Hans Jenny 119 



corresponds to a band having a width of about 1 mm.* Now, one 

 square millimeter of root surface will accommodate 10 s clay platelets 

 having an edge length of 100 mu. On each of these clay particles the 

 surface facing the root contains 6000-7000 exchangeable monovalent 

 cations. Accordingly, in a clay soil the ion concentrations surrounding 

 one square millimeter of root surface is tremendous and contact feeding 

 cannot be a limiting factor in supplying ions to the root surface. 



The second objection of Wanner, the existence of an inert cellulose 

 membrane between the clay surface and the outer surface of the 

 cytoplasm, is also voiced by Lundegardh (29). 



The thickness of the cellulose wall is quite variable. Lundegardh 

 quotes a range of thickness of 0.1-3.0U or 1000-30,000 A. Frey-Wyssling 

 (10, 11) gives a value of 0.5U for roots. It is not inconceivable that the 

 electric double layers of the cytoplasm and the clay particle are suffici- 

 ently diffuse to penetrate a thin cell membrane from both sides and 

 intermingle within the intermicellar spaces. It is questionable, however, 

 whether this direct contact exchange could bridge cellulose walls which 

 exceed i|j in thickness. 



Considerable evidence is on hand which refutes the idea of an inert 

 cell membrane. First of all, pure cellulose is not inactive. It has definite, 

 though small, cation exchange properties associated with acidic groups 

 (43). Second, it is very unlikely that the cellulose wall consists of pure 

 cellulose. Frey-Wyssling in his detailed discussion of properties of cell 

 walls reports that the cellulose membrane is permeated by intermicellar 

 spaces which contain pectic substances, lignin, hemicellulose, and 

 mineral substances. Of these, pectic substances are probably most 

 abundant. They have pronounced cation exchange properties. It is also 

 possible that active protoplasmic strands extend into some of the chan- 

 nels of cell membranes of growing root tips. 



In the light of these considerations, the fine structure of cell walls 

 favors contact exchange rather than disfavors it. 



The barley roots used by Jenny and Overstreet have a cation exchange 

 capacity (ammonium acetate method) of 11.0 m. e. per 100 grams of 

 dry roots. The precise seat of the exchange spots is not known. How- 



* According to personal communication by Overstreet, at room temperature 

 nutrient absorption is not restricted to the meristem region. 



