168 



PHYTOHORMONES 



TABLE XIV 



Attempts to relate the phototropic curvatures quantita- 

 tively to the auxin distribution have been few. In the upper 

 zones, auxin is not the limiting factor, so that an increase 

 of auxin on the shaded side can cause little or no increase 

 in growth rate. Thus Went (1928a) concluded that most 

 of the curvature in the upper zones must be due to a decrease 

 in the growth of the lighted side, and calculated that, in 

 order to explain the strongest phototropic curvatures from 

 the observed auxin distribution, the growth of the lighted 

 side must almost cease. This was confirmed kinematograph- 

 ically by du Buy and Nuernbergk (1929, 1930). They 

 found only slight acceleration of the dark side, and a very 

 marked slowing down, or complete cessation, of growth on 

 the lighted side. These results are the opposite of those of 

 Boysen Jensen (1911, 1913) who concluded that the curva- 

 ture was caused by a change in growth rate of the dark side. 

 On the other hand, in the lower zones, where auxin is lim- 

 iting growth there is both acceleration on the dark side 

 and retardation on the lighted side. 



Unlike geotropism, in which Dolk found that within an 

 hour after returning to the vertical position the auxin dis- 

 tribution was again normal, in phototropism the unequal 

 distribution persists for several hours after illumination 

 (Went, 1928). Correspondingly, if the plants are rotated 

 on a clinostat so that gravity does not interfere with the 



