170 PHYTOHORMONES 



increased markedly. The free or redistributable auxin dis- 

 appears first from the upper zones and persists longest in 

 the base (u). 



The response of tip and of stump thus both rest on the 

 same phenomenon, namely the redistribution of free auxin. 

 The two responses differ, however, (1) in the amount of 

 light needed to bring them about, (2) in their spectral sensi- 

 tivity, and (3) in their reaction times (Haig, 1935). 



Different phototropic sensitivity in different parts of the 

 spectrum must be due to unequal absorption of light by the 

 light-sensitive system in the coleoptile. Because of the role 

 of auxin in the production of phototropic curvatures, it has 

 been assumed that the greater sensitivity to blue and violet 

 light is due to the absorption of these shorter wave-lengths 

 by auxin. This, however, is not the case; the auxins have 

 no pronounced absorption spectrum in the visible, while 

 the phototropic sensitivity curve shows maxima at about 

 440 and 475 m/>t. Wald and du Buy (1936) have pointed out 

 that the spectral sensitivity of Avena corresponds very well 

 with the absorption spectrum of carotene, of which they 

 showed small amounts to be present in etiolated coleoptiles. 

 Castle (1935) has made a similar comparison for the photo- 

 tropic sporangiophores of Phycomyces. 



The bulk of the curvature in the Avena coleoptile can 

 thus be explained as due to the asymmetrical distribution of 

 auxin, although more extensive data are very desirable. 

 Just how the light can produce this distribution, and what 

 role is played by the different pigments remains to be 

 found out. 



Our knowledge of auxin redistribution under the influence 

 of unilateral light is even scarcer in other objects than in 

 Avena. Only in Raphanus has a complete set of data been 

 collected (van Overbeek, 1933). These may be summarized 

 here. The distribution of auxin diffusing from the apical 

 10 mm. of the hypocotyl in darkness was symmetrical; 

 after 3 hours' continued illumination, however, the light 

 side gave off only 15 per cent, the dark side 85 per cent of 



