114 THE ROYAL SOCIETY OF CANADA 
to “growth” simply which may take place at a lower temperature. 
For example, as an indication of the first signs of renewed activity 
in plants, maple sap was dripping from a broken branch at Ottawa 
on March 23rd, 1915, although the maximum and minimum tempera- 
tures for that day were 48° F. and 25° F. respectively. 
A similar definition will hold for the end of the period of active 
growth. The dates for a number of places throughout the Dominion 
as well as the length of the active growth period are given in Table III. 
TABLE III. 
PERIOD OF ACTIVE GROWTH, 1910-12. 



Average 
Active growth begins. Active growth ends. number 
of days. 
Dawson... 26 Apr. to v1 May 15 Sept. 140 
Victoria: 2224046 11226 Feb} to 20Mar. 5 Nov. to 31 Dec. 268 
Barkerville....... 1 May to 26 May 20 Aug. to 20 Sept. 119 
Edmonton....... 6 Apr. to 1 May 20 Sept. to 25 Oct. 176 
Qu’Appelle....... 16 Apr. to 6 June 20 Sept. to 25 Oct. | 148 
Winnipeg ........ 16 Apr. to 1 May 25) Sept; to 25) OCt- 00 113 
Port Arthur... 6 May 20 October | 168 
Horonton te et... 21 Mar. to 11 Apr. Si'Oct. “to 15 Nove Ml 216 
Montreal... ;.2..):|, slGApr. tof21-Apr, 25 OCE Osi Oct: 193 
SÉOnn eme 16 Apr. to 1 May.. 31 Oct. to 5 Nov. 192 
SITE RENTE 16 Apr. to 1 May 31 Oct. to 10 Nov. 197 
SE TONNES 21 Apr. to 6 May 14 Oct. to 15 Nov. 182 





Instead of estimating the total number of heat-units for the 
period May to October it might be even more satisfactory to estimate 
the number of heat-units above 41° F. during the period of active 
growth, but this has not been attempted at present. 
As light frosts may occur during the period of active growth 
a separate table will be necessary to show the dates of the last frosts 
in spring and the first frosts in autumn with the number of days 
between. A frost is taken to mean a temperature of 32° F. or lower: 
the term “‘killing frost’”’ is ambiguous, as a frost that would be sufficient 
to kill tomato plants would have no effect on a turnip crop. The 
frostless period for the places mentioned is shown on Table IV. 
