214 CONTROL MECHANISMS IN CELLULAR PROCESSES 



the final effect one seeks to explain? In practice, one tries to limit 

 the second question to those processes following fairly immediately 

 upon light absorption, since otherwise its answer can, and often 

 does, involve almost every aspect of physiology. This discussion 

 will deal largely with the first question. 



A twofold approach is often used in trving to identifv a photo- 

 receptor pigment. It consists in determining an action spectrum 

 for the process in question, and then demonstrating that a compound 

 obtainable from the organism has an absorption spectrum similar 

 enough to the action spectrum to justif\^ the conclusion that it is 

 absorbing the effective light. This procedure, of course, is rarely 

 as straightforward as it sounds. Even action spectra alone, how- 

 ever, can often tell a great deal. 



One general principle— though a negative one— seems to be that 

 an effect of light on the same— or apparently the same— physiological 

 process in different organisms will not necessarily be mediated by 

 the same pigment. Let us trespass on the topic of the next paper 

 for a brief example. In higher plants, the initiation and phase-con- 

 trol of rhythmic phenomena by light is probably (e.g., Lorcher, 1958) 

 exerted through tlie red, far-red reversible system which we will 

 discuss next. In the dinoflagellate Gonijaidax, however, Hastings 

 and Sweeney ( 1960 ) have found no evidence at all for such control. 

 Rather, the action spectrum follows the absorption spectrum of the 

 whole cells, with prominent peaks in both the red and the blue. 



A related general principle is that apparently quite different phys- 

 iological processes in a given organism may be controlled through 

 the same light reaction. This is well illustrated by the red, far-red 

 reversible system. Since studv of this system also represents a tri- 

 umph, or at least an impending triumph, for the action-spectrum and 

 extraction approach to photoreceptor identification, let us review its 

 essential features here. 



Shortly after the discovery of the photoperiodic control of flower- 

 ing, it became clear that pliotoperiodism was a response to the dura- 

 tion, or timing, of light and darkness, with little dependence on total 

 light energy during each cycle. Thus, a long-day plant can flower 

 only on daylengths longer than a certain critical value— which is to 

 say, only if the dark periods are sufficiently short, but flowers even 

 with long nights if these are interrupted by a brief flash of light. 

 Conversely, flowering in a short-day plant, requiring sufficiently long 

 nights, will be inhibited if these nights are interrupted by light- 



