THE SOURCE AND NATURE OF VITAL ENERGY 299 



THE LIFE OF A PLANT 



Sunlight furnishes the earth with practically all its energy. 

 There have been many attempts to make efficient sun engines, 

 which will utilize the rays of the sun to serve directly as a 

 source of energy sufficient to run engines. Sun engines have 

 been made, but as yet they are cumbersome, unwieldy, and im- 

 practical. But it seems that the time must come, after the ex- 

 haustion of the coal supply, when sun engines will be a necessity. 

 A plant growing on the surface of the earth is a perfectly efficient 

 sun engine, devised by nature to utilize the rays of the sun 

 and then to transfer the energy thus received to the rest of 

 the living world. The life of the ordinary green plants consists 

 of two features: (1) the utilization of the sun's rays and the 

 storing away of these rays in a form of potential energy; 

 (2) the liberation of this energy and its subsequent use by 

 the plant. These two processes will be considered in turn. 



Energy Stored by Plants. All green plants have the power 

 of absorbing the sun's rays and, by the means of energy thus 

 obtained, of building up chemical compounds of great complex- 

 ity which will contain the energy thus absorbed, stored away in 

 a potential form. Their method of accomplishing this is in part 

 as follows: In Chapter VI we have learned that plants have 

 the power of manufacturing starch out of carbon dioxid and 

 water. This process involves the manufacture of complex 

 molecules (C 6 Hi O 5 ) out of simple ones (H 2 O and CO 2 ), and 

 hence requires the expenditure of energy. Since it can take 

 place only in sunlight, it becomes evident that (1) the sun's 

 rays are the source of energy used, that (2) the starch manu- 

 factured will contain in a potential form the energy used in 

 building it, and that (3) this energy may be liberated in an 

 active form if the starch molecule is broken down. 



Stored Energy Utilized by Plants. The energy stored in 

 the starch is the primary source of energy for nearly all the 

 activities on the earth, except water power. The plant uses it 

 for two distinct purposes: L While plants do not in their 



