ABSORPTION AND MOVEMENT OF WATER 137 



able to control the rate and the amount of evaporation. 

 Because evaporation from the body of the living plant is 

 controllable within certain limits by the plant itself, and to 

 this extent is a physiological process, it has been given the 

 separate name of transpiration. With this idea of evapora- 

 tion controlled by the living organism has been coup- 

 led the notion that water is vaporized by the chlorophyll 

 grains illuminated by sunlight and manufacturing carbo- 

 hydrates, and that this water-vapor, produced by physical 

 means acting through the living organs of the plant, is 

 an important part of the total volume of water given off. 

 This process Van Tieghem* distinguishes from evaporation 

 by the name of chlorovaporization. Assuming that water is 

 set free in combining carbon-dioxide and water into carbo- 

 hydrate, it is hard to conceive that the liberation of water 

 in a water-containing cell would be in the form of escaping 

 vapor any more than in the familiar reactions carried on in 

 solutions in the laboratory. The water molecules liberated 

 at the temperatures prevailing in cells photosynthetically 

 active would mix with the water in the cell-sap both in the 

 protoplasm and in the vacuoles. Because light is absorbed 

 in chlorophyll-containing cells, the temperature of these cells 

 may be (but not necessarily will be) higher than of other 

 ceUs not absorbing light. If this is the case, and if these 

 cells are warmer than the air, evaporation will of course 

 take place. Transpiration is, therefore, a physical process 

 controlled but not carried on by the living plant. Accord- 

 ing to circumstances it may be more or less rapid than 

 simple evaporation. 



Plants living in regions where the rain-fall is slight or is 

 very unequally distributed through the year, and where the 

 soil is not an efficient reservoir of water, are forced not only 

 to store water in their bodies (see p. 124) but also to 

 check the loss of water by every possible means. The 

 greatest economy of water is shown by the Cacti. In the 

 most condensed forms we have nearly spherical plants, the 



* Van Tieghem, Ph. Traite de Botanique, t. I., p. 185, 1891. Also 

 Transpiration et chlorovaporisation. Bulletin de la Soc. Bot. de France, 

 1886. 



