564 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
contact of plant roots with nutrient solutions of certain definite composition 
has not as yet been fully explained, it is generally recognized as being largely 
the result of differential ion-absorption by the plant roots. 
In the solutions used in the present experiments the plants were able 
to change the reaction from an initial pH value of 4.6 to one quite near the 
neutral point. The data for the sand cultures will not be considered here 
since they were found to be in quite close agreement with those for the solu¬ 
tion cultures. The latter are presented in graphic form in figure 2. Since 
aeration was found to have no effect upon reaction change, the solid line in 
figure 2 represents the averages of the pH value of corresponding cultures 
of the two series with intermittent solution renewal, while the broken line 
represents the averages of the pH values of corresponding cultures of the 
two series with continuous solution renewal. 
Fig. 2. Graphs of pH values of the culture solutions in contact with the plant roots 
at intervals during the growth period. 
The graph representing the pH values of the cultures in which the 
solutions were continuously renewed indicates that the capacity of the 
plants to produce reaction change in the nutrient solutions increases gradu¬ 
ally but uniformly to a maximum at five weeks. This was approximately 
the time when the plants of these cultures reached their highest vegetative 
activity. Following this, the power of the plants to produce reaction 
