THE LEAF AND ITS FUNCTIONS 169 



pits or in a dense forest of minute hairs; or the leaf may be curled so that 

 it will partly protect the lower surface from air currents. 



THE FUNCTIONING OF THE LEAF 



The food of plants. The inorganic materials used by plants are com- 

 monly called plant foods; in popular usage this term has become associated 

 with fertilizers. It is, of course, a matter of definition as to what shall be 

 regarded as a food. According to one commonly accepted definition, a 

 food, whether used by animals or plants, is a substance which, either im- 

 mediately or after a digestive process, may be oxidized to furnish energy 

 or may be used in the building of protoplasm. By this definition, carbon 

 dioxide, water, and the mineral constituents of the soil and of fertilizers 

 can scarcely be classed as foods. The true foodstuffs are carbohydrates, 

 fats, and proteins and possibly, by stretching a point, the vitamins. This 

 definition of food has the advantage of emphasizing the fact that nutrition 

 in plants and animals is an identical process. The two kinds of organism 

 differ, not in the nature of the food that they use but in the method of 

 obtaining it. We have already stressed the fact that the plant manufac- 

 tures its food from inorganic substances, whereas the animal cannot do 

 this and therefore must obtain its food directly or indirectly from the 

 plant. 



The manufacture of food by photosynthesis. The basic food of the 

 green plant is the simple sugar called glucose, which has the chemical 

 formula C 6 Hi 2 6 . From glucose the plant can manufacture other carbo- 

 hydrates, and from carbohydrates and minerals it can make the fats, 

 proteins, enzymes, vitamins, and plant hormones that it requires. 



Glucose is synthesized in the green plants and only in them. It is formed 

 by the chemical combination of water and carbon dioxide through the 

 agency of chlorophyll and by means of the energy of sunlight. If we con- 

 sider only the end products of the reaction, it appears quite simple, as is 

 shown by the following equation: 



6 molecules + 6 molecules + Light energy (yields) 1 molecule + 6 molecules 

 of water of carbon (through agency of glucose of oxygen 



dioxide of chlorophyll) 



6H 2 + 6C0 2 + Energy -> C 6 H 12 6 + 60 2 



It is well established that in the process of photosynthesis carbon dioxide 

 and water are the raw materials, chlorophyll and light are necessary, sugar 

 and oxygen are formed, and for each volume of carbon dioxide used an 

 equal volume of oxygen is liberated. For an elementary understanding of 

 plant functioning no more than this is needed; but the actual photosyn- 

 thetic process is not this simple. In fact it is so complex that no one has 

 succeeded in duplicating it in the laboratory. Nevertheless great progress 



