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 F z 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\, F% and later 

 generations and in guiding his selection of FZ 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 Fi 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 Fi 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 

 Fi 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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