METABOLISM 57 



moving, although its movements may not always 

 be evident. Says Huxley, referring to the constant 

 ^streaming and circulation of the protoplasm in the 

 cell : ' * The wonderful noonday silence of a tropical 

 forest is after all due only to the dullness of our 

 . hearing ; and could our ears catch the murmur of 

 . these tiny maelstroms as they whirl in the innumer- 

 able myriads of living cells which constitute each tree 

 we should be stunned as with the roar of a great city." 

 All these phenomena, as we know, are but mani- 

 festations of energy. What is its source? From 

 careful experiment we have learned that it is not only 

 the breaking down of the circulating food substances, 

 but may be the disintegration of the protoplasm 

 itself. Made up of complex aggregations of matter 

 held together by the power of chemical affinity, the 

 protoplasm is a storehouse of potential energy that 

 may be translated into kinetic energy by the disinte- 

 gration of the unstable compounds composing it. In 

 proportion, then, as the plant does work of any sort, it 

 draws on its own substances for the energy requisite. 

 Here we have the direct reverse of the building-up 

 process just described. The circulating food sub- 

 stances or the living tissue itself is constantly break- 

 ing down and as constantly being renewed. This 

 continuous flux and flow is called Metabolism, the 

 tearing down process, Katabolism. 



Metabolism in Animals. The whole animal world 

 is dependent upon the plant world for its existence, 

 since even the flesh-eaters depend ultimately upon 



