U.S. Department of the Interior

U.S. Geological Survey

A Preliminary Study on the Variation of Trapa in Japan

By Yasuro Kadono 

Department of Biology, College of Liberal Arts, Kobe University, Kobe 657

角 野 康 郎 *: 日本産ヒシ属の変異に關する予察的研究

* 神戸大学教養部生物学教室

Translated from the Japanese by V. Chintu Lai, U.S. Geological Survey

English translation edited by Nancy B. Rybicki, U.S. Geological Survey 

This English translation is released with the permission of Yasuro Kadono and the publisher of the original article, 

which was published in final form in Japanese as follows:

Kadono, Y., 1987, A preliminary study on the variation of Trapa in Japan: Acta Phytotaxanomica et Geobotanica, 

v. 38 (September), p. 199–210, https://doi.org/10.18942/bunruichiri.KJ00002992255. 

Open-File Report 2018–1075
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Suggested citation:

Kadono, Yasuro, 2018, A preliminary study of variation of Trapa in Japan (translated into English from 

the Japanese by V. Chintu Lai): U.S. Geological Survey Open-File Report 2018–1075, 15 p., 

https://doi.org/10.3133/ofr20181075. [The translation was edited by Nancy B. Rybicki. The article was 

originally published in Japanese with English summary as Kadono, Y., 1987, A preliminary study of 

variation of Trapa in Japan: Acta Phytotaxonomica et Geobotanica, v. 38 (September), p. 199–210,

https://doi.org/10.18942/bunruichiri.KJ00002992255.] 

ISSN 2331-1258 (online)
1 

A Preliminary Study on the Variation of Trapa in Japan

By Yasuro Kadono1

角 野 康 郎 : 日本産ヒシ属の変異に關する予察的研究

Translated from the Japanese by V. Chintu Lai2, 3

English translation edited by Nancy B. Rybicki2

Trapa is the genus of annual aquatic plants, growing in lakes, marshes, irrigation ponds, stagnant 

waters and pools in the river and waterways, throughout Japan. Growing in poorly nourished 

(mesotrophic) waters to increasingly rich, nourished (eutrophic) waters, it is one of the most common 

aquatic plants in Japan. 

The study about classification of the genus Trapa was initiated by Makino (1908), and advanced 

by Nakano (1913, 1964) and Nakai 中井 (1942). In particular, Nakano’s study was made with attempt to 

clarify the heredity and the phenotypic variation of vegetative morphological characters through 

cultivation experiments, and is worth a special mention. 

When making classification of the genus Trapa, one of the important vegetative morphological 

characters is the morphology of the nut. The genus Trapa can be divided into two major groups: after 

flowering, the first group has all four sepals forming projections, and is called “four-thorn type.” The 

second group has two sepals fallen off and the remaining two protruding, and is called “two-thorn type.” 

The former has big Onibishi (Monster Hishi: Trapa natans) and small Himebishi (Little-princess Hishi, 

Trapa incisa), and the latter has Hishi (Trapa japonica). In the current Pictorial Books of the Flora or the 

like (of Japan), although researchers may notice that some discrepancies in adoption of scientific names 

(cf. table 1) exist, all seem to be basically in agreement of recognizing the above three kinds of Trapa

species, except in only a few remaining unsettled problems of treating their variations. 

However, as investigations of populations in the genus Trapa make progress, it has become 

noticeable that diverse variation forms or in-between forms, with their appropriate belonging classes 

uncertain, begin to appear in high frequency (Toda 戸田 1979; Sakurai 桜井 1981; Hamajima 浜島 1981; 

Sakaguchi 坂口 1982, 1983; Takada 高田 1983). Because investigations hitherto depended on a small 

number of individuals (specimens), the number was insufficient for us to fully understand the diverse 

variations that exist in the genus Trapa. Thus, when faced with handling of the plant group with such 

diverse variations as that of the genus Trapa, it was essential to first explicate fully the variation within 

the population and the different variation patterns with the population as a whole. Under such problemconsciousness, I promoted investigations of the genus Trapa populations at various places. In the present 

writing, the account of variations in the genus Trapa populations, which were based on the data obtained 

 

1

Currently at Department of Biology, Graduate School of Science, Kobe University, Nada, Kobe 657-8501, Japan. 

email: kadono@kobe-u.ac.jp 2

U.S. Geological Survey.

3

V.C. Lai translated all parts of the journal article that were in Japanese except for the addresses and names of water 

bodies listed in table 2. The English table titles, figure captions and explanations, and the “Summary” section from

the original journal article were reproduced without change. 
2 

mostly from Hyogo Prefecture, will be reported. These will hopefully serve as defining a problemfocusing point for the future study. 

Literature 1. Hara (1954), 2. Makino (1961), 3. Kitamura and Murata (1961), 4. Nakano (1964), 5. Ohwi 

(1965), 6. Ohtaki and Ishido (1980), 7. Kitagawa (1982) 

Material and Methods

Because the most notable variation in the genus Trapa is manifested in its nuts, the analysis of the 

variation was carried out by focusing on the nut morphology. 

From the populations at 21 locations shown in table 2, 100 pieces of nuts, in principle, were 

collected from each population by random sampling. (For the U population, only nuts of Himebishi were 

collected.) For collection, one of the following three methods was appropriately used. (1) Fully ripe nuts 

were collected during the autumn (referring only to those either already fallen away from the parent plant 

or, if even not separated from it, a separation layer was formed between the nut and the stem). The outer 

rind was removed from the nut after allowing it to rot away; the nut was then measured. (2) From spring 

to early summer, by carefully pulling out the budding plant, the nut attached to it was collected. (3) We 

collected nuts that drifted to the waterside. More than one-half of those germinated before the previous 

year, and thus, turned to nutshells, with many of them having been broken. Hence, only those that were 

still retaining the measurable parts with them were selected. In the following analyses, regardless of what 

collection method is used, all nuts are treated uniformly. 

Parts for measurement and types of lower projections are shown in figure 1. The lower 

projections include lower spines and pseudohorns. The former is a sharp-headed projection being 

developed from a sepal, and has, except in the case of Himebishi, fine reverse barbs on its apical part. We 

classified this as Type IV. The latter is a bulge that forms after the sepal falls off, for which quite a variety 

of forms and sizes can be identified. For the study conducted this time, Types I–III have been set up 

according to length of the horn. Also, the nuts of the genus Trapa are commonly asymmetrical in front 

against rear sides (belly vs. back), and thus the projections at the front side and the rear side are often of 

different lengths, for such cases the classification was made along the longer projection.
3 

Table 2. Location of populations investigated. 

Population Location and name (or type) of water body

Prefecture Address Name (or type) of water body

A Hyogo 兵庫県小野市王子町 (Irrigation pond)

B Hyogo 兵庫県加東郡社町沢部 (Irrigation pond)

C Hyogo 兵庫県神戸市西区岩岡町古郷 (Irrigation pond)

D Hyogo 兵庫県加東郡東条町黒谷 (Irrigation pond)

E Hyogo 兵庫県三木市興治 (Irrigation pond)

F Hyogo 兵庫県明石市西島 大池

G Hyogo 兵庫県加西市野田町 (Irrigation pond)

H Hyogo 兵庫県加西市田原町 (Irrigation pond)

I Hyogo 兵庫県加古川市平岡町 潰目池

J Hyogo 兵庫県加古郡稲美町加古 入ケ池

K Hyogo 兵庫県加西市倉谷町 アシガ池

L Hyogo 兵庫県明石市大久保町 上池

M Hyogo 兵庫県加古郡稲美町天満 天満大池

N Hyogo 兵庫県加古郡稲美町天満 琴池

O Hyogo 兵庫県明石市中尾 (Irrigation pond)

P Hyogo 兵庫県加古郡稲美町母里 (Irrigation pond)

Q Hyogo 兵庫県神戸市西区神出町 和合成池

R Hyogo 兵庫県小野市高田町 (Irrigation pond)

S Hyogo 兵庫県神戸市西区岩岡町岩岡 竜ケ池

T Hyogo 兵庫県加古郡稲美町加古 (Irrigation pond)

U Fukuoka 福岡県遠賀郡芦屋町山鹿 (Irrigation channel)
4 

In addition, such features as the relative height of top crest to nut shoulder, the degree of bulging 

of “trace of petal”4 and, in the case the lower spine exists, its extending direction (horizontally, or 

downward), were recorded. Also, for the name of each part and its morphological origin, the reader is 

referred to Miki (1952a) (In this writing, “upper horns” and “lower horns” were each renamed as upper 

spines and lower spines.) 

For morphology of other parts than the nuts, attention was paid to the form of leaves, the 

conditions of hair growing on the leaf-back and on the petiole. For observation of these, besides the 

materials of 21 populations shown in table 2, the specimens of Kyoto University as well as those in 

possession of Kobe University, Liberal Arts School were extensively examined.

Results

1. Variation in the Lower Projections

Figure 2 shows through the investigated populations the frequency with which each type (I–IV) 

of lower projections had appeared. Population U was supposed to be a mixed group of Hishi and 

Himebishi, but only the Himebishi nuts were actually collected, and for this reason U was omitted in 

this figure.

Populations A–D were supposed to be the populations consisting of only two-thorn-type nuts, yet, 

in any case (population), there were mixings of nuts with undeveloped pseudohorns (I) and that with 

developed pseudohorns (II, III). This trend was also noticed among the populations that appeared together 

with nuts of four-thorn type. There were also populations like L, R, and S consisting only of nuts with not 

well-developed pseudohorns. On the other hand, there were examples like K and M, in which welldeveloped pseudohorns prevail over the greater part of two-thorn type nuts.

 

4

In Miki (1959), this part is considered to be a swelling of an attached piece lying between sepals.
5 

Among populations that include nuts of four-thorn type, it was also found, though small in 

number, nuts of two-thorn type. As for T, the nuts collected during one autumn were all of four-thorn 

type, but then in the following spring nuts of two-thorn type were found being dashed against the 

waterside. This fact allowed us to judge clearly that the population of interest was mixed. This results in a 

finding that as far as the investigation conducted for this time, there were no nut populations consisting 

purely of four-thorn type alone. 

2. Variation of Size 

As the value of the measure of each part is proportional to the size of the whole body of nut, there 

should have existed, more or less, mutual correlations. Among these, although the width (W: width of the 

nut across the upper spines) varied by the angle between upper spines (A), it was deemed as the best 

parameter for indicating the size of the nut itself in the mode of measurement employed this time. 

Figure 3 shows the mean widths and the standard deviations of the nuts, based on the types of 

lower projections. Nuts of two-thorn type will fall into the size class of approximately 30–50 mm (to be 

called middle-size hereafter) except the case of big nuts that are to be stated later. No difference was 

found between those with the pseudohorns well developed and those with not well developed (I–III) 

within the same populations. 

Strikingly big-size nuts with two-thorn type were found mixed in populations N and Q. For the 

nuts in these populations, it was found that there existed frequency distribution of two-crest shape in 

regard to width. That is, these can be divided into two groups, the middle-size class and the greater-than50-mm-size class (to be called big-size class hereafter). These two groups are clearly distinguishable by 
6 

the height of an ovary projection or by the shape of a crest-top. For the big-size nuts, pseudohorns of notwell-developed form (I–II) and broad-width pseudohorns of well-developed form (III) were found.

In case of the nuts of four-thorn type, there existed large variations in size by population. The bigsize nuts with width of greater than (45 ~) 50 mm and those belonging to the middle-size class with width 

30–50 mm were each found; and there was a population consisting of in-between size nuts (L), also for 

those populations within each of which the middle-size nuts and the big-size nuts were mixed, the inbetween size nuts too were found (such as J and M). The U samples had around 20 mm of width, of 

exceedingly small and regular shape (with little variation in size). 

Shown in figure 4 are the measurement results of body thickness (D) of the nuts. According to the 

size-class that is divided by the width, the thickness of the small-size nuts was about 5 mm, that of 

middle-size nuts was 8–13 mm, and that of big-size nuts was 13–23 mm; the classification by thickness 

into three size classes of small, middle, and big, can be equally made. 

Concerning the height (H, fig. 5), in contrast to the middle-size nuts for which the ovary 

projection grows quite well, for the big-size nuts the ovary projection will not grow so well and the 

central part of the nut body is depressed. The difference in its value is not as great as the difference of the 

body sizes of nuts. For the middle-size nut it gets up to 20 mm, and for the big-size nut it could 

commonly exceed 20 mm. 
7 

The width across lower projections (L) made a large difference in its value depending on the 

existence of lower spines or the extent of pseudohorn development, and also on the direction in which 

they extend, horizontal or downward. For those types in which pseudohorns do not develop well, L turned 

out to be about the same value as that of the thickness; for big-size nuts with lower spines, this value 

reached to 45–60 mm (data not shown).

3. Variation of Open-Angle of Upper Spines; Others 

The angle between upper spines (table 3) varies from -30° (oriented inwardly) to 190° for the nuts 

of two-thorn type, and does so in a range of 60° to 220° for the nuts of four-thorn type. The ranges of 

variation were rather large whether within or between populations. 

When the nuts of two-thorn type and that of four-thorn type (excluding Himebishi of U) were 

compared, it was found that the latter exhibited a tendency of larger angle between upper spines in most 

of the populations, yet there was also found a case of reverse situation in L. 
8 

The ovary projections of the middle-size nuts of two-thorn type generally grow fairly well and 

their top-crown often reaches higher than the shoulder of the body of nut. On the other hand, in the case 

of the nuts of four-thorn type, it has become clear that a tendency has existed such that projections of 

small-size nuts protrude high, and that of big-size nuts has been depressed around the ovary projection, 

making the top-crown lower than the shoulder. For the middle-size nuts, there were ovary projections that 

would grow and that would not. 

Although there were mixes of some number of nuts on which surfaces exhibited distinctly 

different forms of projections and protuberances, none of these would make their appearance in any 

specific populations as a group. 

4. Variation of Leaf 

Concerning the form and size of leaves, no quantitative analysis has so far been made, only those 

points that were noticed during qualitative observations will be written down herewith. Moreover, these 

observations were limited to the plants which were determinable to be Himebishi, Hishi, or Onibishi 

based on the morphology of nuts. 

The width of a Himebishi leaf reaches up to about 20 mm, but its length often grows longer than 

its width, and teeth at the edge of the leaf looked conspicuous. However, as Hishi also sometimes has the 

same shape of small leaf, it was rather risky to identify Himebishi only by the shape of leaf. The leaves of 
9 

both Hishi and Onibishi have extremely rich variety of shapes and sizes, and it was thought impossible to 

distinguish one kind from the other. 

About the hairs on leaf-back or on leafstalk (petiole), those of Himebishi, except the undeveloped 

leaves, were of mostly inconspicuous kinds, but specimens of leaves with hair were also found. As to 

those of Hishi and Onibishi, considerable variations in growing conditions of hair were noticed. As 

against the Hishi grown in the West Japan, for which many leaves grow thick hair on leaf-back or 

leafstalks, it was noteworthy to mention that very little hair was found for specimens of Hokkaido 

(northern Japan).

Discussion

1. How to Comprehend the Variations 

It has been clarified through the study made this time that there exists a great breadth of variation 

among the populations of the genus Trapa, and that the mode (or pattern) of variation differs from 

population to population. Hence, the examination into the background of such variations will be made 

first herein.

Among the nuts of the genus Trapa, the factors that make striking variations and cause confusion 

in taxonomy are the size of the nuts and the form of or the difference in degree of development of 

pseudohorns. For the size of the nuts, it was pointed out by Staszkiewicz and Wojcicki (1981) that the 

size could be influenced by the environmental conditions of the place in which the nuts grew. With this in 

mind, the author would like to advance his study further in future. Not only an incidental factor attendant 

with the falling of calyxes foster the development of pseudohorns, but, it is certain, the heredity 

background does affect the development of pseudohorns also. The trend in pseudohorn development 

shows clear difference by population. 

 Thereupon, what becomes the problem is the origin of such hereditary variations. Plants that bear 

the nuts with well-developed pseudohorns were considered to be the in-between forms that intervene the 

four-thorned species (of Trapa) (Onibishi) and the two-thorned one (Hishi), and to be regarded as the 

results of hybridization (Sakurai 桜井 1981; Sakaguchi 坂口 1982). However, Nakano (1913) maintains a 

negative opinion to the theory of hybrid origin of intermediate forms. Among those populations 

investigated this time, the fact of witnessing complete absence of the in-between form in populations such 

as L, R, and S, where in spite of mixed growing of Hishi with poorly developed pseudohorns and

Onibishi with four spines, might suggest the limited chance of crossing between Hishi and Onibishi. 

In order to explore the possibility of hybrid formation, it is necessary to clarify the breeding 

system of the genus Trapa. Kadono and Schneider (1986) state that for Onibishi the main mode of 

pollination is by self-pollination, and cross pollination occurs only contingently. They also show that 

bearing nuts by apomixis is attainable. By this matter, their statement throws a doubt upon the 

viewpoint expressed by Staszkiewicz and Wojcicki (1979) that frequent hybridization is taking place in 

the genus Trapa. 

It also looks necessary here to examine whether or not an entity yielding nuts of well-developed 

pseudohorns possesses the character of crossbred species. A preliminary investigation has been made at 

the K population hitherto as to the fruit set and the situation of pollen grains. As a result, it was found that 

there was no such thing as to the “in-between form” generating inferior nuts, and also that the generated 

pollen grains were quite normal (Kadono, unpub. data). These facts do not by themselves negate the 

possibility of the “in-between form” to be of mixed breed, it could show the limitation in an approach 

from such a side.
10

On the other hand, there exists a fact that suggests a possibility of the “in-between form.” When 

Toda 戸田 (1979) sowed nuts having well-developed pseudohorns, the second-generation plants yielded a 

great variant of nuts, ranging from those having no pseudohorns to those with well-developed 

pseudohorns. This fact may allow us to infer that for those populations in which the pseudohorns may 

develop, the introduction of the gene from the plant of four-thorn type takes place first, after which

character separation may occur. In such case, it could be interpreted that the populations like L, R, and S 

have not had a chance for cross breeding. In any case, it is necessary to give a rigorous examination 

on this matter from the viewpoint of cytogenetics. It is also of help to conduct artificial cross breeding 

experiments.

The fact that every population has different pattern of variation may be explained by the 

dominance of self-pollination, which might facilitate preservation, within that population, of an entity 

variation resulting from some key factor or other. Furthermore, the irrigation pond or the waterway, in 

which Trapa plants grow, each becomes an island isolated from each other. In addition, the seeds of 

the genus Trapa will adapt themselves by sinking down and then sprouting at the same location where 

the parent plants had grown up. Because the nuts may be unsuitable for extensive seed scattering

(Miki, 1952b), it is understandable that the chance for exchanging genes with other populations will be 

much limited.

Although presently it cannot be argued as to when the genus Trapa changed its mode of 

pollination to self-pollination, it is shown, according to a study of fossil by Miki (1952b), that hereditary 

isolation has played an important role in the evolution of the genus Trapa. Miki has reported from Japan 

16 fossil species of the genus Trapa, where each individual species ceased to exist in a relatively short 

period and is replaced by a new species. He also has pointed out finding of numerous “local species.” For 

the genus Trapa, new variation forms continue to appear both temporally and spatially, and these 

(variation forms) each speciate to new “species” on one hand, yet, many other populations were 

exterminated because of the poor migratory nature: such a history of differentiation is repeated and 

today’s complex variation patterns were considered to be brought about in this way.

2. The Type and Classification of the Nuts 

One of the reasons for confusion in classification of the genus Trapa is, as stated earlier, in the 

variation of nut morphology. However, by considering together its size and configuration of the lower 

projections, it is possible to recognize some entities for the shape of nuts. An attempt is made herein to 

examine the present results with the previous way of classification.

Among the middle-size nuts of two-thorn type, there were the nuts without well-developed 

pseudohorns (fig. 6-B) and those that had pseudohorns well developed (fig. 6-C, D). According to the 

degree of pseudohorn development, Nakano (1964) has divided these into three varieties, called 

Akamonbishi (which was called as Akababishi in (Nakano (1913)), Hishi, and Ibobishi. However, not 

only do many of these “variants” grow mingled within one population, but they could also coexist often 

on one same plant shoot. Nakano (1913) maintained the degree of pseudohorn development to be 

hereditary, but it is clear from the cultivation experiments of Toda 戸田 (1979), as mentioned earlier, this 

form would not necessarily be considered to be a stable character. (Furthermore, although it was not 

included in the report this time, there were also some populations in which all nuts had extremely stable, 

well-developed pseudohorns; thus, before we make any conclusion on the matter of pseudohorns it 

seemed necessary to conduct a broader investigation.) I think that this type of variations is, as stated by 

Toda 戸田 (1979) or Takada 高田 (1983) also, better treated as the polymorphic variation within the 

species Hishi (Trapa japonica). 
11

Among the big-size nuts of the two-thorn type, one type having little developed pseudohorns 

(fig. 6-F) and another type having well-developed broad pseudohorns (fig. 6-G) can be found. The former 

grows in India or China, and is possibly imported to Japan by the name, T. bispinosa ROXB. (トウビシ). 

This matter will be left as a study subject hereafter, but because of its low frequency in the fields, it is 

conjectured not to have a particularly great fecundity, or competitive power. The latter type found in the 

M population showed especially little variations. This type of nut is said to grow also in irrigation ponds 

of Kagawa Prefecture (Sakaguchi 坂口, private communication). Whether or not to recognize the said 

type as a new taxon is also a task awaiting decision.

Among the nuts of four-thorn type, the small-size Himebishi (fig. 6-A) has a 16- to 23-mm-wide 

gap between the upper spines, and about 10-mm-tall height; the nut itself is rather long in the vertical 

direction, and is clearly identifiable as a distinct species. According to the specimen, the Himebishi has 

been widely collected from everywhere in Japan, yet it has become exceedingly rare species presently. 

[It is to be called attention herein that there are often examples of misidentifying Koonibishi (コオニビシ) 

—to be mentioned later—with Himebishi (ヒメビシ).] 

The big-size nut (fig. 6-H) corresponds to Onibishi (オニビシ). However, for the lower spines, 

there are those having broad width with thick spine and those with thin one, which show rich variations. 
12

As to the orientation of the lower spines, there are those directing downwards, or those growing 

horizontally, or even those bending upwards, and all these can be found within the same population. The 

difference in forms of the lower spines may probably be treated as the variation within a species.

Among the big-size nuts bearing four thorns, there are those having petioles tinged with red. 

Makino (1913) classified these as a Mebishi (メビシ var. rubeola), being a variety of the Onibishi. 

However, the fact that the petioles, or sometimes the laminas themselves, become tinged with red in the 

early autumn is more or less a common feature for the genus Trapa, and is not a phenomenon seen in a 

particular population or for some limited entities. Because there is no difference in the morphology of 

nuts between the “Onibishi” whose leafstalk does not turn to red, and the “Mebishi” that does turn to red, 

the writer cannot agree to regard the Mebishi as an independent taxon. 

The middle-size nuts of the four-thorn type (fig. 6-E) were made as an independent variety

named Koonibishi コオニビシ (a variant of Onibishi) by Nakano (1964). Among the data collected 

this time, the nuts considered to be corresponding to Koonibishi had made quite a frequent appearance. 

Yet, they always mingle with Onibishi and, moreover, are continually shifting their sizes. Looking at this 

fact, a question might be thrown in as to if the Koonibishi is nothing but the ill-developed nut of the 

Onibishi nuts. 

When this question is considered, it is important to examine the germination ability of immature 

seeds. In groups of the genus Trapa, even during the summer to early-autumn growing season, 

Hishihamushi (Galerucella nipponensis Labois) bites the leaves of plants, and sometimes the whole 

population would get a destructive blow. In such a case, most of the still small, premature nuts will fall 

away and die in the water. Had these nuts grown up to a certain point, could they have survived to the 

next year and functioned rightly as seeds? Thus, in population M, in which many Koonibishi nuts were 

found, the nuts of various mature stages were collected, and they were left submerged in water till the 

next spring in order to observe their germination ability. The results were as follows: The nuts taken out 

of individuals identified as the Onibishi have the minimum size of 32-mm width (which amounts to 

47.6% that of the largest Onibishi nuts in the M population), the inner pericarp became hard, and the 

sprouting has been observed. In the study made this time, the true characteristics of what we thought to be 

the nut of Koonibishi could be questionable. They may be the immature seeds of Onibishi. This doubt is 

further intensified by the fact that the writer had not yet observed the state of the Koonibishi nut attaching 

to a parent plant. It probably demands our reexamination of the true identity of Koonibishi.

The Onibishi has also large variations between populations as the Hishi does. The variations are 

particularly notable in nut size. There are populations (such as S, T) with average width exceeding 60 mm 

on the one hand, there found a population (L) with that being less than 50 mm on the other. For the dry 

weight, which is not affected by the shape of nut, it has been confirmed that there exist statistically 

significant differences between populations (Kadono, unpub. data). The writer thinks that in the 

Onibishi case, it is best at this point to comprehensively treat the Onibishi as a species with considerable 

degree of variations.

The writer hopes to deepen understanding of the identity of varied types and to reexamine 

classification ranks and (valid) scientific names by collecting additional data throughout the country in 

the future.

Acknowledgment: Ms. Mutsuko Ohno (大野睦子) of Kita-kyushu City kindly collected nuts of 

Himebishi from Fukuoka Prefecture, and also guided the writer through his field visit. The writer would 

like to express his most sincere appreciation. This study is a part of the research grant by the Japan

Society of the Promotion of Science, No. 60740381. 
13

Summary

Frequent occurrence of intermediate forms and poor knowledge on the variability of characters 

have caused some difficulties in the taxonomy of Trapa in Japan. Thus I made a preliminary analysis on 

the variation of nuts collected from 21 populations in Southwestern Japan. Attention was paid to some 

morphometrical characters of the nut and development of lower spines or “pseudohorns.”

Each population usually contained different forms of nuts. Among them, however, several entities 

could be recognized based on the shape of nut as follows.

1) Two-spined form: This included nuts of middle size (width 30–50 mm) and ones of big size 

(width over 45 mm). In case of the former ones (fig. 6-B, C, D) the nuts with pseudohorns of varying 

degree of development usually occurred together within one population and even on a single plant. I 

propose to treat them as one taxon, Trapa japonica, sensu OHWI (1965), without inventing varieties. But 

at the same time it was remarkable that the tendency of development of pseudohorns was apparently 

different from population to population.

The bigger ones included two types, that is, one without pseudohorns (fig. 6-F) and the other with 

pseudohorns (fig. 6-G). The former one may be identified as T. bispinosa ROXB., but the latter one has not 

been described in literature.

2) Four-spined form: The nuts of small size (width of about 20 mm, fig. 6-A) were well definable 

and thought to be T. incisa SIEB. et ZUCC. The nuts of bigger size showed some variations with respect to 

their size and/or stoutness of lower spines. The big ones (width over 45 mm, fig. 6-H) may be treated as 

one taxon, T. natans or its variety. The nuts of middle size (fig. 6-E) have been named T. natans var. 

pumila NAKANO. But so far as present materials were concerned, its entity seemed dubious. They might be 

immature nuts of bigger ones.

The different patterns of variation among populations were ascribed to genetic differentiation. 

Predominance of self-pollination and isolation of habitats were thought to promote genetic isolation and 

preservation of genetic variations which occurred in each population. But the possibility of hybridization 

cannot be excluded.

References

Hamajima, S. 1981. Investigation of water plants in irrigation ponds. 3. Irrigation ponds in Mie and Gifu. 

Plants and Nature 15 (13): 27–30 (in Japanese).

Hara, H. 1954. Enumeratio Spermatophytarium Japonicarum. Vol. 3. Iwanami Shoten, Tokyo.

Kadono,Y. and E.L. Schneider, 1986. Floral biology of Trapa natans var. japonica. Bot. Mag., Tokyo 99: 

435–439. 

Kitagawa, M. 1982. Trapaceae (Hishi Family) in “Wild Plants of Japan II” edited by Y. Satake 佐竹義輔,

J. Ohwi 大井次三郎, S. Kitamura 北村四郎, S. Watari 亘里俊次, and T. Tominari 富成忠夫:

262, Heibonsha, Tokyo (in Japanese).

Kitamura, S. and G. Murata, 1961. Colored illustrations of herbaceous plants of Japan. Vol. 2 

(Choripetalae). Hoikusha, Osaka (in Japanese).

Makino, T. 1961. Makino’s new illustrated flora of Japan. Hokuryukan, Tokyo (in Japanese). 

Makino, T. 1908. Observations on the flora of Japan. Bot. Mag., Tokyo 22: 171–172.

-------------, 1913. Observations on the flora of Japan. Bot. Mag., Tokyo 27: 251–252.

Miki, S. 1952a. Morphological character of remains of Trapa and adaption to water life. Bull. Soc. Pl. 

Ecol. 2: 111–116 (in Japanese). 
14

Miki, S. 1952b. Trapa of Japan with special reference to its remains. J. Inst. Polytech. Osaka City Univ.,

Ser. D 3: 1–29, pls. 2 

---------, 1959. Evolution of Trapa from ancestral Lythrum through Hemitrapa. Proc. Jap. Acad. 35:

289–294.

Nakai, T. 1942. Notulae ad Plantas Aseie Orientalis (XXI). Jour. Jap. Bot. 18: 421–437 (in Japanese).

Nakano, H. 1913. Beitrage zur Kenntnis der Variationen von Trapa in Japan (Contributions to the 

cognizance of the variations of Trapa in Japan). Bot. Jahrb. 50: 440–458 (in German). 

---------, 1964. Further studies on Trapa from Japan and its adjacent countries. Bot. Mag. Tokyo 77: 

159–167. 

Ohwi, J. 1965. Flora of Japan. Revised Edition. Shibundo, Tokyo (in Japanese).

Ohtaki, S. and T. Ishido, 1980. Illustrated Japanese water plants. Hokuryukan, Tokyo (in Japanese).

Sakaguchi, S. 1982. A report of the genus Trapa in Kagawa Prefecture. Japan (1). Kagawa Seibutsu, 10: 

13–17 (in Japanese).

----------------, 1983. A report of the genus Trapa in Kagawa Prefecture, Japan (2) Kagawa Seibutsu, 11: 

35–39 (in Japanese).

Sakurai, Y. 1981. Changes of flora, vegetation area and biomass of aquatic plants in the recent progress of 

eutrophication in Lake Kasumigaura—Research Report from the National Institute for 

Environmental Studies 22: 229–279 (in Japanese).

Staszkiewicz, J. and J.J. Wojcicki, 1979. Biometrical analysis of Trapa L. nuts from Poland. Fragmenta

Floristica et Geobotanica 25 (1): 33–59 (in Polish with English summary).

--------------------- and ---------------, 1981, Variability of the metrical characters of nuts of the genus Trapa

L. from central Europe. Fragmenta Floristica et Geobotanica 27 (3): 415–431.

Takada, J. 1983. Taxonomic notes of the plants in Akita Prefecture (1) Tohoku Shokubutsu Kenkyu

(Plant Research in Tohoku Region) 1: 7–12 (in Japanese).

Toda, H. 1979. On the Hishi (Trapa) of western Shizuoka Prefecture. Biology of Shizuoka Prefecture: 

330–333 (in Japanese).