[Vor. 11 
96 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
physiological conditions other than those in these experiments, 
the results as here given would be found to vary to some extent. 
The Ру limits, optimum Ру zone, and direction of change in the 
active acidity of the substratum vary with the environmental 
conditions. 
The writer wishes to take this opportunity to express his 
indebtedness to Dr. B. M. Duggar for suggesting this problem 
and for many kindly and helpful criticisms and suggestions, 
and to Dr. George T. Moore for the privileges and facilities of 
the Missouri Botanical Garden. 
Graduate Laboratory, Missouri Botanical Garden. 
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