IMPORTANCE OF WATER 115 



This plasmolytic method of determining the concentration of 

 the cell sap is simple and expedient and therefore widely used. It 

 is especially valuable when one has to deal with single cells or 

 with tissues of small volume, from which it is difficult, or even 

 impossible, to obtain the amount of sap required for the usual 

 physicochemical methods of determination of the concentration of 

 solutions. When the osmotic pressure is to be ascertained in 

 sufficiently massive organs yielding much sap, then, instead of the 

 plasmolytic method, the cryoscopic method may be used. The 

 sap is squeezed from the minced or frozen tissues by means of a 

 strong press, and the freezing point is determined. Knowing that a 

 gram-molecular solution of any non-electrolyte lowers the freezing 

 point of water by 1.86°, it is easy to calculate the molar concentra- 

 tion of the sap from the freezing-point depression. Both methods 

 give the same results, thus confirming the correctness of the values 

 obtained by plasmolysis. 



The absolute magnitude of osmotic pressure in the cells is 

 rather high. In land plants it most frequently ranges from 5 to 10 

 atmospheres, in submerged fresh-water plants, it is considerably 

 lower — from 1 to 3 atmospheres. In plants native to saline soils 

 or dry habitats the osmotic pressure is very high, reaching 60 to 80, 

 or even 100 or more atmospheres. The magnitude of pressure 

 depends chiefly on the accumulation in the cell sap of soluble 

 substances of small molecular weight. In the majority of halo- 

 phytes with an extraordinarily high pressure the substances are 

 most frequently either sodium chloride or other mineral salts. In 

 some plants, osmotic pressure is induced chiefly by sugars and 

 organic acids. 



Such immense magnitudes of the osmotic pressure in the cells 

 appear improbable at first sight and give rise to questions as to 

 how the thin cell walls are able to withstand the tension. A more 

 detailed study of the question shows, however, that the magni- 

 tudes of the osmotic pressure, calculated according to the concen- 

 tration of the cell sap, considerably exceed the actual hydrostatic 

 pressure on the cell walls. The solution absorbed from saline 

 soils, surrounding the cells of halophytes, shows a very high con- 

 centration, and the cell wall is subjected to the excess of the inner 

 over the outer pressure. If these plants are placed into pure water, 

 the abrupt increase of hydrostatic pressure of the contents leads 

 to a strong distention of the cell walls and sometimes t 



LilLIB 



