112 W. GORDON WHALEY 



growth. Into this category fall the enzymes, the auxins, and the other "phys- 

 iological key" substances. 



Many heterotic hybrid plants seem to gain their advantage within the first 

 few hours after germination. This advantage may not be shown as statistical- 

 ly significant until it has been further heightened by subsequent growth. 

 The primary growth activities during this period are those involved in the 

 unfolding of the enzymatic pattern; the mobilization, transformation, and 

 utilization of stored materials, and the building up of active protoplasmic 

 synthesis. It seems definitely to be here that the hybrid advantage lies. By 

 the time growth is well under way, the hybrid advantage is already well 

 developed. 



Structural diflferences between inbreds and heterotic hybrids shown by the 

 studies of Burkholder and McVeigh (1940), Weaver (1946), and the members 

 of our laboratory (Whaley et al., 1950; Heimsch et al., 1950; Rabideau ef al., 

 1950) are apparently to be regarded as results of heterosis rather than as 

 causal factors. The evidence suggests that heterosis is concerned primarily 

 with growth processes and that differentiation activities are most likely in- 

 volved secondarily rather than primarily. What seems to be indicated is the 

 assignment of the physiological basis of heterosis to the activity of one or 

 more of the so-called physiologically active substances involved in early 

 growth. 



Much of the apparent hybrid vigor is assignable to these activities only in 

 a secondary fashion. Once the advantage of a larger number of growing 

 centers or of heightened meristematic activity is established, the greater 

 availability of nutrients, the greater amount of protoplasm involved in 

 further protoplasm building, and other general advantages tend to increase 

 the initial differences. To the general evidence in favor of this supposition 

 can be added the specific evidence of the few cases in which the physiological 

 action of particular alleles is known. Where these alleles in combination are 

 responsible for heterosis, they have — when studied in sufficient detail — 

 invariably been shown to be alleles whose action involves basic enzyme or 

 other growth substance activity. 



If we are to make significant headway in understanding the physiological 

 mechanism of heterosis, we shall have to concentrate on a detailed study of 

 the developmental physiology of early growth. Much of the general knowl- 

 edge we already have can contribute toward this understanding if we trans- 

 late it into terms signifying that when we speak of quantitative differ- 

 ences — size, yield, or of rate difTerences — we are really concerned with differ- 

 ences in the level of metabolism. We must recognize that these differences in 

 the level of metabolism are bound to vary against different environmental 

 backgrounds, and where the particular genes involved are associated with 

 different genetic backgrounds. 



Our approach to the heterosis problem has been complicated by common 



