[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 433 



lected pure-line plants ( Fig. 204 ) . Environment affects the development 

 of a pure-line plant just as it does that of any plant; but changing the 

 environment does not change its characteristic of breeding true to tvpe. 

 Compared with plant selection, the controlling of plant breeding is a 

 very modern occupation. Prehistoric man, by the introduction and culti- 

 vation of closelv related species and \'arieties of plants in the same 



Fig. 204. Uniformity of height and maturity in hybrid corn of similar genetic com- 

 position. Photo by G. H. Stringfield, Ohio Agricultural Experiment Station. 



locality, was unconsciously responsible for cross-pollination and the con- 

 sequent hybrid varieties that developed as a result of cross-fertilization. 

 On the other hand, the ancient practice of collecting pollen-bearing 

 branches from staminate trees of dates and tying them among the flowers 

 of pistillate trees to insure the development of fruit resulted in some 

 wholly unintended limitation of cross-pollination. In the wild state, the 

 staminate trees of dates are as abundant as the pistillate trees, and the 

 pollen is distributed by wind. More than 5000 varieties of dates are now 

 under cultivation. 



Intelligent efforts to control plant breeding through pollination de- 

 pended upon the discovery of sex in plants and of the dependence of 

 sexual reproduction in seed plants upon pollination. The experiments of 

 Camerarius from 1691 to 1694, by which he showed that embryos do not 

 develop in seeds in the absence of pollination, were the first definite 



