520 
R. B. HARVEY AND R. H. TRUE 
below a certain point, concentrations of the nutrient solution might 
be so low that the root could not absorb enough salts from the solution 
simply because of the dilution of the latter." This surmise seems 
entirely correct in view of the data here presented. 
To maintain so low a value for the ion content of the solution about 
it for three weeks, as is here seen in sweet corn, either the cell membrane 
must possess almost perfect directional semipermeability and be able 
to hold the electrolytes during life, or the ions present must be chem- 
ically combined into compounds which are but little dissociated. The 
variation in the deviation from the value for CO2 saturation indicates 
specific differences in the rate of ion production. Since upon the death 
of the plant leaching occurs, the assumed directional permeabiHty is 
to be ascribed to some living membrane of the plant, or the leach is 
due to the breaking down of ion compounds of the cell substances upon 
the death of the protoplasm. 
Summary 
The equilibrium concentration of electrolytes established by the 
squash, peanut, soy bean, and sweet corn grown in water culture was 
found to be specific for each plant. 
The equilibrium concentration value is independent of the kind 
of nutrient salt used, the concentration of the electrolyte, Or the volume 
of the solution; provided, that the concentration is below the toxic 
limit for the plant, and that the quantity of the salt is within the re- 
quirement of the plant during the period of growth. 
At the point of equilibrium between the plant and the solution, 
the electrolyte content of the solution is determined, first, by certain 
factors which are constant for different plants under the same con- 
ditions, such as the CO2 equilibrium with the air; and, second, it is 
determined by the rate of cleavage of ion-producing compounds of the 
cell and the reabsorption of the ions produced. 
Office of Plant Physiological and Fermentation Investigations, 
Bureau of Plant Industry, 
U. S. Department of Agriculture. 
