34 Rhodora [Marcu 
cal or chiefly chemical. The subsequent demonstration by Gillespie! 
that the reaction (acidity or alkalinity) of a soil can be directly meas- 
ured by the use of indicators—that is, dyes which change their colors 
with variations in reaction furnished a means for obtaining definite 
information upon the matter. The method was first tried out in the 
laboratory, on soil samples representing various geological formations 
as well as different species of plants which were supposed to be sen- 
sitive to soil reaction; and the preliminary results on one group, the 
Orchidaceae, have already been published. The method was later 
modified for use in the field, as recently described.’ On learning of 
this method, Mr. Frederick V. Coville of the Bureau of Plant Indus- 
try, U. S. Department of Agriculture, suggested to the writer that 
since the Ericaceae are apparently especially sensitive to soil reac- 
tion—for the most part requiring definite acidity—it would be desir- 
able that tests be made on a number of members of this family. 
Accordingly, with the aid of funds from the Bureau of Plant Industry, 
several trips were taken for this purpose; and in the present paper 
are recorded the results obtained on one of these trips in June, 1919, 
at certain points in northern New England. While the Ericaceae 
were studied primarily, data were obtained on other plants growing 
in the same regions; although only plants which for one reason or 
another are inferred to be decidedly sensitive to soil reaction are con- 
sidered, and no attempt is made to list all the species growing in the 
places visited. The nomenclature of Gray's Manual, 7th edition, 
1908, is followed throughout, synonyms being introduced in certain 
cases. Pressed specimens of the plants studied have been deposited 
in the U. S. National Herbarium. 
'The acidity and alkalinity of the soils studied are described in 
terms recommended for the purpose elsewhere.* To summarize the 
plan here, omitting technical physical-chemical terms,—pure water, 
which is neutral in that equivalent amounts of acid and alkaline con- 
stituents (ions) are present in it, is taken as the unit of both “specific 
acidity" and “specific alkalinity.” A solution containing up to 10 
1! Gillespie, L. J. The reaction of soil and measurements of hydrogen-ion concen- 
tration. Journ. Wash. Acad. Sci. vi. 7 (1916). 
? Wherry, Edgar T. "The reactions of the soils supporting the growth of certain 
native orchids. Journ. Wash. Acad. Sci., viii. 589 (1918). 
? Wherry, Edgar T. Determining soil acidity and alkalinity by indicators in the 
field. Jorn. Wash. Acad. Sci., x. (April, 1920). 
4 Wherry, Edgar T. The statement of acidity and alkalinity, with special refer- 
ence to soils. Journ. Wash. Acad. Sci., ix. 305 (1919). 
