148 THE OSMOTIC QUANTITIES OF PLANT CELLS 



true of cells in the tissues of xerophytes and some water plants (Ernest, 

 1934 b). 



The fundamental relations between the osmotic pressure, wall pressure, 

 turgor pressure, and diffusion pressure deficit of a plant cell are expressed in 

 the following simple equations: 



Diffusion pressure deficit = Osmotic pressure — Wall pressure 

 Wall pressure = Turgor pressure 



For example, cell B (Fig. 31) after attaining an equilibrium with a solution 

 of 6 atmos. osmotic pressure, had an osmotic pressure of 10 atmos., and a 

 wall pressure of 4 atmos. Hence its diffusion pressure deficit was 6 atmos. 

 and its turgor pressure was 4 atmos. 



The quantity to which the name "diffusion pressure deficit" has been given 

 in this discussion has been variously termed by writers in the field of plant 

 physiology (Ursprung, 1935). Among the terms which have been used are 

 "suction," "suction force," "suction pressure," "suction tension," "water- 

 absorbing power," "turgor deficit," and "net osmotic pressure." All of these 

 terms are likely to be encountered in any extensive reading upon the subject 

 of the water relations of plants. The authors of this book are not convinced 

 that any of these terms is entirely free from criticism nor that any has as yet 

 gained the sanction of even approximately universal usage. Hence we shall 

 adhere to the term "diffusion pressure deficit" as a name for this physical 

 quantity throughout this book. Those who have a decided preference for 

 any other term need only to substitute it for "diffusion pressure deficit" 

 wherever it occurs in the text. 



The osmotic pressure, diffusion pressure deficit, turgor pressure, and wall 

 pressure are collectively called the osmotic quantities of plant cells. As pre- 

 vious discussion has shown a full evaluation of these quantities also requires 

 a consideration of the volume changes of plant cells. 



In plant tissues many of the cells are under a pressure imposed upon them 

 by the surrounding cells. In addition to its own wall pressure the water in 

 such a cell is also subjected to this pressure of external origin. This pres- 

 sure is just as much a factor in determining the diffusion pressure deficit 

 of a cell as its own wall pressure. The true diffusion pressure deficit of such 

 a cell is therefore less than that of the cell considered as an individual unit 

 by the amount of this added pressure. Hence the equation representing the 

 factors determining the diffusion pressure deficit of a cell under such condi- 

 tions becomes: 



Diffusion pressure deficit = Osmotic pressure — Wall pressure — 

 Pressure exerted by surrounding cells. 



