CONTROL BY LIGHT 221 



addition, however, one must note that even the major proponent of 

 the flavine hypothesis, Galston (1959), taking the position tliat the 

 l)ulk of the evidence is against carotene, points out that such evi- 

 dence does not automatically favor the fla vines. Other yellow com- 

 pounds such as the pteridine?;— which have been suggested as visual 

 photoreceptors in some organisms (Forrest and Mitchell, 1954) — 

 have been relatively little investigated but cannot be excluded from 

 consideration. 



Compare for a moment the success of the search for the photo- 

 periodic pigment with the failure of that for the phototropic. The 

 approaches have been similar— wherein lies the difference? This is 

 easily answered. Investigators of the photoperiodic pigment soon 

 learned that thev had the good fortune of dealing not with one but 

 ^^'ith two related action spectra— the red-peaked spectrum for the 

 reaction in one direction and the far-red-peaked spectrum for its 

 reversal. Basically it is this circumstance that has made further 

 advances possible, and nothing like it seems to be available for 

 students of blue action spectra. Since one cannot very well suggest 

 that reversibilitv be discovered where it does not exist, there are 

 few if any lessons for the future here. Faced with a multiplicity of 

 yellow compounds, what is the blue-light physiologist to do? 



There is still, of course, room for more action spectra, for more 

 absorption spectra, and for careful studies of the responses corre- 

 lated with varying pigment contents. Work of this kind may yet 

 prove adequate to the task. But it is now time to recall that the 

 light physiologist has another interest besides the identity of the 

 absorbing pigment and that is the nature of the events immediately 

 following light absorption. If one knew what the immediate con- 

 sequences of light absorption were, one might then be able to decide 

 wliicli of the several possible pigments was capable of bringing 

 them about. This approach, while not yet particularly fruitful, has 

 had a strong influence on the study of phototropism. 



It has been generally accepted for many years that phototropic 

 bending in higher plants is due to an unequal distribution of growth 

 hormone— auxin— in the illuminated axis. This is probably not an 

 immediate result of light absorption. It has not been demonstrated 

 to the satisfaction of all concerned whether it results from an un- 

 equal destruction of auxin, from a light-induced movement of auxin, 

 from an unequal photoinhibition of auxin synthesis, or from some 

 combination of these or other mechanisms. That auxin is involved, 



