66 PLANT BIOLOGY 



tion is changed into electrical energy, and this in turn may be 

 converted into light or heat energy in our houses or into energy 

 of motion, as, for instance, in the running of a trolley car. 



77. Source of the energy developed in living things. 

 In all these marvelous transformations of energy that we have 

 just enumerated no new energy is created and none is de- 

 stroyed. Whence, then, comes this abundant supply of 

 energy? We shall find the probable answer to this question 

 in considering again the processes carried on in the leaves of 

 plants. From the sun comes the radiant energy that is abso- 

 lutely essential for the activity of the chlorophyll bodies, by 

 which carbohydrates are formed. In the formation of these 

 compounds, the sun's radiant energy is used and apparently 

 disappears. 1 In reality, however, it has only been stored in 

 the chemical compounds formed thereby, since, as we have 

 said above, energy cannot be destroyed. 



78. Oxidation as a means of liberating energy. Let us 

 now refer once more to the processes that take place in an 

 electric power plant. We have just said that the energy de- 

 rived from the rays of the sun is stored up in the wood, coal, 

 and other fuel. In order to set free from these compounds 

 the energy they contain, the wood or coal must be burned, or 

 in other words combined with oxygen. Every one knows that 

 when we wish to secure a large amount of heat from our fuel 

 we open wide the drafts in order to secure a plentiful supply 

 of oxygen. We demonstrated in our studies in oxidation 

 that whenever a substance is burned, heat energy is liberated 



1 The authors are indebted to Mr. Paul B. Mann of the Morris 

 High School Department of Biology for the following demonstration 

 of the effectiveness of the radiant energy of the sun. Place a 

 radiometer (usually found in the physics equipment of schools) in 

 direct sunlight, and then remove it from the sun's rays. Try also 

 the effect of an electric light by bringing the radiometer near the 

 light bulb, and then slowly removing it to a distance. 



