Sulcorebutia or Rebutia?
GÜNTER and KARIN HENTZSCHELEver since Backeberg's description of thegenus Sulcorebutia in 1951, the question has
existed whether the genus is justified. Cárdenas (1966, 1968) never accepted it. Buxbaum(Endler & Buxbaum, 1974) at first could not
fit it into his syste m but then decided to add it
as a "primitive side-branch” of his subtribe
Gymnocalyciinae. Brandt (1977) placed it into
Weingartia. Donald (1971, 1977) at first supported the independence of Sulcorebutia andemended it with Brederoo (Donald & Brederoo, 1972). Later he adopted Hunt's opinion
and preferred merging Sulcorebutia and Weingartia with Rebutia.
The reason for this brief but confusing
history of Sulcorebutia was the superficial,inco mplete, and unconvincing description by
Backeberg. Except for Donald and Buxbaum,
who looked into the characteristic features of
the genus, most authors only made statements
or made new co mbinations without giving
reasons for doing so (Hunt & Taylor, 1986).
Hentzschel (1999a, 1999b, 1999c) exa mined the distinguishing characters of Sulcorebutia, together with those of other possibly
related groups of species, and emended thegenus once again. In order to have an unambiguous reference, a neotype was deposited(Gertel, 1996; Leuenberger, 1989). Hentzschelquoted and commented on the original description (Backeberg, 1951) and the emendation by Donald & Brederoo (1972). He wrote a
new description of the genus that took into
account the many recent discoveries.
The following re marks are a summary of
all the observations made so far on Sulcorebutia and its possibly related allies. They will becompleted later after we discuss hybridizationand the geographic distribution of theseplants. In order to carry out a critical comparison with Rebutia K. Schum., the most important characters of the genus Sulcorebutia
must first be presented.

Characteristics of the genus Sulcorebutia
Back. emend. Hentzschel
Bodies single or offsetting, spherical toshortly elongated, mostly small but sometimes
reaching exceptional sizes of more than 10 cm
in dia meter. They have a sunken apex and the
body is divided into spirally-arranged, rho m-boidal tubercles that never meet to form
straight ribs. The roots are fibrous or swollen,
forming a cylindric or conical taproot sometimes narrowed near the body of the plant.
The areoles are sunken into the upper partof the tubercles and merge towards the top
into a distinctive, pronounced, slightly slanting epidermal fold, which sometimes may bevery short. The areoles are elongated to very
extended, thus bearing slightly radiating, but
mostly pectinate, spines. Radial and central
spines cannot always be distinguished andsometimes centrals are lacking. The spines are
hard but straight to curved but never hooked.
Their surface is s mooth to so metimes very
rough.
The flowers are formed from externally
naked buds that emerge from the upper part ofolder areoles but never fro m near the ste mapex. The open flower is funnel-shaped, rarely
campanulate. The color of the flowers is
usually magenta, yellow, red or multicolored,
rarely white. The base of flower is often magenta. The petals are spatulate to lanceloate,with partially jagged edges. The pericarp andreceptacle have fleshy, heart-shaped, differently colored, sometimes slightly protruding
scales, which towards the apex of the tube
become equal in shape and color to the petals.
In the axils of the lowest scales there aresome axillary hairs and so metimes a few small
bristles or spines, but very rarely are there
spiny areoles (such as occur when lateral offsets beco me converted to flowers). On theinside of the receptacle the sta mens are usually uniformly distributed, rarely arranged in
two groups. The fila ments vary in color andoften are even bicolored. The anthers are yellow. The style bears the stigma with its 3 to
13 lobes, which can be white, yellowish or
greenish. The style usually reaches the sameheight as the upper sta mens, though in somespecies it can be considerably shorter or evenextend far beyond the stamens and perianth.
The fruit is a spherical or flattenedspherical, fleshy, false berry when mature, bursting open near the middle from the pressure of
the swelling funiculi. Sometimes, instead of
opening, it dries, appearing almost leathery. Thebase of the fruit adheres firmly to the body ofthe plant so that during the dry period the entire
fruit or only the lower part, with the seeds enclosed as in a bag, remains protected until therainy season.
The funiculi are solitary and densely arranged in usually five flattened placentas,
which sometimes may be once-branched basally.
The seeds are globular to irregularly
oblong to ovoid, ranging in size between 1
and 2 mm, laterally broadened, dull brownish
black, with an irregular surface formed mostly
by a thick cuticle of very variable structure.
The hilum and micropyle are surrounded by ajoint wall of very small, flattened testa cells.
These cells are more or less isodiametric,
brownish black, and have finely warted, slightly convex, outer cell-walls. The hilummicropylar region (HMR) is covered with aslack, yellowish white tissue. The mature seedcontains a very simplified embryo without
distinctly visible cotyledons. Endosperm andperisperm are not detectable.
Further details can be found in Augustin
et al. (2000).
All this shows that Sulcorebutia is a veryvariable genus, but it is possible to recognize
certain consistent characters needed for adifferential diagnosis:
1. The oblong to very elongated areoles
are clearly shifted apically and somewhat sunken on top of the rho mboid, spirally arranged tubercles.
Ribs are never formed.
2. The buds are covered completely withfleshy, heart-shaped scales. Hairs
cannot be found in the bud stage.
3. These characteristic scales, often auriculate at the base, later cover the
pericarp and the lower part of thereceptacle of the unopened flower.
They are still visible on the mature
fruit. Usually so me axillary hairs andbristles can be found underneath themore basal scales.
4, The flowers always originate fromolder areoles, often even near thebase of the ste m, but never fro m theapex.
5. The fruit consists of a fleshy, multilayered, outer and inner pericarp. At
maturity, fruits split equatorially tosubequatorially because the area of
the inner pericarp that will later open
had earlier become partially dissolved. In cases where the fruit dries off
and appears leather-like, the innerpericarp did not dissolve and theouter pericarp always re mains.
6. The funiculi are solitary, only at theirbase somet i mes beco ming o nc e

branched.
7. The seeds are spherical to oblongovate, 1-2 mm long, with a defined
cuticular fold and always with planoconvex, periclinal testa cell-walls.
By means of these seven groups of characters, Sulcorebutia can be easily distinguished fro m all other genera of cacti.

Differentiation
other generaof

Sulcorebutia

fromThe essential differences from Rebutia
sensu lato will next be discussed, as well as
possible relatives or ancestors of Sulcorebutia
and cacti that have been confused with it.
1. Differences from Rebutia sensu lato
Before the treat ment by the I.O.S Working Group (Hunt & Taylor, 1986; Hunt,
1999), the genus Rebutia K. Schumann was
already a conglo meration of different groupsof cacti. Therefore not all of the following
comparisons of characters is correct for all the
Rebutia species.
Sulcorebutias always have rhomboid tubercles, whereas they are roundish in Rebutia.
Sulcorebutias always have an elongated areole
positioned and sunken in the upper part of the
tubercle (adaxial). In Rebutia the mostly
round areole is positioned at the apex of the
tubercle. Some populations of the Rebutia
padcayensis-margarethae group also haveadaxial, sunken areoles. A distinct epidermal
fold also exists. To distinguish those plants
clearly fro m Sulcorebutia, one needs to especially examine the flowers, fruits, and seeds.
The spines of sulcorebutias are hard but
flexible, while in rebutias they are brittle andbreak easily. Investigations of the fine structure of the spines that could explain thesedifferences have not yet been made.
The flowers of Sulcorebutia and Rebutia
are shaped quite differently, according to thegroup they belong to. Those of Sulcorebutia
have fleshy bud scales located on the apex of
tubercles; these scales have a broad base andoften bear lateral auricules. Rebutia, on theother hand, has fragile, acute, triangular bud
scales that develop axillary hairs and spines.
Both of these characters result in the protection of the buds: in Sulcorebutia the buds areprotected by the scales and in most species of
Rebutia by the axillary hairs.
W i t h i n t h e Re b u t i a p a d c a y e n s i s —margarethae group and in R. minuscula and its
allies, relatively broad scales are formed, butthey almost never have hairs or spines. The
fruits and the testa structure of the seeds
clearly show features typical of Rebutia sensu
stricto.
The structure of the fruits is also important. In Sulcorebutia both cellular layers of
the outer pericarp are co mpletely intact at thetime of maturity. At this time the central portion of the inner pericarp is already dissolved,
so that the fruit tears open due to the pressureof the swelling funiculi. In Rebutia the innerpericarp, the funiculi, and the placentas areconverted into a sticky pulp at quite an earlystage of development, while the seeds are stillbrown. Also the outer pericarp decomposesand disintegrates, except in the Rebutia aureiflora group. In the latter group the fruit tears
open transversely, but the plants are easily
recognized as rebutias by their narrow scaleswith tufts of axillary hair.
2. Differences from Rebutia fidaiana, R.
neumanniana and R. neocumingii
These plants are better known as membersof the genus Weingartia Werd. One group canbe distinguished as a northern group (Rebutia
neocumingii (Back.) Hunt and related taxa)
and another as a southern group (R. fidaiana(Back.) Hunt, R. neumanniana (Werd.) Hunt).
During their juvenile stage the northern
forms resemble certain Sulcorebutia species,
having rho mbic, spirally-arranged tubercles.

They only form ribs, if they do so at all, whenvery old. The southern forms soon develop
broad flat ribs similar to those of Gymnocalycium.
The structure of the tubercles is comparable to that of Sulcorebutia. The areoles aremostly round to ovate, with tougher spines
than those of Sulcorebutia, and have elongated areoles and mostly pectinate spines.
Buds, bud scales and petals are very similar to those of Sulcorebutia but on the whole
are more fleshy. Bud scales and petals have abroader base than in Sulcorebutia and theirbasal lateral auricules are even more pronounced; the structure of these auricules resembles
those of Gymnocalycium.
The inner and outer structure of the flower and fruit of the southern forms is comparable to that of Sulcorebutia. The northern
forms differ from the former in several significant characters. The fruits of the southern
weingartias tear open equatorially by the pressure of the swelling whereas the fruits of thenorthern forms decompose by the dissolvingof the pericarp and funiculi.
I t i s a l s o i n t e r e s t i n g t h a t i n We i n g a r t i a , a s i n S u l c o r e b u t i a , mo s t l y u n b r a n c h e d
f u n i c u l i a r e fo r me d , n o r ma l l y i n f i v e p l a centas. The placentas of the northern
fo r ms a r e l i n e a r , t h o s e o f t h e s o u t h e r n
o n e s a r e b r o a d e r . T h e fu n i c u l i a r e o ft e nplaced so close together that a branching

of funiculi is simulated, though not actual(Hentzschel, 1999a; Augustin et al., 2000).
The northern Weingartia species form more
numerous funiculi than the southern ones andthe sulcorebutias. Therefore they develop
more numerous seeds. which are considerably
smaller than those of sulcorebutias and thesouthern weingartias.
3. Differences from Echinopsis sensu lato
All cacti grouped together in the genus
Echinopsis Zucc. normally differ considerablyfrom Sulcorebutia and Weingartia in body,flower and fruit. Therefore there have beenvery few cases of confusion.
In s o me p la c e s fo r ms o f Ec h i n o p s i s
c i n n a b a r i n a (Ho o k e r) La b o ure t g r o w t o get he r w it h S u l c o r e b u t i a p ur p u r e a (D o na ld &La u) Bre d e ro o D o na ld a nd ha ve b e e n
c o n fus e d d ue t o t he ir s i mi l a r a p p e a ra nc e
d ur i n g t he re s t p e rio d , b ut w it h t he fi rs t
o ns e t o f flo w e rs t he e rro r b e c o me s o b vio us .
So me l u x ur ia nt l y s p ro ut i n g a nd ra re l y flo w e ri n g fo r ms o f E. s c h i e l e a n a (Ba c k . ) Hu nt
a nd S u l c o r e b u t i a k r u g er a e va r. ho f f m a n n i iAugustin &Hentzsche l weres o me -times confused as well. In this case, too, themistake was soon realized by observing thelarge, hairy flowers of E. schieleana with
their triangular bud scales.
Generally one can say that cacti withwoolly buds and acute triangular scales cannever be Sulcorebutia or Weingartia species
but according to their other characters could
be Echinopsis (sensu lato) species, matucanasor Chilean globular cacti.
Observations from hybridization
The importance of cross-pollination experiments with cacti is generally underrated.
Unsuccessful hybridization of cacti may have
many causes and cannot be assessed conclusively. A successful hybridization on the other
hand, if the offspring are able to reproduce,means there is a high genetic similarity of themating partners—an indication of close relationship.
As self-fertility is to be avoided in hybridization, experiments with Sulcorebutia and Weingartia are made easier because of their selfsterility. In Rebutia sensu stricto self-fertility is widespread. But whether this differente has tax-

ono mic significance is not known at present.
Hybridization experiments by the author
with Sulcorebutia, Weingartia, Rebutia andEchinopsis have produced the following results:
1. Most, perhaps all, Sulcorebutia species
can hybridise with each other.
2. All experiments of crossing Sulcorebutia and Weingartia have resulted in
an F . 1 generation capable of surviving. The plants grew normally, butsome were partially chlorotic.
3. Cross-pollinating experiments of Sulcorebutia and Echinopsis chamaecereus H. Friedrich & Glaetzle werecarried out in various combinations
but did not produce an Fl-generation.
From time to time, fruits developedwith barren seeds. On the other hand,hybridization between Rebutia and E.
chamaecereus succeeded many times.
4. Hybridization failed between Sulcorebutia and rebutias of the R. padcayensis group, which closely rese mble Sulcorebutia. With several other
Rebutia species it was also not possible to produce an F1 -generation. The
only exception, a hybrid produced byJohn Donald between Sulcorebutia
krahnii and Rebutia gracilispina, wasreported by Gröner (1997).
Although investigations have not been concluded, the following conclusions can be made:1. The genetic distance between Sulcorebutia species is very small.
2. Sulcorebutia and Weingartia are closely related.
3. The genetic distance between Sulcorebutia and Echinopsis is large.
4. The genetic distance between Rebutia
and Echinopsis is s maller than between Sulcorebutia and Echinopsis.
5. There is a relatively large genetic distance between Sulcorebutia and Rebutia.
Geohistorical distribution
In obs erv in g p la nts a nd t he ir are aso f p rese nt d ist rib ut io n, o ne has t o bea r inmi nd t ha t t hese a re as a nd t he p l a nts t hat
no w occ ur t he re a re t he res ult o f maj or geo lo gic a l c ha nge s. This is es pe c ia ll y t rue fo r
are as w it h s uc h ra p id a nd int e nse tec to nic
c ha nge s as i n t he And es. As i n Euro pe, t heflo ra a nd fa u na o f No rt h a nd So ut h A me ric a,es pe c ia l l y t hro u g ho ut t he An de s a nd

Patagonia, have been strongly influenced by
the ice ages. It is evident by terminal moraines of the cordilleras that glaciation occurred
four times during the quaternary. Throughout
the area of lakes and salt lakes, a four-foldchange of glacial and interglacial periods isobvious. During the strongest glaciation theshield of ice extended fro m the Andes over
Chile as far as Tierra del Fuego and in theeast as far as the Atlantic Ocean.
The whole region of southern South America and especially that of the central andsouthern Andes, except for a few refuge areas,
was scoured of plants and was repopulated
again between and after the ice ages. The
repopulation ca me fro m different refuge areas.
At the same time, inevitable genetic exchange
between closely related groups occurred in theoverlapping areas, leading to an increase ofvariability and to a strengthening of the genetic drift.
The extre mely variable Sulcorebutia populations are the result of these postglacial
introgressions, a process that can also be observed in a multitude of other Andean, Patagonian and North American plant populations.
This also applies to Rebutia and particularly
for a possible genetic interchange between
Rebutia and Sulcorebutia.
On the basis of the differing main distribution areas of these two genera, one canassume that they each immigrated from different refuge areas. In spite of geographicalintrogression of both genera, no genetic interchange with consequent hybrid populations is
known up to now between Sulcorebutia andRebutia. This fact lets us assume that only aminor relationship exists between these genera.
Because of the morphological similarities
of Sulcorebutia and the R. padcayensis group,
the southern overlapping region of both genera should be investigated even more intensively.
Conclusions
On the basis of important shared or similar characters in Sulcorebutia and Weingartia
it is justified to unite both with the older genus Weingartia. Both genera can be regarded
as extre mely neotenic, highly derived plantgroups. They have a lot of characters in common with Gymnocalycium as well, so it would
be conceivable to integrate Sulcorebutia andWeingartia as subgenus Weingartia into thegenus Gymnocalycium.
On the whole, the genus Rebutia has moremorphologic characters in co mmon with Echinopsis and could be regarded as a highlyderived group within that genus.
Acknowledgments
I am grateful to Willi Gertel, Germany,for h is support and the translation of this paper, and to Paul Hoffman, USA, for his further
improvements to the text.
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All illustrations are by the authors.
Contact details:
Günter and Karin Hentzschel, Institut für Allgemeine
Botanik und Botanischer Garten, Ohnhorststraße
18,22609 Hamburg, Germany.
E-mail: pelzschafkaktus@foni.net

Originally published in C.& S.J. USA Vol. 73 2001, N° 5 (p. 237-242)
Reproduced with the permission of the the author and the publisher