POLLINATION AND FERTILIZATION 



141 



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

 better results than cross-fertilization. 1 



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. Many highly successful weeds owe their 

 predominance partly to the 

 fact that they produce good 

 seed after self-pollination. 



Since cross-fertilization at 

 intervals appears to be suffi- 

 cient to keep up the strength 

 and fertility of many kinds 

 of plants, there might be some 

 advantage in uniting the cer- 

 tainty which characterizes self- 

 pollination with the renewal 

 of strength which comes from 

 cross-pollination. Violets and 

 many other less familiar plants 

 unite the two methods by pro- 

 ducing ordinary showy flowers 

 and also inconspicuous closed, 

 or cleistogamous, flowers. In 

 violets the latter are borne 

 on flower stalks close to the 



caps 



FIG. 124. A violet with cleistogamous 



flowers as seen in late July or early 



August, after the conspicuous flowers 



have disappeared 



cl, cleistogamous flowers; caps, cap- 

 sules produced by earlier flowers of 

 the same sort 



ground (fig. 124) and usually, 



before maturing, become partially buried in the earth. Pol- 

 lination occurs within the closed flower, the pollen tubes 

 developing within the anthers and making their way to the 

 stigma. The cleistogamous flowers produce many more seeds 

 than the showy ones, but the latter insure at least occasional 

 cross-pollination since they are freely visited by bees and 

 other flying insects. 



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

 Dept. Agr., 1907. 



