Page:Popular Science Monthly Volume 43.djvu/523

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507
HOW PLANTS AND ANIMALS GROW.

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 properties. 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 composed 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 substances, 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 carbohydrates enter into the composition of its complex molecules, and it gives rise to all three of these groups of nutrients in the processes 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 dominant 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