254 



R. M. Mitir (mil C. Hanscli 



10 



X 



h- 



$ 



o 



ir 

 o 



10 



15 



20 



25 



30 



X 10^ 



MOLAR CONCN. 2,4-D 



Fig. 2. Inhibiting effect of 4-ethyl-3-mercaptobenzoic acid at 1 x 10"^M and 2 X 

 lO-'M on tlie growth of Avena coleoptile sections induced by 2,4-dichlorophenoxv- 

 acetic acid. 



dole-3-acetic acid at 2 X lO'^M and EMBA at 5 X 10-^M were re- 

 placed after 5 hrs. by EMBA at the same concentration and indole-3- 

 acetic acid at 4 X 10-^M. The higher concentration of indole-3-acetic 

 acid resulted in a growth rate nearly the same as that of the tissue in 

 the absence of EMBA. 



To evaluate the chemical grouping responsible for the inhibition 

 brought about by EMBA, the effects of several related compounds 

 were examined and the data are given in Table 3. The S-meihyl eilier 



Table 3. Percentage inhibition of elongation of Avena coleoptiles produced by 

 indole-3-acetic acid in presence of substituted benzoic acids. 



Substituted Benzoic Acid 



3-Mercapto-4-ethyl- 



3-Methyimcrcapto-4-ethyl-.. . . 



2-Mercapto- 



3-Mercapto- 



4-Mercapto- 



4-Ethyl- 



4-Chloro- 



2,4-Dichioro- .... 



Molar 

 Concn. 



5 X 10-6 



5 X 10-6 



2 X 10-^ 



2 X 10-'' 



2 X 10-4 



2 X 10-4 



1 X 10-" 



1 X lO-* 



Molar Concn. of lAA 



2 X 10-7 



65 

 20 

 10 

 15 



50 

 35 

 35 



15 



5 X 10-7 



45 



20 



5 



10 



40 

 30 

 30 

 20 



2 X 10-6 



35 



10 



5 



10 



40 

 20 

 30 

 20 



