DIURNAL VARIATIONS IN OSMOTIC QUANTITIES 293 



movement of water can occur as is the case when water is supplied by 

 means of auto-irrigators. 



In herbaceous plants and probably also in many woody species the lag in 

 the rate of absorption behind the rate of transpiration is largely due to the 

 resistance of the living cells of the root to the passage of water (Kramer, 



1938). 



Diurnal Variations in the Osmotic Quantities of Plant Cells. — Deter- 

 minations of daily variations in the water content of leaves and other plant 

 organs have been valuable in elucidating the principle that development of an 

 internal water deficit is a phenomenon of almost daily occurrence in most 

 plants during their growing season. However, such determinations alone are 

 inadequate to give a complete picture of the dynamic aspects of the internal 

 water relations of plants. The same change in water content, for example, 

 which induces a certain shift in the turgor pressure of the leaf cells of one 

 species may have a very different effect upon the turgor pressure in the leaves 

 of another species. In general the influence of fluctuations in the water 

 content upon internal movements of water and upon physiological processes 

 can only be fully interpreted if the status of the water present is expressed 

 in terms of diffusion pressure deficits or other dynamic units. 



The daily variation in the osmotic pressure of leaves has already been 

 discussed (Fig. 28). The daily variation in the average diffusion pressure 

 deficit of the leaf cells follows a somewhat similar trend. Under "standard 

 day" conditions (Chap. XIII) the diffusion pressure deficit of the leaf cells is 

 usually low throughout the early morning hours, rises until late afternoon, 

 then decreases during the night hours. During the early morning hours the 

 leaf cells often approach their maximum turgidity. In the late afternoon 

 their turgidity is often low (incipient wilting) and their diffusion pressure 

 deficit approaches the osmotic pressure of the cells (Herrick, 1933). When 

 the diffusion pressure deficit of the leaf cells becomes equal to their osmotic 

 pressure, the turgor pressure of the leaves is zero, and the leaves are in a dis- 

 tinctly wilted condition. As subsequent discussion shows in at least some kinds 

 of plants it is possible for the diffusion pressure deficits of leaf cells to exceed 

 their osmotic pressures. 



The increase in the diffusion pressure deficit of leaf cells which is usually 

 observed during the forepart of the daylight period results from the simul- 

 taneous operation of the factors of the increasing osmotic pressure and de- 

 creasing wall pressure, the latter in turn being due to the gradual diminution 

 in the volume of water in the cells. Similarly the decrease in diffusion pres- 

 sure deficit which usually begins sometime during the afternoon, and continues 

 during the night hours, is due to the concurrent effect of a decreasing osmotic 



