FACTORS AFFECTING SALT ABSORPTION 65 



absorption without any appreciable effect on respiration (see p. 89). 

 An account of the Hght which the use of inhibitors has shed upon the 

 mechanism of salt absorption is given below. 



B. Inherent Factors 



1. Surface-volume Relations 



Since the rate at which salts are transported across surface 

 membranes may be a controlling factor in absorption it is not 

 surprising that uptake tends to be related more closely to surface 

 area than to volume. The problem is presented in its simplest form 

 in cultures of micro-organisms where small cells exhibiting a high 

 surface area to volume ratio absorb salts more rapidly per unit of 

 volume (although not necessarily per unit of surface area) than do 

 large cells. The same relationship holds also between cell volume 

 and a variety of other metabolic activities, including respiration. 



When cells are packed together in a tissue, as in a root or storage 

 organ, an additional surface becomes important, namely the surface 

 of the tissue in contact with the external medium. In general, when 

 other things are equal, there is a close relationship between the 

 surface area of a root system in contact with the soil, and the rate 

 of sah absorption. Steward and Harrison (1939) observed a 

 correlation between surface area and absorption of salt in experi- 

 ments with potato slices of varying thickness (Fig. 22a). In this 

 material, cells near the surface which are in an actively metabolizing 

 and absorbing state represent a greater proportion of the whole 

 number in thin slices than in thick ones, and hence absorption is less 

 active per unit of tissue volume in the latter than in the former, 



2. Internal Salt Concentration 



As the concentration of salt in a tissue rises, absorption occurs 

 more slowly. Hoagland and Broyer (1936) found that roots grown 

 under conditions of minimal nutrient supply (low-salt roots) possess 

 a much greater capacity for subsequent salt absorption than those 

 grown under normal conditions. In non-growing cells of storage 

 tissues, the rate of salt uptake decreases as the internal concentration 

 rises and eventually stops (Fig. 22a). This effect of internal con- 

 centration is apparently directly on the absorption mechanism, 

 rather than the result of changes in the rate of passive leakage 



