THE PLANT BODY 89 



ing through the plant entering the root with various sub- 

 stances in solution and emerging through the stomata as 

 water vapor. The fact that much more water usually is 

 evaporated from a forest than from an equal area of a lake, 

 affords some conception of the part played by vegetation not 

 only in returning water to the atmosphere but also in 'con- 

 suming' heat energy cooling the summer air. The dynam- 

 ics of the circulation through the xylem, however, are prob- 

 ably by no means so simple as might appear from the theory 

 just outlined; and, moreover, there is no satisfactory explana- 

 tion of the causes of food distribution in the phloem, further 

 than that osmotic phenomena play an important role. 



3. .Food Utilization 



The food which the plant has constructed and distributed 

 to the various parts of its body must be employed by the 

 individual cells in supplying the material and energy for their 

 life processes. It is important not to lose sight of the cell in 

 the larger organization of which it is a part, for, in the final 

 analysis, the life of the individual plant is but the life of the 

 multitude of units which cooperate toward its make-up. 

 Although the cells suppress their individuality in the cor- 

 porate whole which they form, the life of the plant is as truly 

 the life of the protoplasmic units which form it as is the life 

 of a human community resident in the individual citizens. 



The cells select from the food stream not only the materi- 

 als essential for their individual life, but in addition those 

 which they require for the performance of their particular part 

 in the economy of the whole. But doing this implies work, 

 and work means expenditure of energy the same energy of 

 sunlight which was stored in the food during its construction 

 by the chlorenchyma cells of the leaf. In order to release 

 this energy RESPIRATION must occur. Carbohydrates, fats, 



