Plant Qrowth'Suhstances 



Four other furane derivatives tested by Thimann were 

 found to be completely inactive by the two Avena tests. 



For synthesis, and effect on plants, of coumaryl-2(3 ?)— 

 acetic acid, see V. Titoff, H. Miiller, and T. Reichstein, 

 Helv. Chem. Acta, 1937, 20, 883 (from B.C. A). 



A number of discrepant results have been reported about 

 the "activity" or "inactivity" of substances, in relation to 

 their effects on growth. Substances judged as "inactive" or 

 only slightly active, by workers using one test, have been found 

 highly active by later workers using a different test-plant and 

 technique, and possibly evoking a different type of physiolo- 

 gical effect on cell-growth, proliferation or differentiation. 



For this reason, amongst others, it has not seemed worth 

 while to list and classify the numerous substances of which 

 the physiological activity has been tested on plants. 



A comparison of three plant tests (oat-coleoptile, etiolated 

 pea stem, and "cylinder") has been made by Haagen-Smit 

 and Went (1935), using a large number of synthetic sub- 

 stances, mostly of the indolyl and phenyl types. No naphtha- 

 lene or higher aryl compound was tested in this series. AllO' 

 cinnamic acid, ethyl wo-propyl indole- acetate, and 5-methyl- 

 indole-acetic acid were reported to be equally as eflFective as 

 3-indole-acetic acid in the (Went's) pea test. Examined by 

 the other two tests, no substance tried was more than a fifth 

 as effective as indole-acetic acid, and only indole-3 -pyruvic 

 acid, i-methyl-indole-3-acetic acid, 5-methyl-indole-acetic 

 acid, and a//o-cinnamic acid reached that degree of effective- 

 ness. Though so eflFective in the pea and cylinder tests, allo- 

 cinnamic acid and i-methyl-indole-acetic acid were only 

 weakly effective in the Avena coleoptile test, in which they 

 produced an "abnormal" type of bending. Ordinary cinnamic 

 acid was inactive in all three tests. 



These authors wrote: "That steric conditions play an 

 important part in the activity of these substances was con- 

 firmed by the activity of m-o-methoxy-cinnamic acid, whilst 

 the trans compound was inactive. In the meantime, the sub- 

 stitution with the methoxy groups has greatly diminished the 

 activity." Cw-o-methoxy-cinnamic acid, however, was 



84 



