Grafts et al. 



102 



Water in Plants 



but at all other states the hydrostatic pressure or turgor of the cells lowers 

 the D'PD of the water in the cells. At full turgor, or what is termed water 

 saturation, the diffusion pressure of water in the cell sap is equal to that of 

 pure water at the same temperature, and the DPD = zero. Cells in the 

 normal state are neither at full turgor nor at limiting plasmolysis ; proce- 

 dures employed in estimating DPD must be applicable to cells in a state of 

 partial turgor. The following methods have been used in DPD measure- 

 ments. In all of them, the water absorbing power of cells, tissues, or organs 

 is balanced by various means, such that there is no change in size, weight, 

 turgor, or vapor pressure. 



5^ ^o.osY 



*0.06 



*0.04- 



■^O-OZ - 



\ 



\ 



I 



I 



U e 4 6 S /O /2 /4- /6 /8 £0 22 24 26 28 30 J2 



Os/7?o//c Pressure faf/n. of £0° C.J 



Fig. 34A. — Diffusion pressure deficit values determined by the vol- 

 ume method. (Meyer and Wallace, 1941). 



The Cell Method: — Although the idea was suggested by de Vries (1884), this 

 method was first employed in actual measurements by Ursprung and Blum (1916fc). 

 A thin section of tissue, a bit of thin leaf, or any cells that may be observed under the 

 microscope may be used. The tissue is placed in paraffin oil on a slide to prevent 

 evaporation of water. One or more cells are selected, and their surface outlines are 

 drawn on paper by means of a camera lucida. The tissue is then placed in sucrose 

 solutions of varying concentration until one is found in which the cells do not change 

 in volume. The OP of the solution causing no change in volume is recorded as equi- 

 valent to the DPD of the cells. 



This method has certain weaknesses. Ernest (1931) has pointed out that upon 

 removing a tissue section from a plant organ, a change in equilibrium conditions occurs 

 and volume measurements may become invalid. In fact, gradients in DPD may be 

 reversed due to changing intercellular pressures and to the osmotic effect of sap liberated 

 by sectioning on the cells. She further demonstrated that water-saturated tissue, on 

 sectioning, can absorb yet more water due to partial removal of intercellular pressure. 

 Thus DPD values measured may be too high. Ernest (1931, 1934a, 6) used an epi- 

 dermal strip method, whereby intact mesophyll cells adhere to the epidermis, and found 

 leayes of Iris and Crocus, among others, to be suitable material. It has been pointed 

 out by Oppenheimer (1936), however, that even here there is a release of intercellular 

 pressure. This author (1930a, \9Z2a,h) further criticises the cell method. Volume 

 changes are difficult to detect and are often so small as to be insignificant within the 

 range of experimental error. Changes in the three dimensions are often disproportional, 

 so that volumes determined by surface measurements are erroneous. 



