Aquatic Plants 241 



the depth hmitation is at a much greater level. Animals can respond 

 to the difference between day and night at somewhat greater depths. 

 Below this level no perceptible light from the surface penetrates and 

 the water is completely dark except for light provided by the lumi- 

 nescent organs of deep-sea animals. Terms used in deep bodies of 

 water for zones based on the light factor are as follows: 



Euphotic Zone: Sufficient light for photosynthesis. 



Disphotic Zone: Insufficient light for photosynthesis but sufficient light for 



animal responses. 

 Aphotic Zone: No fight of biological significance from the surface. 



The depth limits of these zones differ widely according to trans- 

 parency. 



The discussion in this chapter has revealed the many important 

 roles played by light in the world of life. Although there is gen- 

 erally sufficient light on land for plants and animals, the special cir- 

 cumstances of this factor exert a wide control over the activities of 

 many terrestrial organisms. In the aquatic environment light is even 

 more crucial since, in addition to its various periodicities, its rapid 

 changes with depth impose serious limitations on the lives of both 

 animals and plants. 



