224 CONTROL MECHANISMS IN CELLULAR PROCESSES 



on the circumstances. From this statement of the problem, one 

 might be more surprised that, in a relative sense, so few light effects 

 are known, rather than so many, in view of the many colored com- 

 pounds with biological roles. 



Probably the most important point so far omitted that should 

 now be emphasized, is that many of the known effects of light occur 

 at extremely low energies. This is evident in both the phototropic 

 and photoperiodic literature, as well as elsewhere. In terms of in- 

 tensities, the threshold for certain morphogenetic responses through 

 the red, far-red system is about 10"'' /xw ( 10~' ergs/sec) per square 

 centimeter, while the threshold for phototropic responses is about 

 10"^. For comparison, the lower limit of rod vision in the dark- 

 adapted eye is about 10"'' /xw/cm-— lying between the phototropic 

 and photoperiodic thresholds— and the intensity of full moonlight 

 is far higher than any of these, being more than 10~^ (Withrow, 

 1959a ) . For this reason, the absence of a light effect on an organism 

 or reaction cannot be assumed unless the dark control represents 

 absolutely uninterrupted total darkness for an extended period of 

 time. There is the additional possibility, observed, for example, in 

 Amoeba (Bendix, 1960), that light effects may be observable only 

 within a short time following a change in intensity. The wider 

 recognition of these simple principles could in itself result in the 

 observation of new light effects, which might also help to explain 

 further those we know today. 



References 



AsoMANiNG, E. J. A. 1960. Phototropic behavior and pigment content of grass 

 coleoptiles. Ph.D. dissertation, Yale University. 



Baker, J. A. 1938. Light as a factor in the production of pigment by certain bac- 

 teria. /. Bad. 35: 625-631. 



Ballard, R. C. 1958. Responses of Stomoxi/s cdcitrans (L) to radiant energy and 

 their relation to absorption characteristics of the eye. Ann. Entom. Soc. America 

 51: 449-464. 



BANBxmY, G. H. 1959. Phototropism of lower plants. Encijcl. Plant Physiol. 17 

 (Part 1): 530-578. Springer-Verlag, Berlin. 



BARTHOLOMEVi^, G. A. 1959. Photoperiodism in reptiles. In Withrow, 1959b. Pp. 

 669-676. 



Beinert, H., and F. L. Crane. 1956. The function of the electron-transferring 

 flavo-protein in tlie first oxidative step of the fatty acid cycle. In Inorganic Ni- 

 trogen Metabolism, eds. W. D. McElroy and B. Glass. Johns Hopkins Press, 

 Baltimore. Pp. 601-624. 



Bendix, S. W. 1960. Phototaxis. Bot. Rev. 26: 146-208. 



Bonner, B. A. 1960. Partial purification of the photomorphogenic pigment from pea 

 seedhngs. Plant Physiol. 35 ( Suppl. ) : xxxii. 



I 



