PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 31 



(in this case, the cations) will be in a higher concentration inside 

 than outside the membrane; while the ions similarly charged, 

 the anions, will remain at a lower concentration inside than 

 outside. According to Donnan's investigations, the product of 

 the concentrations of the anions and cations of the freely diJEfusing 

 salt must be equal on both sides of the membrane. In case 

 there is a considerable excess of colloidal ions inside the mem- 

 brane, there will be a nearly complete disappearance of cations 

 (in this case, of potassium) from the surrounding medium. A 

 corresponding amount of sodium will diffuse from the sac into 

 the surrounding medium. On the other hand, anions (in this 

 case, chlorine) will be almost entirely absent from solution inside 

 of the membrane. This accumulation will take place in spite of 

 the fact that both ions diffuse easily through the membrane and 

 do not undergo any chemical reactions inside the membrane. 



The protein substances, always present in cells, represent the 

 colloidal ions mentioned above. Capable of carr3dng either a 

 positive or negative charge, they can induce the accumulation 

 of both cations and anions. And as the protoplasm is imper- 

 meable not only to colloids but also to many electrolytes, e.g., 

 to organic acids, these substances may cause the establishment 

 of Donnan's membrane equilibrium between the cell and the 

 surrounding medium and contribute to the accumulation of 

 many cations in the plant in concentrations considerably exceed- 

 ing those of the surrounding medium, e.g., of the soil solution. 



However, in connection with the continuous changes both 

 quantitative and qualitative in the composition of the colloid 

 substances, the conditions of Donnan's equilibrium are very 

 complicated in cells and are far from being sufficientlj^ under- 

 stood. The attempts of several authors to reconstruct the whole 

 problem of absorption and assimilation of substances in plant 

 cells on the basis of the laws demonstrated by Donnan must be 

 recognized as premature. During the last few years, there has 

 been a gradual accumulation of data indicating that the process 

 of penetration and accumulation of substances does not simply 

 represent a purely physical process of penetration by osmosis 

 but is rather an active physiological process, which requires the 

 expenditure of a definite amount of energy by the organism. 

 The recent works of Hoagland and his pupil Steward (1932), 

 as well as those of Lundegardh, Pirschle, Petri, and others, have 



