ASPECTS OF SALT ABSORPTION IN CELLS 97 



centration of salt in the sap eventually exceeded that in the proto- 

 plasm if the plants were kept under aerobic conditions. In the 

 absence of oxygen, salts appeared to move into protoplasm but not 

 into the sap, leading Hoagland and Broyer to the conclusion that 

 active transport occurs from the protoplasm into the vacuole, and 

 uptake into protoplasm may take place passively. 



2. Apparent Free Space 



Attempts have been made to determine more exactly the location 

 of permeability barriers in plant cells by estimating the volume of a 

 tissue which is passively penetrated by salts or organic solutes. 

 The principle of the method is as follows: if an unknown quantity of 

 water is added to a known amount of an aqueous solution of known 

 concentration, the volume of water added can be calculated from a 

 measurement of the change in concentration. Similarly, if a piece of 

 tissue is placed in a given volume of solution of known concentration, 

 under conditions which inhibit metabolism, the volume of tissue 

 penetrated passively can be calculated on the assumption that the 

 concentration of solute attained in this region is the same as that 

 in the external medium. To the space within a tissue into which 

 salts move passively to a concentration equalling that of the medium, 

 the terms "water-free space" (WFS) and "outer space" have been 

 applied. 



The method was used successfully by Conway and Downey 

 (1950) with yeast, and they calculated that about 26 per cent of the 

 volume of a packed cell suspension is penetrated passively by 

 inulin, gelatin and peptone. This volume is close to the theoretical 

 space between closely packed spheres of the size of yeast plants, 

 suggesting that these substances do not enter the cells at an appreci- 

 able rate. The volume of yeast suspension penetrated by some other 

 substances, including potassium and sodium chlorides, was found 

 to be 33-34 per cent of the total, and the additional volume was 

 identified as water-filled spaces in wet cell walls. In yeast, therefore, 

 it seems that there is a barrier to diffusion and exchange of ions at 

 the outer surface of the protoplasm, immediately within the cell 

 wall. Using the same technique, Mitchell (1954) established that a 

 barrier to diffusion of phosphate is present, at or near, the surface 

 of the protoplasm of Staphylococcus cells. 



Cowie et al. (1949) concluded, on the contrary, that the outer 



