194 Outline of Genetics 



fern prothallium frequently produces antheridia only, 

 while a mature prothallium produces archegonia only. 

 In attempting explanation it is usually stated that more 

 ''nutrition" is required for the production of archegonia 

 and eggs than for the production of antheridia and 

 sperms. During the flowering season, monoecious angio- 

 sperms (e.g., Begonia) will sometimes produce the male 

 flowers distinctly earlier than the female flowers, or the 

 reverse. In connection with such cases botanists usu- 

 ally feel that the potentialities for both sexes are at all 

 times present in all the tissues of the individual, and that 

 it remains for some unknown complex of physiological 

 conditions to call out one or the other sex in any given 

 region of the plant. Surely no sex chromosome mecha- 

 nism can be at play to account for sex differentiation 

 here! Only by assuming a reduction division some 

 time during somatogenesis or a regular and peri- 

 odic ''chromosome-elimination" could such cases be 

 brought in line with the sex chromosome mechanism 

 of sex determination. It is much more reasonable 

 (for the present at least) to regard bisexual plants as 

 "outside the scope" of the sex chromosome mechanism. 

 In unisexual plants one is confronted by a different 

 situation, and sometimes, as discussed above, a sex chro- 

 mosome mechanism seems to be determining sex. Even 

 here, however, it would doubtless be possible to cite more 

 evidence favoring the physiological theories. Angio- 

 sperms that are normally dioecious have frequently 

 produced bisexual plants that might well be regarded as 

 "intersexes." Considerable work has been done to indi- 

 cate that various environmental conditions may either 

 modify the sex ratio or result in the production of inter- 



