OSMOSIS 195 



chloride) or 5 to 20 per cent of sugar (sucrose), the cells that are 

 in 1 or 2 per cent salt or 5 to 8 per cent sugar will show no change, 

 while those in 3 per cent salt may, and those in 4 per cent are 

 certain to, show a slight shrinking away from the walls within a 

 few minutes. Those in 5 or 6 per cent salt will show a pronounced 

 shrinking away (B, Fig. 104), while those in 10 per cent salt (or 

 20 per cent sugar) will, in a few minutes look like spheres within 

 boxes much too large for them (C, Fig. 104). The protoplasts 

 have shrunk owing to reduction in turgor (but not in osmotic 

 pressure; this has increased, owing to increase in concentration of 

 sap from loss of water). 



A solution that has the same osmotic pressure as has the cell 

 sap (in the preceding experiment, this was true of 3 per cent 

 potassium chloride) is isosmotic or isotonic with the cell sap. 

 As we have said, solutions of lower concentration are hypotonic, 

 and those of higher concentration are hypertonic. The last 

 named alone cause plasmolysis. The concentration of an iso- 

 tonic solution (one which just does not cause plasmolysis) is 

 known as the critical plasmolytic concentration. The osmotic 

 pressure of a salt or sugar solution which is of a critical plasmo- 

 lytic concentration is the osmotic pressure of the contents of the 

 cell vacuole, for the two are isosmotic. In this way is the osmotic 

 pressure of plant cell sap determined. It tells nothing of the true 

 pressure or turgor within the cell. The turgor within plant cells 

 differs greatly. That this is true is likely from the great differ- 

 ences in osmotic pressures of cell saps. A common average is 

 10 atmospheres. The osmotic pressure of the cell sap of molds 

 (Penicillium) which grow on very concentrated solutions or in 

 the fumes of highly concentrated acid may reach maximum 

 values of over 100 atmospheres. This must be true if the living 

 plant is to draw water from solutions of high concentration, i.e., 

 of low water content. But this is osmotic pressure and not 

 turgor. The actual pressure within such cells need not be great. 

 In fresh and crisp plants, turgor is high; in wilted plants, it is 

 very low, but the osmotic pressure may not vary in the two cases ; 

 if the same plant is involved, the osmotic pressure will be greater 

 in the wilted plant of lower turgor. (Loss of water means higher 

 concentration and therefore higher osmotic pressure of the cell 

 sap.) High osmotic pressure suggests high turgor but is no 

 proof of it and is certainly not a measure of it. 



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