Growth Substances 387 



buds farthest from the apex are most inhibited, which does not seem 

 logical if auxin from the terminal bud is the inhibitor. Furthermore, 

 inhibition may sometimes pass upward, although auxin typically passes 

 downward. Snow believes that auxin is either changed into an inhibiting 

 substance (for which Libbert, 1955, has evidence) or stimulates the pro- 

 duction of one and that the greater the distance from the apex, the 

 greater the amount of the inhibitor. Such a substance is not polar in 

 movement and can thus pass into lateral buds as auxin presumably can- 

 not. 



Champagnat ( 1955 and others ) has studied especially the effects upon 

 bud growth of the cotyledons or leaves that subtend them and comes to 

 the conclusion that both inhibitory and stimulatory substances are in- 

 volved. 



Thimann (1954£>) is inclined to think that auxin itself is the major 

 inhibiting influence and that the various apparent objections to this 

 hypothesis may be met by assuming that auxin may sometimes move in 

 an apolar direction and that its effects may be different under different 

 circumstances. He does not rule out the possibility of the existence of 

 other and specifically inhibiting substances. 



Meanwhile other workers have emphasized the importance of nu- 

 trition in apical dominance. Van Overbeek (1938) observed that after 

 removal of the terminal bud the auxin content of the stem decreases. He 

 believes that in some way auxin blocks the passage of nutrients to the 

 lateral buds, which have only a poor vascular connection with the main 

 cylinder. When the amount of auxin is reduced a better connection is 

 established, nutrients enter the buds, and thev begin to develop. 



The nutrient theory of apical dominance is strongly supported by 

 Gregory and Veale (1957). They suggest that the degree of dominance 

 is proportional to the supply of available carbohydrate and nitrogen and 

 that competition among the various buds explains the difference in relative 

 bud growth. Auxin is concerned in this competition since a high auxin 

 content prevents the formation of the vascular connections between bud 

 and vascular cvlinder. 



The underlying mechanism in such an apparently simple phenomenon 

 as bud dominance is still by no means clear. The general fact of domi- 

 nance is established, however, and helps toward an understanding of 

 some of the differences among plants in their bodily patterns, for what- 

 ever determines bud growth determines the shape of the plant. There are 

 usually a great number of potential growing points on an axis but most 

 of them do not develop into shoots, apparentlv because of inhibitory' 

 action mediated in one way or another by growth substances. Plants differ 

 in the degree of this inhibition. For example, in Aster novaeangliae there 

 is one main stem with only a few floral branches at the top. Aster 



