110 : THE ROYAL SOCIETY OF CANADA 
low. With high air temperatures the growth rate was relatively 
much greater than can be accounted for by the ‘“‘temperature indexes”’ 
alone. The “temperature index” is taken as the mean daily tempera- 
ture above 40° F. 
The above mentioned schemes relate to degrees of heat but very 
few attempts appear to have been made to measure cold. Angot? 
proposed a method of classifying winters by the sum of the minimum 
monthly temperatures below 0° C. 
Of all the starting points that have been suggested the freezing 
point of water appears to be the most appropriate. The actual 
temperature at which a plant begins to carry on its functions varies 
with the species and with the climate to which the plant has been 
accustomed. Vinson$ states that the rate of growth of the date 
palm throughout the entire year is in most casesin proportion to the 
heat time units over 50° F. and Swingle® says that a daily mean 
temperature of 64-4° F. is the zero point of activity of the date tree 
as far as flowering and fruiting functions are concerned. 
On the other hand in the case of many species of plants to assume 
that the temperature of 6° C. (41° F.) inhibits their growth is not 
in accordance with the facts. Johnson’ states that the minimum 
temperature for the germination of the uredospores of Puccinia gram- 
inis is 2°C. A species of fungus grows vigorously on the surface 
of grass and decaying vegetation under melting snow and bacteria 
increase in numbers in frozen soil. Kjellman! mentions many 
species of Algz which bore re-productive organs at temperatures 
of—1-8° to 0° C. 
Livingston’s proposal to regard the growing season as coter- 
minous with the frostless period cannot be applied rigidly. For 
example, at Ottawa, tulips were one inch high above the ground when 
the temperature fell to 10° F. on March 30, 1915, after which they 
continued their growth without injury. In fact, if we adhered 
strictly to the frostless period as the boundary of the growing season 
of plants we should be led to the conclusion that no plant life is possible 
in regions which we know to be covered with vegetation. Day” 
argues as follows:—In the elevated mountain regions and on some 

7 Bull. Soc. Nat.-Agric. France, 73: 186-8, 1913. 
8 Vinson, A. E.—The effect of climatic conditions on the rate of growth of date 
palms. Bot. Gaz. 57: 324-7, 1914. 
® Swingle, W. T.—The date palm and its utilization in the Southwestern States, 
U.S., Dept. Agr. Bur. Pl. Indus. Bull., 53, 1904. 
10 Phytopathology, 1912. 
11 Kjellman, F. R.—The Algæ of the Arctic Sea. 1883. 
2 Day, P. C.—Frost data of the United States and length of the crop-growing 
season. U.S. Dept. Agric. Weather Bur. Bull. V. 1911. 
