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| − | Protoplasm. The living or organized cell-content. The difference between living and non-living things, so far as it has been possible to study it, consists in the fact that the former are characterized by the possession of protoplasm, "the physical basis of life." This protoplasm is a most complex material, the seat of diverse chemical reactions and physical changes, and at the same time a material having a wonderful capacity for correlation and growth. When the cell or living organism is killed, there is no loss of substance, and the material originally constituting this protoplasm remains, but there is left relatively little to suggest living protoplasm. So far as is known, this non-living residue can never be reendowed or activated with those characteristic properties of correlation and growth, and many other properties less complex, which are the potential or kinetic possessions of the living. It is in some ways unfortunate to call the dead material by the same name as the living.
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| − | In the living plant or animal, the protoplasmic unit is the cell, usually microscopic in size, and an association of cells of the same form, or with similar functions, constitutes a tissue. The spores of many fungi and of mosses and ferns are single cells. In plants the protoplasmic unit is usually surrounded by a resistant mem-
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| − | brane, or cell-wall, resulting in a high degree of rigidity and strength. In some cases, as in "woody" tissues, the cells become highly modified, the walls may be much thickened, and the protoplasm may disappear, leaving only the non-living cell-walls.
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| − | Since the protoplasm is the seat of the greater part of the chemical reactions and physical changes even in the more complex living plants, with it must be associated the absorption, digestion, and assimilation of foods, respiration, and excretion, as well as growth, reproduction, and heredity capacities. In the simplest plants, such as many of the lower algae (pond-scums), consisting of but a single cell, this cell must perform all the functions of the organism; but in complex plants there is a certain amount of differentiation of labor or function of the various protoplasts, or cell units. Thus the various tissues are more or less seats of different physiological processes; for example, the nectar-glands are "organs of excretion, the green tissues are the seats of organic food-making (see Photosynthesis).
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| − | The protoplasm of the cell is itself differentiated into various structures, important among which are (1) the cytoplasm, or general protoplasm, within which are (2) the nucleus, and (3) the plastids (in green plants).
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| − | Protoplasm is generally regarded as a viscid semi-fluid material, and commonly it behaves as a liquid (an emulsion colloid). When killed, protoplasm is "set," that is, it becomes a jelly-like matrix, and it is a study of such fixed material upon which have been based the earlier views regarding structure. There is strong evidence that much of what is called the finer structure of protoplasm is a result of fixation, and that there is actually little real "structure" in the living material, although certainly the gross appearance may change more or less with the diverse activities of the cell. Protoplasm cannot be expressed chemically; indeed, the view which is today most widely accepted is that it consists of numerous substances physically related, rather than of complex molecules of a definite "substance." B.M. Duggar.
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| − | Prumnopitys: Potocarpus.
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| | #REDIRECT [[Special:Whatlinkshere/Protoplasm]] | | #REDIRECT [[Special:Whatlinkshere/Protoplasm]] |