EN V IRONIC FACTORS 



421 



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 sub- 

 stratum 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 gelatin- 

 ous mass of the plant ramifies as does 

 the lower, but in this case into flat- 

 tened organs or leaves. Here, as well 

 as everywhere, on the external sur- 

 faces of the plant evaporation of water 

 takes place in a manner modified by 

 the specialized character and struc- 

 ture of the surface as well as by the 

 relative humidity, temperature and 

 movements of the air. This tran- 

 spiration, 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 devel- 

 opment 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 De- 

 vice, as frequently arranged. A rubber 

 stopper in the bottom bears besides the sup- 

 ply 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. 



