322 



FLOWERS 



requiring the plant to depend upon more or less uncertain 

 outside agencies for its performance. In some cases (some 

 cleistogamous flowers) it allows the placing of the seeds in 

 a position favorable to germination. Evidently, if equally 

 good seeds can be produced by close-pollination, then this 

 method has distinct advantages over the other. 



But cross-polHnation is the more common method. 

 Dichnous plants necessarily use it, and the great majority 

 of monocUnous ones not only appear to take pains to secure 

 it, but also appear to take pains to avoid the other kind 

 of pollination. With corn it has been proved that even 

 geitonogamy results in diminished vigor ; com requires 

 xenogamy. Wheat, on the other hand, appears to do 

 very well with close-pollination, and some of our com- 

 mon weeds make use of this method constantly and suc- 

 cessfully. Pigweed, knotgrass, chickweed, and maUow are 

 examples. The flowers of trillium and geranium are 

 usually self-pollinated. The evening primrose, though 

 often cross-poUinated, may be self-polHnated even before 

 the flowers open; in its yellow corollas the stamens and 

 the pistil may be seen growing together in such manner 

 that the pollen is fairly rubbed off on the stigma. 



It is evident, as stated in Cowles's Ecology, that " the 

 benefits of cross-polUnation and the disadvantages of 

 close-poUination have been too much emphasized. Close- 

 pollination and its essential equivalent, geitonogamy, are 

 extremely common in nature, nor must it be forgotten, 

 also, that many of the important plant and animal races 

 utilized by man have reached their present state of com- 

 mercial perfection by the most careful inbreeding," that 

 is, by careful prevention of cross-polUnation. 



