162 PHYTOHORMONES 



makes it clear in what direction we must look for an ex- 

 planation of the geotropism of roots. Unfortunately the 

 facts are still partly in dispute. The experiment of Cholodny, 

 mentioned above (1924), indicates that auxin is necessary 

 for the response of the root to gravity (cf. Keeble, Nelson, 

 and Snow, 1931; Keeble and Nelson, 1935). On the other 

 hand, isolated roots in culture appear neither to contain 

 nor to produce any auxin, and yet they are still able to curve 

 geotropically (Fiedler, 1936). A possible explanation for 

 this discrepancy may be sought in Fiedler's observation 

 that his roots developed traces of chlorophyll when in agar 

 media. This, as shown in IV A and B, would lead to the 

 continuous production of traces of auxin, which might suf- 

 fice for geotropic response, although not enough auxin would 

 accumulate to be detectable by extraction methods. Since 

 it is always difficult to prove a negative, i.e. the absence 

 of auxin, we may for the present consider only the positive 

 evidence. 



It is important that, although Keeble, Nelson, and Snow 

 (1931) state the contrary, the growth rate of the root does 

 not depend on its position in regard to gravity, i.e. there is 

 no ''geo-growth reaction" (Cholodny, 1932; Navez, 1933). 



Hawker (1932) investigated the distribution of auxin in 

 geotropically stimulated root tips. Zea Mays roots were 

 placed horizontally and after 3 hours the tips were cut off 

 and separated into upper and lower halves. These halves 

 were then applied to half the cut surface of unstimulated 

 root stumps, and the curvature of the stumps measured. 

 The half of the tip which had been the lower during stimula- 

 tion gave three times as big a curvature as that which had 

 been the upper. Hence auxin passes to the lower side of 

 the root under the influence of gravity and thus the auxin 

 distribution in roots is the same as that in coleoptiles. 

 Direct confirmation of this by the diffusion method was 

 obtained soon after by Boysen Jensen (19336). He allowed 

 the auxin from the upper and lower sides to diffuse into 

 separate blocks of dextrose agar. These, when tested on 



