198 Light 



changes in illumination in the aquatic environment are therefore fre- 

 quently very much greater than in the terrestrial environment. Any 

 water body that displays a much higher transparency in winter than 

 in summer will have less light available during the summer in its 

 deeper layers and thus suffer a reversal of the usual seasonal change 

 in the light factor. The foregoing instances are sufficient to show 

 that in the aquatic environment light becomes profoundly altered 

 quantitatively and qualitatively and that its changes may go far be- 

 yond those experienced by plants and animals on land. 



BIOLOGICAL EFFECTS OF LIGHT 



Before concerning ourselves with the influence of the light factor 

 on growth, reproduction, locomotion, and other activities of the or- 

 ganism as a whole, we shall consider certain general effects of radia- 

 tion as it strikes the surface of the plant or animal. In addition to 

 heating the tissues, the absorbed radiation affects biological processes 

 in the exposed tissues, including particularly their pigmentation. 



General Effects 



Among green plants light is required for the production of chloro- 

 phyll in the chloroplasts. Plants germinated under insufficient il- 

 lumination will not develop their normal green color. Normal plants 

 become etiolated in the absence of light; that is, they lose their pig- 

 ment and develop abnormal form. On the other hand, excessive il- 

 lumination causes the destruction of chlorophyll. In some plants ex- 

 cessive absorption of light by the deeper tissues is prevented by the 

 screening action of thickened chloroplasts or of increased numbers of 

 chloroplasts near the surface. In some species when the light be- 

 comes too bright, the chloroplasts line up one behind another so that 

 a larger proportion of the radiation passes through the leaf between 

 the chloroplasts. When the light becomes weak, the chloroplasts 

 spread out and absorb a maximum percentage of the incident illu- 

 mination. 



Light may influence thq pigmentation of animals in several ways. 

 Skin color may be indirectly affected by light through the mediation 

 of the eyes or other receptors. In other instances differences in color 

 may have arisen as a result of selective survival. The abundance of 

 pigment exposed in the chromatophores is sometimes directly con- 

 trolled by the intensity of the light received. The characteristic lack 

 of pigment in cave animals is associated with darkness, and certain 



