The New Student's Reference Work/Absorption

Absorp′tion (in plants), the method by which almost all plants take materials from the outside into their bodies. Plants absorb water and a great variety of substances soluble in water. Because the protoplasm of plant cells forms about each a continuous covering, the cell wall having no visible openings, and because both cell wall and protoplasm hold among their particles large amounts (50–98 per cent) of water, all substances, whether solid or gaseous, must be dissolved in water before they can enter the plant. When so dissolved the particles are free to move through the water, and tend to distribute themselves uniformly. As the water outside is continuous with that forming part of the body, the particles may migrate into the plant almost as readily as in other directions. They will enter it if of suitable size, and if the water inside contains less of that substance than the water outside. The movement of each sort of material is independent, and a substance will continue to enter until it becomes equally distributed. If it is being used or stored, it may be absorbed in large amounts. Similarly water moves from the places where there are fewest particles of all the dissolved substances, i.e., where there is most water, toward the places where there is relatively less water. As water is constantly evaporating from land plants large quantities of water must be absorbed to balance this loss. The absorption of all substances is subject to regulation by the living protoplasm. Not all substances soluble in water are permitted to enter, nor at all times.