the plant is practically continuous with that in the substratum. The absorption of water by the plant, or rather the movement of water from the soil into the plant is influenced by the agencies which affect diffusion, osmosis and adsorption anywhere. In other words, the properties of protoplasm, as a mixture of colloids, cause water to move from the substratum into its body. The further movements of water, such as the ascent of sap, may be taken to result similarly from physical conditions. The uppermost part of the gelatinous mass of the plant ramifies as does the lower, but in this case into flattened organs or leaves. Here, as well as everywhere, on the external surfaces of the plant evaporation of water takes place in a manner modified by the specialized character and structure of the surface as well as by the relative humidity, temperature and movements of the air. This transpiration, or loss of water from the exposed surfaces, is a process of such importance that it is impossible for the plant to maintain growth to any extent, or carry out normal development without it. Consequently it has come in for a great deal of attention during the last century. Much of the work has been of a
Fig. 3. Sectional Diagram Showing the Essentials of the Spherical Porous Cup Atmometer with Non-rain-Absorbing Device, as frequently arranged. A rubber stopper in the bottom bears besides the supply tube to the first mercury valve, a larger tube for filling. The latter has a mask 10 serve as zero point and is covered by a loose-fitting cap. Suction through the open tube at extreme left removes the air from the system and fills the whole with water. Mercury in the valves is shown as if evaporation were in progress; when rain occurs the column rises in the right-hand valve and falls in the other.
