p. W. ZIMMERMAN 183 



Some but not all of the substituted phenoxy acids have formative 

 effects when applied to Kalanchoe plants. Two striking differences ap- 

 peared when 4-chlorophenoxyacetic acid and 2,4-dichlorophenoxyacetic 

 acid are compared. The former causes a change in correlation of organs, 

 hyponasty of leaves, modification of leaf pattern, and pronounced mon- 

 strosities at the terminal part of the plant. In contrast, 2,4-D causes 

 epinasty of leaves and root formation of treated stem tissue but no 

 monstrosities. Similar differences appeared when these two chemicals 

 were tested on Turkish tobacco plants. When applied to tomato plants, 

 however, the effects are nearly indistinguishable (14). Such cases as these 

 make it appear that the molecular configuration of the substance must 

 in some way interlock with the mechanism of the species to bring about 

 a given response. The genetic constitution of the species — perhaps the 

 gene — appears to be the determining factor. This is mentioned because 

 tobacco and tomato are closely related species, and yet they respond 

 differently when tested with these substances. 



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