190 PHYTOHORMONES 



C. Nature of the Root-Forming Substance 

 The use of this method made it possible to investigate 

 the chemical nature of the root-forming hormone (Thimann 

 and Went, 1934). In the first place, tests on a number of 

 pollens and other natural products showed that the root- 

 forming hormone occurs almost always together with auxin 

 (the latter being determined on Avena). In some cases 

 there was even a good quantitative parallehsm between 

 root-forming and growth-promoting acti\dty; the extract of 

 Rhizopus medium, rich in auxin, was also very rich in root- 

 forming hormone, and was therefore worked up. They 

 found that the root-forming substance was extractable by 

 organic solvents only from acid solutions and is therefore 

 an acid; its dissociation constant, determined by shaking 

 out from buffered solutions, was about 2.10-^. The dis- 

 tribution between different solvents was the same as that 

 of auxin. The activity was readily destroyed by oxidizing 

 agents and followed that of the auxin throughout the various 

 stages of purification, even through vacuum distillations. It 

 was therefore clear that the substance was either identical 

 with, or very closely related to, auxin itself. 



It was then found that auxin b and, later, synthetic indole- 

 3-acetic acid (Thimann and Koepfli, 1935; Kogl, 1935) were 

 as active in root formation as the purest Rhizopus prepara- 

 tion. This provides final proof that one, at any rate, of 

 the hormones causing root formation is identical with auxin. 

 The names ''rhizocaline" and "rhizogene," in so far as they 

 really refer to the action of auxin, can therefore be dropped. 

 The evidence against the identity of the root-forming sub- 

 stance with auxin was that the root-forming and growth- 

 promoting activities of various natural preparations were 

 not quantitatively parallel. The explanation for this must 

 lie in the influence of secondary factors (c/. XI E). 



As to the path of transport of the hormone in cuttings, 

 it seems that it moves through the living cells of the phloem, 

 since Cooper (1936) showed that ringing, after appUcation 

 of auxin at the top, prevents rooting at the base. The move- 



