THE SOURCE AND USES OF FOOD 



139 



directly, is accomplished by energy released in the decompos'tion 

 of these foods or substances made from them. 



The green leaves, therefore, are the organs where the sun's 

 energy is transformed so that it can be stored for use afterward 

 where and when it is needed. They are the organs where the 

 materials of the inorganic world are assembled in such a form 

 that they can be used in the construction of the living body. 



How energy is obtained in the decomposition of food will 

 appear from a specific example: Wherj a definite amount of 

 sugar or starch is made within the leaf from carbon dioxide 

 and water a certain amount of sun's energy is employed and 

 transformed from the active or kinetic to the passive or poten- 

 tial state. This energy is not destroyed or lost; it is simply 

 changed from one condition to another. To illustrate: a cer- 

 tain amount of energy is required to lift a brick from floor to 

 table. The energy spent in 

 doing this now rests quiescent 

 in the brick in virtue of its posi- 

 tion above the floor, and reap- 

 pears as active energy when 

 the brick falls. So when sugar 

 or starch is transformed into 

 carbon dioxide and water from 

 which it was made the energy 

 from the sun employed in com- 

 pounding it is set free (com- 

 parable to the falling of the 

 brick) and a part of it can then 

 be used by the plant for other 

 purposes, as in the construction 

 of proteids from carbohydrate, etc., the synthesis of protoplasm, 

 the overcoming of resistance, and in other ways. 



The sun's energy made potential in sugar and the like is, by the 

 translocation of these substances, distributed to all parts of the 

 body where every living cell can make use of it, and where, if not 

 immediately wanted, it can be stored for use at some future time. 



Fig. 69. — Cross section of a portion of 

 the blade of a leaf, showing upper epidermis 

 at a, lower epidermis at b, palisade paren- 

 chyma at c, spongy parenchyma at d, and 

 tracheids from the end of a vein at e. 



