Photoreception 



409 



pigment of a photosensitive system is highly efficient in absorbing blue light, 

 then the same color of light will be most effective in producing a response 

 in that system. For example, the phototropic bending of the oat shoot, 

 Avena, is most effectively stimulated by blue light of 440 fx wave length. 

 Other regions of the visible spectrum, adjusted so that the intensity is equal 

 to that of the blue light, are less effective in producing bending. If these 

 data are quantitated and plotted as in Figure 117 (lower curve), the result 

 depicts the action spectrum of the phototropic bending. Extraction of the 

 carotenoids of the oat coleoptile C Avena') yields an absorption spectrum in- 

 dicated in the upper part of Figure 117. The correspondence between the 

 two cur\ es is indeed striking. 



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400 SOD 



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Fig. 117. Upper, the absorption spectra of the total extracted carotenoids of the oat 

 coleoptile, Avena, and of spore-bearer cells of the mold, Phycomyces. Lower, the action 

 spectra of phototropic bending in Avena and spore bearers of Piloholiis, known to have 

 carotenoid content similar to that of Phycomyces. From Wald.'"* 



Similar corespondence exists in the spectral sensitivity of the photic re- 

 sponse of certain species of green flagellates and the absorption spectra of an 

 extracted carotenoid, which has been identified as astaxanthin. 



For the remainder of the invertebrates there is no conclusive evidence for 

 the participation of carotenoids in photoreception, but several lines of evi- 

 dence suggest such carotenoid function. The polyps of certain colonial co- 

 elenterates bend toward light. This effect is sharply maximal in the blue at 



