GROWTH METABOLISM 75 



go on at a fairly rapid rate, and in connection with them 

 there is a using up of materials, and so, of necessity, 

 there must be a replacement of this material. Living 

 things require food, therefore, simply to enable them 

 to go on living, even though they may not do anything 

 else. 



Grov^th Metabolism. — Living things grow by mak- 

 ing new protoplasm which is added to that which they 

 already possess, and by manufacturing such non-living 

 materials as are necessary to growth. For example, the 

 cellulose walls of plant cells, and the hard parts of the 

 bones in animals, are non-living materials which are made 

 by living protoplasm in connection with growth. In 

 plants, growth metabolism goes on more or less continu- 

 ously throughout life; in many of the lower animals the 

 same thing is true ; but in all the higher animals, includ- 

 ing man, growth metabolism (except in the skin and in 

 the reproductive and blood-forming tissues) goes on only 

 during the early part of life. When the animal or human 

 being has, as we say, gotten its growth, this metabolism 

 for the most part comes to an end. 



The Manufacture of Material. — In addition to 

 those manufacturing processes connected with growth, 

 which have just been spoken of, many kinds of cells, 

 both plant and animal, make special kinds of substances, 

 some of which serve the organisms which make them, 

 as the saliva, made by the cells of the salivary glands, 

 serves in the digestion of food; others, like the opium 

 which the poppy plant makes, have no function that 

 we know of in the life of the organisms which pro- 

 duce them. When one recalls the long lists of animal 

 and plant products that are used in industry, and, 

 as drugs, in medicine, the importance of this sort of 

 metabolism is clear. 



The Metabolism of Active Function. — Practically 

 all kinds of animals and a good many kinds of simple 

 plants perform active motions of one kind or another. 



