ABSORPTION, one of the first and most essential of the functions of animal and vegetable tissues. Both animals and plants grow and perform other vital functions through the agency of materials derived from without. The passage of all substances from the exterior to the interior of their bodies is effected by the function of absorption. This function is performed in all cases by the aid of animal or vegetable, membrane. This membrane is always in the form of the walls of cells or the walls of vessels formed out of cells. Whether the function of absorption be performed in animals or plants, there are certain general conditions of the membrane or cells through which it takes place, that are necessary in all cases. In the first place, as liquids are found to pass through the walls of cells and membranes, it is necessary that they should be permeable. This is found to be the case in all organised bodies, and in proportion to the permea bility of the tissue is the activity with which absorption is performed. In certain parts of plants, as well as animals, the cells become almost impermeable, and these are the parts which cease to grow or to perform active functions. Such are the duramen or heart-a ood of trees, and the nails, hairs, horns, and teeth of animal bodies.
During the performance of the various functions in which absorption is required, both liquids and gases pass through the cell-membrane or cell-wall. Liquids containing salts in solution pass into the plant and animal in the supply of food for nutrition. Gases, including the vapour of water, are also absorbed by the cells of plants as a nutritive process, and by those of animals during the performance of the respiratory function. This transmission of fluids through organic membranes is sometimes referred to as a peculiar vital property of animal and vegetable tissues; but it seems to depend considerably on the physical properties of the fluids and tissues. Organic membranes, when separated from the living structure, have the power of absorbing fluids, and if two fluids of different densities are separated by a mem brane, the flow through tho membrane will be greater from the thinner fluid to the thicker than the contrary. This action, which has been
Called WM106108131, seems to be a modification of that very general law of attraction by which solids are attracted tower's each other, as well as liquids and and which lios at the foundation of those phenomena attributed to capillary attraction. Although it would appear as the result of this law that there must be two currents, the one peening out and the other in, this does not always take place, as the perpetual removal, for the purposes of the system either of the plant or of the animal, of the matter absorbed, prevents the action of the outgoing current, which has been called Ereamesis.
The cells and surfaces which carry on absorption in the regdebre .Kingdom vary according to the circumstancee of the plant. In the simpler plants, such as the lower forms of A tylr, which consist of one or only a few cells, the whole of the cells ore equally employed in absorbing. But as we ascend to plants where the vegetative and reproductive organs are distinct, there we find absorption performed more abundantly by the former. In the higher forms of phanerogamic plants the active duties of absorption are performed by the roots ; the louse tissue at the ends of the fibrils of these organs being remarkably adapted for the performance of this function. The same power is also possessed by the recently formed tissues in the stems of these plants, and thus the food—the sap—is carried from the soil to the branches of the plant, which are covered with leaves. The cells of the leaves are adapted to the exhalation of the fluid which has been adsorbed below, and thus a perpetual demand for new supplies is created_ Not that the leaves are always exhaling ; in moist states of the atmosphere and at night they probably also absorb. This function is also undoubtedly performed by the stems of the leafless Euphor biarem and by the Cadatsce, which possess very small routs, and will even grow without them.