690 PLANT GROWTH AND PLANT COMMUNITIES 



meability. The transpiration stream imposes an electrical potential 

 difference upon the pore capillaries— the well-known streaming poten- 

 tial. 



In the small pores the counter-ions intermingle with one another, 

 and they may do it so completely that they form a "cation solution." 

 Cation diffusion— essentially an exchange diffusion ( Lopez-Gonzales 

 and Jenny, 1958)— readily proceeds in either direction: into the root or 

 out of it, according to the concentration gradients. The pores act as 

 ionic sieves, keeping out anions because of repulsion forces. Iron ions 

 acquired at the root surface may readily diffuse into the interior with- 

 out being precipitated, as there are few soluble anions present. This 

 portion of the cell wall acts as an ionic membrane. 



Whenever the electric double layer of one surface impinges upon 

 that of another— as when a root surface presses against a clay surface— 

 the intermingling of the positive ion swarms produces a localized in- 

 crease in ion concentration, in osmotic pressure, and in associated en- 

 ergy changes. The inner layers— uis., the negative charges— of the 

 double layers of root and clay may become disarranged, resulting in 

 momentary changes in permeability, in influx, and in outHux. The 

 marked effect of resin particles upon releasing non-exchangeable po- 

 tassium from soil minerals (Arnold, 1958; Jenny, 1952) might perhaps 

 be viewed in this light, the ion swarms of the resin prying open or dis- 

 rupting the layer lattice of micaceous plates and initiating ion exchange 

 diffusion. 



No account has been taken of pinocytosis (Buvat, 1958; Holter, 

 1959 ) , the gulping type of uptake of material by way of invaginations 

 of the surface. If it occurs in roots, as has been suggested (Jensen and 

 McLaren, 1960), it would be expected to operate at the cytoplasm 

 boundary rather than at the outer fringes of the cell wall. 



Summary 



Between root and soil a two-way flow of substances, into the root 

 and out of the root, continually takes place. 



The significance of the interactions involving exchangeable cations 

 between the root surface and the soil surface has been analyzed and 

 reviewed on the basis of two-phase experiments with roots, clays, resin 

 particles, and ion-exchange membranes. Ion transfer is a rate problem. 



In controlled experiments on the availability of iron in calcareous 

 media (high pll), it was found that the rate of uptake of iron by 

 alfalfa plants is directly proportional to the number of contact sites 

 between individual iron-oxide particles and the root surface. 



H-roots are able to decompose iron-oxide (Fe203) particles, even 

 in the presence of CaCOa. Iron thus acquired by the root surface rap- 



