210 



PHYTOHORMONES 



tially purified auxin from Rhizop'us, but were later (Skoog 

 and Thimann, 1934) confirmed with pure auxin b and 

 indole-3-acetic acid. 



A clear picture of the effect of auxin in bud inhibition is 

 given by Figure 59. The terminal bud was removed from 



etiolated Pisum seedlings, 

 and instead pure lanoline 

 or lanoline containing vari- 

 ous amounts of indole-acetic 

 acid was applied to the apical 

 cut surface. The swellings 

 produced by the auxin are 

 negligible in comparison 

 with the growth of the buds 

 in the controls without 

 auxin. 



As to the quantitative re- 

 lations, Thimann and Skoog 

 found it necessary, in order 

 to obtain complete inhibi- 

 tion, to use an auxin concen- 

 tration several times that 

 which could be obtained 

 directly from the terminal 

 bud, but this difference is 

 doubtless due to the inac- 

 tivation of the applied auxin 

 at the cut and the loss of 

 auxin in non-transporting 

 tissues. In any case the amount of auxin necessary for bud 

 inhibition in Vicia Faba is considerably larger than that 

 which causes maximal stem elongation, for, after decapita- 

 tion, sufficient auxin for stem elongation, but not sufficient 

 for bud inhibition, is produced in light by the remaining 

 leaves. Thus only a growing bud, which is a very powerful 

 source of auxin, can exert appreciable inhibition of lateral 

 buds. 



I 2 3 4 3 6 7 a 9 10 II 12 days 

 Fig. 58. Growth of axillary buds of 

 Vicia Faba. Ordinate, length of bud in 

 mm.; abscissa, time in days. A, plants 

 decapitated, plain agar applied; B, 

 plants intact; C, decapitated, 1600 

 plant units (=650 AE) of auxin ap- 

 plied in agar every 6 hours to apex — 

 application stopped at arrow. (After 

 Thimann and Skoog, Proc. Roy. Soc. 

 B. 114: 317-339, 1934.) 



