386 Morphogenetic Factors 



Dominance, Inhibition, and Stimulation. Whatever the mechanism for 

 the differential distribution of growth substances may be, there is no ques- 

 tion that it has much to do with the effect of one part on another and 

 thus with growth correlation in general (Chap. 4). 



Meristematic regions are commonly centers of auxin production, and 

 from them this substance moves to others where it may either stimulate 

 or inhibit growth. The most familiar example of this is the so-called 

 dominance of apical buds. It is well known that in most cases, if such a 



Fig. 18-10. Curved primary root of a lu- 

 pine seedling, showing lateral roots grow- 

 ing from convex sides only. (After Noll.) 



Fig. 18-11. Cross section of a curved root 

 like that in Fig. 18-10, showing that the 

 lateral root, in its course through the 

 cortex, bends toward the convex side, X. 

 (After Noll.) 



bud is present, the buds below it will not grow but that they will do so 

 if the apical bud is removed. Thimann and Skoog (1934) showed that if 

 this apical bud is cut off and paste containing indoleacetic acid placed 

 on the stump the lower buds will not develop, though in the controls, 

 with paste alone, they do so. Such a result has been confirmed many 

 times and has led to the widely held view that auxin inhibits growth of 

 the lower buds and thus produces the dominance of the apical one. 



Snow, in a series of papers (1937, 1940, and others), calls attention to 

 some facts that are not easily explained on this hypothesis. He finds that 



