Chemical Nature of Growth Regulators 1 55 



(1949). They established that for this family of auxins at least one 

 position ortho to the side-chain must be unfilled for effective auxin 

 activity. Thus while 2,4-D is highly active, 2,4,6-T is essentially in- 

 active. They pointed out that the substitutions in the 2 and 4 positions 

 would activate the 6 or ortlio position, and concluded that the ring 

 of the auxin must combine with some substance at that position. Sub- 

 sequently it was discovered that certain benzoic acid derivatives also 

 exhibit auxin activity (Bentley, 1950), and it was startling to find that 

 in these substances the ortho positions must be filled by chlorine, 

 methyl or other groups for auxin activity to exist. The strongest auxin 



10 X 10'* M 



CONC. OF AUXIN 



Fig. 69. The effects of various concentrations of some chlorinated phenoxyacetic acids 

 upon growth in the Avena straight-growth test (from Muir et al, 1949). 



