Hybrid Vigor 165 



the limit when the perfectly homozygous condition is 

 reached for the whole population, and then there can 

 be no more loss of vigor. 



3. A cross between sister and brother eiTects nothing. 

 — This is evident, for it introduces no heterozygosity. 



4. "A cross between plants belonging to two self- 

 fertiUzed families results in a progeny of as great vigor, 

 size, and productiveness as are possessed by families 

 which have never been self-fertilized." — Heterozygosis 

 explains this by showing that a cross between two pure 

 Unes may bring into the hybrid a maximum number 

 of heterozygous sets, quite as many as are normally 

 present in normally cross-fertilized families. 



5. Reciprocal crosses are equivalent. — This obviously 

 follows from the theory of heterozygosis. 



In coimection with this work East was under- 

 taking to discover the nature of hybrid vigor. In what 

 respects are such hybrids vigorous ? In reply to this 

 question East offers the following analysis of hybrid 

 vigor. 



Primarily it is an increase and acceleration of cell 

 division; in other words an increase in the power of 

 assimilation. One can early observe a slight increase 

 in the size of the cotyledons. The more rapid growth 

 and earlier maturity of the seedlings is quite noticeable. 

 Then one sees a distinct increase in the size of the roots. 

 In the stem there is no increase in the number of nodes, 

 but the internodal development is striking. Usually the 

 stem growth is greater than the leaf growth, but the 

 increase of the latter can be definitely traced. The size 

 of the flower is usually not affected, nor is there any 

 change in the size of small fruits, such as tobacco. In 



