Page:Popular Science Monthly Volume 43.djvu/521

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505
HOW PLANTS AND ANIMALS GROW.

rented the essential constituents of the cell and active elements in plant nutrition.

The identity in essential features of Dujardin's sarcode and Von Mohl's protoplasm was pointed out by Cohn in 1850, and fully demonstrated by Max Schultze in 1858, who, adopting the term protoplasm, defined the cell as "a unit-mass of nucleated protoplasm, with or without a cell wall," and vegetable and animal physiology were thus placed on a common correlated basis. The original cell theory was materially modified and, in fact, superseded by the conception that the units of organized structure are masses of protoplasm, more or less intimately related, from and by which organic matters, including the cells and various tissues, were formed.

In 1868 Prof. Huxley translated protoplasm into the significant phrase, "the physical basis of life," and all vital activities were assumed to be the result of its inherent properties. While admitting the general pertinence of this assumption, we should not fail to notice that many of the inferences from the facts then known have not been verified in the progress of knowledge, and recent investigations have materially modified our views as to the real composition and constitution of protoplasm.

From what was known in regard to protoplasm twenty years ago, it appeared to be reasonable to assume with Huxley that there is "one kind of matter which is common to all living beings, and that their endless diversities are bound together by a physical as well as an ideal unity"; that vegetable and animal protoplasm are strictly identical; that "an animal can not make protoplasm, but must take it ready made from some other animal or some plant"; or, in other words, that the protoplasm made by plants from mineral matters is, in fact, the physical basis of animal life.

At the present time we may look upon protoplasm as the physical basis of life in the sense that some form of it is the essential and active constituent of every living cell or tissue, whether vegetable or animal, and that it is only formed through the physiological activities of living organisms. In the absence of life, protoplasm can not be formed, and, so far as we can perceive, there are no manifestations of life without it; but we can no longer assume that it is a substance of the same chemical composition and constitution in all the varied conditions under which it appears in the different groups of plants and animals, or even in the different organs of the same individual. Protoplasm is a convenient name for living substance, but we must bear in mind that it is the most complex and unstable of organic substances, and varies widely in structure, specific properties, and probably in chemical composition.