THE LEAF AND ITS FUNCTIONS G9 



tiniious with the main veins of the blade above and which enter 

 directly into the vascular cylinder of the stem below. 



Photosynthesis. — The primary activity of green leaves is the 

 manufacture of food from certain simple inorganic materials — 

 carbon dioxide and water — by energy derived from light. This 

 process of pJwtosynthesis is fundamental in organic nature, for it 

 is not only an essential function of green plants themselves but 

 is of the utmost significance to animals and man, because it 

 constitutes the sole ultimate source of food in the world. Food 

 is primarily a storehouse of energy and of hody-huilding materials 

 for living things. In the green parts of plants, and nowhere else 

 among organisms, is active or kinetic energy — in this case the 

 energy of light — converted into a latent or 'potential form, readily 

 available to living organisms for use in maintaining their vital 

 activities; and, moreover, in green plants alone are produced 

 those fundamental organic materials out of which plant and animal 

 bodies are constructed. All the complex metabolic changes which 

 later take place in the organic world are simply elaborations or 

 simplifications of the primary products of photosynthesis. A 

 more detailed account of the various types of foods and their 

 uses, and of the energy-relations of the plant, will be given in our 

 chapter on Metabolism. 



Materials. — The materials combined by the plant in this proc- 

 ess are but two — water and carbon dioxide. Water is absorbed 

 from the soil by the roots, passes upward through the stem, the 

 petiole, and the veins of the leaf, and thence enters the mesophyll 

 cells. None is obtained by the leaf directly from the atmosphere. 

 It should be remembered that only a relatively small portion of 

 the water taken in by the plant is used in food-manufacture; for 

 much the larger part soon leaves the plant again, passing out of 

 the leaf into the air by transpiration. The carbon dioxide used 

 in photosynthesis is derived entirely from the air. Here it is 

 always present, but in such small quantities that it constitutes 

 only about three parts in ten thousand of the atmosphere or three 

 hundreths of 1 per cent. Experiments have shown that a higher 

 concentration would be advantageous to plant growth, since up to 

 a certain point the rate of photosynthesis rises if the proportion 

 of carbon dioxide in the air is artificially increased. It is through 

 this comparativc^ly rare gas alone that the plant derives its supply 

 of carbon, that element so vitally necessary to all living organ- 

 isms. No other carbon compounds, not even the abundant 



