PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 19 



made, indications being simply given for each object as to the 

 isosmotic (i.e., giving equal pressure) concentration of sugar in 

 gram molecules. This magnitude, computed in atmospheres, 

 is designated as '^ osmotic concentration." 



This plasmolytic method of determining the concentration 

 of the cell sap is simple and convenient and therefore widely 

 used. However, it possesses many serious disadvantages. One 

 of the main drawbacks is that the very procedure of plasmolysis, 

 i.e., immersion of the cells in concentrated solutions, causes a 

 series of changes in them; in particular, it may produce a con- 

 siderable increase of osmotic pressure. Furthermore, the 

 plasmolyzing agent frequently penetrates into the cell, espe- 

 cially when solutions of nitrates or sodium chloride are applied. 

 Another serious difficulty is the fact that the adjoining cells, 

 even of the same tissue, frequently deviate in the magnitude of 

 their osmotic pressure, and still more so the cells of different 

 tissues, e.g., the epidermis and the mesophyll of the leaves. 

 This subject will be discussed in detail later on. For this reason, 

 the plasmolytic method is being more and more often replaced 

 by the cryoscopic method, i.e., by determination of the freezing- 

 point depression of the expressed sap. Since a gram-molecular 

 solution of any nonelectrolyte decreases the freezing point of 

 water 1.86°C. and since its osmotic pressure equals 22.4 atmos- 

 pheres, there may be computed from the data of the freezing- 

 point depression the total molecular and ionic concentration 

 and the osmotic pressure in atmospheres. In the case of precise 

 determinations, however, it is better to use special tables; for 

 the actual relations in concentrated solutions, such as are repre- 

 sented by the plant sap, deviate rather widely from ''ideal" 

 magnitudes. 



The absolute magnitude of osmotic pressure in the cells is 

 quite high. In land plants, it most frequently ranges from 5 to 

 10 atmospheres; in submerged fresh-water plants, it is consider- 

 ably 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 

 osmotic pressure depends chiefly on the accumulation in the cell 

 sap of soluble substances of low molecular weight. In the 

 majority of halophytes with an extraordinarily high osmotic 

 pressure, the substances are most frequently either sodium 



