KESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 1 7 
up to about 60° F., and reaches a maximum at, perhaps, 80° F. These 
points will be at least suggestive of the temperature ranges and tem- 
perature groups to be considered. It will, however, probably not be 
satisfactory to merely note that a given growth was secured in A 
hours in temperatures between T and T r MacDougal (27) sug- 
gests the summation of temperatures from what may be called the 
base of each temperature range (say, about 60° F., but not more than 
65° F.), and as the simplest means of obtaining hour-degrees in each 
temperature range has used the planimeter to measure the area on 
thermograph traces included between any two given lines. 
The study of leaf temperatures is not a study of the environment, 
but will be at least a means to a better understanding of the action 
of the environment and will, perhaps, lead to more comprehensive 
and expressive measurements of the environment. A good deal of 
rough work has been done in measuring the temperatures of leaA^es, 
usually by wrapping them about the bulb of a thermometer or placing 
the latter in close contact with them. Such methods, however, are 
wholly inadequate for treating the needles of conifers, and are of 
doubtful value elsewhere. E. Shreve (36) has made use of the great 
sensitivity and possibly small bulk of a thermocouple, to devise an 
apparatus which will readily reflect the temperature of any part of a 
leaf with which it is brought into contact. The whole equipment 
seems sufficiently compact and practical to furnish great usefulness 
in the field as well as in the laboratory. 
With this sketchy consideration of the problems which should be 
faced, the ordinary means of accumulating temperature records may 
now be mentioned. 
Exposure OF Thermometers. 
Comparisons of air temperatures under different conditions can, 
of course, be made only if the measurements are made in such a 
manner as to eliminate radiation influences. Radiation measure- 
ments or " sun temperatures " undoubtedly have their places, but 
are not to be confused with the present subject and they will be 
discussed later. 
To measure correctly the temperature of the air, direct or reflected 
sunlight must be excluded from thermometers as fully as possible. 
At the same time, the shelter which affords this protection must not 
itself absorb the radiation sufficiently to become heated within. 
This danger is largely overcome by allowing free circulation of air 
through the shelter, and the danger is still further lessened when 
the air circulation is naturally strong. Such radiation is particu- 
larly to be guarded against in any kind of shelter placed on or near 
the ground. The standard type of shelter is double-roofed and has 
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