32 PLANT GROWTH SUBSTANCES 



TABLE 1 



List of compounds whose activity is here quantitatively compared for the 



first time 



Name Activity* 



(Indoleacetic acid = I go) 



Benzofulvenecarboxylic acid 0.5 



Benzofulvenehydroxycarboxylic acid o 



Indene-3-gIycolic acid ethyl ester o 



Phenylthioacetic acid O 



2-methyl-4-chlorophenylthioacetic acid 200 



Methylbenzoquinonethioacetic acid 0.2 



Trimethylbenzoquinonethioacetic acid o 



Phenoxyacetic acid ca. o 



2-chIorophenoxyacetic acid 4 



4-chlorophenoxyacetic acid 200 



2,4-dichlorophenoxyacetic acid 800-1,200 



2,4,5-trichlorophenoxyacetic acid 500 



2,3,4,6-tetrachIorophenoxyacetic acid I 



Pentachlorophenoxyacetic acid ca. 'y 



Bis- (2, 4-dichlorophenoxy) -acetic acid 1.5 



2,4-difluorophenoxyacetic acid 12 



2,4-dichlorophenylacetic acid 15 



Racemic 2,4-dichlorophenoxy-Q:-propionic acid . . 600 



D-2,4-dichlorophenoxy-Q;-propionic acid .... i , 200 



i,4-naphthoquinone-2-butyric acid O 



4-bromophenylbutyric acid 15 



4-hydroxyphenyl-7-ketobutyric acid o 



Naphthalene-i-acetamide (approx.) 10 



*Based on slit pea stem curvatures. 



and activity is reduced (15). It seems to the present author that the 

 evidence for this conception is far from complete though it is in principle 

 reasonable. 



Enhancement of Auxin Action. — To complete the factual picture one 

 other phenomenon should be mentioned. Many compounds, inactive 

 by themselves, increase the apparent activity of the auxins. Went, who 

 discovered this phenomenon, studied it by pretreating the pea sections 

 with the inactive substance and subsequently placing them in a low 

 concentration of the auxins (16). He views the growth process as taking 



