PLANT PHYSIOLOGY 237 



characteristic difference may be stated in another way. Ani- 

 mals utilize food that is brought to them from the outside, 

 they cannot synthesize their own food. Plants, on the other 

 hand, manufacture or synthesize their own food. 



PHOTOSYNTHESIS 



We shall now study the way in which plants store energy in 

 building up complex compounds out of simple ones, these 

 complex compounds so produced being the food of the plant 

 itself. The compounds concerned in this energy-storing process 

 are, as we have repeatedly said, the three classes, carbohydrates, 

 fats and proteins, and the material out of which they are con- 

 structed consists of the two common substances, carbon dioxide 

 and water, together with nitrogen obtained mostly from the 

 soil. How, then, is the plant enabled to carry out this won- 

 derful synthesis? 



Source of Energy. — As we have previously explained, the 

 endothermic reaction involved in the synthesis of carbohydrates 

 from carbon dioxide and water requires the addition of energy. 

 Where does the plant get this energy? The energy by means 

 of which plants are able to carry out this reaction comes directly 

 from the sun. Through this endothermic reaction the kinetic 

 energy of the sun is converted into the potential energy oi complex 

 organic compounds and stored in the plant. This kinetic energy 

 reaches the plant through the action of the sunlight and acts, 

 therefore, only during daylight and only upon those parts of the 

 plant which are above ground and exposed to the light. 

 Furthermore, generally speaking, only those plants known as 

 green plants, or phanerogams, are able to thus utilize the energy 

 of the sun. It has been shown experimentally that other 

 forms of energy can produce this synthesis in plants, but it 

 is not necessary to discuss them. In normal living green 

 plants it is certain that much the greater part, if not all, of the 

 energy connected with this synthesis is the radiant energy of 

 . the sun. 



