122 Fundamentals of Auxin Action 



produce indistinguishable epinastic responses on the leaves of tomato 

 (Zimmerman, 1951). 



It is difficult to say whether the epinastic activity of a growth 

 regulator is proportional to its auxin activity. There are certainly 

 cases where there is no such relationship, for many compounds which 

 are not auxins at all are rather strong epinastic agents. However, 

 strong auxins are almost always rather strong epinastic agents, de- 

 pending of course on the plant material used for testing. It seems that 

 the phenoxyacetic acids are sometimes stronger epinastic agents than 

 the homologous forms of alpha-proprionic acids or alpha-butyric acids 

 (Zimmerman, 1951). For this reason, the proprionic and butyric acids 

 have been suggested as more appropriate agents for inducing fruit-set 

 in tomatoes than the acetic acid derivatives because fruit-set can be 

 obtained with less epinasty of the leaves (Hewlett, 1950). 



Besides the epinastic curling of leaves, there is commonly a dis- 

 tortion in the outline of leaves which grow out after auxin applica- 

 tions. Such distortions of subsequently expanded leaves aie owing to 

 the inhibition of normal cell division and cell enlargement in the 

 young expanding leaves. 



An especially interesting study of the effects of growth regulators 

 on the meristematic growth of bean leaves has been made by Burton 

 (1947). He observed that when 2-chlorophenoxyacetic, 4-chlorophe- 

 noxyacetic, and 2,4-dichlorophenoxyacetic acids are compared, the 

 2-chloro acid inhibits most strongly the meristem which produces the 

 epidermis and consequently causes a pronounced decline in the size 

 of the intercellular spaces. The 4-chloro acid is most toxic to the plate 

 meristem which gives rise to the islets of mesophyll between the veins, 

 and consequently the development of these areas is retarded. 2,4-dichlo- 

 rophenoxyacetic acid on the other hand, is as toxic to the epidermal 

 meristem as the 2-chlorophenoxyacetic acid, and is as toxic to the 

 plate meristem as the 4-chlorophenoxyacetic acid. Thus the 2,4-D 

 characteristically causes both types of leaf distortion in bean plants. 



The greatest effects from auxin sprays appear on organs which 

 are growing rapidly at or shortly after the time of application. In 

 leaves, the type of distortion resulting from the treatment will depend 

 in part upon the stage of development of the leaf at the time of 

 treatment. The various leaf patterns of bean plants developed after 

 2,4-dichlorophenoxyacetic acid application have been described in de- 

 tail by Eames (1951). Many other authors have found the same types 

 of distortions in other plants (Watson, 1948; Mcllrath and Ergle, 1953; 

 Loustalot and Muzik, 1953). 



Figure 55 illustrates the sequence of leaf distortion patterns as 



