JOURNAL 
Vo. II. July, 1902, No, 31. 
THE TEMPERATURE OF THE SOIL. 
A committee on the ‘“ Relation of Plants to Climate” was 
appointed at the New York meeting of the American Associa- 
tion for the Advancement of Science, in June, 1900, and this 
committee immediately undertook the consideration of current 
methods of measuring temperatures with regard to their influence 
upon vegetation. The actual investigations planned by the com- 
mittee were carried out by the author, and a paper was presented 
at the Denver meeting of the Association in August, Igor, 
describing a method of estimation of the total temperature expo- 
sure of a plant. This method implied the use of thermographic 
curves, and the total heat exposures (temperature above zero centi- 
grade, freezing point) and cold exposures (temperature below zero 
centigrade) were calculated in hour-degree units. Thus it was 
found that the flowers of the silver maple (Acer saccharinum) in 
the New York Botanical Garden were mature and ready for 
fertilization on March 26, 1901, after 1,100 hours exposure to air 
temperatures above the freezing point with a total heat exposure 
of 3,466.6 hour-centigrade-degree heat units. Draba cwcerna 
(vernal whitlow grass) reached the same stage in 974 hours with 
1,644 hour-centigrade-degree units of heat exposure. 
The above method of measuring the temperatures to which 
plants were exposed was applicable only to the thermometry of 
the air. The basal portion of the axis of a plant, often the larger 
part of its body, is imbedded in the soil, however, and no adequate 
study of the subject could be made until some graphic method 
was devised for obtaining continuous temperature records of the 
soil. The committee was given a second grant by the Associa- 
