PLANT HORMONES 



153 



to the diffusible auxin; only after the inhi- 

 bition is released does the growth reaction 

 come into play. In the case of induction 

 of parthenocarpy, it is also primarily the 

 preparatory reaction that is important. 



But in root formation the situation is 

 different. It has been known for a long 

 time that the formation of roots on stems 

 of plants is a correlation phenomenon. It 



the subsequent rooting response of the 

 stem. This can be compared with the pre- 

 paratory reaction in growth. But later the 

 auxin takes part in the more specific root- 

 forming reaction. This reaction is more 

 specific than the growth reaction, so that 

 certain substances like phenyl acetic acid 

 can produce growth but not root formation. 

 This leads to the following scheme : 



— COOH 

 H • CH2 • COOH and || | Acetic and benzoic acid 



No reaction 



CH3 • COOH Cyclohexane acetic acid Preparatory reaction only 



^ ''^— CH2 • COOH Phenyl acetic acid 



// \ 



\, 



-CH„ • COOH Indole acetic acid 



Preparatory reaction and growth 

 reaction 



Preparatory reaction and growth 

 reaction and root forming reac- 

 tion 



^ 



N 



/ 



was shown that some specific hormone-like 

 substance which was present or formed in 

 leaves and buds was responsible for this 

 effect (Went 1929), and finally it was 

 found that in many cases this hormone-fac- 

 tor is identical with auxin (Thimann and 

 Koepfli 1935). Now root formation is a 

 typical organization involving induction of 

 meristematic activity and re-differentiation 

 of cells. Therefore it was of special in- 

 terest to investigate in what way auxin 

 affects root formation. 



As a first point I must mention that a 

 large number of factors proved to be in- 

 volved in root formation. We can list the 

 following : sugars, auxin, biotin, vitamin Bi, 

 amino acids, carotene, rhizocaline. They 

 are all necessary in one or another link of 

 the chain of reactions leading towards root 

 formation. If the conditions are carefully 

 chosen, any one of the preceding substances 

 may increase root formation. Still, under 

 conditions of auxin deficiency all but one of 

 the plants investigated respond to the ap- 

 plication of auxin with root formation. 



It was found that the effect of auxin on 

 root formation is dual just as in the case of 

 growth (Went 1939c). During the first 

 hours of application auxin only increases 



It shows that with increasing complexity 

 of the molecule more and more functions 

 can be performed by these substances. 

 Therefore it can be predicted that the com- 

 plexity of the auxin-a molecule is connected 

 with still other functions inside the plant, 

 which have not been discovered as yet. 



In all cases which have been analyzed so 

 far: cell elongation, bud inhibition, and 

 root formation, the preparatory reaction 

 could be identified with the effect of dif- 

 fusing or free-moving auxin on the translo- 

 cation of other growth factors necessary for 

 the respective reactions. In this way the 

 specificity of the effect of auxin is only 

 connected with the specificity of the other 

 or co-growth factors and the specific re- 

 activity of the tissues. 



Once we have proceeded so far with the 

 analysis of the auxin effect, I am tempted 

 to outline an hypothesis concerning the 

 reaction in which auxin participates to 

 produce growth. Is it possible to come to 

 some concrete picture of part of the chain 

 of reaction which leads to growth ? At one 

 end of the chain we feed in simple sub- 

 stances, chemically definable, like auxin, 

 sugars, vitamins, etc. At the other end we 

 obtain growth, which is not as yet de- 



