216 PHYTOHORMONES 



why some buds have a greater tendency to develop than 

 others. Thus in woody twigs after decapitation, it is fre- 

 quently the first bud below the terminal which develops 

 (c/. Reed and Halma, 1919), while in herbaceous plants the 

 lowest bud often develops under these conditions. It is easy 

 to see how this might be related to the distribution or storage 

 of bud-growth promoting factors. Snow (1931) considered 

 that, in Pisum, the inhibiting action of the terminal bud is 

 greatest on the buds farthest from it, i.e. the inhibition in- 

 creases with distance. The fact is, more probably, that the 

 tendency to grow out is greatest in the basal buds, so that 

 these buds show the greatest difference in growth between 

 decapitated and intact plants. In this connection reference 

 may again be made to the poorly-transported auxins, such as 

 indene-3-acetic acid; Thimann (19356) has shown that the 

 inhibition caused by this compound decreases very rapidly 

 with increasing distance from the point of application. In 

 callus formation, the activity even of readily transported 

 auxins like auxin a and indole-acetic acid decreases very 

 rapidly with distance (of. XIIIc). 



The mechanism of bud inhibition can probably not be 

 understood until we know more about the fundamental 

 mechanism of auxin action on the cell, and the role of other 

 factors in bud growth. It is clear that many of the factors 

 necessary for bud development, such as water, food, auxin, 

 and the other substances, could, under the right conditions, 

 become limiting factors in bud growth (cf. e.g. Denny, 1926; 

 Moreland, 1934). However, the experiments in this chapter 

 are concerned principally with the effects of auxin and it 

 will be clear to the reader that the typical ''apical dom- 

 inance" is an auxin phenomenon. 



