On form and function of plant growth substances 



In the pea test a-methylene-phenylacetic acid (XIX) at lower concentra- 

 tions is somewhat more active than phenylacetic acid, whereas the iso- 

 propylidene-derivative is distinctly more active than the parent acid (see 

 Figure 7). That these activities cannot be explained by a possible hydro- 

 genation in the tissue to the corresponding alkyl-derivatives must be 

 deduced from the fact that then in the case of «Vopropylidene-phenylacetic 

 acid the antagonistically active uopropyl-derivative would result. So 

 apparently an a-hydrogen atom is not essential here. 



The great difference in physiological effect of the z'^opropyl- and isopropyl- 

 idene-phenylacetic acid illustrates once more how sensitively the primary 

 active site responds to seemingly small differences in strvicture. Judging from 

 the ultra-violet absorption spectra, the double bond in the unsaturated acid 

 is co-planar with the benzene nucleus and, in that case, as to its spatial form 

 the acid is to be compared with the active 2-substituted a-naphthoic acids 

 (Veldstra, 1952). We have no plausible explanation to offer, however, for 

 the fact that the ethylidene- and «-propylidene- phenylacetic acids are 

 practically inactive. Both of them may exist as cis- and trans- forms. These 

 have not been separated as yet, and a more conclusive discussion of these 

 interesting relations will have to be postponed for that reason also. 



DITHIOCARBAMATES 



A most interesting and important contribution to the study of structure- 

 activity has recently been given by van der Kerk, van Raalte, Kaars 

 Sijpesteijn, and van der Veen (1955), when during work on fungicides a 

 notable growth substance activity was found for carboxymethyl dimethyl- 

 dithiocarbamate (XXIII). 



^N— C— S— CH2COOH ^N— C— O— CHoCOOH 



H3C I H3C I 



XXIII XXIV 



HgC HgCg. 



^N— C— S— CHoCOOH ^N— C— S— CH.COOH 



H3C ^ H5C2 ^ 



XXV XXVI 



CH, 



H3C. 



H3C 



^3 



I 

 ;N— C— S— C— COOH 



II I 



S CHg 



XXVII 



The activity first reported concerned only epinastic curvature and leaf 

 deformation in tomato plants, whereas the compound also proved to be 

 weakly active in the pea test and in the straight growth test. It was con- 

 vincingly shown that activity in this type of compound is connected with 



V 127 



