SELECTIVE PERMEABILITY AND THE PLASMA- ^'^ 



MEMBRANE 



J. DAVIDSON 

 Cornell University, Ithaca, New York 



A necessary condition for plant nutrition and, consequently, 

 for plant growth is that substances from the outside must enter 

 the plant. Some of these substances come from the atmosphere 

 and some from the soil through the medium of the soil solution. 

 The substances which come from the atmosphere enter the 

 plant by simple diffusion. The case, however, is not so definite 

 and clear with reference to those substances which come from 

 the soil. It is not definitely known whether they enter by 

 diffusion or whether there are other forces involved — physical, 

 chemical, or forces which lie beyond our present knowledge 

 which are generally designated as vital. 



The diffusion alternative would simplify matters very much. 

 The limited nmnber of mineral elements found in plants could 

 be accounted for in three ways: 



1. The elements found in plants are those which make up 

 principally the soil solution. 



2. The metabolic processes of the cell may exert a regulatory 

 function with reference to the intake of the mineral elements; 

 those which are found in plants enter in chemical combinations 

 with the organic products of metabolism and are thus accumu- 

 lated, while the others, if they do enter the plant, are present 

 in too insignificant quantities to be detected by chemical analysis. 



3. Only those elements which are found in plants can enter 

 the plant cells, because of selective diffusion. 



The diffusion possibility, however, is not borne out by facts 

 obtained in experiments concerned with measurement of the 

 permeability of plant cells to different substances. ^Vhen, for 

 instance, seedlings are grown in a balanced solution of electro- 



331 



THE PLANT WORLD, VOL. 19, NO. 11 

 NOVE.M£?ER, 1916 



