966 A. 

gentlei was scattered over the area at a density of 

about 10 reproducing adults per acre. There had 

clearly been enough seed and seedlings present to 

do this, despite the fact that the average density 

of A. gentlei in this type of forest, when undisturbed, is about 0.1 tree per acre. 

Reproductive details of A. gentlei have not been 

recorded, but it sprouts very well from cut stumps. 

Seedlings grow well in very dense shade but appear 

to require a slight break in the general canopy before they can grow large enough to reproduce. 

Seed crops are large and therefore comparable in 

size with the other dry-land acacias when fully 

insolated and mature. Adults are difficult to age 

because they show the least clear growth rings of 

any of the swollen-thorn acacias. Based on size, they 

probably live up to about 20 or 25 years and rarely 

have a trunk diameter in excess of 15 cm. 

Trees of A. gentlei are usually very heavily oc- 
98 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

h 

fi 

B - 150 

FIGURE 84.-Acacia gentlei: A, type A thorns from a lateral branch from the crown of a mature 

tree (Guatemala, site 11); B, type A thorns from the crown of a mature tree (British Honduras, 

site 2); c, type B thorns on the base of a lateral branch (British Honduras, site 2). 

X 

t-r 

t3 

2 

w 

cl 

2 

0: 

0 

X 

t-r 

130 

140 i 

a* 

a 

I I I I 

0.5 1.0 1.5 2.0 

TOTAL THORN VOLUME (cc) 

FIGURE 85.-Distribution of 15.4 cc of thorn volume among 

14 very large type A thorns from one A. gentlei (Guatemala, 

site 13). 

cupied by P. ferruginea or one of the two undescribed species of obligate acacia-ants found within 

its range. The young single-queen colonies appear 

to have little trouble becoming establishedthough in roadside areas of British Honduras, an 

unusually aggressive black morph of Pseudomyrmex 

gracilis often takes over young acacias. While the 

dark green foliage of A. gentlei closely resembles 

that of A. chiupensis in texture, color, and taste, A. 

gentlei lacks the herbivore damage so characteristic 

of A. chiupensis. This lack of herbivore damage is 

apparently because the ant colonies grow very large 

in A. gentlei; a single-queen colony of 15,000 workers is not uncommon in a 3-m-tall tree. 

Acacia gentlei shows that thin thorns (Figure 83~, 

84) alone do not lead to poor growth of an ant 

colony. As can be seen in Figure 85, even the very 

longest thorn has a very small thorn volume. Many 

thorns are only 1.5 to 3 mm inside diameter, barely 

allowing the worker ants to pass through. As a 

consequence there is a very small amount of brood 

per thorn, but the very large number of thorns per 

tree probably means that the total thorn volume 

available to the colony is about the same in equal- 
NUMBER 13 

FIGURE 8G.-Football-sized carton nests of Azteca ants in the 

crowns of 12-m-tall A. gentlei (Guatemala, site 12). 

sized trees of A. gentlei and other swollen-thorn 

acacias. The other thin-thorned species, A. cookii, 

does not appear to have an excessive number of 

swollen thorns, but the slightly larger diameter of 

A. cookii thorns may compensate for the lower 

number as contrasted with A. gentlei. 

Large senescent trees of A. gentlei growing in 

undisturbed (by humans) habitats are occasionally 

heavily tenanted by an unidentified species of ant 

in the genus Azteca. This ant genus, better known 

for its species occupying Cecropia and Cordia, is 

arboreal, predaceous on insects, and very aggressive 

to animals that shake the foliage near the nest. In 

A. gentlei the ants live both in the thorns previously 

opened up by the obligate acacia-ants and in hanging carton nests (Figure 86). While they do not 

systematically patrol the acacia leaves, they do 

forage on them, which probably results in the 

senescent tree living longer than normal. The obligate acacia-ant colony generally does very poorly in 

a senescent acacia since the ants have to have 

Beltian bodies and nectar in large quantities to 

maintain a colony large enough to effectively patrol 

the acacia. 

11. Acacia collinsii Safford 

Acacia collinsii Safford, 1910:677. 

Acacia yucatanensis Schenck, 1913:361. 

Acacia neisonii Safford, 1914:3G3. 

Acacia costaricensis Schenck, 1913:361. 

Acacia penonornenscs Safford, 1914:363. 

Acacia panamensis Schenck, 1913:362. 

Myrmecodendron collinsii (Safford) Britton and Rose, 1928: 

Myrmecodendron costaricense (Schenck) Britton and Rose, 

92. 

1928:95. 

Acacia collinsii is easily distinguished from all 

other swollen-thorn acacias by its unique broadly 

dome-shaped petiolar nectaries. It has elongated 

inflorescences and fully dehiscent pods (which it 

has in common only with A. hindsii). A. hindsii 

and A. collinsii may be distinguished at almost all 

localities by their thorns, since type A thorns from 

mature A. hindsii are always strongly flattened 

while type A thorns of A. collinsii are round in 

cross section. Of all the swollen-thorn acacias, A. 

collinsii and A. cornigera have the widest geographical and ecological distributions (Figure 87) . 

Safford’s tjpe specimen of A. collinsii offers a 

classic example of the inadequacy of type specimens 

at representing an entire population. While Safford’s type specimen is representative of the population of A. collinsii at site 1, the Chicoasen 

population of A. collinsii is quite different from A. 

collinsii over the remainder of its range. The type 

specimen appears to have been taken from a large 

adult tree in a partially shaded riparian site. Its 

leaves are as long as any found on A. collinsii (up 

to 16 cm long with the 10 pinnae spaced up to 3 

cm apart). The leaves have the unusually large 

number of 5 to 6 petiolar nectaries with the distal 

ones being as far as 2 mm apart (this makes one 

suspect a bit of introgression with A. globulifera, 

which also occurs at this site). The sexual branchlets are up to 8 cm long (normal maximum) and 

the peduncles of the mature inflorescences range up 

to the exceptional length of 33 mm. The peduncles 

are also nearly 2 mm thick, while on most A. 

collinsii they are 1 mm or less thick. The thin type 

A thorns are round in cross section but twisted 

around the branch; this is characteristic of the A. 

collinsii population only in Chiapas, Mexico. 

Schenck’s type of A. yucatanensis (site 2) is representative of most of the population of A. collinsii 

in drier habitats, and especially the Yucatan portion 

of the population. The small leaves are specially 

characteristic of A. collinsii. His type of A. costaricensis is not available (apparently destroyed in 

Berlin) but a paratype from Costa Rica (site 66) is 

clearly drawn from that portion of the A. collinsii 

population at dry intermediate elevations in Costa 

Rica. If it were useful to designate a lectotype, this 
20. 

8’ * 0 

would be the appropriate specimen. Schenck’s description of A. costaricensis applies unmistakably to 

A. collinsii and is very different from the other 

Costa Rican swollen-thorn acacias (A. cornigera, A. 

allenii, A. ruddiae). Britton and Rose (1928) refer 

to the type locality of A. costaricensis as “Lagarto, 

Costa Rica” but do not give the source of these data; 

there is a “Lagarto” in Puntarenas Province and A. 

collinsii is abundant there (sites 67 and 75). 

Acacia panamensis is clearly based on two species 

of plants. The description of the thorn and leafbearing branch clearly applies to A. collinsii (not 

to A. melanoceras, the only other central Panamanian species of swollen-thorn acacia). Schenck 

(1913) notes that he is not sure that the inflorescences belong to the sterile branch; the globular 

inflorescences could be from A. melanoceras (unlikely owing to their very thin peduncle) or, more 

likely, from one of the other nonant-acacias that 

grow commonly in the drier parts of Panama along 

with A. collinsii. A final complication is that the 

exact locality is given as “Aljahuela” and I cannot 
SUMBER 13 101 

locate such a site in Panama. This site most likely 

represents Alajuela in Alajuela Province, Costa 

Rica, which was a favorite collection site for German collectors around the turn of the century. A. 

collinsii is the only swollen-thorn acacia in the 

Alajuela region (sites 68, 69, 74). 

Safford’s type of A. nelsonii (site 3) is representative of the A. collinsii population in the area north 

of the Isthmus of Tehuantepec, Mexico, but differs 

from more southerly populations in having large 

pinnules (up to 11 mm long and 3 mm wide) and 

somewhat flattened thorns (2 to 3 times as wide as 

deep in cross section). Flattened thorns are, however, even better developed in the A. collinsii population from central Costa Rica to Panama. The 

inflorescences of the type of A. nelsonii are normal, 

but the thorns on the same branch have been 

weathered gray rather than being the usual beige 

color of younger thorns from western Mexico. 

Safforcl’s type of A. penonomensis (site 76) differs 

from the remainder of the A. collinsii population 

on the dry Pacific side of Panama in that its type A 

thorns are not quite as flattened as are those on 

many A. collinsii at sites 82 to 84. The seed pods 

on the type of A. penonomensis are representative 

of those found in the drier parts of the range of A. 

collinsii; these pods have stiff woody valves that 

only curl a slight amount during dehiscion. 

KEY COLLECTION LOCALITIES 

Mexico: 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

Type of A. collinsii: (between San Fernandino and) 

Chicoasen, Chiapas, Mexico (320 m). 1-13-1907, G. N. 

Collins and C. B. Doyle, no. 180 (US 692159). 

Type of A. yucatanensis: Yucatan, Mexico. 1895, G. F. 

Gaumer, no. 353 (US 571618). 

Type of A. nelsonii: Acapulco, Guererro, Mexico (sea 

level). IV-30-1903, E. W. Nelson, no. 7024 (US 399366). 

3.6 mi NW, border station between Mexico and Guatemala (La Mesilla) to 8.4 mi S La Trinitaria on Hgy. 

190, Chiapas (680-720-600-1060 m) . VIII-7-1966, DHJ, 

nos. 441, 443; VIII-17-1967, DHJ, no. 1713 (very common to occasional) . 11.8 mi E (near turnoff to Villa Hermosa) Chiapa de 

Corzo to 13.8 mi SW Tuxtla Gutierrez on Hgy. 190, 

Chiapas (10904.50-1090 m). VIII-17-1967, DHJ, nos. 

424, 43.5 (common to occasional). 

13.8 mi SW, Tuxtla Gutierrez to 17.9 mi SW Las Cruces 

(turnoff to Arriaga) on Hgy. 190 (1090-530-820 m), 

Chiapas. VIII-18-1967, DHJ, nc (common to exceedingly rare). 

1-2 mi N Gabriel Zamora on Hgy. 37, Michoacan (820 

m) . VI-10-20-1968, DHJ, nc (very common). 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

13.4 to 23.5 mi E Zanatepec on Hgy. 190, Oaxaca (50 

m) . VIII-18-1968, DHJ, nc (very rare to common), fl. 

42.1 mi E Zanatepec on Hgy. 190, Oaxaca (60 m) . VIII18-1967, DHJ, nc (1 plant). 

3 mi N Soyalo on Tuxtla Guticrrez.Villa Hermosa Hgy., 

Chiapas (- 1100 m) . VII-10-1966, DHJ, nc (common), 

fl. 

10 to 14 W Tchuantepec (Hgy. 190), Oaxaca (100 m). 

1-10-1965, DHJ, nos. 1845, 1950. IX-3-1965, DHJ, nos. 

1779, 1780, 1783 (common). 

Pucrto Escondido to 3.5 mi N on road to Oaxaca, 

Oaxaca (10-120 m) . VI-19-1967, DHJ, nc (common to 

very rare) . Puerto Escondido to 18.4 mi RTV (Hgy. ZOO), Oaxaca 

(10-80 m). VI-19-1967, DHJ, nc (common to very 

rare). 

Hills in and around Acapulco, Guerrero (0-100 m). 

1’1-18-1967, DHJ, nc (occasional) . Acapulco to 1.3 mi SE Papanoa, Guerrero (0-30 m). 

VI-16-1967, DHJ, nc (common on dry hillsides, absent from swamps). 

48 mi NE Isla Aguada (across from Puerto Real, Campeche) to Merida (Yucatan), on Hgy. 180 (0-30 m), 

VI-22-1966, DHJ, nos. 428, 431, 433, 438, 606 (occasional 

to very abundant.) 

Ruinas Edzna north to Hgy. 180 (east of Campeche), 

Campechc (15-40 m). VI-25-1966, DHJ, nc (rare to 

occasional). 

Becal south to Hgy. 180, Campcche (1540 m) . VI-26- 

1967, DHJ, nc (rare to occasional). 

Merida to 14.2 mi E (Hgv. 180), Yucatan (10-20 m). 

VI-28-1966, DHJ, no. 455 (rare). 

Mum (Yucatan) to Felipe Carillo Puerto (road to Cd. 

Chetumal) , Quintana Roo (10-20 m). VI-29-1966, DHJ, 

nc (very rare). 

Felipe Carillo Puerto to Cd. Chetumal, Quintana Roo 

(20-0 m). VI-30-1966, DHJ, no. 453 (very rare to occasional) . 22. 9.6 mi E San Marcos on Hgy. 200, Oaxaca (50 m). VI18-1967, DHJ, nc (1 plant). 

23. La Ventosa, 7 km E Salina Cruz, Oaxaca (sea level). 

VII-3-1959, K.M. King, no. 1302 (US 2362102), fl. 

24. Petatlan, Montes de Oca, Guerrero. VI-17-1937, G. B. 

Hinton, no. 10333 (US 1805541, F 959109). 

25. Tuxpena, Campcche. 11-2-1932, C. L. Lundell, no. 1279 

(US 1494445), mp. 

26. Sudzal, Yucatan. IV-21-1956, 0. G. Enriquez, no. 564 

(US 2448079). 

27. Chichen Itza, Yucatan. V-30-1929, J. Bequaert, no. 2 

(US 1477467), ff., mp. 

28. Isla Cozumel, Quintana Roo. XII-5-1966, V. E. Rudd, 

no. 2036 (US 2518023). 

29. San Vicente, Chiapas. IV-16-1904, E. A. Goldman, no. 

862 (US 470663), mp. 

30. Berriozabal, on road to San Fernando, Chiapas (950 m). 

XII-18-1949, F. Aliranda, no. 5847 (US 2367591), mp. 

31. Chorreadcro de Tuxtla, 5.6 mi E Chiapa de Corzo (800 

ni). 11-16-1965, D. E. Breedlove, no. 9018 (F 1627187), 

fl. 
102 SMITHSOKIAN COSTRIBUTIONS TO BOTANY 

32. Chichen Itza, Yucatan. V-30-1929, J. Bequaert, no. 2 

(F 710846). 

33. Izamal, Yucatan. 11-21-1906, J. M. Gaurner, no. 379 (F 

189844), fl. 

34. Salina Cruz, Oaxaca (sea level). IV-28-1910, Orcutt, no. 

3451 (F 280154), mp. 

British Honduras: 

35. 23.6 mi S Orange Walk to Belize, Belize Prov. (10-0 m). 

VII-2-1966, DHJ, nc (rare to common). 

36. 10.1 to 34.4 mi SIV Belize on road to El Cayo, Belize 

Prov. (30-100 m). VII-3-1966, DHJ, nc (rare to occasional). 

37. Duck Rum, Pine Ridge, El Cay0 Prov. 11-17-1931, 

H. H. Rartlett, no. 11546 (US 1493376, US 1493375), 

mP* 

Guatemala: 

38. 2.6 mi S\V Puente Rio Hondo on Hgy. CA-9 (near 

turnoff to Zacapa), to 1.8 mi NE Pucnte El Lob0 on 

Hgy. CA-9, Depto. Zacapa (200-100 m) . VIII-9-1967, 

UHJ, nos. 1794, 1797 (very common to occasional). 

39. “District of Peten.” IV-8-1933, C. L. Lundelf, no. 2614 

(F 738050). 

Honduras: 

40. El Espino (Sicaragua) to 0.4 mi W on HondurasSiraragua border on Interamerican Hgy., Depto. Choluteca (880-930 m). VII-29-1967, DHJ, nc (4 plants). 

41. 21.6 mi I$’ San Marcos de Colon on Interamerican Hgy., 

to Choluteca, Depto. Choluteca (450-10 m). VIII-31- 

1965, DHJ, 110. 1788; V-11-1965, DHJ, no. 1807; VII29-1967, DHJ, nc (very common). 

42. Choluteca (Depto. Choluteca) to Jicaro Galan (Depto. 

L’alle) on Interamerican Hgy. (10-40 m) . VII-29-1967, 

DHJ, no. 1803 (common to rare). 

43. Jicaro Galan to 3.8 mi NE (Hyg. 1) , Depto. Valle (40- 

80 m). VII-29-1967, DHJ, nc (rare). 

44. Comayagua to 3 mi S on Hgy. 1, Depto. Comayagua 

(550 m). VII-31-1967, DHJ, nc (rare to common), fl. 

43, Jicaro Galan to El Amatillo (Honduras-El Salvador 

border) on Interamerican Hgy. (40-200 m) . 1-15-1965, 

DHJ, no. 1841; V-12-1965, DHJ, nc (occasional to rare), 

fl. 

46. Juticalpa, Depto. Olancho. 111-5-16-1949, P. C. Standley, no. 17580 (F 1378910), mp. 

47. Rio Guayainbre, Valle Jamastran, Depto. El Paraiso 

(350 in). 111-13-1956, A. Molina, no, 7364 (F 1599625). 

48. Danli, Dcpto. El Paraiso (700-800 m) . 11-11-23-1949, 

P. C. Standley, no. 16506 (F 1378924). 

El Salvador: 

49. El Amatillo (Honduras-El Salvador border) to 7.1 mi 

IV (on road to La Union), Depto. La Union (200-190 

in). V-12-1965, DHJ, no. 1801 (rare), fl. 

Xicaragua: 

50. Penas Blancas (Depto. Rivas) to Jinotepe (Depto. 

Carazo) (Hgy. 2) (50-500 m) . V-9-1965, DHJ, no. 1822; 

51. 

52. 

53. 

54. 

56. 

57. 

58. 

59. 

60. 

61. 

62. 

63. 

64. 

65. 

1-16-1965, DHJ, nos. 1830, 1932; VII-26-1967, DHJ, nc 

(occasional to very common). 

La Virgin to San Juan del Sur (Hgy. 16), Rivas Prov. 

(50-0 m). V-10-1965, DHJ, nos. 1814, 1815, 1819, 1921 

(common), fl, mp. 

Masachapa to 14.7 mi NE (Hgy. 8), Depto. Managua 

(0-480 m) . VII-26-1967, DHJ, nc (common to rare) . 15.2 mi SW Interamerican Hgy. (Depto. Managua) on 

road to Chinandega (Hgy. 12) to 5 mi SE Leon (Depto. 

Leon), (30-450 in). VII-27-1967, DHJ, nc (occasional 

to rare). 

Puerto Somoza to 6.4 mi SE (junction with Hgy. 12), 

Depto. Leon (0-30 mi S Tipitapa on Hgy. 1, Depto. 

Managua (50 m). V-10-1965, DHJ, no. 1812; VII-28- 

1967, DHJ, nc (1 unoccupied plant). 

4.2 mi N Tipitapa on Hgy. 1, Managua (75 m). VII28-1967, DHJ, nc (1 unoccupied plant), fl. 

San Benito (N of Tipitapa on Hgy. 1) to 16.7 NE 

(Hgy. y) (100 m). VII-28-1967, DHJ, nc (extremely 

common). 

San Bcnito (Depto. Managua) to Sebaco (Depto. Matagalpa) on Hgy. 1 (100-420-350-500 m). VII-28-1967, 

DHJ, nc (very common to very rare at 420 m to common) , fl. 

Sebaco to 1.6 mi N Matagalpa (Hgy. 3), Depto. Matagalpa (500-730 m) . VII-28-1967, DHJ, nc (common to 

occasional), fl. 

Sebaco (Depto. Matagalpa) to Esteli (Depto. Esteli) on 

Hgy. 1 (500-1040-850 m). VII-28-1967, DHJ, nc (corn. 

mon to very rare at 1040 m to common). 

Esteli (Depto. Esteli) to El Espino on the HondurasNicaragua border (Depto. Madriz) 011 Hgy. 1 (850-530- 

880 m) . 1-15-1965, DHJ, no. 1825; V-11-1965, DHJ, no. 

1808, 1810; VII-29-1967, DHJ, nc (common). 

Laguna de Masaya, Depto. Masaya (300 m) . VII-6-1923, 

W. R. Maxon, no. 7739 (US 1181452), 8. 

Granada, Depto. Granada (250 m) . VII-2-1923, W. R. 

Alnxon et al., no. 7397 (US 1181308) ; IV-21-1916, J. D. 

Mandragou (US 764653 and CS 764655), fl; and II-12- 

1903, C. F. Baker, no. 2546 (US 862954), fl. 

La Libertad, Depto. Chontales, V-29-VI-1-1947, P. C. 

Standley, no. 8826 (F 1288080). 

Braggman’s Bluff (Puerto Cabezas), Yulu Village, Depto. 

Zelaya (100 m) , (F 585006). 

Costa Rica: 

66. Potential lectotjpe of A. costaricensis: Alajuela, Alajuela, Costa Rica (920 m). 1896, J. D. Smith, no. 6488 

(cf. Schenck, 1913). 

67. 18.4 mi to 21.4 mi E Palmar Sorte, Puntarenas Prov. 

(90 m) . V-23-1963, DHJ, no. 1852; VI-26-1967, VIII23-1965, V-15-1965, 111-21-1966, VII-21-1963, DHJ, nc 

(occasional), in fl. 

68. Escazu to 3.6 in 11’ Santaigo de Puriscal (Hgy. 7), Alajuela Prov. (1000-600 m). VII-17-1967, 11-15-1965, 

DHJ, no. 1862; 111-2-1965 (fl), DHJ, nc (occasional to 

rare). 

69. Along the electric railroad from Antonio to Orotina, 

Alajuela Prov. (900 to 50 in). VIII-1963, DHJ, nc (occasional). 
SUMBER 19 103 

70. 

71. 

72. 

i3. 

74. 

75. 

15.0 mi NTV turnoff of Interamerican Hgy. to Puntarenas, to Peiias Blancas (Border of Costa Rica and 

Nicaragua), Guanacaste Prov. (10-100 m) . JanuarySeptember, periodically in 1963-1967, DH], nos. 1757, 

1759, 1762, 1763, 1826 (rare to exceedingly common), 

fl. January to April. 

Liberia to Playa COCO (Hgy. 21), Guanacaste Prov. 

(100-0 m) , January-September, periodically in 1963- 

1967, DHJ, no. 1858 (occasional to very common), fl. 

January to April. 

Hda. Palo Verde (about 18 mi S. Bagaces), Guanacaste 

Prov. (sea level). IV-2-1966, DHJ, nc (common), fl. 

Canas to 6 mi NE Guanacaste Prov. (80-300 m) . IV-15- 

1965, DHJ, nc (occasional), fl. 

Alajuela Prov. (920 m) . 111-1896, J. D. Smith, no. 6488 

(US 943274), fl. 

Lagarto, Puntarenas Prov. (sea level). XII-1891, Tonduz, no. 4810 (US 943276). 

Panama: 

76. 

77. 

78. 

79. 

80. 

81. 

82. 

83. 

84. 

85. 

86. 

87. 

88. 

81). 

90. 

Type of A. penonomensis: Penome, Cocle, Panama (15- 

300 m). 11-23-1908 to 111-22-1908, R. S. Williams, no. 

113. 

4.8 mi NE Tucuman Airport on Panama-Chepo Hgy., 

Panama Prov. (20 m). VII-1-1967, DHJ, no. 1704 (very 

rare), gp. 

Vicinity of Madden Dam, Madden Lake (50 m). VII-3- 

1967, DHJ, no. 1653 (common), gp, fl. 

Miraflores Locks to Paraiso and Corozal area, Canal 

Zone, VII-8-1967, DH] and Wheeler (1942), nc (rare). 

Arraijuan to 4.1 mi E on Interamerican Hgy., Canal 

Zone. VII-9-1967, DHJ, nc (occasional). 

3.2 mi SIV La Chorrera, Panama Prov. (100 m) , VII-I1967, DHJ, nc (rare). 

5 mi N. Bejuco (Panama Prov.) to Santiago (Veraguas 

Prov.) along the Interamerican Hgy. (10-50 m) . VII-1- 

1967, DHJ, nos. 1698, 1700, 1702 (rare to occasional). 

Interamerican Hgy. (Herrara Prov.) to 3.1 mi S Pedasi 

(Los Santos Prov.) on Azuero Peninsula (5-25 m) . VI30-1967, DHJ, no. 1695 (occasional to common). 

Santiago (Veraguas Prov.) to David (Chiriqui Prov.) 

along Interamerican Hgy. (50-200 m). VII-11-1967, 

DHJ, nc (very rare to occasional). 

2.1 mi SE Sona to Santiago, Veraguas Prov. (50 m). 

1'1-29-1967, DHJ, nc (rare). 

Remedios (Chiriqui Prov.) to 19.9 mi StV Jorones 

(Veraguas Prov.) on Sona-Remedios Road. VI-29-1967, 

DHJ, no. 1666 (occasional). 

David to Pedregal, Chiriqui Prov. (0-30 m). VI-29- 

1967, DHJ, nc (rare). 

David to 8.5 mi S Boquete, Chiriqui Prov. (30-620 m) . VI-28-1967, DHJ, nc (rare to occasional), gp. 

David to 2.7 mi NIV on Interamerican Hgv., Chiriqui 

Prov. (90 m) , VI-28-1967, DHJ, nc (rare to occasional). 

Along Quebrada Ocho Paso, Rio Tonosi, Tonosi, Los 

Santos Prov. (20 m). 11-26-1963, W. L. Stern et nl., no. 

1831 (US 2490276), fl., mp. 

Colombia:* 

* Colombian localities are not included in Figure 87. 

91. 

92. 

93. 

94. 

95. 

96. 

97. 

98. 

99. 

100. 

101. 

Str-ams of Usiacuri and Pital, in mountains SIV of 

Baranquilla, Depto. Atlantico. V-26-1928, E. Nienzeyer 

(US 1496168). 

Malambo to Sabanagrande, Depto. Atlantico (60 m) . 

1-1936, A. Dugand, nos. 117, 404 (US 1619541). 

Megua, Depto. .4tlantico (50 m) . 1-13-1941, A. Dugand 

and Jaramillo, no. 2796 (US 1799137). 

Usiacuri, Baranquilla, Depto. Atlantico (250 m). VII1927, B. Elias, no. 245 (US 1342935), fl. 

Arroyo Jubilado, Depto. Atlantico (50-60 m) . XII-24- 

1933, A. Dugand, no. Ilia, 40u (US 1573571). 

Cartagena, Depto. Bolivar. 1919, B. Heriberto, no. 39 

(US 1041093). 

Isla de Providencia (Caribbean Sea). IV-29 to V-9-1948, 

G. R. Proctor, no. 3430 (US 1979268), fl., nip.; I-24- 

1967, S. Anderson; VIII-23 to VIII-29-1970, DHJ, nc 

(very common). 

Providence Island (Caribbean Sea). VII-19-26-1966, 

.I. H. Torres R., no. 238 (COL 112189). 

,Tuanmina, Baranquilla Dist., Depto. Atlantico (20 m). 

VII-5-1963, H. S. McKee, no. 10433 (COL 98405). 

Valencia de Jesus, Valledupar, Magdalena (30 m). II14-1948, R. Roinero-Castaneda, no. 793 (COL 44486). 

Arroyo de Megua, Depto. Atlantico (50 m) . 1-13-1941, 

A. Dugand and R. Jni-amillo, no. 2769 (COL 05500). 

NATURAL HISTORY 

Over its wide geographical and ecological range 

(Figure 87), A. collinsii is found in habitats ranging 

from moderately wet to very dry, but the majority 

of the population is found in habitats that appear 

generally too dry for other swollen-thorn acacias 

(except A. sphaeroccpJiala with its peculiarly local 

distribution). These habitats ale often too dry for 

agriculture on hillsides (8 month dry season), and 

as a result, the majoiity of the contemporary population of A. colliiisii lives in shrubby vegetation 

that is overgrazed by cattle and goats (Figure 88). 

In the moister paits of its habitat, the natural 

vegetation has been almost entirely destroyed to 

plant cornfields, but where this is done with a 5 to 

25 year fallow period (e.g., sites 67 and 88), A. 

collinsii is a common part of the regeneration. In 

the very dry parts of its range, such as the east side 

of the Yucatan Peninsula and the Tehuantepec 

area, A. collinsii probably grows in vegetation similar to that occupied before the advent of European 

agriculture except for graiing by goats. 

TVherever A. collinsii and othei species of swollenthorn acacias occur in the same general area, A. 

collinsii is always in the driest subhabitat (and thus 

mixed in with several nonant-acacias). This is particularly striking at a place such as site 15 where it 
104 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 88.-Typical pasture habitat of A. collinsii (Nicaragua, site 60). 

is common on the dry ridges that extend into the 

swampy coastal flatland occupied by A. cornigera. 

Also, in the very dry lowlands in northeastern 

Guatemala (site 38), A. collinsii occupies the dry 

rocky pastures, while A. hindsii, A. gentlci, and 

(rarely) A. cornigera occupy the swamps and stream 

bottoms and banks. Since much of the Yucatan 

Peninsula lacks surface watercourses, it would lack 

swollen-thorn acacias in most of the eastern drier 

area if it were not for A. collinsii. While A. collinsii 

is apparently sympatric with either A. cornigern 

(e.g., site 17) or A. globziliferu (e.g., site 16), the 

latter two species are always in a local wet site, 

such as a lagoon running along the beach or the 

site of a cenote or well. 

Acacia collinsii appears rarely to be moved about 

by contemporary human activities, and the geographic distribution recorded in Figure 87 is probably little changed since Europeans began disturbing Central American vegetation. Like A. cornigera 

and A. hindsii, however, A. collinsii is probably 

much more common now than in pre-Colombian 

times, since it grows very well in the dry and 

shrubby pastures that have replaced much of lowland Central America's deciduous forests. The failure of humans to move the seeds of A. collinsii is 

probably due in part to the fact that the pods 

dehisce so that the edible pulp is either taken by a 

bird or drops to the ground (in contrast to the 

indehiscent pods of A. cotnigera that can be easily 

harvested, and carried about). Also, A. collinsiz lives 

in habitats that are so dry that earth-moving equipment is less likely to pick up mud with seeds in it 

than is the case for either A. cornigein or A. hindsii. 

Finally, and perhaps most important, A. collinsii 

appears to occupy L irtually all the suitable habitats 

in the dry lowlands on both coasts of Central 

America. The conspicuous exceptions to this statement are the dry northern portions of the east and 

west coasts of Mexico, occupied by A. cornigera and 

A. sphaerocephala, and by A. hindsii, respectively; 

human-movement patterns in these pal ts of hlexico 

are not at ,all favorable to contemporary transport 

of A. collinsii further north than at present. 

The failure of A. collinsii to spread throughout 

the di y lowlands of Colombia is the most perplexing 

aspect of its distribution. There are two possibilities, neither of which can be discussed fully with 

the data at hand. It may be a relatively recent 

introduction from Panama or further north along 

the coast; old coastal ports such as Acapulco, La 

Libertad, Puntarenas, and Panama City are well 

within the range of A. collinsii. On the other hand, 

the twin dry seasons in Colombia may be inimical 

to the development of A. collinsii. Jesus Idrobo 

(personal communication) reports that it is most 

commonly found in swamps in Colombia, a habitat 

rarely occupied by A. collinsii in Central America. 
SUMBER 13 105 

FIGLIRE 89.--Acncia collinsii: A, left, unoccupied sucker regeneration and on the right, occupied 

sucker-shoot regeneration of the same age (Costa Rica, site 70); B, occupied lateral branch end; 

contrast with Figure c (Costa Rica, site 67); c, unoccupied lateial branch end growing near 

Figure B. 

IVhile swollen-thorn acacias may reach immediately offshore islands (eg., occupied A. collinsii on 

Isla Co/umel, site 28), there is only one case where 

the ant >< acacia interaction appears to have become n:iturall~~ established on an island far from 

the Central ;\merican coast. A. colliiisii is a comiiion member 01 the flora of Isla Providencia (sites 

9i antl 98) where it is occupied by Pseudoiugmzex 

ferruginen. This island is about 100 miles off the 

Caribbean coast of Nicaragua, and even though the 

acacia is completely wild, there is no guarantee that 

it was not introduced bl, postColombian human 

activities. Isla Providencia has been in contact with 

Europeans for at least 200 years. Even if the acacia 

was recently introduced, it is most likely that the 

ants got there of their own accord; even the most 

avid gardener is unlikely to bring an acacia with an 

ant colony on a boat from the mainland to Isla 

Providencia. The Isla Providencia population of 

A. collinsii is morphologically representative of the 

mainland population, with the exception that the 

aeed pods are the smallest 1 have seen. Sot only are 

they very hea\.ily ~voody (like those on the dry 

Pacific side of Panama) but range from 2 to 3 cm 

long and 4 to 8 inm thick. Almost all of the vegetation on Isla Pi-ovidencia has been cut many times 

and is repeatedly disturbed by livestock antl cultivation; however, even in equally disturbed sites on the 

mainland, the population density of adult A. colliiisii is higher on this island than I have ever 

encountered in mainland sites of comparable land 

use. This high abundance is probably attributable 
106 SMITHSOSIAS COSTRIBLTIOSS TO BOTAXY 

FILVRE SO.--Accccic~ collinsii (Panama, site 83) : A, crown of adult with clumps of type A thoins 

caused by an unknown disease; B, close-up of the thorn cluster from the crown. 

FIGURE 91.--Acacia co11imii: A, two seedlings (compound leaces) with their first pair of leaves 

(Costa Rica, site 70); B, sucker spiout from burned stump (Costa Rica, site 70); c, unoccupied 

2-4-year-old seedlings (Panama, site 88). 
NUMBER 13 107 

to several factors: (1) Based on two small seed 

samples taken in August 1969, the bruchids that 

kill a high percentage of the mainland seeds have 

not arrived on the island; each of the seeds produced is therefore still alive when dispersed. (2) 

Based on sweep samples taken during the rainy season (late August 1969) the herbivorous insect density 

on the island is far lower than found in any mainland lowland habitat during the rainy season; this 

means that the primary environmental challenge 

that unoccupied seedlings face is competition from 

other plants, (3) Since the number of plant species 

is very low on the island, most of the usual competitors with A. collinsii are absent from the 

habitat; in particular, vines are missing. The ants 

appeared to be as aggressive as their mainland 

counterparts and attacked vegetation contacting the 

A. collinsii crown as usual. The only major herbivore on A. collinsii that appears to have arrived on 

Isla Providencia is the noctuid moth (Cocina hadenoides) whose larva can protect itself against the 

ants and eats the shoot tips of occupied swollenthorn acacias (Janzen, 196713). On the mainland, 

this larva invades the acacia often enough to cause 

occasional multiple branching in the shrub's development. On Isla Providencia, its damage was 

intense, causing nearly all the acacias to take on a 

shrubby life form. 

'M'hile A. collinsii lives in a drier habitat than A. 

cornigern, it appears to have a dependence on 

obligate acacia-ants as great as that of A. cornigern. 

A brief test of this was conducted with the hilltop 

portion of the A. collinsii population two miles 

southeast of Canas, Guanacaste Province, Costa 

Kica (site 70). While there are three species of 

obligate acacia-ants in the area (P. ferruginca, P. 

belti, P. nigrocincta), only those plants occupied 

by P. belti were used in tlie experiment. The hilltop site was covered with regenerating pasture 

shrubs (Baziliinin, Ci,oton, Cassia, Lithen, Acacia, 

etc.) and about 40 adults of A. collinsii to the acre. 

On February 16, 1967, 52 plants of A. collinsii in 

the 2 to 3 m class (3 to 4 years of occupied growth) 

were cut off 60 cm above the ground. The acacia 

crown was left next to 25 of the stumps, and for 

27 stumps tlie crown was dragged at least 10 m 

away. After the usual movements of P. belti colonies 

following this disturbance, 9 of the stumps in the 

control plot lost their colonies and none of those 

in the experimental plot gained one (if this exFIGURE 92.--Acncia collinsii: A, ten-year-old adult (Panama, 

site 83); B, crown of adult (Sicaragua, site 54); c, left, 7-yearold clump occupied by Pseudoniyrrnex belti; right, 7-year-old 

tt'w occupied by Pseudomyrmex ferruginea (Sicaxagua, 57). 

periment had been conducted during the rainy 

season, when colonies are larger and the ground is 

not so hot, 5 to 10 of the stumps in the experimental 

plot would have regained their colonies despite the 
108 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 93.-Acacia collinsii (Panama): A, central crown of adult (site 78); B, crown of 10-m-tall 

tree (site 88) : c, senescent crown of 10-m-tall tree (site 88). 

distance to which they were removed). By July 19, 

1967, about five months later, the mean length of 

the longest sucker shoot from each of the 25 stumps 

in the control plot was 25.1 cm (s.d. 10 cm) while 

the same measurement was 5.1 cm (s.d. 4 cm) in 

the experimental plot. These differences are highly 

significant (Mann-Whitney U test) and differ in no 

major way from the results obtained with occupied 

and unoccupied sucker regeneration of A. cornigera 

and P. ferrziginea in eastern Mexico (Janzen, 

1967b). If we consider only the 16 members of the 

control plot that were occupied, the results are more 

dramatic; the mean length of the longest sucker 

shoot is 36.4 cm (s.d. 14 cm). The 9 unoccupied 

suckers of the control plot also had a mean length 

of 5.1 cm (s.d. 12 cm). Figure 89~ illustrates a 

representative shoot from each of the occupied and 

unoccupied stumps of this date. Figures 898 and c 

compare the branch ends of large lateral branches 

from naturally occupied and unoccupied adult 

acacias in a much wetter habitat in southern Costa 

Rica (site 67). 

Since nearly all the regeneration was during the 

dry season, this low total amount of sucker-shoot 

development was expected. That no stump died 

during the course of the experiment suggests 

strongly that substantial energy reserves were stored 

in the lower stump and root system. One year 

later, on July 12, 1968, only 4 of the stumps unoccupied on July 19, 1967, were still alive; all the 

plants that had been occupied in 1967 were still 

alive. By July 14, 1969, all the occupied shoots had 

approximately regained their original height (1967) 

or grown taller, and there was no trace of the 36 

stumps unoccupied on July 19, 1967. 

Acacia collinsii has a small number of leaf and 

stem galls just as do other swollen-thorn acacias. 

At site 86 in Panama, however, the population of 
NUMBER 13 109 

FIGURE 94.-Acacia collinsii (Costa Rica, site 68): A, adult 

growing in open pasture, August 1, 1963; B, same tree crown, 

July 1, 1967; the lack of change is due to lack of interplant 

competition. 

A. collinsii is afflicted with an outstanding disease. 

Almost all the acacias have thick clusters of swollen 

thorns scattered thiougli the crown (Figure 90). 

These are the consequence of some agent that 

causes extreme branch proliferation, yet each internode is extremely shoit. The number of thorns 

produced by the acacia is far greater than the ants 

can use, with the consequence that many of the 

thorns in the cluster have no entrance holes. The 

potentially negative effect of this excess of thorns 

on seed production has not been examined. Young 

seedling and sucker shoot development of A. collinsii 

is very similar to that of A. cornigera and A. hinclsii. 

The bird-dispersed seed germinates at the beginning 

of each rainy season (Figure 91~) and, if fully insolated, forms into a small bush that produces its 

first swollen thorn on the 12th to 17th leaf (about 

two months old). Suckers from stumps in recently 

FIGURE 95.-Acacia collinsii: A, crown of adult growing in 

area with very severe dry season (Nicaragua, site 54); B, 

crown of tree growing in area with very mild dry season 

(Costa Rica, site 67). 

burned areas do the same (Figure ~IB), but the 

larger the stump, the more the first foliage resembles 

that of the mature plant. As the months pass without a new colony developing in the young plants, 

they gain in height at the rate of about 10 cm per 

year and suffer very heavy damage from defoliating 

insects (Figure 91c). In most open habitats, A. 

collirisii grows into a bushy shrub once occupied 

(Figures 92~, B). If it is in dense regeneration, it 

may become a small tree that gradually loses crown 

size and senesces when 15 to 20 years old (Figure 

93). It may retain this shape for many years (Figure 94). 

There is considerable variation in branching pat- 
110 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 96.-Acacia collinsii (Nicaragua): A, dense crown of adult caused by shoot damage by 

Pseudomyrmex ferruginea (site 57) : B, diffuse crown of adult occupied by Pseudomyrmex nigrocincta (site 51). 

terns of A. collinsii in fully insolated habitats. In 

the very driest sites, where A. collinsii may almost 

be regarded as a member of the “climax” forest, 

the plant has only a few major branches, and the 

new leaves each year are produced in axillary tufts 

or on very short new branches (Figure 95~). In 

the wettest sites, the canopy is uniformly dense 

owing to the production of a large number of small 

lateral branches (Figure 95~). A second type of 

crown variation is produced by the interaction between the eastern Nicaraguan portion of the 

Pseudomyrmex ferrziginea population and A. collinsii. This species of ant in this area prunes the 

shoot tips of the swollen-thorn acacia it occupies. 

The consequence is that two adjacent A. coZlinsii 

of the same age-one occupied by P. fer7ziginea 

and the other by P. belti or P. nigrocincta-will be 

extremely different in height (Figure 92c). If the 

shoot is not occupied by P. ferruginea until it is 

1 to 2 m tall, it produces a ‘‘lollypop’’ life form 

(Figure 96). As discussed in Janzen (1969c), one 

possible explanation for the ants’ behavior is that 

birds preying on ant brood cannot get into the 

dense crown to get the queen-thorn; in this area, A. 

collinsii does not have the well-developed type B 

thorns which are normally adequate to protect the 

queen froni birds. 

Acacia collinsii, like A. cornigera, bears its first 

flowers at the end of three years of occupied growth. 

Seeds therefore appear in the fourth dry season 

after the plant is occupied, meaning that A. collinsii 

can exist as a reproducing population in cultivated 

sites with only four years fallow. Its long inflorescences are normally born in short axillary clusters 

(Figure 98~) but in very wet sites may be produced 

on long sexual branchlets (Figure 97A). There 

appears to be a general cline in increasing inflorescence length from the northern to the southern end 

of the range of A. collinsii. The woody pods are 

usually born 1 to 6 per inflorescence; upon 
SUMBER 13 111 

FICL‘RE 97.--Acacia collinsii: A, exceptionally well-developed sexual branchlet (Mexico, site 4); 

B, type A thorn at base of leaf with three droplets of nectar from petiolar nectaries (Costa Rica, 

site 65); c, newly grown type B thorns and fully developed branch on tree in Figure B. 

dehiscing, the seeds drop partly out and are 

wrapped in sweet yellow pulp which is picked off 

by birds in flight. This dispersal mechanism is 

common to the entire population, but there is 

considerable variation as to how woody the pods 

are and how much they curl upon opening. In no 

areas do the pods curl as extremely as those of A. 

hindsii and A. globulijern. 

Type A thorns on A. collinsii are easily distinguished from those of all other swollen-thorn 

acacias, but the differences are difficult to characterize. A. collinsii bears the shortest type A thorns 

to be found on a mature swollen-thorn acacia. They 

are usually round in cross section and give the 

impression of being “fat” (Figures 98, 99). When 

flattened (Figures 98~, 99~) like A. hindsii, they 

still have much thicker walls than would an A. 

hindsii thorn of the same size. .4. collinsii probably 

shows the most extreme variation on type A thorn 

morphology of any of the swollen thorn acacias. 

Most of the population of A. collinsii bears thorns 

that are ivory to beige, but in wet or high elevation 

sites they are often dark brown with a maroon or 

reddish cast. There is usually a prominent bump 

on the inner wall of each thorn near the base; this 

may aid the ants in making use of the large volume 

of space at the base of the “fatter” thorn types 

(e.g., Figure 98~). 

Type B thorns of A. collinsii (Figures 100-103) 

are easily as variable in shape as those of A. 

cornigera and A. hiiadsii. As is the case with these 

latter two species, there is little intrapopulation 

variation in this trait, but of course there is rather 

great variation associated with the age of the acacia. 

Type B thorns are generally produced at the expense of the leaf they subtend (Figure 97c) and are 

produced very rapidly once branch elongation 

starts. It is common to find all the type B thorns 

on a short branch to be at approximately the same 

stage in hardening. While type B thorns may appear 
112 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 98.--Geographic variation in type A thorns from occupied adult A. collinsii: A, Costa 

Rica, site 68; B, Nicaragua, site 61; c, D, Mexico, site 16. 

FIGURE 99.-Geographic variation in type A thorns from occupied adult A. collinsii: A, Mexico, 

site 11; i%, c, Mexico, site 16; D, Nicaragua, site 61; E, Panama, site 86. 
NUMBER 13 

113 

FIGURE 100.-Type B thorns around main trunk of A. collinsii: A, Panama, site 88; B, Costa Rica, 

site 67. 

to converge on those of A. coriiigera or A. hindsii, 

A. colliiisii is not sympatric with those species 

where this occurs. 'Cl'hile the very largest type B 

thorns to be found on A. collinsii (Figure 104) 

have ii large volume and are therefore similar to 

the largest on A. hiiidsii, in general both type A 

and type 13 thorns of A. colliiisii are small in 

volume when compared with swollen-thorn acacias 

in general. 

12. Acacia hindsii Bentham 

Acacia hindsii Bentham, 1842:504. 

Acacia bwsaria Schenck, 1913:363. 

Amcin tepicnna Safford, 1914:366. 

Acacia sinaloemis Safford, 1914:365. 

Myrrttecodendron lzindsii (Bcntham) Britton and Rose, 1928: 

91. 

A mature Acacia hindsii is distinguished from all 

other swollen-thorn acacias by its very flat and distended thorns. On occasion the thorns of A. collinsii 

display superficially similar distension, but A. 

hindsiz has small diameter petiolar nectaries rather 

than broad dome-shaped ones as on A. collinsii. 

A. Iiiizdsti, A. collinsii, and A. cornigera are the 

three most widely distributed swollen-thorn acacias. 

A. hiiidsii, however, is essentially a plant on the 

Pacific lowlands and foothills (Figure 105). 

1 was unable to examine the type specimen of A. 

hindsii, but A. hindsii is the only species of swollenthorn acacia in the Nanzanillo Bay area (site l), 

and the t) pe description agrees closely with the 

swollen-thorn acacia population in that area. The 

northernmost record of A. collinsii (A. collinsii site 

7) is from slightly to the south of site 1, but at this 

site it has cylindrical thorns and is not sympatric 

with A. hindsii. A. hindsii is not to be confused 

with its hybrids with nonant-acacias such as A. 

pennatula, e.g., A. oaxacanum, A. hirtipes, and A. 
114 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 101.-Geographic variation 

in type B thorns of occupied adult 

A. collinsii: A, B, Mexico, site 4; c, 

Nicaragua, site 51; D, Costa Rica, 

site 67; E, Mexico, site 11. 

FIGURE 102.-Geographic variation in type B thorns of occupied adult A. collinsii: A, Costa Rica, 

site 67; B, Nicaragua, site 51; c, Mexico, site 4. 
NUMBER 13 

140 - 

130 - 

2120 - 

g110 - 

2 100 - 

2 90- 

6 

X 

z 

W 

4 

I 

0 

X 

II0 

I115 

FIGURE 103.4eographic variation in type B thorns of occupied adult A. collinsii: A, Panama, 

site 82: B, Mexico, site 11: c, Guatemala, site 38. 

c 

1- I I 

TOTAL THORN VOLUME (cc) 

2 4 6 8 10 

FIGURE 104.-Distribution of thorn volume among the largest 

type B thorns of A. collinsii (Costa Rica, site 67). 

standleyi. None of the hybrids has a full complement of Beltian bodies, large nectaries or fully developed swollen thorns. 

Shenck's description of the type of A. bursaria 

applies without doubt to the A. hindsii population 

at site 49, rather than to A. cornigera, which is the 

only other swollen-thorn acacia at the type locality. 

The thorns on the type specimen of A. bursaria are 

exceptionally large type A thorns, of the kind often 

found on large and very healthy A. hindsii in the 

wetter parts of Guatemala. 

The vegetative branch chosen for the type specimen of A. tepicana is a representative young sucker 

shoot or central axis from a young seedling before 

it became occupied by obligate acacia-ants. The 

thorns are not flattened, but this is the case with 

most young A. hindsii. Their very light color is 

also representative of young A. hindsii in areas 

where the adults have very dark colored thorns. 

The peduncle with mature fruits accompanying the 

type specimen was taken from a representative 

mature A. hindsii. A. hindsii is the only swollenthorn acacia found as far north as Acaponeta in 

western Mexico (site 2). 

Safford's type specimen for A. sinaloensis is a 

representative branch from the upper crown of a 

normal mature A. hindsii of the form common at 

the type locality (site 3). Type A thorns of all degrees of flattening occur in such a crown and the 
116 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FICIIRF. 105.-Postulated general distribution (stippled area) and collection localities of A. hindsii. 

type specimen has thorns representing the smaller 

end of the spectrum. The seed pod accompanying 

the type is a normal A. hiizdsii pod. A. hindsii is 

the only swollen-thorn acacia found as far north 

as Villa Union, Sinaloa, Mexico. 

KEY COLLECTION LOCALITIES 

Mexico 

1. Type of A. hindsii: Shore of Manzanillo Bay, Colima, 

Mexico (sea level). 1836-39 (or 1841; Sir George Taylor, 

in litt.), R. C. Ilinds, no. 248. 

2. Type of A. tepicana: Acaponeta, Nayarit, Mexico (30 m). 

IV-10-1910. J. N. Rose, P. C. Standley, and P. G. Russell, 

no. 14357. 

3. Type of A. sinaloensb: Villa Union, Sinaloa, Mexico (10 

m). IV-2-1910, J. N. Rose, P. C. Standley, and P. G. Rus- 

$ell, no. 13972. 

4. Arriaga to 13.2 mi N on Hgy. 200, Chiapas (10-550 m) . 

1-9-1965, DHJ, nc (common). 

5. Tehuantepec to 21.2 mi E on Hgy. 190, Oaxaca (10-30 

m). 1-10-1965, DHJ, no. 1041; IX-2-1965, DHJ, nc; 

VIII-7-1966, DHJ, no. 415 (occasional to rare). 

6. Near Salina Cruz, Tehuantepec, Oaxaca (sea level). IXl-1965, no. 1934 (occasional to common). 

7. Tapilula to 3 mi E, Chiapas (700 in). VII-10-1966, DHJ, 

nc (occasional). 

8. 12.2 mi SW El Camaron to 10.2 mi W turn into Texquisistlan on Hgy. 190, Oaxaca (925-735 m). 1-10-1965, 

DHJ, nos. 1033, 1034. 

9. Puerto Escondido to 36.9 mi N on Hgy. to Oaxaca, Oaxaca (sea level to 940 m). VI-19-1967, DHJ, nc (occasional to ra:e) . 10. Las Cruces (just inland from Acapulco) to Puerto Escondido on Hgy. 200, Gucrerro-Oaxaca (20-300-10 m) . VI-18-1967, DHJ, nc (common to occasional). 

11. 14.0 mi S Chilpancingo to Las Cruccs, Guererro (1175-30 

m) . 1-7-1965. DHJ, nos. 1031, 1035; DHJ, nc (common). 

12. Playa de Pie de la Cuesta (just W of Acapulco) to 13.6 

mi SE Papanoa on Hgy. 200, Guererro (sea level). VI17-1967, DHJ, nc (common to very rare, only along upper edges of swamp sites). 

13. 21.G mi SW Autlan to Barra de Navidad, Jalisco (450-0 

m). 1-5-1965, DHJ, nos. 1026, 1027, 1028, 1029, 1030 

(common to rare, only in wetter sites) . 14. Barra de Navidad to Tecoman, Jalisco-Colima (50-0 m.) 

1-5-1965, DHJ, nc (rare-common) . 

15. 12 mi NW Tepic to 28.4 mi SE Mazatlan on Hgy. 15, 
NUMBER 13 117 

Nayarit-Sinaloa (700-5-70 m). IX-14-1963, DHJ, no. 

1745; 1-2-1965, DHJ, no, 1011; VI-8-1967, DHJ, nc (rare 

to common to rare) , fl. 

16. Hgy. 12 to San Blas on Hgy. 46, Nayarit (200-0 m). I13-1965, DHJ, nos. 1019, 1020, 1021, 1022, 1023, 1024, 

1025. 

17. 23.5 mi SW El Palinito to 9.0 mi SW, Concordia on Hgy. 

40 (1089-10 m). 1-2-1965, DHJ, nos. 1005, 1006, 1012, 

1013, 1014, 1015, 1016, 1017; VI-6-1967, DHJ, nc (common to rare). 

18. Palo Blanco, San Ignacio, Sinaloa (340 m). IV-18-1918, 

M. N. Montes and H. E. Salazar, no. 286 (US 1035197). 

19. Puerto Angel, Oaxaca (sea level), IV-12-1933, C. V. 

Morton and E. Makrinius, no. 2614 (US 1585606), fl. 

20. Cafetal Concordia, Oaxaca (400-650 m) . IV-15-1933, 

C. V. Morton and E. Makinius, no. 2427 (US 1585440), 

fl. 

21. Pochutla, Oaxaca (50 m) . IV-19-1917, no. 3146, C. Conzatti (US 763852), fl. 

22. Huatulco, Oaxaca (150 m). 11-1941, B. P. Reko, no. 6010 

(F 1510963), fl. 

23. 13.7 mi NE La Mira to Playa Azul and to Melchor 

Ocampo on Hgy. 37, Michoacan (200-0 m). VI-12-1968, 

DHJ, nc (common). 

24. Guatemala-Mexico border (near Ciudad Cuauhtemoc) 

to 3.6 mi N on Hgy. 190, Chiapas (720-680 m). VIII-6- 

1966, DHJ no, 416; VIII-16-1967, DHJ, nc (common to 

rare). 

25. 5.1 mi SW Cintalapa (Chiapas) to 20.9 mi W Tapanatepec (Oaxaca) on Hgy. 190 (610-820-110 m) . VIII-18- 

1967, DHJ, nc (common to rare). 

25a. 23.3 mi S Pichucalco, on Villa Hermosa-Chiapa de Corzo 

road, Chiapas (1000 m) . VII-10-1966, DHJ, no. 420, 564. 

26. 29.3-35.0 mi W Tapanatepec, Oaxaca (100-20 m) . VIII18-1967, DHJ, nc (occasional). 

27. 6.8-4.1 m S of turn into Matias Romero, on Hgy. 185, 

Oaxaca (230 m) . VIII-18-1967, DHJ, nc (3 plants). 

28. Guatemala-Mexico border (Pte. Talisman) to Arriaga 

on Hgy. 200, Chiapas. 1-10-1965, DHJ, nc (common). 

Guatemala: 

29. 6.8 mi SW (1370 m) Guatemala City to Escuintla, Depto. 

Esquintla on Hgy. 2. 1-12-1965, DHJ, no. 1050; VIII-12- 

1967, DHJ, nc (common), 

30. Shores of Lake Amatitlan and surrounding hills (1300- 

1350 m). Depto. Guatemala. VIII-11-1967, DHJ, nc 

(common). 

31. Escuintla, Depto. Escuintla (330 m) to GuatemalaMexico border (Pte. Talisman), Depto. San Marcos (50 

m) , 1-12-1965, DHJ, nos. 1048, 1049 (rare to common). 

32. Retalhuleu SW to Champerico, Depto. Retalhuleu (200- 

0 m) . VIII-1-1966, DHJ, nc (occasional to rare). 

33. Retalhuleu to 15.2 mi NE on Hgy. 9S, Depto. Retalhuleu 

(200-1000 m). VII-18-1966, DHJ, no. 400; VII-16-1966, 

DHJ, no. 404; VII-28-1966, DHJ, nc (common to rare). 

34. 16.5 mi SE to Guatemala-Mexico border (Ciudad 

Cuauhtemoc), Depto. Huehuetenango (900-1000-720 m) 

on Interamerican Hgy. VIII-16-1967, DHJ, nc (common). 

35. 9.5 mi SW to 6.2 mi NE Puente El Lob0 on Hgy. 4, 

Depto. Zacapa (150 m). V-14-1965, DHJ, nos. 1929, 

1930; V-20-1965, DHJ, no. 1931; VIII-9-1967, DHJ, nc 

(very rare except as clumps on river banks). 

36. Guatemala-El Salvador border (25 m; Pijije) , Depto. 

Jutinpa to Escuintla (330 m), Depto Escuintla on Hgy. 

6E. 1-13-1965, DHJ, nc (rare to common), fl. 

37. Guatemala.El Salvador border (San Cristobal), Depto. 

Jutinpa to 15.8 mi SE Guatemala City on Hgy. 2, Depto. 

Guatemala (520-1250 m) , VII-7-1967, DHJ, nc (common 

to rare to common). 

38. Tikal, Depto. Peten (100-500 m). IV-2-1922, 0. F. Cook 

and R. D. Martin, no. 170 (US 1084640), mp. 

39. San Carlos, Depto. Retalhuleu (650 m) . 111-22-1921, 

Rojas, no. 241 (US 1014948), fl. 

40. Santa Barbara, Depto. Solola (400 m) . VIII-1891, W. C. 

Shannon, no. 578 (US 943298). 

41. Pancajche, Depto. Alta Verapaz (360 m). IV-5-1939, 

P. C. Standley, no. 70648 (F 978766), fl. 

42. narranca Hondo, lower slopes Volcan de Fuego, Depto. 

Sacatepequez (1800 m) , XII-16-1938, P. C. Standley, no. 

60262 (F 994870), gp. 

43. Ocos, Depto. San Marcos (sea level). 111-15-1940, J. A. 

Steyermark, no. 37779 (F 105266). 

44. Between Finca Pirineos and Finca Soledad, lower southfacing slopes of Volcan Santa Maria, between Santa 

Maria and Calahuache, Depto. Quezaltenango (1300- 

1400 m). 1-5-1940, J. A. Steyermark, no. 33599 (F 

1051210). 

45. Near Calahuache, Depto. Quezaltenango (1020 m) . III1-1939, P. C. Standley, no. 67107 (F 987673), mp. 

46. Rio Chiquimula, 1.5 mi NE Chiquimula, Depto. Chiquimula (400 in). X-21- 1939, J. A. Steyermark, no. 30123 

(F 1038583). 

47. Finca El Porvenir, Rio Cabus, SW slope Volcan Tajumulco, Depto. San Marcos (1000-1300 m) . 111-12-1940, 

J. A. Steyermark, no. 37550 (F 1049161). 

48. Along Rio Gaucalate, Depto. Escuintla (500 m). XI-28- 

1938, P. C. Standley, no. 58188 (F 993751), gp. 

49. Type of A. bursaria: San Felipe, Retalhuleu, Guatemala 

(300 m). February, 1878, Bernouilli and Cario, no. 1129 

or Laguna Amatitlan, Depto. Guatemala, Guatemala 

(1100 m). 1890, J. D. Smith, no. 2304 (Sir George Taylor, 

in litt.) . 

Honduras: 

50. 13.1 mi W San Marcos de Colon (850 m) to 11.6 mi NE 

Choluteca (330 m) on Interamerican Hgy., Depto. Choluteca. 1-14-1965, DHJ, no. 1053; V-11-1965, DHJ, nos. 

1928, 1933; VII-29-1967, DHJ, nc (rare to common to 

rare). 

51. 9.4 mi NE Choluteca on Interamerican Hgy., Depto. 

Choluteca (300 m) . VII-29-1967, DHJ, nc (1 plant). 

52. 4.3 mi NE Jicaro Galan, Depto. Valle (70 m) to 21.6 mi 

S Tegucigalpa, Depto. Francisco Morazan (1000 m) on 

Hgy. 1. VII-30-1967, DHJ, nc (rare to common to.rare). 

53. Honduras-El Salvador border (El Amatillo) to 15.5 mi 

E on Interamerican Hgy., Depto. Valle (50-100 m). I-15- 

1965, DHJ, nc (very rare). 

54. 13.1 mi W Zambrano, Depto. Francisco Morazan (830 m) 

to 7.8 mi SE Villa de San Antonio, Depto. Comayagua 
118 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

55. 

56. 

57. 

58. 

59. 

60. 

61, 

62. 

63. 

64. 

65. 

66. 

67, 

68 

(750 m) on Tegucigalpa-Puerto Cortes Hgy. VII-31-1967, 

DHJ, nc (common). 

7.8 mi SE to 4.7 mi N (550 m) Villa de San Antonio on 

Tegucigalpa-Puerto Cortes Hgy., Depto. Francisco Morazan. VII-31-1967, DHJ, nc (occasional to very rare). 

5.3 to 6.3 mi NW Comayagua on Tegucigalpa-Puerto 

Cortes Hgy., Depto. Comayagua (510 m). VII-31-1967, 

DHJ, nc (rare). 

13.2 mi NW Comayagua on Tegucigalpa-Puerto Cortes 

Hgy. (850 m). VII-31-1967, DHJ, nc (1 plant). 

10.7 mi NW (820 m) Siguatepec, Depto. Comayagua to 

16.4 mi S (50 m) San Pedro Sula, Depto. Cortes on 

Tegucigalpa-Puerto Cortes Hgy. VIII-2-1967, DHJ, nc 

(common to occasional). 

4.6 mi SW (90 m) to 12.9 mi SW (150 m) San Pedro 

Sula, Depto. Cortes on road to Santa Rosa. VIII-2-1967, 

DH], nc (occasional). 

24.4 mi SW (240 m, bank of Rio Chamelecon) San Pedro 

Sula, Depto. Cortes to 10 mi N Santa Rosa. VIII-2-1967, 

DHJ, nc (occasional to very rare). 

4.4 mi SW Santa Rosa, Depto. Copan (940 m) , VIII-3- 

1967, DHJ, nc (1 plant). 

4.4 mi NE (1310 m) Nueva Ocotepeque to Honduras-El 

Salvador border (850 m) on Santa Rosa-San Salvador 

Hgy., Depto. Ocotepeque. VIII-4-1967, DHJ, nc (rare to 

common). 

Amapala, Isla Tigre, Depto. Valle (less than 250 m) . II14-1922, P. C. Standley, no. 20738 (US 1136566, US 

1136557), fl. 

Siguatepec, Depto. Comayagua (1080-1400 m). II-14-27- 

1928, P. C. Standley, no. 56333 (US 1409186), fl. 

Las Limas, Depto. Comayagua (1000 m) . IV-16-1932, 

J. B. Edwards, no. P-89 (F 713842), fl. 

Vicinity of Siguatepeque, Depto. Comayagua (1080-1400 

m) . 11-14-27-1928, P. C. Standley, no. 56333 (F 581757), 

fl. 

La Lima, Depto. Cortes (30 m). IV-11-20-1947, P. C. 

Standley and J. Chacon, no. 7548 (F 1268590), fl. 

Near Lancetilla, Depto. Atlantida. VIII-10-1934, T. G. 

Yuncker, no. 5099 (F 749502). 

El Salvador: 

69. 

70. 

71. 

72. 

73. 

74. 

Honduras-El Salvador border (850 m) to 10 mi S (1170 

m), Depto. Chalatenango on Santa Rosa-San Salvador 

Hgy. VIII-4-1967, DHJ, nc (occasional to very rare). 

11.7 mi S (1130 m) Honduras-El Salvador border (Depto. 

Chalatenango) to 9.3 mi N San Salvador (580 m) on 

Santa Rosa-San Salvador Hgy. VIII-4-1967, DHJ, nc 

(rare to very common to rare). 

Intersection just E of La Libertad (sea level) to 11.8 mi 

N on Hgy. to Interamerican Hgy. (910 m), Depto. La 

Libertad. VIII-5-1967, DHJ, nc (rare to very common). 

La Libertad, Depto. La Libertad to La Union, Depto. La 

Union (sea level to 30 m) on La Libertad-ZacatecolucaUsulutan-La Union Hgy. VIII-6-1967, DHJ, nc (common 

to occasional to rare), 

La Herradura to La Libertad-Zacatecoluca Hgy., Depto. 

LaPaz. VIII-6-1967, DHJ, nc (very rare) . 41.5 mi SE San Miguel to La Union on San Miguel-La 

n” 

13. 

76. 

77. 

78. 

79. 

80. 

81. 

82. 

83. 

84. 

Union Hgy., Depto. La Union. V-12-1965, DHJ, no. 1932; 

1-15-1965, DHJ, nc (rare), 

La Libertad, Depto. La Libertad to El Salvador-Guatemala border (La Hachadura) , Depto. Ahuachapan. 1-16 

1965, DHJ, no. 1051 (common to rare). 

Sonsonate (280 m) SE to Interamerican Hgy#, Depto 

Sonsonate. VIII-6-1967, DHJ, nc (common to accasional). 

Sonsonate through Armenia to San Salvador-Guatemala 

Hgy. (280-920-450 m). VIII-6-1967, DHJ, nc occasional 

to rare). 

Intersection of San Salvador-Guatemala City Hey., with 

Sonsonate-Armenia Hgy. to Guatemala-San Salvador border (San Cristobal) , Depto. Santa Ana (450-810-520 m) . VIII-6-1967, DHJ, nc (very rare to common to occasional) , 

San Martin de Laguna Ilopango, Depto. San Salvador. 

IV-1-1922, P. C. Standley, no. 22622 (US 1138360), fl. 

2 km N7V Monte Cristo, 15 km XE San Miguel, Depto. 

Morazan (150 m). XII-7-1941, J. iM. Tucker, no. 486 

(US), mp. 

Laguna de Olomega, Depto. San Miguel (75 m). II-20- 

1922, P. C. Standley, no. 21027 (US 1136842). 

San Vicente, Depto. San Vicento (350-500 m) . III-2-11- 

1022, P. C. Standley, no. 21291 (US 1137447), fl. 

Ahuachapan, Depto Ahuachapan (700-1100 m) . I-16- 

1925, P. C. Standley and E. Padilla, no. 2497 (F 1256801). 

2 km NW Monte Cristo, 15 km NE San Miguel (150 m) . XII-7-1941, J. M. Tucker, no. 486 (F 1492748). 

Nicaragua: 

85. 3.4 mi SE Leon on Hgy. 12, Depto. Leon (20 m). VII27-1967, DHJ, nc (30 acre patch), gp. 

86. 4.9 mi SE Leon on Hgy. 12, Depto. Leon (20 m). VII-27- 

1965, DHJ, nc (1 plant). 

87. Ameya (near Pto. Morazan) Depto. Chianandega (sea 

level). VI-19-21-1923, W. R. Maxon et a!., no. 7176 

(US 1180890). 

NATURAL HISTORY 

Being widespread geographically (Figure 105) on 

soils and in climates very suitable to seasonal agriculture, A. hindsii is difficult to characterize 

ecologically since nearly all the semideciduous and 

deciduous forests that it occupied before the advent 

of European agriculture have been destroyed. As 

McVaugh (1966) has stressed, when both of these 

forest types are cleared, A. hindsii is a common 

member of the shrubby regenerating vegetation. In 

contrast to those swollen-thorn acacias that live in 

natural disturbance sites in wet or rain forests (A. 

allenii, A. melanoceras, perhaps A. mayana), the 

A. hindsii population density has very likely increased greatly since the advent of European agriculture, except in those areas where Indian slashburn agriculture was common on a system with at 
NUMBER I3 119 

least ten years fallow per field. Based on examination of A. hindsii throughout its contemporary 

range, I suspect that its pre-Colombian ecological 

distribution was the following. The plant was extremely common along rivers and mangrove swamps 

in deciduous and seinideciduous forests and formed 

large thickets where occupied by multiple-queen 

species of obligate acacia-ants (Janzen, 1967a, 1973). 

Scattered large trees surrounded by sucker shoots 

from lateral roots occurred on dry hillsides, becoming progressively rarer the more intense and 

the longer the tlry season. Along the wetter margins 

of its range (usually higher as well) it was found 

only along watercourses and other natural disturbance sites. In the very driest parts of its range 

(e.g., where the Tropic of Cancer crosses western 

Mexico), it occurred only along watercourses and 

in the foothills where the slightly cooler weather 

compensated for a lack of rain equal to that in the 

well-drained parts of the coastal plain. In no places 

did it occur as a shrub in the semidesert conditions 

as A. collinsii may do on rare occasions. In short, 

A. hiizdsii was a plant of locally wet sites in the 

drier habitats of the Pacific side of Central America 

between Nicaragua and the Tropic of Cancer. 

Throughout this range it invaded wet sites, as wet 

as those occupied by A. gentlei, A. cookii, A. mayana, A. globulifera on the Atlantic coast. Like these 

four swollen-thorn acacias, it was restricted to 

natural disturbance sites such as river banks in the 

wetter habitats. The wetter sites on the Pacific side, 

however, tend to experience a much more severe 

tlry season than do sites on the Atlantic side with 

roughly equal annual rainfall. That the plant had 

to be able to survive this dry season, and thus could 

move into drier sites fairly readily, had been instrumental in preventing the very widespread population of A. hindsii from differentiating into many 

species. In other words, drying of the general 

weather would not be so likely to split that population of A. hindsii into as many small and isolated 

populations as it would a species widely distributed 

along the wetter Atlantic side of Central America. 

The restriction of A. hindsii to the Pacific side 

of Central America is very puzzling. It has been 

collected east of the Continental Divide at only 7 

sites, and 5 of these (sites 7, 27, 35, 55, 56) could 

well be the result of seed dispersal via road building 

machinery; at these sites the population is restricted 

in the area within a mile of the highway. At Tikal 

and Lancetilla (sites 38 and 68) there has been 

much traffic by biologists from other parts of Central America, and the collections could well be 

recent introductions. A. hiizdsii is certainly not the 

common swollen-thorn acacia at these sites; however, there were reproducing trees at each of the 5 

sites that I examined, and if they are not very recent 

introductions, it is most enigmatic that they have 

not spread from these sites over much of Caribbean 

Central America. Even if these 7 sites represent 

recent introductions, it is puzzling that the many 

natural introductions that must have occurred in 

the p.ast have not resulted in established populations of A. hindsii on the Atlantic side of Central 

America. Being bird dispersed, the seeds surely 

must have been carried across the Continental Divide. Since isolated greenhouse plants of A. hindsii 

never set seed despite the fact that they flower 

profusely, it is likely that they are self-sterile. Successful establishment across the Continental Divide 

would then be exceedingly difficult except with 

simultaneous multiple-seed dispersal as could easily 

occur via the mud caked on road-building equipment. Low-income workers on the roads also eat 

the sweet pulp around the seeds and may have dispersed some seeds in this manner. 

The dependency of A. hindsii on obligate acaciaants for survival to reproductive maturity appears 

no different from that of A. cornigera, though experiments have not been conducted. Naturally unoccupied plants of A. hindsii are heavily damaged 

by herbivorous insects except under two circumstances: one natural and the other artificial. At 

the upper elevation limits of their distribution (e.g., 

sites 8, 37) in windy cool habitats, it appears that 

herbivorous insect pressure is low enough so that 

only during the warmest months of the year are 

unoccupied A. hindsii subject to severe defoliation. 

A. hindsii is one of the small number of species of 

woody second-growth plants that has managed to 

survive along the well-traveled dirt roads on lowland coastal El Salvador (site 72). These roads lie 

between cotton fields that are frequently sprayed 

with parathion and other very toxic insecticides. 

Not only have all the insects been eliminated from 

the roadside strips of vegetation by the combination 

of dust and insecticides, but the colonies of obligate 

acacia-ants have been gone so long that a large 

A. hindsii can show no sign of ever being located 

by searching queens. The thorns only stay on the 
120 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 106.-Acacia kindsii: A, unoccupied 1-year-old sucker shoot (Mexico, site 6); B, unoccupied 

2-year-old seedling (Mexico, site 9); c, occupied 2-year-old sucker shoot (Honduras, site 59); D, 

lateral branch from crown of occupied adult (Mexico, site 6). 
NUMBER 13 121 

acacia for a maximum of about 4 years, so this 

means that the ants have been missing at least that 

long. Since these fields are 10 to 30 years older than 

that, I suspect that the ants and most other insects 

have been missing equally long. The dust along 

other Central American roads is generally not as 

bad, since those with as much traffic as El Salvadoran roads are usually paved. Despite the absence 

of herbivorous insects, the A. hindsii do very poorly 

because they are frequently used as vine standards 

by wild plants. At site 85 in Nicaragua, the southernmost local population of A. hindssii displays an 

extreme case of insecticide protection. This 30-acre 

patch of nearly pure A. hindsii (with scattered A. 

FIGURE 107.-~, B, Acacia hindsii (Mexico, site 6) : A, pair of 2-m-tall subadult occupied sucker 

shoots; B, adult 4-m-tall occupied tree, with the life form characteristic of open pastures; c, 

brushy pasture with a 3-m-tall occupied A. hindsii on the left and A. pennatvla at right of 

center (Guatemala, site 37). 
122 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE lOS.-Acacia hindsii: A, looking up through the crown of an occupied adult (Mexico, 

site 17) ; B, fully leafed crowm of an occupied subadult (Guatemala, site 35). 

cornigera) was almost certainly established from 

seed brought by man and is an island in the center 

of many hundreds of acres of cotton. The acacias 

are of all ages antl many are reproducing. Three 

species of obligate acacia-ants are in the foothills 25 

miles east, but after searching for 2 hours at site 85 

I could find no evidence that any searching queen 

had located the patch during the past 4 years. If 

any did, they were killed by insecticide before they 

could attempt to cut an entrance hole in a thorn. 

There were almost no other insects in this patch of 

vegetation, antl almost every plant had no herbivore 

damage; cotton in this area is almost uneconomical 

to grow because of the severity of insect damage. 

Seedlings of A. hindsii in natural disturbance 

sites, and in brushy pasture, tend to attain a height 

of 10 to 40 cm over 1 to 3 years of growth unoccupied by obligate acacia-ants. Such unoccupied 

seedlings are very stunted in appearance (Figures 

106~, B) with damaged leaves and many short 

branches that grew a few cm before having the 

shoot tip eaten off by an insect. In contrast to the 

heavily shaded unoccupied seedlings of A. allenii 

and A. melanoceras, whose slow growth is as much 

from a lack of sunlight as defoliation, the slow 

growth of unoccupied A. hiudsii seedlings can be 

attributed almost entirely to defoliation by insects. 

It appears that somewhat less than 1 percent of the 

seedlings of A. hindsii ever become occupied; most 

die within the first 4 years. Cursory examination of 

habitats where they are common indicated that the 

fully insolated unoccupied seedlings live longest, 

despite the dryness of the site. This indicated that 

once established, energy relationships during the 

wet season are more important than desiccation 

during the dry season. 

Once occupied by a single-queen colony, the 

young seedling often produces a small intact bush 

(Figure 1OGc) with multiple branches but quickly 

one of the vertical branches becomes the central 

trunk (Figures 107~, c). The central trunk grows 

rapidly upward; there may be 1 to 2 m of vertical 

elongation during a 5-month iainy season. During 

the second and third years of occupied growth, the 

crown begins to fill in with small branches (Figures 

107c, 108~) ifnot under strong lateral competition 

from the crowns of other plants. If there is much 

adjacent woody vegetation, the vertical elongation 
KUMBER 13 123 

of the central trunk may continue until the tree is 

6 to 12 m tall. Lateral branches are long and bear 

large leaves and swollen thorns (Figure 106~). Old 

A. hindsii almost invariably have dense crowns 

(Figures 1 0 7 ~ , 1 0 8 ~ ) . The development described 

for A. hindsii is very similar to that for A. cornigera. 

In areas where niultiple-queen species of obligate 

acacia-ants are primary occupants of A. hindsii, the 

development is somewhat different. Here, most new 

FIGURE 109.-A clump (clone?) of adult 

A. hindsii occupied by a multiplequeen species of obligate acacia-ant 

(Mexico, site 16). 

plants are sucker shoots from lateral roots of older 

acacias and are occupied by part of the colony in 

the parent shoot. This produces a dense thicket 

(Figure 109) of A. hindsii that can cover as much 

as several acres. In a new habitat, where the A. 

hirtdsii is becoming established through seeding, 

the early stages of shoot development are not much 

different from the case with single-queen obligate 

acacia-ants, but within a few years, the thicket be- 
124 

40 - 

35 - 

- 

v 2 30- 

SMITHSONIAN CONTRIBUTIONS TO BOTANY 

W 

u 

z 

W 

V 

V) 

W 

E 

0 

25' 

I+ 

8 

0 

0 

n 

0 n 

.... 0 

0 

0. 0 

0 

0 

0 

PI, , , , W 

cl 

150 200 250 300 

NUMBER OF FLORETS PER INFLORESCENCE 

FIGURE 110.-Distribution of 4314 florets among 20 inflorescences from one adult A. hindsii (Mexico, site 5). 

gins to form. The multiple-queen situation prevails 

in areas with particularly severe dry seasons coupled 

with wet seasons very favorable to acacia growth, as 

in western Mexico. 

Acacia hindsii does not bear flowers until the 

fifth to seventh year after becoming occupied, and 

v) 

W 

40 

u 

v) 

W 

0 

I4 

a 30 

k 20 

a 10 

W 

B 

2 1234 6 

many trees occupied by multiple-queen species do 

not produce their largest seed crops until they are 

10 to 30 years old. In contrast, A. cornigera and 

A. collinsii bear flowers in the fourth to sixth year 

of occupied growth and have their heaviest seed 

crops when 6 to 15 years old. The inflorescences of 

A. hindsii, like those of other dry-land acacias, are 

visited by many species of small wild bees. The 

inflorescences vary greatly in length, but not so 

greatly in the number of florets per inflorescence, 

(Figure 110) , but a maximum of 8 pods per inflorescence (Figures 11 l~, 112~) isobserved in most pod 

crops. When more than 2 pods develop on a single 

inflorescence, they become so tangled when they 

dehisce (Figure 14~) that not all the seeds are externally visible to the birds that disperse them. The 

number of seeds per pod (Figure 111~) is representative of acacias in general and swollen-thorn 

acacias in particular. As with other dry-land acacias, 

these seeds are heavily attacked by bruchid beetles. 

The swollen thorns of A. hindsii show very high 

variation in volume, as do those of A. collinsii. The 

volume of type A thorns (Figure 113) tends to be 

about the same throughout the range. In areas 

where there are only multiple-queen ant colonies, 

all thorns are type A and rather monotonous in 

form. Where single-queen colonies are present, however, and A. hindsii is represented primarily by 

well-defined individual trees, there is great variation in thorn volume. The volume for some of the 

largest type B thorns encountered are graphed in 

6 

I4 

0 

62 

B 

E 

A NUMBER OF PODS PER INFLORESCENCE B NUMBER OF SEEDS PER POD 

FIGURE 111.--Acacia hindsii (Guatemala, site 29): A, distribution of 187 mature seed pods among 

100 inflorescences on on,e tree; B, distribution of 268 seeds among 22 of the mature pods. 
NUMBER 13 125 

FIGURE 112.-Infructesccnccs with immature pods: A, A. globulifera (Guatemala, sitc 13) ; R, A. 

hindsii (Honduras, site 59). 

Figure 114, and in contrast to the large volumes 

for the dune ecotype of A. sphncrocephala, the 

large space is all useful to the ants because the thorn 

is flattened such that the thorn walls are only 2 to 

4 mm apart. 

The thorns of A. hiiidsii are highly variable in 

shape, as are those of A. cornigera and A. collinsii. 

Almost all seedlings have thorns that are cylindrical 

or only slightly flattened in cross section (e.g., 

Figure 106~). Thorns on young sucker shoots are 

the same (Figure 115~, middle). This difference 

between young and old thorns is probably associated with maximizing thorn volume and wall 

strength with a fixed small amount of energy to be 

expended on the thorn. Type A thorns in the 

crowns of mature A. hindsii show strong geographic 

variation (Figures 82~, 83c, 115, 116, 117, 118~, C) 

but have low variation at any particular locality. 

Their geographic variation in morphology is as 

great as that displayed between different species of 

more geographically restricted swollen-thorn acacias. 

- 130 

E 

B 

3: 

I-. 

0 

z 

1 

120 

5110 

g 100 

2 

LL: 

I I I I 1 1 I 

0.8 1.0 1.2 1.4 1.6 1.8 2.0 

TOTAL THORN VOLUME (cc) 

FIGURE llS.-Distribution of thorn volume among type A 

thorns from rcprescntative latcral branchcs in the crown of 

an adult occupied A. hintlsii (Mexico, site 17). 
126 

170 - h 

8 

E 

X 

b 

W 

z 

w 

cl 

- 150- 

E 130- 

0 

X + 

cl 

0 

b 

;s 110- 

SMITHSONIAN CONTRIBUTIONS TO BOTANY 

I I I I 

150 

h 

Y 

170 - h 

8 

E 

X 

b 

W 

z 

w 

cl 

- 150- 

E 130- 

0 

X + 

cl 

0 

b 

;s 110- 

I I I I 

n 

< 110- 

k 

0 

** 

8. 

B 

FIGURE 114.-A. hindsii (Mexico): A, distribution of 84 cc of thorn volume among 17 of the 

largest type B thorns on an occupied adult (site 9); B, distribution of 62 cc of thorn volume 

among 11 of largest type B thorns on an occupied adult (site 11). 

FICIRE ll5.-Geographic variation in typc A thorns from occupied adult A. hindsii: A, Mexico, 

site 17; R, Guatemala, site 33; C, Honduras, site 50; D, Guatemala, site 25A. 
NUMBER 13 I27 

FIGURE 116.-Geographic variation in type A thorns from occupied adult A. hindsii: A, 

Mexico, site 11, B, Mexico, site 

13; c, Guatemala, site 33. 

FIGURE 117.-Geographic variation in type A thorns from 

occupied adult A. hindsii: A, 

Mexico, site 8; B, Guatemala, 

site 35; c, Mexico, site 13; D, 

Guatemala, site 35. 
128 SMITHSONIAN CONTRIBUTIONS TO BOTANY 

FIGURE 118.-Acacia hindsii (Mexico): A, type B thorns on short lateral branches from main 

trunk of an occupied adult (site 9) : B, white type A thorns on main trunk and lateral branches 

of occupied subadult (site 17); c, black type A thorns on main trunk and lateral branches of 

occupied subadult (site 17). 

Much of the variation is probably adaptive in the 

context of the developmental physiology of the 

lateral branch, rather than the ant colony. It is 

possible that the details of the thorn shape are 

nonadaptive, but unlikely, since so much of it is 

clinal in nature. The production of type B thorns 

appears highly adaptive in the context of multiplequeen colonies; the single-colony queen lives in the 

type B thorns. There is, however, a great deal of 

geographic variation in type B thorn shape (Figures 

118~, 119) that probably had the same basis as that 

displayed by type A thorns. 
NUMBER 13 

FIGURE 119.-Geographic variation in type B thorns from occupied adult A. hindsii: A, B, F, Mexico, site 11; c, Guatemala, site 31; D, Mexico, site 13. E, Type B thorns of A. collinsii that are 

convergent in shape on those of A. hindsii. 

129 
130 

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