60 CARNEGIE INSTITUTION OF WASHINGTON. 



Permeability in Plant Cells, by D. T. MacDougal. 



Two main features may be taken to determine the permeability of the wall, 

 membranes, or outer layers of protoplasts: the chemical composition of these 

 layers and the changes which may be induced by the substances which pass 

 through them. The weight of evidence now points to the predominance of 

 the pentosans and of the lipins in the outer layers of plant cells, in which the 

 proteins may play only a subordinate part. The material of both groups of 

 substances may be arranged in such condition as to allow the passage of both 

 fat-soluble and water-soluble material. Some writers are disposed to attribute 

 interferences to the chemical combinations which one base may make with 

 the material of the membrane, thereby altering its permeability to another 

 kation. Thus, True rests all variations in permeability of root-hairs upon the 

 replacement of one base by another in the pectates of the wall and membranes. 

 It is not at all clear that such replacements actually occur or that the com- 

 pounds implied are present. The pectins are very weak acids, and it is not 

 known definitely that calcium, potassium, and magnesium pectates are 

 formed when these bases in the form of salts are infiltrated. 



It may be plainly demonstrated that these bases cause the material 

 of the wall to assume different degrees of permeability, varying density, etc. 

 That these changes rest upon chemical reactions between pentosans and 

 salts alone does not seem probable in view of the established presence of 

 lipins in the walls and membranes. Furthermore, the change from a calcium 

 pectate to a sodium or potassium pectate would not explain the wide range 

 of variation or abruptness of changes in permeability. More adequate ex- 

 planations may be based upon the colloidal condition of the material, which 

 is in the form of a heterogeneous system of emulsoids. 



The density or viscosity of such colloids is of course determined by the 

 proportion of water present at any given temperature. The difference 

 between the rates of diffusion of such substances as the ions of sodium, 

 potassium, and magnesium in gelatine or agar with a small proportion and 

 with a high proportion of water is so small as to be of but little importance 

 in permeabilities of the kind discussed by the physiologist. 



When salts are added to the water in which agar or gelatine may be hydrated 

 the combined action of the kations and anions determines the degree of 

 hydration. Thus, it has been shown that the hydration reached by agar, by 

 biocoUoids in solutions of potassium, sodium, and calcium, are widely different 

 and the colloid would in each take on a certain pertinent viscosity. 



These hydration values may be safely taken to rest upon the adsorption 

 of the different kations by the molecular aggregates of the colloid, and such 

 aggregates then have the specific capacity indicated by the final hydration 

 value. The formation of these adsorption compounds may be tentatively 

 offered as an alternative to the theory of True, that definite salts are formed 

 between the substances in the colloids and the bases. Furthermore, when 

 the colloid has adsorbed any base, it is found that its presence will retard 

 the diffusion of another kation which might give the colloid a higher hydra- 

 tion capacity. This is in fact the essential feature in the interferences which 

 have become known as "antagonisms" to the physiologist. 



So important is the direct action of the kation on the colloid that when 

 solutions of potassium, sodium, and calcium are placed inside an artificial 



