CHAPTER 11 



Photoreception 



INTRODUCTION 



T 



I HE ENTIRE GAMUT of behavioral adjustments which an organism or 

 I its component parts make to changes in the external environ- 

 B ment is the direct result of sense organ activity. Of the numerous 

 types of sense organs which regulate the activity of animals in their external 

 environment, those that are sensitive to Hght, called photoreceptors, serve 

 in a capacity which equals, and perhaps exceeds in importance that of all 

 other sense organs. 



The ability to perceive light usually resides in a well defined structural 

 entity, called a photoreceptor, although this is not necessarily true. Some 

 echinoderms, for example, possess light sensitivity in the absence of any 

 known discrete structural photoreceptors, as do many protozoa. Photorecep- 

 tors present a great diversity of structure. Despite these morphological differ- 

 ences, there seems to exist a functional similarity in all of the photoreceptors 

 studied. 



PHOTORECEPTORS AND ORIENTATION 



Throughout the animal kingdom, as well as in the plant kingdom, light 

 plays an important part in organismic orientation. Utilization of the direct- 

 ional properties of light resides in the morphological and physiological rela- 

 tions which obtain in the photoreceptor proper as well as in the central ram- 

 ifications of the visual pathway, and in the functional organization of the 

 central nervous system with respect to the visual and effector systems. The 

 mere presence of a photoreceptor does not indicate the nature of the orien- 

 tation. 



Many studies have been performed on the orientation of organisms to 

 light. Much of this material is summarized in the monograph of Fraenkel 

 and Gunn.'*^ According to their definitions, the word tropism is restricted 

 to orientation of sessile animals or plants, taxis to directed orientation of ani- 

 mals which move in their environment, and kinesis to increase in velocity of 

 movement on illumination in those animals which lack a directional orienta- 

 tion. 



Diffuse Light Sensitivity 



It has long been known that plants utilize light not only for photosyn- 

 thesis but also for a type of orientation called phototropism. Many plants 

 bend toward the source of illumination. This phototropism is particularly 

 marked in blue light, which results in relatively little photosynthesis, and 

 is associated with yellow pigments (carotenoids) present in plants^^" (see 

 page 408.) 



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