HYBRID VIGOR OR HETEROSIS 153 



reached. The resulting inbred strains would have about 

 the same amount of development as the. original inbred 

 strains, but would probably differ from them in appear- 

 ance through the possession of different combinations of 

 characters. The principal point is that the vigor and size 

 lost by inbreeding are immediately restored by crossing, 

 but lost again on further inbreeding. It is a transitory 

 effect, for the most part, impossible of fixation. 



Increases in yield of grain are also frequently ob- 

 tained when ordinary commercial varieties of maize are 

 crossed. Rarely are the increases greater than 10 per 

 cent., however, and even this is more commonly to be 

 expected when varieties of somewhat different type are 

 used ; for example, flint and dent. Most varieties of corn 

 are now so widely crossed and furthermore are so near 

 the limit of production that great advances are not to be 

 expected. Collins ^* has obtained especially large incre- 

 ments in yield by hybridizing types of com from different 

 geographical regions. Three different varieties of com 

 from the southwest — Hopi, Brownsville and Hairy Mexi- 

 can — each gave an increase of 100 per cent, or more when 

 crossed with a variety from China having seeds with a 

 different type of endosperm. 



Even before the plants are obtained there is a striking 

 effect of crossing in an immediate increase in the size of 

 seed. This was noted by Roberts,^®^ and established very 

 clearly by Collins and Kempton ^ through pollinating 

 ears of maize with a mixture of the plant's own pollen and 

 of a different sort. By taking advantage of the phenom- 

 enon of double or "endosperm" fertilization, the 

 experiment was so designed that the outcrossed seeds 

 could be distinguished by differences in endosperm color. 



