894 LIGHT AND LIFE 



a major peak at 445 m^, minor peaks at 472 and 425 m^, and a 

 broad peak in the near ultraviolet (SSOm/x) , the latter being less 

 than half as effective as the blue light in evoking the response. The 

 major peaks agree well with the absorption spectrum of a-carotene, 

 and there is in fact a carotenoid extractable from the coleoptile and 

 microscopically visible in polarized light within it; but a-carotene 

 lacks the broad absorption band in the near ultraviolet. A cis- 

 configuration in the molecule would produce a band between 300 and 

 350 m^A, but so far no c/^-carotenoid has been found to duplicate the 

 natural action spectrum very closely. Another view, represented by 

 Galston, and supported by him in the general discussion, is that the 

 photoreceptor might be riboflavin or some other flavin compound. 

 Thimann discounts this possibility because, though riboflavin has an 

 absorption band at 365 m^u, this band is as strong as the major 

 peak at 450 m/^, and nothing can be seen of the secondary peaks of 

 the action spectrum for phototropic curvature. However, the ribose- 

 free compound lumiflavin, and even more strikingly 3-methyl lumi- 

 flavin, when dissolved in benzene, exhibit absorption curves very like 

 the action spectrum. Yet the peak in the ultraviolet is at too short 

 a wavelength; and in water, where the band shifts to a position in 

 agreement with the action spectrum, the details in the main part of 

 the spectrum disappear. Thimann accordingly prefers to look, at 

 the present time, for some c?'.y-carotenoid conjugated with a protein; 

 but Galston believes that in the natural medium of the cell the ab- 

 sorption spectrum of a flavin might shift sufficiently to accord Avith 

 the action spectriun. Moreover, he points to a number of cases — 

 albino corn, a white sunflower, and Phycomyces grown on lactate — 

 in which the carotene content was imdetectable or greatly reduced, 

 but which nevertheless exhibit a phototropic response. Thimann ques- 

 tions both the complete absence of carotene and the normality of 

 phototropic sensitivity in these cases; and he adds that pre-exposine to 

 red light, which doubles the concentration of FMN, decreases or 

 leaves unchanged the photosensitivity. 



In addition to the positive cmvature elicited by low intensities 

 of light, there is also a negative response which is evoked by higher 

 intensities. At very high energies there is again a positive curvature, 

 but of a different kind. It appears mainly at the base, and is responsi- 

 ble for most of the ordinary bending of plants toward the daylight. 

 This "base response" may also be elicited by ultraviolet illumination, 

 and the action spectrum has peaks at 280 and 297 wxjx. This action 

 spectrum ajjpears to be that of indole acetic acid (lAA) itself, shifted 



