426 PRACTICAL BOTANY 



which do not quickly revert to the original type, unless as a 

 result of miscellaneous crossing. Plant breeding, as a science, 

 is much too young to enable us as yet to answer the question 

 how far varieties tend to " run out " and what plants are most 

 subject to this reversion. 1 It is probable that most cultivated 

 plants grown from seed will be found to be decidedly less 

 constant in maintaining their character for years than are 

 the grains. 



390. Hybridizing. Hybridizing, as the term is now gen- 

 erally used, means the production of seed by the action of pol- 

 len of one variety or species on the ovule of another variety or 

 species. In order to produce seed that will grow, both species 

 must usually belong to the same genus. Frequently different 

 species of the same genus hybridize with difficulty; that is, 

 the result of the attempted cross is to produce no seed, or seed 

 that does not grow well. The offspring produced by hybridi- 

 zation are known as hybrids. 



It has long been known that hybrids are often extraordi- 

 narily variable, but the law (Mendel's law) which in many 

 cases, though not in all, determines their characteristics and 

 their mode of variation, was not discovered until 1865, 2 and 

 did not become well known until some thirty-five years later. 



Recently much use has been made of hybridizing in order 

 to set plants to varying, and the most desirable varieties thus 

 produced have then been selected and used in breeding as 

 already described. 



391. How hybrids are artificially produced. Hybridizing, 

 or crossing plants, is sometimes an easy, sometimes a rather dif- 

 ficult, process. It is simplest in unisexual flowers, for exam- 

 ple, in those of Indian corn. Here the "tassel" (Fig. 335) is a 

 cluster of spikes of staminate flowers, and the "ear" (Fig. 336) 

 is a spike of pistillate flowers, each thread of the " silk " rep- 

 resenting a stigma and style attached to an ovary (grain of 



1 See Bailey, The Survival of the Unlike, Essay XXIV. The Macmillan 

 Company, New York. 



2 See Bailey, Plant Breeding, chap. iv. The Macmillan Company, New York. 



