158 PHYTOHORMONES 



the geotropism of Bryophyllum stems were tentatively in- 

 terpreted on the basis of "specific geotropic substances or 

 hormones . . . having the tendency to collect on the lower 

 side of a horizontally suspended stem." His subsequent 

 change of view is discussed in II E. 



Some experiments of Gradmann (1925) were at first 

 thought to be in opposition to the above theory. He split 

 stems of Lahiatae longitudinally and placed them horizontally : 

 those with the cut surface uppermost curved normally and 

 showed considerable growth, but those with the cut surface 

 downward showed little growth or curvature. A large num- 

 ber of variations on this type of experiment gave similar 

 results, and his general conclusion was that, in an intact 

 organ, the influence of gra\dty produced geotropo-hormone 

 only in the lower half and this then accelerated growth of 

 this half. Isolated upper halves would thus have no such 

 hormone. His interpretations have, however, been criti- 

 cized both by Cholodny (1927, 1931) and Dolk (1930) on 

 the ground that he completely neglected the effects of the 

 wound, which are very great (see section J). 



It remained for Dolk (1929, 1929a, 1930, 1936) to show 

 that the Cholodny- Went theory applies completely and in 

 its simplest form to the case of geotropism in Avena. He 

 first showed that neither upon rotating coleoptiles hori- 

 zontally nor upon returning them to the vertical position 

 was there any change in growth rate, i.e. there is no ''geo- 

 growth reaction" (see also Cholodny, 1929a). This was 

 afterwards confirmed by growth measurements made during 

 the actual geotropic curvature by Navez and Robinson 

 (1933). Correspondingly the total amount of auxin diffusing 

 out of coleoptile tips is the same whether they have been 

 placed horizontally or vertically. However — and this is 

 the important point — the amount diffusing out of the lower 

 half, when they have been placed horizontally, is 62.5 per 

 cent, and that diffusing out of the upper half 37.5 per 

 cent, of the total. Exactly the same distribution was found 

 for Zea Mays coleoptile tips. (Navez and Robinson [1933] 



