160 PHYTOHORMONES 



that gravity has no effect on the total production of auxin, 

 he attempted to correlate quantitatively the unequal auxin 

 distribution with the unequal growth distribution. The 

 unequal auxin distribution begins half an hour after the 

 plants are placed horizontally and the curvature begins at 

 the same time. He showed that for these objects, as for 

 Avena, straight growth is proportional to auxin applied, 

 and he could thus calculate how much growth difference 

 between the two sides of the geotropically curved organ 

 would be expected from the observed auxin difference. He 

 found that from 70 to 100 per cent of the growth difference 

 may be accounted for by the auxin difference. It is possible 

 that the pH difference (see below), if real, plays also a 

 minor role in such curvatures, but on the whole the con- 

 firmation of the Cholodny-Went theory is excellent. 



Van der Laan (1934), in an investigation on the effect 

 of ethylene, confirmed the older observation that in Vicia 

 Faba seedlings exposure to ethylene causes downward bend- 

 ing of the shoots and found that this is correlated with an 

 unequal auxin distribution such that the upper side has 

 more than the lower. This, of course, is to be distinguished 

 from the increase of auxin in the lower half of horizontally 

 placed stems in pure air. The curious behavior of these 

 seedlings in ethylene is thus explained. 



Boysen Jensen (1936a) has applied the chloroform extrac- 

 tion method of Thimann (see IV ^) to the direct determina- 

 tion of the auxin in the upper and lower halves of geotrop- 

 ically stimulated stems of Phaseolus and Vicia Faba. He 

 finds that the distribution between the two sides is of the 

 same order as that found by Dolk and Dijkman. However, 

 the difference in auxin does not seem numerically great 

 enough to account for the difference in growth. Boysen 

 Jensen explains this by reference to a suggestion of Thimann 

 (1934) that some of the auxin in the plant is present in a 

 bound non-diffusible form, but that this is extracted by 

 chloroform along with the free or active auxin (see discus- 

 sion in VIII F). Hence the real difference between the free 



