RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 17 



and size for the various comparisons. It seems to be quite well estab- 

 lished that growth of most plants begins at about 40° F., is very slow 

 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 leaves 

 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 influence. Radiation measure- 

 ments or "sun temperatures' 1 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 thermometeis as fully a- possible. 

 At the same time, the shelter which affords this protection nxusl 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 

 10163— 22— Bull. 1059 2 



