PHOTOPIGMENTS 



119 



adrenal and renal cortex) is a further point of association between 

 the action of hght and sexual activities ^ (Goodwin, 1950). 



In the vegetable kingdom the predominant carotenoids are 

 j3-CAR0TENE, C40H56, and XANTHOPHYLL, C4oH54(OH)2 — jellow pig- 

 ments absorbing preferentially blue light with absorption spectra quite 

 diflferent from that of chlorophyll. The latter and its relatives are active 

 in the photosynthesis of plant metabolism and have no effect upon 

 phototropic responses ; the former and its derivatives are concerned in 

 photoreception in systems mediating orientation to light, they are 

 pecuharly susceptible to the blue end of the spectrum, and are found 

 only in the photosensitive parts of plants, such as the oat coleoptile 



—t Y — 



A 



^»s 



400 



500 



Way/zlenqt'h ~mu. 



100 



80 



60 



40 



20 







c 



Fig. 77. — Spectral Sensitivity of the Phototropic Bending of Plants, 

 AND THE Absorption Spectra of the Associated Carotenoids. 



Absorption spectrum (extinction) of the total carotenoids of the etiolated 

 oat coleoptile, Avena ; continuous line (after Wald). Relative spectral sensi- 

 tivity of the oat shoot ; broken line (after Johnston). 



(Voerkel, 1933 ; Castle, 1935 ; Biinning, 1937 ; Wald, 1943). Wald 

 (1945-46) brought out this relationship dramatically by a study of the 

 absorption characteristics of the phototropic response ; he found that 

 the active spectrum of the phototropic bending of the seedling of the 

 oat, Avena, was maximal in a blue light of 440 m/x and corresponded 

 very closely with the absorption spectrum of the carotenoids extracted 

 from the coleoptile (Fig. 77). 



The phototactic movements of animals, so far as they have been 

 investigated, are also determined by carotenoids but in these the single 

 maxima of absorption are disjDlaced to wave-lengths considerably longer 

 than those associated with the phototropic bending of plants (473 to 

 534 niju) (Mast, 1917 ; Laurens and Hooker, 1920 ; Luntz, 1931). The 

 pigment responsi])le for phototactic activity in a number of the green 



1 p. 16, 



