BUD INHIBITION 213 



C. Possible Mechanisms of Bud Inhibition 



The auxins thus possess not only growth-promoting and 

 organ-forming ability, but under certain circumstances they 

 may also inhibit growth. Leaving aside those inhibitions 

 which are produced by unphysiologically high concentra- 

 tions, the growth of lateral buds and of roots are both in- 

 hibited by auxin. This raises the question of how all these 

 different actions may be brought about. It seems clear that 

 the auxins control some master reaction in the cell (Thi- 

 mann, 19356), which may then lead to different effects 

 according to the age and position of the cell and the influ- 

 ence of other factors. However, the mechanism whereby 

 any such action could lead to bud inhibition remains com- 

 pletely unexplained. Some hypotheses may be considered. 



In the first place, the production of auxin within the 

 developing bud is probably from some precursor which is 

 stored in it. Thus Went (1934a) found that the presence of 

 buds on etiolated pea seedlings led to the formation of 

 roots, presumably due to auxin storage in the buds. In 

 Solidago the smallest leaf in the bud, weight for weight, pro- 

 duces the most auxin (Goodwin, 1937), and since these 

 small leaves are completely enclosed in the bud their auxin 

 production cannot be directly from photosynthesis (cf. 

 IV B). Now Thimann and Skoog (1934) suggested that the 

 relatively high concentration of auxin reaching the lateral 

 bud from the tip shifts the equilibrium. Precursor ^ Auxin, 

 towards the precursor, thus preventing the lateral buds 

 from forming auxin themselves. Their diffusion experiments, 

 however, showed that very little auxin diffuses out of the 

 undeveloped lateral buds so that the amount actually in 

 them cannot be very large. 



Laibach (1933) and Muller (1935), on the other hand, 

 assumed that the swelling and growth, accompanying appli- 

 cation of high concentrations of auxin to the cut surface, 

 are the cause of the inhibition. According to this view, the 

 inhibition is only a secondary result of the increased stem 



