8 



Control by Light 



William S. Hillman ^ 



Rather than concentrate on a smgle small aspect of light physiol- 

 ogy, I shall try to review two general topics and to point out 

 areas that may have been neglected. 



Let us immediately exclude from consideration both photosynthe- 

 sis and vision. These have an immense literature and are relatively 

 —though only relatively— well understood. At least all biologists, 

 whatever their special interests, are aware that photosynthesis and 

 vision exist, which is more than can be said for many other light 

 effects. For similar reasons let us also exclude ultraviolet effects- 

 wavelengths below 4000 A— altliough they will enter slightly a bit 

 later. 



It has become clear to botanists only relatively recently that 

 many actions of light, even on green plants, bear no close relation- 

 ship to photosynthesis; zoologists occasionally still seem to assume 

 that all effects of light on animals are necessarily mediated through 

 vision, and some microbiologists seem unaware that light can affect 

 many of their experimental organisms. This brief paper will have 

 served its purpose if it merely emphasizes the multiplicity and 

 widespread nature of light effects, as well as our ignorance concern- 

 ing most of them. 



At least two major questions are asked by the light physiologist. 

 The first is: What is the compound absorbing the light involved in 

 a particular process— the photoreceptor pigment or pigments? The 

 second is: What processes intervene between light absorption and 



1 Department of Botany, Yale Uni\ersity, New Ha\en, Connecticut. Supported by 

 a grant from tlie National Science Foundation (NSF G-11304). 



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