Feb., 1918] Sexual Dimorphism 113 



A further advance in the extent of the dimorphism is shown 

 by those plants in which the distinctive staminate and car- 

 pellate flowers are confined to definite parts of the same inflor- 

 escence. A good example of this condition is the wild rice, 

 Zizania aquatica L. This is a monecious species with the 

 flowers in a large panicle. The upper part of the inflorescence 

 is carpellate and the lower staminate. The change in condition 

 usually extends transversely across the inflorescence axis and 

 its branches. The staminate spikelets have a vestigial gynecium 

 with three minute stigmas, while the carpellate spikelets have 

 six distinct vestigial stamens. Here we have a considerable 

 extent of tissue involving a large number of spikelets in the 

 same sexual state. The amount of the vegetative tissue involved 

 is much greater than in cases like the cocoanut. But the most 

 striking peculiarity of this inflorescence is in the central part. 

 Here the spikelets are bisporangiate having perfect stamens and 

 gynecia. (See diagrams, Figs. 10, 11, 12). On the transition 

 zone neither state is established and so no inhibition occurs. 

 The sexual state is brought about in the sporophylls as is usual 

 in bisporangiate plants in general as well as in the lower grasses. 

 Examples like Zizania show plainly that the determination of 

 staminate, carpellate, and bisporangiate flowers takes place in 

 the vegetative tissues by the establishment of a certain physio- 

 logical state and has nothing to do with gross or cellular mor- 

 phology. The fundamental morphological conditions, both 

 gross and microscopic, are the same throughout the entire 

 panicle. 



Zizania aquatica has also a remarkable sex-limited char- 

 acter. The lemmas of the staminate spikelets are awnless while 

 those of the carpellate spikelets are long-awned. The bispor- 

 angiate spikelets in the neutral region have short awns. (Figs. 

 13, 14, 15). There is every gradation of length of awn in pass- 

 ing from the awnless lemmas of the staminate region to the 

 long-awned lemmas of the carpellate region. No matter 

 whether one or many awn factors are involved, the character of 

 the awn is due entirely to the latency or activity of the awn 

 factor or factors under the influence of the sexual state. Here 

 then we have plain evidence of the nature of sex limited char- 

 acters in plant sporophytes. Both awnless and awned lemmas 

 have a common sporophyte heredity, but this heredity expresses 

 itself in all degrees of latency and activity depending on the 



