Feb., 1918] Sexual Dimorphism 119 



The genus Acer presents a very good series, passing from 

 Acer platanoides L. through a number of species to Acer 

 negundo L., the boxelder. So far as the writer's observations 

 go, the boxelder is strictly diecious. Acer platanoides is in a 

 much more primitive condition being but a short distance 

 removed from the normal bisporangiate type. However the 

 plants are staminate and carpellate. The writer has found 

 carpellate trees which developed a few typical staminate flowers 

 with vestigial gynecia and normally developed stamens, although 

 the trees were covered with young fruit from the normal car- 

 pellate type of flower. (Figs. 20 and 21). Staminate trees 

 were also discovered which were producing many carpellate 

 flowers with vestigial stamens (Figures 22-24). These 

 carpellate flowers developed normal fruit. Figure 24 rep- 

 resents a partially developed fruit. In both cases the per- 

 centage of the unexpected kind of flowers was small, but the 

 staminate trees produced a much larger per cent, of carpellate 

 flowers than the carpellate trees did of staminate flowers. 



Rumex is another genus which begins with a partial 

 bisporangiate state and ends in a diecious condition. Rumex 

 crispus L. has bisporangiate flowers and carpellate flowers, 

 the latter containing prominent vestigial stamens. Rumex 

 acetosella L. is strictly diecious. The carpellate plant shows 

 no vestige of the stamens or else these are too small to be seen 

 without making special sections. But the staminate flower 

 shows a small vestigial gynecium with three minute stigmas. 

 (Fig. 25). The persistence of the three vestigial stigmas may 

 be accounted for by the fact that the normal stigmas of Rumex 

 are very large and branched. (Fig. 20). In some of the 

 intermediate species of Rumex, as for example Rumex altissimus 

 Wood, the vestigial stamens are rather prominent and the 

 vestigial gynecium is of fair size with three small slightly 

 branched stigmas. 



A most striking example to illustrate the stages in the 

 evolutionary development of dieciousness is presented by the 

 genus Fraxinus. Fraxinus cuspidata Torr. has flowers with 

 corolla and calyx. They are bisporangiate and very fragrant. 

 Fraxinus quadrangulata Mx. has bisporangiate flowers, but 

 no perianth. The merest vestige of a calyx is present. Fraxinus 

 americana L. has a minute calyx and is strictly diecious. If 

 the antiquated and misleading classification of the flowering 



