108 W. GORDON WHALEY 



revealed by Harvey's studies on tomato inbreds and hybrids. Burkholder and 

 McVeigh (1940) have also noted differences in responses of corn inbreds and 

 hybrids to various levels of available nitrogen. These investigators corre- 

 lated apical meristematic development, and the differentiation of the vascu- 

 lar system with the level of nitrogen nutrition, and the efficiency of different 

 lines and hybrids in utilizing the available nitrogen. Their results indicate 

 that hybrid vigor, involving superiority in the production of dry matter 

 and the differentiation of organs, was not correlated with greater growth and 

 development of the vascular system. 



There definitely are vascular organization differences between the heterot- 

 ic hybrids and the inbreds in the material we have studied. These vascular 

 organization differences seem not to be the result of differences in mineral 

 absorption and distribution, but rather to be one of the factors responsible 

 for the differences in absorption and distribution. All the evidence seems to 

 indicate that the greater absorption of minerals by heterotic hybrids can be 

 referred to better developed root systems in the hybrids, probably also to the 

 presence of more efficient transport systems, and to a generally higher level 

 of metabolic activity. 



Recently we have undertaken a rather extensive analysis of both the 

 morphological and physiological characteristics of a tomato cross in which 

 there is marked heterosis. We have found no significant differences between 

 the inbreds and the hybrids as to total phosphorus content of the leaves, 

 stems, or roots. There is some suggestion that the phosphorus content of the 

 organs of the hybrids reaches a higher level earlier in growth than it does in 

 the inbreds. Neither do the hybrid plants have any consistent advantage 

 with respect to nitrogen content. 



Analyses of the starch content of the leaves and stems suggest that the 

 hybrid plants may have a slightly higher starch content than the inbreds 

 during the early growth stages. In terms of average figures over the whole 

 growth period, however, there are no marked differences between the in- 

 breds and the hybrids. The same appears to be true of the sugar content. 

 The hybrids have a somewhat higher sugar content, at least in the leaves, 

 early in development. During the greater part of the growth cycle the hy- 

 brids do not have significantly more sugar than the inbreds. The only clear 

 difference found between the inbreds and the hybrids is in the catalase ac- 

 tivity of the shoot tips, the hybrids having an appreciably greater index of 

 catalase activity than either of the inbred parents. The catalase activity 

 differences are associated with much more active meristematic growth in the 

 hybrid plants. 



THE ROLE OF SPECIFIC SUBSTANCES IN HETEROSIS 



Evidence for another sort of physiological differences possibly involved in 

 heterosis is furnished by the work of Robbins (1940, 1941a) in assaying the 



