PLANT ROOT-SOIL INTERACTIONS 667 



from the work of Frey-\^' yssling ( 1954 ) and of Scott et al. ( 1956 ) . The 

 diagram emphasizes the following prominent structural features of the 

 root: ( 1 ) The frame structure of the cell wall (one micron thick), com- 

 posed of relatively inert cellulose fibrils ( m ) , about 200 Angstrom units 

 in diameter. (2) The pore spaces, or free spaces (/), of ultramicro- 

 scopic proportions, filled with water, solute, and gases. (3) Interfibril- 

 lar material (/;), composed of pectic substances, hemicelluloses, etc., 

 which possess reactive carboxyl, alcoholic hydroxyl, and possibly other 

 groups. The irregular cross-hatching denotes localized areas of oriented 

 thread molecules. (4) Cytoplasm, with strands into the cell wall. Out- 

 side the root ( right-hand side of drawing ) is a portion of the soil phase. 

 The black parallelepipeds denote colloidal clay particles of various 

 sizes, or iron-oxide grains and rods; the round and oval areas represent 

 a virus (V) and a small bacterium (B). Ordinary molecules and ions 

 are too small to be shown. In the water-saturated state the blank areas 

 represent the solution phase— pore solution in the cell wall and soil 

 solution on the outside. The two intercommunicate freely. 



In the solution phase, entry of ions and molecules into the root is 

 considered akin to conditions that exist in a culture solution. For these 

 the investigations accomplished are legion. They will not be examined 

 here, as excellent surveys have recently been published (Robertson, 

 1956, 1958). Rather, we shall focus attention upon the interaction of 

 the gel frame structures, represented by close-packed stacks and arrays 

 of clay particles, and by the aggregates of intermicellar material in the 

 cell wall. 



Ion-exchange properties of soils and roots 



The knowledge that soils, specifically clays, possess ion-exchange 

 properties is over a century old ( Deuel and Hostettler, 1950 ) . During 

 eleven decades and on tortuous paths, countless soil chemists have ex- 

 plored and clarified the base-exchange process experimentally. They 

 have developed theoretical models on thermodynamic and kinetic 

 grounds and on the basis of Donnan equilibria. With the onset of the 

 resin industry, ion exchange has become a scientific and technical com- 

 monplace. In recent years it has been applied to roots. 



Briefly, ion exchange with clays is stoichiometric and is repre- 

 sented by equations such as the following: 



K-clay -f NaCl , Na-clay + KCl 



Ca-clay + 2 NaCl , 2 Na-clay + CaCb 



At times it is advantageous to stress the colloidal nature of the clay 



