SUNLIGHT AND CHLOROPHYLL 



47 



fore, all animals and all plants that do not contain chlorophyll 

 are dependent either directly or indirectly upon these green 

 plants for their food supply. Different kinds of plants require 

 varying amounts of light and, therefore, we find some plants 

 growing in the direct sunlight, while others are usually found in 

 the shade and will not thrive in the intense sunlight. We also 

 notice that the arrangement of tlie foliage on the plant, and its 



Fig. 33.— Self-pruned twigs of the poplar showing the cleavage planes. 



position during different hours of the day and on cloudy and 

 clear days, are such as to relieve the sunlight in proportions more 

 or less suitable for its own work. Plants gTowing in the desert, 

 or other dry places, may be " leafless " or have the leaves greatly 

 reduced, but the stems of such plants contain the chlorophyll 

 and serve as foliage. Plants may produce more foliage than is 

 necessary for their normal growth. Rueh plants may drop in- 

 dividual leaves or even large shoots during the growing season 

 by a process of self-pruning. These leaves and shoots are not 



