Page:Popular Science Monthly Volume 84.djvu/427

Rh efficiency of the leaf as a food-forming organ decreases notably as the incipient drying stage is reached and long before externally visible, wilting or flagging is shown. The skilful agriculturist will, therefore, irrigate his crops not when they wilt but when the proportion of water in the leaves falls below a certain point.

Still another feature of relative transpiration and incipient drying remained to be detected and measured. Evaporation, of course, tends to render heat latent and hence keep down the temperature of leaves. Variations in transpiration should therefore be accompanied by characteristic temperatures. A calorimeter for the requisite measurements was designed by Mrs. E. B. Shreve. Leaves were put into a chamber filled with turpentine which penetrated the tissues quickly and realized the temperatures at once. The temperatures were found to meet expectancies, even in the stages of incipient desiccation, where the lessened water-loss was accompanied by the development of a degree of heat which might affect the efficiency of the leaf in food-formation.

Returning to the figure of the plant as a cylindrical mass of colloids, it is to be said that the water which enters the plant at its roots does not move as in a tube directly to the upper end where it is transpired. The cylinder may in effect be enlarged or variously developed to such shape that a surplus of water accumulates and if the supply be cut off from below the amount on hand may be such that the plant lives for a season, a decade or even longer upon the water on hand. I have carried out a series of measurements upon this phase of the water relations of plants during the last six years and find that many plants of arid regions in South America, North America and Africa show such accumulation of water. The sap of such plants under normal conditions shows about the concentration which gives an osmotic pressure of three to twelve atmospheres. When the supply is cut off the loss of water continues with the result that the concentration increases four or five times. The desiccation of these plants, however, is not simply that of drying out. The rate of loss decreases much more rapidly than would be justifiable on the facts of amount of water present," and one is led to infer that the plant again to be thought of as a cylinder of jelly undergoes changes of its colloids which tend to prevent transpiration. Whether such changes are reversible as in incipient desiccation or not is a matter yet to be determined. Without going into detail at all it may be said that the continued depletion of the store of water of a succulent is responsible for many important features in the life-cycle of the plant, in growth and reproduction and in survival (see Fig. 4).

The consideration of the facts brought to light in a study of the balances or accumulations of water in plants formed a basis for an analysis of the conditions of parasitism in the higher forms. This is primarily a water-relation. When one plant as, for example, the mistletoe, is parasitic on another, such conditions must be present as to cause water