128 BOTANY: PRINCIPLES AND PROBLEMS 



A machine is supplied with fuel (its "food") by an operator, 

 and an animal obtains its food by seizure, but the food which a green 

 plant consumes must be secured through the plant's own activi- 

 ties. The significance of the food-making process we call photo- 

 synthesis is now more evident than before. The kinetic energy 

 which the plant, through its chlorophyll, absorbs directly from the 

 rays of the sun is here used to do the work of pulling together 

 carbon dioxide and water and uniting them into the simple food 

 glucose (Fig. 67). The energy expended in accomplishing this 

 union is obviously stored up in potential form in the molecules 

 of the glucose and of the other foods or plant materials which may 

 be derived therefrom, just as the energj^ expended in winding a 

 clock is stored up in the compressed mainspring. Under appro- 

 priate conditions this potential energy may be liberated anywhere 

 and at any time to do work in any organism. One of the most 

 important principles of physiology is thus brought out — that food 

 is merely the medium by which energy received from the sun 

 intermittently and only in certain exposed organs is stored up, 

 carried to all parts of the plant, and made available for work at 

 all times and in all places. This conception has been concisely 

 formulated in the metaphor that "food is a 'storage battery,' 

 charged in green leaves by the sun and discharged in the body by 

 respiration". 



The Importance of Photosynthesis. — The importance of photo- 

 synthesis to the organic world lies in the fact that this process is 

 practically the only means whereby living things can store up 

 energy. Green plants, the ultimate providers of all foods, are 

 the sole agencies through which the animals and man can tap the 

 abundant supplies of energy in the universe and obtain therefrom a 

 sufficient quantity to maintain their varied activities. This energy 

 is all stored originally in the molecule of glucose, and before it is 

 converted again into kinetic form may pass through scores of 

 modifications and enter into the composition of the bodies of a 

 dozen successive organisms. Although kinetic energy is stream- 

 ing upon the earth daily in untold quantities from the sun, only 

 the chlorophyll-bearing plants are able to use it directly. 

 Even in industry, man still depends very largely upon photo- 

 synthesis, since the energy which he releases from wood, coal, and 

 oil was originally locked up in these substances, in some cases 

 millions of years ago, by the photosynthetic activity of green 

 plants. 



