THE EXCHANGE OF IONS 
263 
seen in the weakest solution, which may perhaps represent the con- 
centration at which Ca(N03)2 becomes unavailable to the plants 
under the given conditions. 
The behavior of the distilled water check showed a striking con- 
trast to that seen in the calcium cultures. The roots lost ions with 
approximate regularity for about 7 days after which the rate of loss 
decreased for the remaining time. At the close of the experiment the 
distilled water was equal in its ion content to over 3oikf X io~^, that 
is, it was richer in ions than the final concentration of any Ca(N0rf)2 
solution which had had an original concentration less than lOoM 
X io~^. It will be seen that in general the net aKsorption in Ca(N03)2 
solutions was considerably greater in the stronger solutions than in the 
more dilute members of the series, due probably to the fact that there 
was more material present to be absorbed. The maximum absorption 
in comparison with the amount of Ca(N0.3)2 offered was, as might 
be expected, highest in the more dilute solutions (20 to ^oM X lO"''). 
It seems necessary, if active absorption is to take place, that at least 
^oM X io~^ Ca(N03)2 should be present in the solution, and it is 
much better if two or three times that concentration is present. 
Measurements of various dimensions of the seedlings used were 
made at the close of the experiment, but only the growth of the 
primary root seemed to show any perceptible relation to modification 
of the medium. The growth made in distilled water was distinctly 
poorer than that seen in any of the calcium solutions and in a general 
way the total growth in length made by the roots was greater as the 
salt content of the medium increased. In some cases this was not 
true, but in view of the small number of plants in each culture there 
is good reason to believe that these departures were due in considerable 
part to the individual variation of the plants. 
In view of the results seen in the experiment summarized in figure 
I, it was thought desirable to ascertain the effect of further dilution of 
the salt and at the same time repeat the observations as far as the 
lesser concentrations were concerned. Accordingly in experiment 2 
a series of thirteen solutions was made up ranging from 8 to 104.M X 
io~^ with a constant interval of SM X io~^ between adjacent members 
of the series. The experiment ran seventeen days, and is summarized 
in figure 2. 
The record of these solutions shows the same general features 
seen in the foregoing experiment. The lupine roots were always 
