THE GROWTH OF ROOTS 145 



this amount with dextrose agar. With this method he showed 

 that, in Zea "caragua,'' the concentration of auxin decreases 

 rapidly from the tip of the root towards the base, and sub- 

 sequently (19336) also confirmed Hawker's finding that more 

 auxin diffuses out of the lower than the upper half of root 

 tips which had been placed horizontally. 



The presence of auxin in Avena roots was proven in 

 another way by Thimann (1934), who acidified the roots, 

 extracted directly with chloroform, and subsequently tested 

 the extract, freed from chloroform and dissolved in agar, 

 upon coleoptiles. The concentration of auxin decreased from 

 the tip towards the base, as may be seen from Figure 30, p. 68, 

 which summarizes his results. Boysen Jensen (1936a) con- 

 firmed that auxin could be extracted from roots by chloro- 

 form and found that more was extracted from the lower half 

 of horizontally placed roots than from the upper. 



The above experiments show that auxin is present in the 

 root, and especially in the tip, but do not make it clear that 

 it is actually produced there. Thimann (1934), using 10 mm. 

 Avena root tips, compared the auxin obtainable by chloro- 

 form extraction with that obtainable by diffusion into dex- 

 trose agar, and stated that ''the only fair conclusion seems 

 to be that growth substance is not produced in the root tip 

 but merely accumulates there." However, Cholodny (1934) 

 found that, if suppHed with dextrose gelatin, root tips of 

 Zea Mays continue to yield auxin for 5-6 hours and Nagao 

 (1936) obtained the same result with short (2 mm.) tips of 

 Avena roots. The researches of Van Raalte (1936) go far 

 towards clearing up this question. Using Vicia Faha roots, 

 he found that addition of salts alone to the agar gives very 

 good yields of auxin, but addition of dextrose isotonic with 

 the salt is still more effective. Thus the dextrose acts in 

 two ways — osmotically, by increasing the diffusion out of 

 the root tip, and nutritively, by maintaining the root tip 

 during the experiment. Boysen Jensen (1933a) had pre- 

 viously found that mannite was almost as effective as glu- 

 cose and concluded from this that its action could not be 



