40 



WHAT IS BEING ALIVE? 



until now he has spent enormous sums of money to bring pure 

 water to his cities. 



U. S. Forest Service 

 An irrigated ranch. Why is the vegetation unusually thick along the water edge ? 



Light. Light is another important factor of the environment. 

 A study of the leaves on any green plant growing near a window 

 will convince one that the stems of such plants grow toward the 

 light, and that the leaves grow in positions to get a maximum 

 amount of sunlight. All green plants are thus influenced by the 

 sun. We say an organism is positively influenced by a stimulus 

 when it turns or moves toward that stimulus, and that it is nega- 

 tively responsive when it turns or moves away from the stimulus. 

 Other plants which are not green seem either indifferent or nega- 

 tively influenced by the stimulus of light. The direction, as well 

 as the intensity of light, is an important factor. Animals may or 

 may not be attracted by light. A moth, for example, will fly 

 toward a flame ; an earthworm will move away from light. Some 

 animals prefer a moderate or weak intensity of light and live in 

 shady forests or jungles, prowling about at night. Others seem 

 to need strong light. Man himself is most comfortable and works 

 most efficiently in a moderate intensity of light. 



