ABSORPTION AND TRANSMISSION OF WATER 93 



absorption, the roots, and a special region of transmission 

 of water, the xylem, have been developed. 



But the great expanse of exposed surface would render it 

 utterly impossible that the living cells of one of the larger 

 plants be kept saturated with water, were it not for the 

 development of various kinds of thickened walls and even 

 layers of dead cells (e. g., cork) covering the exposed sur- 

 faces. By these means evaporation is greatly reduced. But 

 to carry on the photosynthetic process it is essential that 

 carbon dioxid be freely absorbed from the air. Now the 

 only manner by which this gas can reach the interior of the 

 living cells is by passing into solution in the imbibed water 

 of the cellulose walls and then diffusing through the tissues 

 as a solute. The complicated structures of stomata and air 

 chambers bring about a condition of things such that thor- 

 oughly saturated cellulose walls are exposed to the air, while 

 at the same time a minimum amount of evaporation is 

 allowed to go on. The plant would be a more economical 

 machine, in some ways at least, if it could avoid evapora- 

 tion entirely, but this is impossible if imbibed walls are to 

 be exposed to the atmosphere. By far the greater quantity 

 of the water absorbed through the roots finds its way out of 

 the green plant through the leaves, and is of no direct 

 material use. This evaporation plays an important part in 

 keeping the green parts cool when they are subjected to the 

 direct rays of the sun. It is probable also that much of the 

 energy for raising water in the plant comes from this molecu- 

 lar diffusion, which we call evaporation. The same process 

 produces a current of liquid up the stem and thus aids in 

 the transmission of solutes. 



A comparatively small amount of the absorbed water is 

 used as food material in the processes of photosynthesis, 

 growth, and general metabolism. In the lower forms without 

 chlorophyll the exposure of a wet membrane to the atmos- 



