220 CONTROL MECHANISMS IN CELLULAR PROCESSES 



jection to this is that it also fits the absorption of certain carotenoids; 

 besides which, one cannot assume that all portions of the action 

 spectrum necessarily depend on a single pigment. 



Instead of going into the many objections that various investi- 

 gators have urged against each others' interpretations, let us con- 

 sider a few of the general problems of interpreting such action 

 spectra. First of all, precision, particularly for such a complex re- 

 sponse as phototropism, is limited by the variability of the material. 

 Thus the relative height, position, and even existence of closely 

 placed peaks is not easily ascertained. A second difficulty is that, 

 since phototropism depends on an asymetric distribution of light 

 about the responding organ, it might be affected by pigments, not 

 active photoreceptors themselves, that screen or filter the light pass- 

 ing through the organ. Thus, the action spectrum could reflect the 

 absorption characteristics of both photoreceptor and screen. Rela- 

 tively few quantitative evaluations of this possibility have been 

 made, although Delbriick and Shropshire (1960) have found that 

 the effect is probably negligible in the fungus Plujcomyces. 



Perhaps the most disturbing question is that already implied on 

 page 219. Once an action spectrum is established, to what absorp- 

 tion spectrum of a suspected pigment shall it be compared? It is 

 evident, at least with flavines, that the absorption spectrum is af- 

 fected by the solvent, and there seems to be no obvious way of 

 deciding what solvent— be it water or any of many organic sub- 

 stances—will approximate the state of the pigment in vivo. 



We have tacitly assumed throughout this discussion that both 

 major possible photoreceptors— carotenoids and flavines— are usually 

 present together in quantities sufficient to make a decision on the 

 basis of relative concentration impossible. This is generally true. 

 Some work with albino seedlings having a low carotenoid content 

 has been done, but the results have been interpreted both ways ( cf . 

 Galston, 1959, and Went, 1956) and are quantitatively unsatisfac- 

 tory. Similar work with certain fungi, in which carotenoid content 

 can be artificially lowered, is also equivocal so far. Of course, the 

 chance of ever obtaining flavine-free tissues is very small, consid- 

 ering the number of functions performed by such compounds. 



This necessarily brief summary indicates that, even considering 

 only flavines and carotenoids, we have too many candidates for the 

 blue-light photoreceptor unless we simply assume that both classes 

 of compounds may be active. This is, of course, quite possible. In 



