HOW PLANTS AND ANIMALS GROW. 507 



drogen, and oxygen, with from fifteen to eighteen per cent of 

 nitrogen and a variable quantity of ash constituents, and they 

 present marked differences in their general appearance and prop- 

 erties. The white of an egg, the casein of cheese, the glutin of 

 wheat, and the legumin of peas and beans are often referred to 

 as typical proteids, but they in fact represent several kinds of 

 proteids which differ in many properties, and can not be assumed 

 to have precisely the same physiological significance and value as 

 nutrients. 



The group of fats includes a great variety of compounds com- 

 posed of carbon, hydrogen, and oxygen, without nitrogen, and 

 their properties are various. The carbohydrates are likewise 

 composed of carbon, hydrogen, and oxygen, without nitrogen, and 

 they include starch, sugar, cellulose, woody fiber, and allied sub- 

 stances, differing in form and various properties, so that their 

 physiological value can not be the same. Oxygen is the most 

 abundant element of the animal body as a whole, and it stands 

 next to carbon in the percentage composition of the proximate 

 constituents of the tissues. Its significance as a food element is 

 too often overlooked, but it is undoubtedly as important a factor 

 in tissue-building as any other food constituent. 



Protoplasm was formerly looked upon as a proteid, but it is now 

 generally admitted that its composition and structure are very 

 much more complex than any form of proteid. The chemical 

 composition of living protoplasm, as already pointed out, can not 

 be determined, but there is evidence that proteids, fats, and carbo- 

 hydrates enter into the composition of its complex molecules, and 

 it gives rise to all three of these groups of nutrients in the pro- 

 cesses of destructive metabolism, and it should also be noted that 

 several varieties of proteid matter have been detected in dead 

 protoplasm. 



Energy has been defined as the power of doing work, and it is 

 expended in the work involved in building protoplasm out of the 

 simpler proteids, fats, and carbohydrates from which it is formed. 

 An essential constituent of the complex molecules of protoplasm 

 which is neglected in chemical analysis is the potential energy 

 stored up as a result of the constructive process, which is liberated 

 in the form of heat in destructive metabolism. The properties of 

 living protoplasm, and its role in the vital activities of plants and 

 animals, have been more definitely determined than its chemical 

 constitution, and although it is generally admitted to be the domi- 

 nant factor in nutrition, there is yet much to learn in regard to 

 its properties and specific action in its diverse forms. 



Living protoplasm, or, in other words, living substance, must 

 be looked upon as constantly undergoing changes that vary with 

 the function required of it. These changes, without attempting 



