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. 
(b) 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. 
(b) Flowers hermaphrodite, but self-fertilization precluded on account 
of self-sterility of the plants. Example: sunflower, red clover. 
(c) Moncecious 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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