Monoecious and Hermaphroditic Plants 



511 



by McPhee. As he has pointed out, such changes, of course, 

 do not change the genotype. They merely indicate that a given 

 genotype may have a somewhat different expressivity under dif- 

 ferent sets of environmental conditions in the same way that 

 other genes may produce different phenotypes in different en- 

 vironments. 



Fig. 146. Tassel of maize with a small, well-formed ear. Most of the 

 male flowers in the tassel are sterile but a few are well developed. (Photo- 

 graph by Dr. W. Brooks Hamilton.) 



Any gene that affects the sexual processes, the sex organs, or 

 the gametes themselves may be regarded as a sex gene. A num- 

 ber of such genes have been identified in maize. Some of them 

 produce complete or partial male or female sterility or both. It- 

 may be accomplished by upsetting meiosis or otherwise directly 

 affecting the spores or gametophytes, but it may also be brought 

 about by affecting the organs in which the sporocytes are pro- 

 duced. Thus a gene that prevented the development of anthers 

 might not be thought of as a "sex gene" but would be merely 

 another gene that causes some abnormality in growth in the 



