212 MAIZE 



CHAP, are equal for the development of progeny of homozygous and 

 heterozygous parents. 



But as Shull (i), East (2), and others have pointed out, in 

 maize " self-fertilization, or even inbreeding between much 

 wider than individual limits, results in deterioration ". Again : 

 "Although a study of the injurious effects of self-fertilization 

 was not the aim of the investigation, it was immediately ap- 

 parent in the smaller, weaker stalks, fewer and smaller ears, 

 and the much greater susceptibility to the attacks of the corn- 

 smut {Ustilago Maydis). The results were almost as marked 

 when the chosen parents were above the average quality, as 

 when they were below it, which in itself refutes the idea that 

 the injurious effect is due to the accumulation of deficiencies 

 possessed by the chosen parents" [Shull, 1). 



" Inbreeding in maize gives the same effect as lack of 

 nutrients, while cross-breeding gives the opposite effect. There 

 is retardation or acceleration of cell division, respectively. . . . 

 It is an established fact, although the cause is unknown, 

 that crosses between nearly related types are more vigorous 

 than either of the types alone" (East, 2). 



Collins (2) calls attention to the fact, however, that while 

 it is fully recognized that isolating the pure strains or biotypes 

 will very greatly reduce their vigour and yield, yet bv making 

 a combination of the proper strains, so isolated, it is believed 

 that the degree of fertility of the cross will reach that of the 

 most productive plants in the original mixed strain, and that 

 an increase of the total yield can be obtained in this way. He 

 also quotes an experiment of Dr. Shull, in which two self- 

 fertilized strains which were separated from a common stock 

 in 1904, and continuously self-fertilized since that time, were 

 reciprocally crossed in 1907. In 190S the yields of these re- 

 ciprocal crosses were compared with each other, with the self- 

 fertilized plants, and with cross-bred stocks of the original 

 breed. The yield from the cross-pollinated seed was 30 per cent 

 greater than that from the self-pollinated ear, and 2 per cent 

 greater than the average of the original cross-bred stock. 



By this means it is found possible to isolate a homozygous 

 type, the individuals of which, when mated, are as vigorous 

 and productive as the original mongrel heterozygote, but with- 

 out its objectionable features. 



