196 PHYTOHORMONES 



The mechanism of this facihtating effect is unknown, but 

 it is of considerable interest as indicating a physiological 

 difference between the actions of the two auxins (cf. X G 

 and Y F for another difference). 



These experiments with auxins were carried out by appli- 

 cation at the apical end of the cutting. The transport of 

 the root-forming hormone was earlier shown to be basipetal 

 and this has been confirmed with pure auxin. 



Bios. There are other substances which are like the 

 sugars in that they must be applied to cuttings from the 

 base, but are unlike the sugars in that the amounts of them 

 necessary to influence root formation are extremely small. 

 In contrast to the sugars, addition to the cuttings of various 

 amino-acids does not promote root formation. Tryptophane, 

 however, is an exception; if applied at the base it gives rise 

 to a large number of roots (u). This is doubtless due to its 

 conversion by the plant to indole-acetic acid. 



The clearest example of a special substance, other than 

 auxin, which is highly active when applied at the base, is 

 that of the yeast-growth-promoting substance bios. Bios, 

 which has been shown by Eastcott (1928) and subsequent 

 workers to consist of a complex of at least 3 factors, is present 

 in various sources including yeast extract itself. In a study 

 of the effect of various additional substances on the forma- 

 tion of roots by pea cuttings it was found that yeast extract 

 definitely increases the number of roots formed in presence 

 of auxin (u). On this account and because ordinary amino- 

 acids have no effect, an attempt was made to test the activity 

 of the various constituents of bios. z-Inositol (Bios I) was 

 inactive. Recently, however, one of the most important of 

 the factors, biotin, has been isolated in a pure state by 

 Kogl (1935, 1936) and a sample of this, from one of the last 

 stages of purification, was tested in the same way (u). This 

 work, which the authors carried out in cooperation with 

 Professor Kogl, who supplied the biotin, showed a remarkable 

 effect, one example of which is given in Figure 55 (cf. also 

 Figure 62). Here the cuttings were treated apically with 



