SOIL-PLANT RELATIONSHIPS AND PLANT NUTRITION 377 



are they interdependent. Whereas anion uptake is certainly "active" in the 

 sense that it is an energy-consuming process, there is some controversy as 

 to whether cation accumulation involves the expenditure of energy, though 

 an actively metabolizing cellular system is required. 



It has been found recently that some revision of the classical concept of a 

 root is necessary. Instead of picturing the external cells of roots as forming 

 a continuous semi-permeable membrane through which must pass the nutri- 

 ents, water, and oxygen necessary for plant growth, there is now considerable 

 evidence for the view that an external solution can freely penetrate some 

 parts of the root tissue. This region, accessible by simple diffusion, has been 

 described as the "apparent free space," the "free space," or, somewhat 

 ambiguously, the "outer space." The cells of some micro-organisms have 

 similarly been shown to contain certain areas or space, the liquid in which 

 comes quickly into equilibrium with the external solution. Reversible diffusion 

 studies give information as to the volume of this space in roots. In some roots 

 as much as one-third of the root volume has been shown to be accessible. The 

 most carefully determined value is 23 per cent for the roots of young barley 

 seedlings. This free space has not been defined histologically. The obvious 

 suggestion would be that the free space consists of connecting intercellular 

 spaces forming a ramifying system through the root. There is, however, rea- 

 son for thinking that the accessible regions may involve some part of the 

 cell cytoplasm. 



The importance of this concept of an accessible free space in plant roots 

 Is that the liquid in the free space may be that from which all active ion ac- 

 cumulation takes place. However, this liquid may contain soluble substances 

 of cytoplasmic origin, which then would be capable of "leaking" from roots. 

 Those root exudations, or emanations, that provide the substrate for the 

 rhizosphere population may have this origin. 



If the liquid in the free space is in equilibrium with that outside the roots, 

 then it might appear that the soil-solution theory would be adequate to ac- 

 count for the nutrition of the plant and that interest in the exchangeable base 

 system of the soil colloids would be limited only to its effect on the con- 

 centration of cations in the soil solution with which it would be in equilibrium. 

 This has to be modified, however, because root surfaces are capable of retain- 

 ing cations and have a measurable and rather high cation-exchange capacity 

 comparable to, but not identical with, that exhibited by the clay and organic 

 colloids of the soil. This cation-exchange capacity can be determined by meth- 

 ods similar to those used for soil or by titrating electrodialyzed plant roots. 

 The basis of expression leaves something to be desired, but the figures in 

 table 3 attest to the activity of root surfaces and to the important fact that 

 the roots of different plant species may vary widely in ability to retain cations. 

 Perhaps it is not coincidence that leguminous plants, which are high accumu- 

 lators of cations, have roots of high cation-retaining capacity. It is important 



