94 The Living Plant 



where energy is potential in the form of unsatisfied chemical 

 affinity. The electric current forces apart the tightly-cohering 

 atoms of certain very stable chemical compounds; but these atom.s 

 nevertheless retain all their old attraction for one another, and 

 it is in the form of this unsatisfied attraction that the energy 

 is latent; and this energy is given out again in kinetic form at 

 the moment when the atoms are allowed once more to unite. 

 Now the very same thing is true of carbon dioxide, which is a 

 very stable substance of tightly-cohering atoms. To force apart 

 carbon dioxide into its constituents requires kinetic energy, 

 which then remains in the latent form, as energy of unsatisfied 

 chemical affinity, so long as the carbon and oxygen are held apart, 

 but becomes Idnetic again when the carbon and oxygen are al- 

 lowed to reunite to carbon dioxide. Does the reader see the ap- 

 plication? Surely he must. The kinetic energy of the sunlight 

 splits apart carbon dioxide in the green leaf, the oxygen going 

 out to the air and the carbon combining with the elements of 

 water into grape sugar; so long as this carbon and oxygen are kept 

 apart, that energy is latent in the form of unsatisfied chemical 

 affinity; and when the carbon of the sugar (or of any other sub- 

 stance into which the sugar is transformed) is allowed to unite 

 with the oxygen of the air, as it is in the process of respiration, 

 then kinetic energy is again given out and can be used for the work 

 of the plant. Such is the source of the energy of respiration, — 

 it is energy released from the latent state in food, where it was 

 placed (or ''stored") by the kinetic energy of the sunlight. Food, 

 therefore, is a storage battery charged by the sun, and discharged 

 by respiration. 



The principal function of food must now be quite plain. As a 

 storage battery it has advantage over any that man has yet 

 made in the fact that it can be reduced to very small fragments, 

 or even to solution (by digestion), and thus transported to all 

 parts of plants and throughout the bodies of animals. Then, at 

 the spot w^here work needs to be done, just at the right instant, 



