126 BOTANY: PRINCIPLES AND PROBLEMS 



only to remember that a bent spring weighs no more than an 

 unbent one or a charged battery than an uncharged one. Energy 

 and matter are fundamentally distinct. 



Release of Stored Energy. — The potential energy in an object 

 may at any time, under an appropriate stimulus, become con- 

 verted again into kinetic form and do work, as when the stretched 

 spring moves the mechanism of a watch, the bow moves the 

 arrow, the falling water moves a mill wheel, the battery moves a 

 telegraph sounder, the burning coal converts water into steam 

 which moves a machine, or the explosive moves a projectile. 

 In all of these cases the supply of stored energy is finally ex- 

 hausted and motion ceases. In this process of converting kinetic 

 into potential energy and back again, no energy is gained or 

 lost, the total amount remaining constant. 



A machine is anything which controls and directs the expen- 

 diture of energy so that work of a particular kind is done at a 

 particular place and time. One of its prime necessities is an 

 ample supply of potential energy, and in the machines with which 

 we are most familiar this is available in the form of wood, coal, 

 oil, or stored electricity. The Hving organism resembles a 

 machine in the fact that it, too, directs the expenditure of energy, 

 and it therefore needs a plentiful supply of this energy in potential 

 form which it may liberate, in the process of respiration, at any 

 point throughout its body for the performance of its many 

 activities. The fuel which the organic machine uses in this 

 process we know as food, and the potential energy within this 

 food came originally from the kinetic energy of sunlight and was 

 converted into potential form by photosynthesis in the green 

 cells of the leaf. Food resembles wood, coal, or oil in being a 

 sonicwhat unstable chemical compound which, through the addi- 

 tion of oxygen, will rapidly break down and resolve itself into 

 simpler components, usually carbon dioxide and water, and thus 

 release the potential energy it contains. This process of oxidation 

 is common in nature. In ordinary fuels it takes place only at 

 high temperatures and is then known as combustion. In living 

 organisms it can go on at much lower temperatures and is here 

 known as physiological combustion or respiration. In their 

 essential feature^ — the liberation of energy in kinetic form by the 

 breaking down of complex and unstable chemical compounds 

 into simpler ones through the addition of oxygen — respiration 

 and combustion are precisely similar. 



