MEASURLNG DIFFUSION PRESSURE DEFICITS 151 



substance than in the surrounding water (Chap. IX), We may therefore 

 also speak of the diffusion pressure deficit of an imbibing substance. A block 

 •^f dry gelatin, for example, dropped into a solution with an osmotic pressure 

 of 10 atmos. will, after a d^'namic equilibrium has been established, have a 

 diffusion pressure deficit of 10 atmos. A completely saturated imbibant will 

 have a diffusion pressure deficit of zero. The development of what we have 

 just called the diffusion pressure deficit of an imbibing substance is a more 

 complicated phenomenon than the development of the diffusion pressure deficit 

 of a solution, but further details cannot be considered here. 



Within a plant cell the cell sap, the water in the protoplasm, and the 

 water in the cell wall may each be regarded as possessing a diffusion pressure 

 deficit of its own. The diffusion pressure deficit of the protoplasm and the 

 cell wall are at least in part of im.bibitional origin. In an isolated plant cell 

 from which evaporation of water is prevented the diffusion pressure deficits of 

 all parts of a cell are usually approximately in equilibrium. If immersed in a 

 solution all parts of a cell will more or less rapidly attain an equilibrium, not 

 only with each other but with the surrounding solution. 



If the magnitude of the diffusion pressure deficit of any part of a cell is 

 either increased or decreased the values for other parts of the cell tend towards 

 equilibrium with this new value. For example, if the diffusion pressure 

 deficit of the mesophjll walls of a cell is increased due to evaporation from 

 them during transpirational water loss, water will diffuse toward the walls 

 from the protoplasm and cell sap as long as their diffusion pressure deficit is 

 less than that of the walls. Similarly if the osmotic pressure of the cell sap is 

 increased by a conversion of insoluble starch to sugars, water will diffuse into 

 the cell sap from the wall and protoplasm until a diffusion pressure deficit 

 equilibrium is attained within the cell. 



Methods of Measuring the Diffusion Pressure Deficit of Plant Cells 

 and Tissues. — By the methods at present in use it is possible to measure the 

 diffusion pressure deficit only for cells with elastic walls. Most methods 

 of measuring this quantity' are based on the principle, previously discussed, 

 that if a cell is immersed in a solution with an osmotic pressure equal to the 

 diffusion pressure deficit of the cell, a dynamic equilibrium is immediately 

 established, and no change will occur in the volume of the cell. In solu- 

 tions with a higher osmotic pressure than the diffusion pressure deficit of the 

 cell, it will decrease in volume; in solutions of lower osmotic pressure than 

 its diffusion pressure deficit the cell will increase in volume. 



The first step in making determinations of the diffusion pressure deficits 

 of plant cells is to prepare a series of sucrose solutions graded at appropriate 

 intervals in osmotic pressure. The range of the series to be used will depend 



