POLLINATION AND FERTILIZATION 



133 



125. When self-pollination is advantageous: cleistogamous 

 flowers. Some flowers are usually self-pollinated unless cross- 

 pollinated by accident or by human agency. Wheat is a 

 notable instance of the kind, and apparently self-pollination 

 can go on in this grain for a long period without injury to the 

 fertility or the robustness of the offspring. 1 Experiments in 

 raising selected varieties of tobacco seem to show that in this 

 plant self-fertilization, for several generations at any rate, 

 produces better results than cross-fertilization. 2 



Whenever cross-pollination by the wind or by the agency of 

 animals is impossible, it is evident that self-pollination would 

 be advantageous, since it is infinitely better than no pollination 

 at all. Examples of the impossibility of cross-pollination are the 

 cases of plants which grow isolated or in localities in which the 

 special pollinating 

 animal is not found, 

 as American yuc- 

 cas in European 

 botanic gardens. 

 Many highly suc- 

 cessful weeds owe 

 part of their pre- 

 dominance to the 

 fact that they seed 

 well after self- 

 pollination. 



Since occasional cross-fertilization appears to be sufficient 

 to keep up the strength and fertility of many kinds of plants, 

 it would seem to be an advantageous plan for these to unite 

 the certainty which characterizes self-pollination with the re- 

 newal of strength which comes from cross-pollination. Violets 

 and many other less familiar plants unite the two methods 



1 See "Wheat : Varieties, Breeding, Cultivation," Bulletin 62, University 

 of Minn. Agr. Exp. Sta., 1899. 



2 See "Tobacco Breeding," Bulletin 96, Bureau of Plant Industry, U. S. 

 Dept. Agr., 1907. 



FIG. 123. Lengthwise section of dimorphous flower 

 of bluets 



A, long-styled form; B, short-styled form ; a, anthers; 

 s, stigmas. About three times natural size 



