Page:Popular Science Monthly Volume 63.djvu/246

242 an aqueous solution outside the cell, can there be any absorption. The submersed aquatic has many or all of its cells in direct contact with the water. The land plant has only those cells which touch, or are in, the soil which are regularly in direct contact with water. Except those plants living in swamp or marsh, and except immediately after heavy rain, land plants are able to obtain only those thin films of water held on the surfaces of the soil particles. To reach these films, to bring the solution within the cells into contact with the water (also a solution) on the soil particles, land plants develop hairs—the rhizoids of the lower forms, the root-hairs of the higher. An aquatic composed of a chain or of a film of cells has all its cells directly in contact with the water, which holds in solution oxygen, carbon dioxide, and those mineral salts which constitute its food materials. An aquatic composed of a mass of cells, on the other hand, has only some cells which are able directly to absorb food materials from the water, those cells on the surface. The surface cells constitute the absorbing organ. Under these are other cells, containing chlorophyll, which manufacture the absorbed food materials into foods. If the plant is small, there may be besides only those cells which are used for storing the manufactured product and those concerned with reproduction. If the plant is larger, like the rock weeds and kelps, there must be in addition a system of cells for conducting the foods from the cells manufacturing them to others needing them. In all aquatics, even the largest, unless some are land plants retaining the structures characteristic of land plants even after becoming aquatic, there is only this one system of conducting tissues, the one which distributes food.

As we pass from the submersed aquatics to those only periodically submersed, from these to plants living prostrate on the ground, like most liverworts, and from these to erect plants, we see progressive changes in absorbing and conducting systems. The plants living between the tide-marks, for example the rock weeds and devil's apron (Laminaria), possess a conducting system similar to the submersed kelps, but the absorbing system is reduced in extent to prevent the plant from losing water by evaporation while exposed at low tide. Jn these plants there is need of two sets of qualities, those adapted to life under water, those fitted to life in the air—essentially, enough cells for absorbing water, and enough cells so placed and of such composition as to keep evaporation within safe limits.

The prostrate land plants, for example the liverworts, possess tissues similar to the small though massive algæ living between the tide-marks—an absorbing system and a protective system. But as, for most of the time, the prostrate land plant can absorb water only from the soil underneath it, and lose water by evaporation only from its upper surface, the absorbing and protective systems are