Feb., 1918] Sexual Dimorphism 107 



modern morphological discoveries. We cannot hope to analyze 

 hereditary phenomena unless we have a correct understanding 

 of the morphology and physiology of the parts under considera- 

 tion. In recent years a consistent terminology has been 

 developed by morphologists and there is no excuse for not 

 using it. We have a sporophyte terminology and a game- 

 tophyte terminology. However, when applied to the new 

 science of genetics out language is still inadequate, and one is 

 immediately confronted with the difficulty of expressing 

 hereditary phenomena in relation to sex when the sporophyte 

 of heterosporous plants is involved. The homosporous sporo- 

 phyte having no sexual dimorphism does not raise the issue, 

 but when passing from homosporous to heterosporous spor- 

 ophytes the problem presents itself with the common solution 

 that organs formerly described and defined as without sex 

 now obtain a radically different treatment, to the confusion 

 of both the learned and the unlearned,. It seems to the writer 

 that the way out of the difficulty, at present, is to employ the 

 sporophyte terminology when structures are mentioned, and 

 to limit the sexual terminology, as far as possible, to the states 

 or conditions of special gametophyte phenomena expressed 

 in the sporophyte. Thus a spore bearing fern leaf is a non- 

 sexual structure and a sporophyll of Marsilea is still nonsexual 

 although it produces spores of two sizes. But the difference 

 between the spores is a sexual difference. A carpel is still a 

 megasporophyll and a nonsexual reproductive organ of the 

 same fundamental nature as the megasporophyll of a hetero- 

 sporous pteridophyte, but it is a sporophyll in which the sexual 

 phenomena peculiar to the gametophyte are finding expression. 

 The sexual state is thrown back, so to speak, into a small part 

 of the sporophyte. It is this spreading of the sexual state, 

 with an ever increasing area of the tissue involved, that con- 

 stitutes one of the most interesting aspects of sporophyte 

 evolution in the higher plants. 



So far as the writer knows, Marsilea is the lowest living 

 genus of heterosporous plants in respect to sexual dimorphism 

 of the sporophyte. In external aspect the sporophyte shows 

 no dimorphism, the sporocarps all being alike (Fig. 1). But 

 the sporangia are slightly different, although the same in general 

 shape. The most striking difference is in the stalks. The stalk 

 of the microsporangium is comparatively long and slender. 



