536 MANUAL OF ELEMENTARY ZOOLOGY 



fermentation employed in the manufacture of alcoholic liquors, and from 

 it the term fermentation has been applied to other processes, such as the 

 souring of milk or wine and the putrefaction of dead bodies (p. 548), in 

 which an organism brings about a change in a mass of matter which is very 

 great compared with that of its own body. It has been shown that in 

 many such cases there is formed by the organism an enzyme (p. 50) which 

 brings about the fermentation without itself being destroyed in the re- 

 action. Such an enzyme has been found in the protoplasm of yeast. 

 Quite a different class of fermentation is found in certain bacteria which 

 obtain energy by the oxidation of inorganic substances, such as sul- 

 phuretted hydrogen, sulphur, ferrous salts, and ammonia. It is by 

 an action of the latter class that the ammonia compounds excreted by 

 animals, or resulting from the destruction of their bodies by putrefactive 

 organisms, are converted into nitrates for the use of the higher plants. 



Organic substances 

 of animals. 



Organic substances' 

 of plants. , 



Inorganic substances 



Fig. 394. — A diagram of the circulation of matter through 

 the bodies of organisms. 



The fundamental difference in nutrition between animals 

 and plants has an important result in regard to 

 of Nature their relation with the rest of nature and with 

 one another. In their action upon the in- 

 organic world these two kinds of organisms bring about 

 precisely opposite changes, and do so in such a way that 

 each sets up conditions favourable to the activity of the 

 other. The plant, absorbing the energy of the sun's rays, 

 builds up with storage of that energy complex organic 

 compounds from simple inorganic substances. 1 These 

 manufactured substances it assimilates, partly in repairing 

 the waste of its protoplasm, but mainly in adding to its 

 substance by growth. Its construction of organic materials 

 is in excess of its destruction of them, and the net result of 



1 The storage of energy is of course due to the fact that more is 

 absorbed in splitting the stable inorganic molecules than is freed in 

 forming the unstable organic molecules. 



