Osmosis. The water to supply these absolutely essential needs comes from the soil, often apparently dry, but always containing at least a little moisture which the plant must obtain if it is to live.
This vastly important root function of absorption depends on a physical process called osmosis which may be defined as the mixing or diffusion of two liquids or gases of different densities, through a non-porous membrane — the greater flow being toward the denser substance.
Osmosis is one of the most important biologic processes, and upon it depends not only absorption in roots, but all forms of absorption in plant and animal, all digestive processes, excretion, respiration, and assimilation. Wherever a liquid or gas passes through any tissue, osmosis is the acting cause, controlled sometimes by the living protoplasm that lines the cell.
The essentials for osmosis are a dense liquid, a less dense liquid, and the osmotic membrane. In the root the protoplasmic layer lining the walls of the root hairs, acts as the membrane, the cell sap as the denser, and the soil water as the less dense liquid.
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