[apaMs] CLIMATIC FACTORS IN RELATION TO PLANT LIFE : 109 
TEMPERATURE. 
Various schemes have been devised for measuring the total effect 
of temperature. In most of these schemes a certain starting point 
has been fixed below which it is assumed plant-growth does not take 
place. Hervé Mangon! and Merriam? consider that plant life is dor- 
mant below a mean temperature of 6°C. (or 43° F.) Merriam accord- 
ingly reckons up the total quantity of heat within the period when the 
normal mean daily temperature is 43° F. orhigher. He adds together 
the mean daily temperature for this period after deducting 43° F. 
from each. He also states that ‘the southward distribution is gov- 
erned by the mean temperature of a brief period during the hottest 
part of the year’’ this period being taken tentatively as six weeks. 
MacDougall’ starts with the freezing point and sums up all 
temperatures above this as “‘hour-centigrade-degree” units, basing 
his results on thermographic records. 
Livingston? regards the growing season as the average length 
of ‘‘the frostless season, the number of days which intervene between 
the average date of the last killing frost in spring and the first in 
autumn.’ Elsewhere he appears to regard a killing frost as 32° F. 
or lower. He sums up the result in what he calls temperature effic- 
iencies. ‘“To obtain the daily temperature efficiencies corresponding 
to the various normal daily temperature means it is necessary merely 
to deduce them from our basic assumption namely, that the growth 
rate is unity at 40° F. and that it doubles for each rise of 18° F. above 
this.” By this method of reckoning a mean daily temperature of 
76° F. has twice the effect of a mean daily temperature of 58°F. 
Unstead® takes the temperature of 5° C. (41° F.)as the lower 
limit of growth and calculates accumulated temperatures in terms 
of ‘‘day-degrees.’’ He deducts 5° C. from the mean daily temperature 
and multiplies this by the number of days during which the plant is 
growing; this gives the number of day-degrees. 
McLean® states that ‘‘the growth rate tended to vary almost 
directly with the ‘“‘temperature index” when the air temperatures were 

1 Plant Life of Maryland. 1899. 
2 Merriam, C. H.—Life Zones and Crop Zones of the United States. US. 
Dept. Agric. Div. Biol. Surv. Bull., 10. 1898. 
3 MacDougall, D. T.—The Temperature of the Soil. Journal N.Y. Bot. Garden 
3: 125-131, 1902. 
4 Livingston, B.E. and G. J.—Temperature Coefficients in Plant Geography 
and Climatology. Bot. Gaz. 56: 349-375, 1913. 
5 Unstead, J. F.—The Climatic Limits of Wheat Cultivation with special 
reference to North America. The Geographical Journal, Vol. 39, 1912. 
6 McLean, F. T.—Relation of climate to Plant Growth in Maryland. Mon. 
Weather Review, 43: 65-72, 1915. 
