CHAPTER XX 
UTILIZATION OF HYBRIDS IN PLANT BREEDING 
Although the special uses to which plant hybrids may be put are very 
numerous, they fall into two categories, viz., first, the production of new 
desirable combinations and, second, the production of increased vigor 
in the first hybrid generation. The first category includes all phases of 
the usual purpose of crossing plants, which was briefly discussed in the 
preceding chapter. The new character combinations desired may be 
exclusively morphological or physiological or, as is more often the case, 
they may represent combinations of both kinds of characters involving 
many factors. In the simpler cases involving only a few pairs of inde- 
pendent factors the breeder who is familiar with the Mendelian princi- 
ples of heredity can easily compute the number of F2 individuals that he 
must grow in order to secure the desired combination. Even in the most 
complex cases knowledge of the principles of genetics will be of practical 
value in helping the breeder to understand his results in F'1, Ff. and later 
generations and in guiding his selection of F'2 individuals for further test- 
ing. These principles are discussed in Chapters V to X. It is the 
purpose of this chapter to present some specific results of the increased 
vigor so commonly observed in F; hybrids. This increased vigor, or 
heterosis, as it has been termed by Shull, may manifest itself in greater 
size, more rapid growth, larger productivity, greater hardiness, drouth 
resistance, etc. The theoretical explanations of heterosis have been dis- 
cussed in Chapter XII. In the present chapter we shall consider only 
the utilization of the principle that hybridization of closely related 
varieties or species usually results in heterosis. As the methods used 
with plants grown from seed differ from those which can be used with 
vegetatively propagated plants, the two groups will be considered 
separately. 
Increased Production in F, Maize Hybrids.—This phase of corn 
breeding has come into considerable prominence in recent years. 
Although it has not yet become an important factor in corn growing, 
it presents interesting and important possibilities in the way of increased 
production. The most significant results have been obtained by growing 
F, hybrids between species, sub-species, commercial varieties, local strains 
of commercial varieties and closely inbred strains or biotypes. The 
earliest recorded experiments on increased production are stated by 
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