CHAPTER XVII 

 THE COMPOSITION OF PLANT POPULATIONS 



Before taking up in detail the various methods of plant breeding and 

 considering their effectiveness it is well to enquire as to the nature of the 

 populations with which we are required to deal. By a population in this 

 connection we ordinarily mean a variety as that word is used in the trade, 

 although populations as found in cultivation may be made up of mixtures 

 of varieties. Usually, however, within an established variety, that is, a 

 strain or race bred to a given type until it reproduces that type with a 

 fair degree of accuracy, the variations are of minor consequence and not 

 always readily detectable. But they may be due not only to modifications 

 consequent upon slight differences surrounding the development of 

 individuals in a population; they may also be germinal, that is, they may 

 arise either from Mendelian recombination of germinal differences or 

 by actual new germinal changes. We desire to know, therefore, what 

 sorts of populations exhibit germinal diversity, what kinds of germinal 

 diversity they exhibit, and how the germinal diversity may be related 

 to other characteristics of the populations. 



Reproduction in Plants. In seed plants the important factor which 

 determines the character of the population is the kind of pollination 

 which normally takes place. In the following classification most of our 

 important agricultural crop plants are listed roughly with respect to 

 this factor. 



A. Plants normally self-fertilized. 



(a) Flowers hermaphrodite, but the floral mechanism such as practi- 

 cally to preclude cross-pollination. Examples: wheat, oats, barley, rice, 

 beans, peas, and most of the other legumes. 



(6) Flowers hermaphrodite, but the floral mechanism favorable to a 

 low percentage of cross-fertilization. Examples : cotton, tobacco, tomato, 

 flax, and other plants having a similar floral structure. 



B. Plants normally cross-fertilized. 



(a) Flowers hermaphrodite, self-fertile, but with floral devices favor- 

 able to cross-fertilization. Examples: rye, sugar beet. 



(6) Flowers hermaphrodite, but self-fertilization precluded on account 

 of self-sterility of the plants. Example: sunflower, red clover. 



(c) Monoecious plants, self-fertile, but the floral mechanism such 

 as to favor cross-fertilization. Examples: maize, watermelon, squash, 

 pumpkin, cucumber, and cantaloupe. 



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