388 Inbreeding, Selection, and Heterosis 



Heterosis is by no means confined to maize or even to plants. 

 One of the best examples of heterosis is the mule, and many 

 other examples have been described in both plants and animals. 

 Buchholz has recently shown that hybrid vigor is exhibited in 

 crosses between some species of pines. In seedling stages, the 

 growth of the hybrids is much greater than in seedlings from the 

 wind-pollinated parents. Comparisons of hybrids with the 

 parents during stages in the development of the seeds show 

 that in their general dimensions and in their shape indices the 

 embryos of the hybrids are, stage for stage, intermediate be- 



Inbred Inbred Inbred Inbred 



line B-^ 1 line A line C '^ 



d' Q 



line D 



AB hybrid 



? 



CD hybrid 



AB X CD hybrid 



used to produce 



crop 



Fig. 101. An outline of Jones's double cross method of producing hybrid 

 corn commercially. For explanation, see text. 



tween those of their parents during comparable stages of growth. 

 However, the embryos grow more rapidly in the hybrids so that 

 comparable stages are reached more quickly. The embryos of 

 the hybrids are not larger in the mature seeds because the seeds 

 have a fixed size determined before fertilization which restricts 

 the size of the embryo. The difference in growth rate mani- 

 fested during the early stages of the development of the seed 

 is again apparent in the young seedlings after they are planted. 

 Buchholz concludes that in pines hybrid vigor is definitely a 

 physiological vigor of growth and that its explanation is to be 

 sought in physiological and possibly biochemical investigations. 

 Another explanation that has been advanced to explain hetero- 

 sis is the hypothesis of A. F. Shull that heterosis is the result of 

 an initial stimulation resulting from the entrance of a sperm into 

 a new cytoplasmic environment in a specifically different egg. 

 It has received some support but, like other questions of the role 

 of the cytoplasm, is difficult of experimental proof. 



