652 



LIGHT AND LIFE 



slant between curvature and log dose varies with wavelength will 

 be discussed in a later paper. 



The next question is: to what photoreceptor does this action spec- 

 trum point? For years the sensitivity of phototropism to blue light 

 has been taken to mean that the photoreceptor may be a carotenoid, 

 and this has been felt to be supported by the clear indications that 

 the action spectrum has two peaks in the visible; this was particularly 

 true of the data of Bachmann and Bergann (1930) and Johnston 

 (1934) . In Curve B of Fig. 5 the absorption spectrum of a-carotene 



300 



350 



400 



450 



500 



WAVE LENGTH m>u 



Fig. 5. Attion s|H(trimi of lig. 1 (curve A) (omparcd with absorption spectra of:- 

 15, a-carolcnc in licxanc; 



C, hcxane extract of 'jOO coleopiile lips; 



D, 9-9' ris /a-carotene in liexane; curve of Inlioffen et al. (19r)0); 



E, iii)ofla\in in walci; 



F, 3-inethyl-luniilla\ iu in benzene; curve of Hari)uiy el al. (1959). 



