ZIMMERMAN: FORMATIVE INFLUENCES 101 



Attention has been called to the fact that though /?-naphthoxyacetic acid 

 and its higher homologs have a formative influence on growth, there are quali- 

 tative differences in responses induced with these compounds (6). It should be 

 pointed out also that different species bring out further qualitative differences. 

 y8-Naphthoxyacetic acid, for example, modifies the leaves of Turkish tobacco 

 (Nicotiana tabacum) while ;8-naphthoxypropionic acid has little or no effect 

 on the pattern of leaves of this species. 



Substituted phenoxy compounds. There are many active substituted 

 phenoxy compounds. The activity appears to be related to the kind, number, 

 and position of substituted groups in the benzene ring. In general, halogen sub- 

 stitutions bring about greater activity than methyl, amino, or nitro groups. 

 With a single halogen group substitution the para position is more effective 

 than the ortho. However, 2,4-dichlorophenoxyacetic acid is more effective than 

 either o- or /?-chloro phenoxyacetic acid (4, 7). 



Nitro group substitutions do not activate the phenoxy molecule except in 

 the meta position. The amino group activates the molecule when substituted in 

 the para position. The chlorine atom in the ortho, meta, or para position acti- 

 vates the phenoxy molecule. These comparisons are shown in table 2. 



Table 3 shows the comparative activity of non-substituted and chloro-sub- 

 stituted phenoxy compounds. Phenoxyacetic acid does not modify the pattern 

 of leaves though its propionic and butyric acid homologs do. This is in contrast 

 with ortho, para, and 2,4-dichlorophenoxy compounds where the acetic acid 

 form modifies leaA^es but the corresponding propionic and butyric acid homologs 

 do not. These are in further contrast with w-chlorophenoxy and 2,4,5-trichloro- 

 phenoxy compounds where neither the acetic acid forms nor their higher homo- 

 logs modify leaves though all are active for cell elongation. In all active phenoxy 

 compounds the nucleus of the molecule was linked to the chain at the alpha 

 carbon atom. Comparable beta linkages made inactive compounds. 



2,4,6-Trichlorophenoxyacetic acid and the propionic acid homolog did not 

 cause cell elongation but had a slight capacity to modify organs. 2,3,4,6-Tetra- 

 chlorophenoxyacetic acid and 2,3,4, 5,6-pentachlorophenoxyacetic acid were in- 

 active. 



When the ortho and para positions were substituted with chlorine or a 

 methyl group making 2,4-dichlorophenoxyacetic or 2,4-dimethylphenoxyacetic 

 acids, the compounds were active for cell elongation and for modification of 

 leaves. The higher homologs, however, were different. a-(2,4-Dichlorophe- 

 noxy) propionic and butyric acid were not active for modification of organs 

 while the corresponding methyl-substituted compounds were active. This pe- 

 culiarity is illustrated in table 4. 



The formative influence of 2,4-dichlorophenoxyacetic acid is illustrated in 

 figure 1 B and C. When the Nicandra physalodes plant on the right was ap- 



