A preliminary study of variation of Trapa in Japan

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, 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 and the other with pseudohorns. 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) 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) may be treated as one taxon, T. natans or its variety. The nuts of middle size 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.