BUD INHIBITION 209 



was killed by steaming, axillary buds grew out below the 

 dead zone although the part above remained alive ; transport 

 downwards must therefore take place only through living 

 tissue. A similar result was obtained by Child and Bell- 

 amy (1920), by chilUng a zone of the stem with a brine 

 coil (compare also Cooper's experiment, p. 191.) Snow later 

 (1931a) confirmed Harvey's finding, but also obtained some 

 evidence that the inhibiting influence could — at least to 

 some extent — travel up a lateral branch. Experiments of 

 this kind also show that food relations play a subordinate 

 part, since the axillary buds may develop while the top 

 part is also developing. 



It appears, then, that the inhibition travels mainly from 

 apex to base in living tissue, and is exerted principally by 

 the young buds and leaves ; there is also some evidence that 

 it may cross a cut surface and thus may be due to a diffusible 

 substance. 



B. Bud Inhibition Caused by Auxin 



Thimann and Skoog (1933, 1934) pointed out that if 

 inhibition were really due to a substance, then this sub- 

 stance would appear to behave like auxin in the plant. They 

 determined the auxin production in Vicia Faba, using the 

 diffusion method and the Avena test, and found that the 

 terminal bud was the most active auxin-producing center 

 (see Figure 28, p. 62). The leaves produce smaller amounts 

 of auxin, but their production decreases with age (c/. Avery, 

 1935) ; the dormant axillary buds produce almost none, but 

 as soon as they begin to develop they also begin to produce 

 auxin. These results therefore closely parallel the inhibiting 

 power of the different parts discussed above. Finally they 

 removed the terminal bud and applied auxin, in agar, in its 

 place, the auxin being renewed as soon as it had all diffused 

 in from the agar, so as to duplicate the action of the terminal 

 bud in providing a continuous stream. The laterals were then 

 inhibited as completely as those on intact controls (see 

 Figure 58). Their experiments were carried out with par- 



