The influence of growth substances upon sulphydryl compounds 



acids to the growth regulator activity of halogenated benzoic acids. Dickens 

 (1933) has reported that iodoacetic, bromoacetic, and chloroacetic acids 

 show differential activities against sulphydryls of 100:60:1. Triiodo-, 

 tribromo-, and trichlorobenzoic acids show differential activities in inducing 

 stem abscission of 100:1 1 : 5 (Weintraub et al., 1952). 



</^7^~*M 



2 1- Bx10-''^V\ 



Concn. of TIBA 



Figure 5. Cysteine disappearance with TIBA. Conditions as in Figure 4. 



5x10~'*^ 



I 



Figure 6. Time course of cysteine disappearance with TIBA . Conditions as in Figure 4. 



Looking for a moment at TIBA as a growth substance, there are conflicting 

 lines of evidence as to whether it acts competitively with auxin. In brief, it is 

 not at all clear that TIBA competes with auxin for the same activity site. 

 Hence it would be mistaken to interpret the present experiments with TIBA 

 as evidence concerning the site of auxin attachment. 



275 



