MOTION OF THE JUICES. 335 
influenced by its temperature is a plain deduction from 
the fact that the leaves within were found wilted in the 
morning, while they recovered toward noon, although the 
temperature of the air without remained below freezing. 
The wilting was no doubt chiefly due to the diminished 
power of the stem to transmit water; the return of the 
leaves to their normal condition was probably the conse- 
quence of the warming of the stem by the sun’s radiant 
heat.* 
One mode in which changes of temperature in thé trunk 
influence the flow of sap is very obvious. The wood-cells 
contain, not only water, but air. Both are expanded by 
heat, and both contract by cold. Air, especially, under- 
goes a decided change of bulk in this way. Water ex- 
pands nearly one-twentieth in being warmed from 32° to 
212°, and air increases in volume more than one-third by 
the same change of temperature. When, therefore, the 
trunk of a tree is warmed by the sun’s heat the air is ex- 
* panded, exerts a pressure on the sap, and forces it out of 
any wound made through the bark and wood-cells. It 
only requires a rise of temperature to the extent of a few 
degrees to occasion from this cause alone a considerable 
flow of sap from a large tree. (Hartig.) 
If we admit that water continuously enters the deep-ly- 
ing roots whose temperature and absorbent power must 
remain, for the most part, invariable from day to day, we 
should have a constant slow escape of sap from the trunk 
were the temperature of the latter uniform and sufficiently 
high. This really happens at times during every sugar- 
season. When the trunk is cooled down to the freezing 
point, or near it, the contraction of air and water in- the 
tree makes a vacuum there, sap ceases to flow, and air is 
* The temperature of the aér is not always a sire indication of that of the 
solid bodies which it swrounds. A thermometer will often rise by exposure of 
the bulb to the direct ays of the sun, 30 or 40° above its indications when in the 
shade, 
