TROPISMS 171 



the total. In hypocotyl cylinders supplied apically with 

 auxin agar the same redistribution was observed; the length 

 of these cylinders was only 6 mm. and thus one may calculate 

 that the observed auxin difference (37 per cent on light and 

 63 per cent on dark side) corresponds quantitatively with 

 the greater difference in the hj^pocotyl tips 10 mm. long. 

 Van Overbeek also ascertained that the lateral auxin trans- 

 port is induced only by wave-lengths less than 5460 A.U., 

 i.e. by those causing phototropic curvature. 



The phototropic curvature of Phaseolus epicotyls is also 

 accompanied by asymmetric distribution of auxin. Boysen 

 Jensen (1936a) determined the auxin in the light and dark 

 halves of the epicotyl by the chloroform method and found 

 about one third of the total on the Ughted side and two 

 thirds on the dark side. 



As already mentioned, the way in which unilateral light 

 brings about this lateral transport is not understood. Light 

 has no direct effect on the longitudinal transport of applied 

 auxin through cylinders of A vena or Raphanus (see VI C), so 

 that the explanation of du Buy (1933), based on decreased 

 transport rate on the lighted side, cannot be valid. As 

 with gravity, attempts have been made to ascribe the curva- 

 tures to electric potentials. Brauner (1927) found that 

 unilateral illumination of Hordeum nodes caused the light 

 side to become negative to the dark side. Waller (1900) 

 and Bose (1907) had found similar effects. This potential 

 would be in the right direction to produce an electrolytic 

 transport of the anion of auxin, but there is no evidence 

 that it does so. It is, however, a remarkable coincidence 

 that both in geotropism and in phototropism the auxin 

 moves to the side which becomes positive, as would be 

 expected from its acid properties. 



G. Light-Growth Reactions 



As stated above, the asymmetric distribution of auxin 

 is only one of the factors operating to produce photo- 

 tropic curvatures. There are a number of other effects 



