Leaves. 93 



dioxide all of the time, both day and night. This latter 

 process, called breathing or respiration, is essentially the 

 same in plants as in animals, and is as necessary to the life 

 of the plant as to that of the animal. In the higher plants 

 the oxygen for respiration enters chiefly through the sto- 

 mata and through groups of loosely arranged cork cells, 

 termed lenticels, which break through the epidermis in 

 those parts where cork is being formed near the surface. 

 The lenticels can be seen to good advantage as small 

 rounded or elongated protuberances on many woody 

 branches a year or more old. Since all living cells must 

 respire, intercellular spaces are provided in which oxygen 

 diffuses throughout the plant body. In aquatic plants 

 these spaces are large enough to be seen with the naked 

 eye (see the chapter on Adaptation to Environment). 

 Although plants consume oxygen in their breathing, they 

 give off so much more by the process of photosynthesis 

 that the net result is a large addition of free oxygen to 

 the atmosphere. While the process of respiration is com- 

 mon to both plants and animals, that of photosynthesis is 

 peculiar to green plants. 



71. Supply of Raw Materials. Thus plants make their 

 own food by the wonderful processes of tearing down and 

 building up which take place in the chloroplasts. An im- 

 portant question in this connection is whether there is 

 an unlimited amount of raw materials at their disposal. 

 Water certainly seems unlimited for the larger part of the 

 earth's surface. The necessary salts occurring in naturally 

 rich soils are also practically inexhaustible, although they 

 may not become soluble rapidly enough to satisfy all the 

 demands of agriculture. Carbon dioxide occurs in the 

 atmosphere in very small percentage, namely, between 

 three and four parts in ten thousand of the atmosphere ; 



