HYBRID VIGOR OR HETEROSIS 157 



In general, as noted before, there is similarity between 

 the effect of heterozygosis and that of a good environ- 

 ment. Those characters which are quickest to be modi- 

 fied by external factors also show the greatest change 

 on crossing. A good illustration of this. is a Nicotiana 

 cross which was above the average of the parents in both 

 height and leaf size. The length of the corollas, on the 

 other hand, a character very slightly affected by the en- 

 vironment, was not increased. There is at least one 

 difference between the two, however; in time of maturity, 

 environment and heterosis have somewhat opposite ef- 

 fects. Grenerally speaking, favorable growing conditions 

 tend to delay flowering and maturing, whereas conditions 

 which stunt the plants tend, like heterosis, to hasten them. 



Each of these effects is by no means always present 

 when a cross is made. The usual and of course the most 

 noticeable effect is the increase in size. But crossing may 

 have a stimulating effect upon certain parts and a de- 

 pressing effect on others. This is shown in many species 

 crosses in which reproductive ability is greatly reduced 

 or even totally eliminated, while at the same time vegeta- 

 tive growth is enormously increased. Freeman and Sax 

 have independently obtained se'eds from crosses between 

 common bread wheats and macaroni wheats which were 

 shrunken in appearance and snaall in size, owing to a poor 

 development of the endosperm. The embryos were well 

 developed, however, and the plants produced gave dis- 

 tinct evidence of hybrid vigor. 



In this discussion there has been a noticeable omission 

 of the effect of crossing on animals. Illustrations are not 

 lacking that crossing frequently is highly beneficial to 

 animals ; but animals do not furnish as desirable research 



