[Chap. XXXVI ORIGIN OF PLANTS USED BY MAN 431 



rapid introduction of foreign species from all parts of the world, accom- 

 panied by intelligent control of plant breeding and a clear understanding 

 of the different types of variations that occur in plants. 



The introduction of foreign species is merely an attempt to secure 

 plants with other heredities. The ancient caravan trade routes were 

 important means of distributing plants in the early days. Botanic gardens, 

 and more recently experiment stations, became centers of receiving, 

 testing, and distributing these foreign plants. Plant scientists at the U. S. 

 Department of Agriculture annually obtain numerous varieties of prob- 

 ably desirable plants and distribute them to appropriate testing stations 

 throughout the country. They have secured and tested a total of more 

 than 8000 varieties of wheat collected from more than 50 different 

 countries. 



Plants are now critically selected with reference to a number of quali- 

 tative and quantitative characteristics, such as yield and quality of cer- 

 tain organs and tissues, chemical composition, relative development in 

 different soils and climates, and immunity to causal agents of disease. 



New varieties obtained by control of plant breeding. Man no longer 

 depends entirely upon the selection of chance or fortuitous variations as 

 a means of securing new varieties of plants. He has learned how to 

 control plant breeding and obtain more desirable \arieties ( 1 ) by com- 

 bining the desirable heredities of two or more kinds of plants, (2) by 

 eliminating or preventing the expression of undesirable heredity in 

 otherwise desirable plants, and (3) by first eliminating undesirable 

 hereditary qualities in two or more kinds of plants and then combining 

 the desirable heredities. 



Plant breeding is controlled by means of pollination. Plants, such as 

 wheat, peas, and beans, in which close pollination and self-fertilization 

 naturally occur, have to be manipulated differently than plants, such as 

 corn, in which open pollination and both cross-fertilization and self- 

 fertilization naturally occur. In open pollinated plants the pollen of the 

 stamens of a flower may come in contact with the stigma of the pistil of 

 the same flower, of flowers of the same plant, or of flowers of other plants. 



The term close pollination is used here to refer to the fact that in some 

 species of plants pollination usually occurs before the flower buds open. 

 A very small amount of cross-pollination and cross-fertilization occurs in 

 these close pollinated plants. In some varieties of wheat it may be as 

 much as 4 per cent. Insects that break through the floral envelope may 

 become agents of cross-pollination in closed flowers. 



