ENVIRON 1C FACTORS 423 



efficiency of the leaf as a food-forming organ decreases notably as the in- 

 cipient 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 char- 

 acteristic 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 ex- 

 pectancies, 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. 



Eeturning 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 re- 

 gions in South America, North America and Africa show such accumu- 

 lation 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 con- 

 tinues 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 justi- 

 fiable 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 re- 

 sponsible 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 bal- 

 ances 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 



