TOTAL INFLUENCE OF THE FOREST. 
159 
would have escaped below if the ground had been thawed. In 
this case, although the roots had not thawed the thick covering 
of earth above them, the trunks must have melted the ice in 
contact with them. The trees, when observed by Atkinson, 
were in full leaf, but it does not appear at what period the ice 
around their stems had melted. 
From these facts, and others of the like sort, it would seem 
that “ all vegetable functions are ” not absolutely “ dormant ” 
in winter, and, therefore, that trees may give out some heat at 
that season. But, however this may be, the “ circulation of 
the sap ” commences at a very early period in the spring, and 
the temperature of the air in contact with trees may then be 
sufficiently affected by heat evolved in the vital processes of 
vegetation, to raise the thermometric mean of wooded coun¬ 
tries for that season, and, of course, for the year.* 
Total Influence of the Forest on Temperature. 
It has not yet been found practicable to measure, sum up, 
and equate the total influence of the forest, its processes and its 
products, dead and living, upon temperature, and investigators 
differ much in their conclusions on this subject. It seems 
* The low temperature of air and soil at which, in the frigid zone, as 
well as in warmer latitudes under special circumstances, the processes of 
vegetation go on, seems to necessitate the supposition that all the manifes¬ 
tations of vegetable life are attended with an evolution of heat. In the 
United States, it is common to protect ice, in icehouses, by a covering of 
straw, which naturally sometimes contains kernels of grain. These often 
sprout, and even throw out roots and leaves to a considerable length, in a 
temperature very little above the freezing point. Three or four years since, 
I saw a lump of very clear and apparently solid ice, about eight inches long 
by six thick, on which a kernel of grain had sprouted in an icehouse, and 
sent half a dozen or more very slender roots into the pores of the ice and 
through the whole length of the lump. The young plant must have thrown 
out a considerable quantity of heat; for though the ice was, as I have said, 
otherwise solid, the pores through which the roots passed were enlarged to 
perhaps double the diameter of the fibres, but still not so much as to pre¬ 
vent the retention of water in them by capillary attraction. 
