Chapter VII — 105 — Osmotic Quantities of Cells 



volved, amounting to as much as 71 per cent of the average root pressure 

 (see also Chapter IX, page 150). White (1938) also postulates an active 

 component in absorption of water by roots. Probably the method is more 

 valuable in studies on the physiological activity of roots than in osmotic 

 pressure or DPD determinations. 



Several other techniques have been employed in DPD measurements: 



a) Hard leaf method (Ursprung and Blum, 1927). This method is adapted to 

 such structures as sclerophyllous leaves, conifer needles, grass leaves, and horsetails. 

 Changes in thickness instead of length are determined. 



b) Lever method (Ursprung and Blum, 1930). A modification of the above; 

 changes in thickness are indicated by a delicate pointer. 



c) Tissue tension (de Vries, 1884). Release of tension on splitting a growing 

 shoot tip produces a curvature v^-hich can be sketched on paper (Livingston, 1906) or 

 projected (Bouillenne, 1932). 



d) "Schlieren" method (Arcichovskij and Ossipov, 1931a), whereby the con- 

 centration of the reference solution is balanced against that of the plant sap so that 

 their refractive indices are the same. This method is designed for use on growing 

 stems. 



e) Potometer method (Nordhausen, 1921; Arcichovskij and others, 1931). In 

 this a reference solution is brought into contact with the tissue being studied and change 

 in volume is registered on a potometer tube. This method is adapted to measuring the 

 DPD of water in tissues of attached stems. 



/) Vapor pressure methods. Here the vapor pressure of water in the air in a 

 small enclosed chamber is regulated such that there is no change in volume of a tissue 

 piece (usually area or length is measured) (Ursprung and Blum, 1930; Renner, 

 1932) ; or weight (Arcichovskij and Arcichovskaja, 1931) of cells. The difficulties 

 inherent in the method are related to temperature control of the apparatus and the 

 rather long duration of time required for equilibrium. If the tissue is weighed, con- 

 densation of water in the intercellular spaces may give trouble. 



Methods for DPD determination in intact organs and whole plants will 

 be discussed in more detail in Chapter IX. 



The Magnitude and Fluctuation of DPD Values : — Diffusion pres- 

 sure deficit values lie in the same general range as that found for osmotic 

 pressures. The latter are commonly employed as an index of maximum or 

 potential DPD's which can be developed in cells. Although in most cells 

 the DPD should not ordinarily exceed the OP, instances have been re- 

 ported where this seemed to be true. Pisek and Cartellieri (1931) 

 noted such a result in leaves of several plants, and for the most part in the 

 afternoon. In their opinion, the simplified method, used for DPD, yielded 

 values which were too high, especially when leaves in a wilted or nearly 

 wilted condition were measured. Kerr and Anderson (1944), finding 

 that DPD values exceeded OP values in developing cotton seeds, concluded 

 that imbibitional phenomena were the cause, and that cryoscopic values 

 were too low. 



That a considerable proportion of the DPD may at times be due to the 

 development of high tensions has been emphasized by some. Such negative 

 wall pressures, in intact leaves of trees, especially during periods of ab- 

 normally high water loss, may exceed the influence of osmotic effects on 

 the DPD (seeCuv, 1936). 



The variation, in particular the diurnal variation, is much greater for 

 DPD than for OP. This is made clear by visualizing a cell which at limit- 

 ing plasmolysis exhibits an OP of a given value, say 20 atm. As water is 

 absorbed, the OP decreases from 20 to perhaps 16 atm., while the DPD 

 decreases from 20 to atm. In cells where tensions can develop, a greater 

 range is possible. Thus it becomes evident why OP values obtained by use 



