FITNESS 27 



transforms them into oxygen, which renews 

 the air, and sugar, starch, and other sub- 

 stances, which are the food of the animal. 1 

 These products the animal burns, thereby 

 forming once more carbonic acid and water, 

 which return to the plant and so pass through 

 the cycle again and again. The changes in 

 energy which accompany this process are 

 quite different from the chemical changes. 

 Starch and sugar and oxygen, formed in the 

 leaf of the plant, are compounded of carbonic 

 acid, water, and sunshine. This sunshine, 

 or solar energy, wh'en changed into the 

 chemical energy of the carbohydrate, is pre- 

 served and transmitted to the animal. 2 In 

 his body it is set free as muscular force and 

 heat, and then dissipated. Accordingly, when 

 carbonic acid and water are combined to 

 form sugar and oxygen in the leaf, it is al- 

 ways a new store of solar energy which they 

 bear, and while matter goes round and 

 round, energy is being constantly degraded 

 and lost. The one process is cyclic, the 

 other moves steadily in one direction from 



1 Our knowledge of photosynthesis is largely based upon 

 the classical work of N. T. de Saussure, "Recherches Chi- 

 miques sur la Vegetation." Paris, 18(U. 



2 Only after the establishment of the principle of the 

 conservation of energy was it possible to gain a clear con- 

 ception of the energetics of metabolism. 



