ABSORPTION, IN PLANTS. The process by which substances are taken into the body. A few plants only. being devoid of any external cover to the protoplasm, are able to engulf par ticles of food, !Bay then be digested. The most prominent of these are the Alyxonlycet es (q.v.), or slime molds. which in the period of their vegetative activity consist of a mass of naked protoplasm (called a plasmodium), some times ts large us one's two hands. These plas modia. like huge Anneke (q.v.), creep about and envelop particles of deenying organic matter, etc., on which they feed. The or reproductive bodies, of sonic Alpe and Fungi are also microscopic bits of naked protoplasm, but they probably do not ingest solid food during this period. Inasmuch as the protoplasm of most plants forms on its surface, as the first step of development, a thin jacket of cellulose or some similar material, the taking up of solid substances is thereby absolutely prevented. Whether the body consist of one cell or many, it presents to the surrounding medium a contin uous membrane with no visible openings. Through these eell-walls, therefore. neither solid nor gaseous substances can pass without pre viously undergoing solution. The materials whose absorption is to be explained are (I1 dissolved substances or solutes, and (2) the solvent, water.
( 1 ) Som-ms. The protoplasm itself and its surrounding membrane (the cell-wall) contain a large amount of water (50 to ). This water may be conceived of as lying between the particles of which the substances named are composed, much as it stands between the close set stalks of plants in a marsh. Since water always pervades the structures of plants, sub stances in order to enter tIn• plant body must be soluble in water. When so dissolved they behave essentially as gases: their molecules.
being then free to move apart, tend to distribute themselves throughout the solvent. But the diffusion of solutes is greatly retarded by the molecules of the water, so that it is much slower than the similar diffusion of gaseous bodies. It is also retarded somewhat by the particles of cell-wall when these also are encoun tered in the wafer. But. the distances between the particles of the cell-wall are relatively so great. that most solutes are able to pass freely between them. The structure of the protoplasm, however, is such that many substances cannot readily pass through it. Consequently, some materials which can enter the plant body may travel only through the cell-walls and may never enter the living protoplasm. The protoplasm permits at some periods substances to pass through it which at other times are excluded: probably due to ability to alter its structure on occasion. Such substances as can pass through the invisible spaces in cell-wall and protoplasm are therefore free to travel to any part of the plant body. If any such substances be removed from solution through use or storage, they will continue to be supplied from the regions of greater ahundanee, and consequently of greater pressure, to the regions of lesser pressure. i.e.. whei.e they are being used. The fact that different amounts of a given compound enter plants growing in the same soil is explic able mainly on this basis. Thus, wheat and clover may grow side by side; the ash of the wheat will contain 67.5% of silica, while that of the clover contains only 2.5%. This selective absorption must, however, be in part referred to the power possessed by protoplasm of regulat ing the admission of solutes.