190 
D. T. MACDOUGAL 
Assuming now that it is desired to evaluate the variable tempera- 
ture of any place or of any experimental setting it is first of all necessary 
to secure a reliable thermograph record for the period under investiga- 
tion which might include the entire frostless season or the time in 
which a certain stage of development of selected organisms had been 
accomplished. Next, this record is ruled by lines which will divide it 
into figures, the area of which, measured by a planimeter, represents the 
length of time applied to the intensity of the temperature. The obvious 
procedure is simply to construct regular figures which shall include the 
area of the thermographic diagram as nearly as possible and to make 
these figures of such size that the use of averaged rates of growth 
will include the smallest practicable error (see figure 2). Crude as 
this method may appear in this preliminary form it however deals with 
actual and observable facts and all of its essentials are capable of 
correction. The first step in such improvement would of course con-» 
sist in the re-measurements noted above under conditions approxi- 
mating the daily range with corrections for the age of the material 
and with control of all of the environic factors. It may be expected 
that growth of the upper part of the stem from material newly formed 
in the leaf-blades will be different from that which might take place 
in organs in direct connection with the relatively great stores of easily 
hydrolyzable material in the seeds. 
In the case of the wheat plant it has been found most convenient 
in this preliminary essay to calibrate growth vlaues at the average 
rate between 40 and 65° F., 65° and 70°, 70° and 75°, 75° and 80°, 
80° and 85°, 85° and 92° F., etc. The lowest rate, including a range 
of 25° F., the areas measured on the thermograph sheet, in every case 
should have a similar ideal range to avoid distortion of results. 
The summation of the results of the use of the planimeter on the 
thermograph sheets gives the total hour-degreesj during a week, 
month or any other period during which the temperature stood within 
the limits mentioned and it will then be but necessary to apply to 
this sum the factor expressing the rate of growth to obtain the relative 
value of the exposures as is illustrated by the tables below constructed 
from data obtained at the Desert and Coastal Laboratories. 
