Chapter VII — 81 — Osmotic Quantities of Cells 



posite direction: (a) an "electrostatic valve" effect, a function of the in- 

 tensity of the electrokinetic potential at the pore walls, and (b) a mechani- 

 cal filter effect. It is shown that, as is true for certain cellophane mem- 

 branes, the permeability to water exhibited by plant cells is determined 

 largely by factor (o) above. Permeability is thus greatest at the iso-electric 

 point of the protoplasm ; hydration and viscosity are minimal at this pH. 

 In dense protein membranes, such as calf's bladder, the degree of hydra- 

 tion (swelling) and water permeability run hand in hand, i.e., factor (b) 

 predominates. Contrary to the finding of the above authors, de Haan 

 (1933) found that the water permeability of onion bulb scale epidermis 

 increased as the hydration of the protoplasm increased. There are many 

 factors to consider in interpreting permeability phenomena. 



Permeability of Cell Walls: — It has been conveniently assumed, in 

 discussing the passage of substances into and out of cells that the cellulose 

 walls are truly permeable to water and solutes alike. Actually all cell walls 

 would be expected to offer some resistance to the passage of water and dis- 

 solved materials, but in most instances this resistance is not important when 

 compared to that of the outer surface of the protoplasm. For most par- 

 enchyma cells, with normally hydrated walls, this is certainly true. On the 

 other hand, certain cell walls, those containing cutin or suberin (i.e., cork), 

 and those in seed and fruit coats may be almost completely impermeable. 

 Some are clearly differentially permeable. The work of Brown (1909) 

 showed that barley grains readily absorbed water from a number of salt 

 solutions, but the salt remained excluded. Shull (1913) lists a number 

 of substances to which the seed coat of cocklebur is impermeable, as well as 

 several which can penetrate. Denny (1917) found great variation in the 

 passage of water through seed coats and the outer bulb scale of onion. A 

 difficulty resulting from differential permeability of cell walls has been met 

 in plasmolytic studies (see page 82). 



Osmotic Pressure. Methods of Measuring Osmotic Pressure in 

 Plants. The Plasmolytic Method: — Though discovered previously by 

 Pringsheim and Nageli, the plasmolytic method for determining osmotic 

 pressure in plants was not employed until about 1877, when de Vries and 

 Pfeffer used it in their studies. Because little equipment is required, and 

 because observation of living cells is fascinating, studies on plasmolysis 

 have been popular with cell physiologists ; there is a wealth of literature on 

 the subject. Mention should be made of Ursprung's (1938) monograph 

 on the measurement of osmotic properties of cells, in which practically all 

 important findings are reviewed. Other general works which may be con- 

 sulted are Brauner (1932), Strugger (1935), Kuster (1935), Hober 

 (1945), and the periodical Protoplasma which contains one or more papers 

 on plasmolysis in almost every issue. 



Plant cells immersed in hypertonic solutions exhibit plasmolysis. This 

 phenomenon is characterized by separation of the protoplasm from the cell 

 wall due to loss of water from the cell contents and contraction of the 

 vacuole and surrounding cytoplasm. Movement of the water is caused by 

 the gradient of diffusion pressure established when the tissue is immersed 

 in hypertonic solution. Onset of plasmolysis, due to excess shrinkage on 

 the part of the protoplast over shrinkage of the cell wall, depends upon the 

 relative elasticity and rigidity of these two phases. Cell walls are usually 

 the more rigid. 



