THE EXCHANGE OF IONS 
261 
able to determine the composition of the culture solutions at the close 
of each experiment, in order to learn the magnitude of excretion into 
solutions from which there had been a net absorption, and vice versa. 
Thus far, however, we have been unable to make analyses of residual 
solutions. For the sake of simplicity, we have used the unmodified 
terms absorption and excretion in the detailed accounts of our experi- 
ments, but it should not be assumed that we have ever considered 
either process to have taken place to the exclusion of the other. 
Some of the results do indeed indicate that in certain cases the magni- 
tude of excretion is negligible, for at the close of the absorption phase 
the residual culture solutions have been nearly exhausted of ions. 
If a culture solution is of such concentration and composition that 
absorption by the roots exceeds excretion, the concentration of the 
solution diminishes from day to day, as long as the vigor of the seed- 
lings remains unimpaired. The length of time that they can live in 
the dark is, however, conditioned by the reserve materials in the seed. 
As these materials are depleted the vigor of the seedlings diminishes. 
After about ten to fourteen days, in the case of Lupinus albus, the 
seedlings weaken, and the absorption phase is succeeded by an excre- 
tion phase. The roots rapidly give up their salts to the culture 
solution and the plants die. 
Calcium Nitrate. Experiments i and 2 
In experiment i solutions of Ca(N03)o ranging in concentration 
from 20 to 260M X io~^ were used. The difference in concentration 
between neighboring cultures of the series was 20M X io~^. The 
cultures were under observation 15 days. 
Data covering the concentration changes are shown in figure i. 
The units on the axis of abscissas indicate duration in days. The 
ordinates show the concentrations in gram molecules per io~^ liters 
of distilled water. The course of the curved lines shows concentration 
changes calculated in gram molecules of Ca(N03)2 dissolved in io~^ 
liters of distilled water. The reader, therefore, can ascertain approxi- 
mately the concentration of any solution at any time during the 
experiment and therefore the magnitude of change. The correspond- 
ing values of the control experiment in distilled water are represented 
by the line beginning at zero on the axis of ordinates. It appears that 
in these solutions of Ca(N03)2 the roots of Lupinus albus lost electro- 
lytes to the medium during the first day regardless of original con- 
