THE TEMPERATURE OF LEAVES OF PINUS IN WINTERS 
John H. Ehlers 
I. Introduction 
There is evidence to show that the food reserve of decidous trees 
is at a maximum in autumn at the fall of the leaves, and that there 
is a gradual decrease in reserve material during the winter months 
to a minimum in the spring. For trees with persistent leaves, on the 
contrary, there is evidence to show that there is a gradual increase in 
reserve food during the winter, and that the maximum is not reached 
until just before the swelling of the buds in spring. Accepting the 
evidence as true leads to the conclusion that, while deciduous trees 
maintain their existence during the winter at the expense of the 
reserve food, trees with persistent leaves produce, by photosynthesis 
during the same season, food material not only for their maintenance, 
but in quantities sufficient for an accumulation of reserve. How 
they are able to do this in cold climates where the temperature rarely 
rises far above o° C, and where the mean temperature for the winter 
months is several degrees below o° C. is a problem that has not been 
solved. 
It has been shown by investigators that broad leaves, i. e., leaves 
of deciduous trees, exposed to tropical insolation and leaves exposed 
to summer insolation in temperate regions, may attain in still air a 
temperature i6° C. above the shade temperature of the air. Assuming 
that a similar condition holds for trees with persistent leaves, such 
as the conifers, during the cold days of winter, may not this offer a 
solution to the problem suggested above? 
While a number of investigators have definitely determined the 
effect of solar radiation upon broad leaves for tropical regions, and 
for summer conditions in temperate regions, no such work has been 
done to find its effect upon persistent leaves under winter conditions. 
It was to fill this gap, i. e., to determine the effect of solar radiation 
upon the temperature of persistent leaves under winter conditions, 
^ Contribution No. 145 from the Botanical Department of the University of 
Michigan. 
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