TROPISMS 161 



auxin on the two sides is somewhat greater than that found. 



Another effect may play a role here. It has been found 

 by Gundel (1933) and confirmed by Metzner (1934) that 

 the pH of the lower side of horizontally placed stems is 

 about 0.1 pH unit more acid than that of the upper side. 

 According to Bonner (1934) this would increase the auxin 

 which is in the active, free acid, form, by about 20 per 

 cent, and thus the same total amount of auxin would give a 

 greater growth on the lower side than on the upper. 



There are two apparent disagreements with the theory so 

 far. One is the difficulty raised by Beyer (1932) that Taraxa- 

 cum, Helianthus, and other stems and hypocotyls, just before 

 or after they cease growing altogether, still give geotropic 

 curvature. It might certainly be expected that if the auxin 

 present were not enough to cause growth it would not be 

 enough to cause curvature. Du Buy (1936) has shown, how- 

 ever, that in aging material a certain minimum concentra- 

 tion of auxin must be present before any growth can occur — 

 a "threshold concentration." Beyer's material, just finishing 

 its growth, was evidently in this state, and hence the in- 

 creased concentrations of auxin on the lower side, produced 

 by gravity, would be just sufficient to exceed this threshold 

 and so cause growth to begin again. 



The other disagreement is the fact, discovered by Schmitz 

 (1933) that mature nodes of grasses, which have ceased 

 growing and which yield no auxin, begin to produce some 

 auxin again if they are placed horizontally. This, of course, 

 is connected with the well-known exceptional behavior of 

 mature grass nodes which are still capable of geotropic 

 response. It does not, however, agree with the Cholodny- 

 Went theory because there is here an apparent new forma- 

 tion of auxin upon stimulation. It must be left for the present 

 as an unexplained curiosity. 



D. Geotropism of Roots 



The fact that auxin, as has been shown in IX B, inhibits 

 the growth of roots (in all but the minutest concentrations), 



