Sterility i t,2> 



evolutioiuiry progress in the dirccLion of cslablisliiiig partheno- 

 genesis. (The most notable example is i)rovi(le(l by the C'om- 

 positae, although it would certainly be unsafe to conclude 

 therefrom that the ultimate goal of plant evolution was 

 parthenogenesis. Quite on the contrary, one feels that the 

 Compositae have seriously handicapped themselves, so far as 

 future possibilities are concerned, by a ''freakish'' evolutionary 

 maneuver.) 



C. Self-sterility. 



In effect evolution among angiosperms has achieved self- 

 sterility through the perfection of devices favoring cross- 

 pollination. Here should be cited: floral adjustments insuring 

 cross-pollination by insects; protandry and protogeny; dioecism. 



(B. Semi-sterility. 



This class has little more than a theoretical existence here, 

 although doubtless some forms might be found exhibiting the 

 ''incompletion" of some of the above-mentioned evolutionary 

 tendencies, and thereby exhibiting what is in effect semi-sterility.) 



III. Phenomena of genetics. 



Here are included cases where the mechanism underlying the 

 phenomenon is affected by breeding operations. 



A. Sterility. 



I. Wide crosses. 



Crosses between distantly related parents may be effective 

 in producing first generation hybrids which may be notably 

 vigorous individuals (see chapter on ''Hybrid vigor"), but quite 

 sterile. A notable example of this is the cabbage-radish hybrid, 

 which achieves astounding proportions, but is completely sterile 

 (Gravatt 8). 



Here the loss in efficiency in the reproductive system is dis- 

 tinctly not accompanied by loss in efiiciency in vegetative develop- 

 ment. This peculiarity is clarified by the following idea. Wide 

 crosses involve the fusion of relatively ''inharmonious" gametes, 

 w^hich might be expected to produce disturbances in the ontogeny 

 of the resulting individual. The grosser mechanism which regu- 

 lates vegetative development can evidently weather such disturb- 

 ances, while the more finely balanced mechanism of gamete 

 formation is upset. 



