Feb., 1918] Sexual Dimorphism 111 



specialization to a greater or less degree toward the monecious or 

 diecious state. In tracing from the bisporangiate to the 

 monosporangiate flowers one finds a most remarkable display 

 of vestigial structures, which are the result of suppression of the 

 opposite hereditary factors by the distinctive sexual condition 

 set up in the tissues from which they should develop. This 

 inhibitory influence is of every degree of intensity in various 

 species, ranging from cases where the organ is almost normal 

 to its entire disappearance. In the more primitive, or rather 

 less specialized species, reversions are very common. 



The strictly diecious state is comparatively rare. There are 

 no original monosporangiate flowers among the Anthophyta. 

 The few examples, in which all vestige of a bisporangiate con- 

 dition has disappeared, in nearly every case show a direct 

 relationship to species or groups with the opposite structures 

 present either as vestiges or in a normal condition. A few 

 examples will be given to show the general trend of development 

 to a typical diecious condition. It is well, however, to caution 

 against the notion that the monecious condition is a step in the 

 evolution of the diecious. It may be in some cases but mostly 

 the development of diecious plants comes about directly 

 through a succession of more extreme vestiges. 



Sagittaria latifolia Willd., a low species of the Helobiae and 

 closely related to the bisporangiate genus, Echinodorus, is an 

 example of a plant which has taken but a slight step toward the 

 monecious condition. In typical cases the inflorescence bears 



Explanation of Figures 1-l.j. 



Sporocarp of Marsilea quadrifolia L. 



IVIicrosporangium from the same. 



Megasporangium from the same. 



Microsporophyll of Selaginella kraussiana (Kunze). 



Megasporophyll from the same strobilus. 



Stamen of Aquilegia canadensis L. 



Carpel from the same flower, showing hairs on the ovulary, a sex-limited 



character. 

 Staminate flower bud of Cocos nucifera L. 

 Carpellate flower bud from the same inflorescence. 

 Diagram of carpellate spikelet of Zizania aquatica L. 

 Diagram of staminate spikelet from the same inflorescence. 

 Diagram of a bisporangiate spikelet from the middle zone of the same 



inflorescence as Figs. 10 and 11. 

 Staminate spikelet of Zizania aquatica L., showing lack of awn. 

 Bisporangiate spikelet from the same inflorescence, showing awn of 



intermediate length. 

 Fig. 15. Carpellate spikelet from the same inflorescence as Figs. 13 and 14, 



showing long awn, a sex-limited character. 



