GROWTH SUBSTANCES FOR GEOTROPISM 207 



amount of growth substance as the excised tip of the Avena 

 coleoptile. 



A further ckie as to the chemical nature of the growth hormone 

 present in the roots of Yicia Faba was obtained by Heyn (1935) 

 from determinations of the coefficient of diffusion of the sub- 

 stance through a series of agar blocks in contact. The coefficient 

 of diffusion was computed to be 0.391 as compared with a similar 

 value of 0.409 for the growth hormone extracted from Avena 

 coleoptiles and a theoretical value of 0.416 for auxin a. This 

 would indicate that "the hormone is almost certainly identical 

 with ordinary auxin." 



Extraction of Growth Substance from Roots. — Other investi- 

 gators have obtained data which throw doubt upon the presence 

 of growth substance in the root tip. Biinning (1928) carried out 

 numerous growth measurements on different roots and concluded 

 from his experiments that the stimulus of a wound can have a 

 retarding as well as a promoting effect upon the growth of the 

 root and, furthermore, that the root tip does not give off growth 

 substance. Gorter (1932) came to the same conclusion as a 

 result of experiments designed to demonstrate its presence in root 

 tips of Zea mays and Pisum. Tests were made by placing root 

 tips unilaterally upon decapitated Avena coleoptiles or by the 

 method of extraction of growth substance from the root tips into 

 agar blocks or moist sand. It was not possible by these methods 

 to demonstrate that any growth substance was given off by the 

 root tips. 



Boysen Jensen (19336) discovered that the extraction of growth 

 substance from the root tip is easily accomplished when 10 per 

 cent dextrose is added to the agar. He thought originally that 

 the sugar in the agar had a plastic nutritive effect upon the root 

 tip. However, the dextrose may be replaced by mannite, which 

 is scarcely detectable in a root tip. The effect might be a physical 

 one, influencing diffusion in some way, though Cholodny (1934) 

 is of the opinion that the nutritive idea is correct, because the 

 excretion of growth substance into nutritive agar is continued 

 for a relatively long time. Further work by van Raalte (1936) 

 using Vicia Faba root tips has shown that not only does more 

 auxin diffuse out into the agar made up with 10 per cent glucose, 

 but that there is 3.3 times as much auxin present in the root tips 

 which have been in contact with glucose agar (chloroform extrac- 



