28 MINERAL SALTS ABSORPTION IN PLANTS 



The term permeability was formerly used to describe the facihty 

 with which ions or molecules pass into, or through tissues, 

 irrespective of the mechanism of transport (Stiles, 1924). In the 

 text below, permeability is used synonymously with conductivity, 

 that is, as the reciprocal of physical resistance, and thus it refers to 

 the ease with which ions or salts can move passively through a 

 membrane. The membrane in question may be a pauci-molecular 

 lipoprotein layer, a layer of cytoplasm, or a layer of cells, and the 

 structure involved should be indicated in any discussion of perme- 

 ability. It is often important to distinguish between the ease with 

 which ions penetrate into a membrane or cytoplasm {intrability- 

 Hofler, 1931) and their rate of passage across it {transmeability- 

 Arisz, 1945). Permeability can be expressed as grammes penetrating 

 per hour per cm^ of membrane surface for a given concentration 

 gradient and temperature. 



A number of physical mechanisms are involved in salt absorption, 

 and an outline of these processes follows as a prelude to con- 

 sideration of their importance in the plant. 



B. Diffusion 



If a pinch of common salt is placed in a beaker of water it 

 dissolves, and in time becomes uniformly dispersed throughout the 

 solution. The mechanism of dispersion, i.e. diffusion, is the random 

 thermal movements of solute and solvent. Across any plane in a 

 solution, particles (ions and molecules) diffuse in both directions, 

 and it is the difference between the numbers of individual particles 

 moving in opposite directions in a given time, or net diffusion, that is 

 usually measured. Gross diffusion rates can be determined using 

 isotopically labelled substances. When net diffusion is zero, particles 

 continue to move in the solution at rates determined by temperature, 

 and the nature of solute and solvent, but they move equally in all 

 directions. Only at absolute zero (-273 °C) does diffusion stop 

 completely. 



The relationship between diffusion and various factors which 

 affect it is summarized by Pick's law: 



dw= ~D.A. (dc/dx)-dr 

 where dm= the amount of substance diffusing in time df; 



