258 
R. H. TRUE AND H. H. BARTLETT 
solution is ever so completely depleted of ions that none remain 
except those resulting from the dissociation of carbonic acid. 
Stock solutions (from which the culture solutions were made by 
dilution with distilled water) were accurately prepared from Merck's 
best ''blue label" reagents. 
In our experiments a range of concentrations was used comparable 
to the range from distilled water to the less concentrated soil solutions. 
The more concentrated culture solutions, for example, contained less 
mineral matter than the water from the Potomac River, but more 
than the water of the Great Dismal Swamp of Virginia. In the case 
of the simple salts the range of concentration in the culture solutions 
was greater than the range of the same single constituent among 
average soil solutions. In other words, we made an endeavor to 
study the reaction of the plant toward solutions similar in concentra- 
tion to those which it might encounter under natural conditions. 
It will be seen from the accounts of the particular experiments 
that the cultures were not kept at constant temperature. In order 
to avoid the deleterious effect of illuminating gas on the cultures, 
they were set up in a dark cabinet at the greenhouse. Although 
danger from gas leakage was thus avoided, it was found impossible, 
with the means at our disposal, to eliminate a considerable tempera- 
ture fluctuation. The temperature of each culture was read daily 
when the conductivity was determined. It will be seen that the tem- 
perature curves, which are plotted parallel with the concentration 
curves for each experiment, show no consistent influence of tempera- 
ture upon absorption or excretion. As a matter of fact, the most 
perfectly executed experiments, such as those charted in figs, i and 2, 
show no temperature effect whatever. PovSsibly this interesting fact 
may be correlated with certain observations made by Eckerson.^ 
This investigator has found, for example, that the primary roots of 
Phaseolus show no increase in permeability from 13° to 25° C. Similar 
phenomena were observed with other plants. 
In our experiments concentrations were determined in the following 
manner. On the day that a series of cultures was prepared, the con- 
ductivity of each solution was determined at room temperature before 
the roots were placed in it. As long as the experiment lasted, the 
conductivity was read daily, each time at room temperature. It was 
impracticable to bring the culture solutions to constant temperature 
* Eckerson, Sophia, Thermotropism of Roots. Bot. Gaz. 58: 254-263. 1914. 
