[ADAMS] CLIMATIC FACTORS IN RELATION TO PLANT LIFE 111 
of the higher plateaus of the western country freezing temperatures 
are liable to occur in all months of the summer but their effect on 
vegetation is not so pronounced as in the districts east of the moun- 
tains. Owing to the dryness of the air, frost does not always form 
with a temperature of 32° F. or even several degrees lower and in 
addition plant life subjected to such variations in temperature becomes 
more hardy and lower temperatures are required to cause serious 
injury. As an example of the above, Barkerville in British Columbia 
may be cited where frost may occur in every month of the year. 
In the present paper a heat-unit is taken to mean a temperature 
of 1° F. above 32° F. continued for one hour. Similarly temperatures 
below 32° F. are reckoned as cold-units. As a basis of these cal- 
culations the daily maximum and minimum temperatures are taken. 
If both are above 32° F. the mean is taken, 32° F. are deducted and 
the remainder multiplied by 24 gives the number of heat-units. 
Similarly when both maximum and minimum are below 32° F. the 
number of cold-units is calculated. When the maximum is above 
32° and the minimum below 32° the number of heat and cold-units 
must be ascertained separately. In this case it is assumed that the 
temperature remain above 32° F. for 12 hours and below 32° for the 
other 12. Thus the maximum and minimum temperatures at Ottawa 
on April 3rd, 1916, were 39 and 20 and the number of heat and cold- 
units registered would be 33 and 6 respectively. This appears to 
be a preferable method to taking the mean temperature 293 deducting 
it from 32 and expressing the result as 23 cold-units. It has to be re- 
membered that the maximum temperatures are those in the shade 
while the temperature in the sun may be sufficiently high for a certain 
amount of photosynthesis to take place. Thus at Ottawa on March 
14th, 1915, the shade temperature was 343° at the same time that the 
temperature in the sun was 82°F. Calculations based on the maximum 
and minimum temperatures of each 24 hours to determine the number 
of heat and cold-units are only approximately correct. Much more 
accurate results are obtained from thermographic records for each 
hour of the day. The amount of error in the result arrived at by 
the former method is considerable. Thus at Toronto the figures 
for January and July 1912, are as follows:— 

os : — 


Heat units. | Cold units. | Heat Units. 
Jan. Jan. July. 
Thermographic record as basis. ......... 54-6 16,190-4 27, 304-8 
Maximum and Minimum as basis....... 144-0 13 610-4 27 ,974-4 


