654 LIGHT AND LIFE 



365 iTiya in aqueous media. In other words, the presence of the clearly 

 defined peaks in the visible is associated with an ultraviolet peak too 

 far to the lelt; but when the ultraviolet peak is in the right place to 

 fit that of the action spectrum the three resonances in the visible are 

 damped out. Hence the action spectnnn in the ultraviolet is not 

 really well accounted for by any of the carotenoids or flavins known 

 at present. However, the ratio of heights at 365 and 445 m^ in the 

 action spectriun is much nearer that of the carotenoid than that of 

 the flavins. 



Thus even the most detailed action spectra for phototropism in 

 the visible and near ultraviolet unfortunately do not lead to an un- 

 equivocal conclusion about the photoreceptor. By analogy with 

 vision, in which the photoreceptor is a complex of a c/5-carotenoid 

 and a protein, the participation of a r?i-carotenoid in phototropism 

 is obviously an attractive hypothesis; however, it remains to be proven 

 (a) that such a r/5-compound is present, (b) that the solvent or com- 

 bined form in which it exists has the effect of shifting the wave- 

 length of the ciVpeak about 20 m/^ towards the red without affecting 

 the peaks in the visible. As against this, the possibility that the photo- 

 receptor is a ribose-free flavin derivative in a non-polar solvent 

 cannot be excluded, though, curiously enough, this is subject to 

 exactly the same reservation, namely (b) above, as for a carotenoid. 



There is, of course, some supporting evidence. Lactobacillus assays 

 show that riboflavin is present throughout the coleoptile, with no 

 particular concentration at the tip. The presence of /^-carotene or 

 lutein in the hexane extracts of the tip was shown in Fig. 5,C. The 

 absence of the r/Vpeak in these extracts might be due either to a 

 great preponderance of the trans form, or to loss of the c/^-configura- 

 tion as a result of extraction (cf. the paper of Wald in this sym- 

 posium) . Chromatographic analysis of these extracts shows at least 

 three carotenoids present. 



Since carotenoid-protein complexes do exist in plants (Nishimura 

 and Takamatsu, 1957) it seemed worth-while to examine the absorp- 

 tion spectrum of the coleoptile tij) in vivo and to compare this with 

 the absorption of the protein-free extract in iiexane. This was done 

 with the ojjalescent plate method of Shibata (1956), with the results 

 shown in Fig. 6. The steej)ly rising "end-absorption" can be approxi- 

 mately corrected for by drawing a smooth curve between 390 and 

 520 m|u, and subtracting this from the overall curve, with the result 

 shown as the lower curve. The three peaks are brought out very 

 shar])ly, and it can be concluded that their positions are probably 



