334 HOW CROPS GROW. 
is closely connected with the changes of temperature that 
take place above ground. The sap begins to flow from a 
cut when the trunk itself is warmed to a certain point, 
and, in general, the flow appears to be the more rapid the 
warmer the trunk. During warm, clear days, the radiant 
heat of the sun is absorbed by the dark, rough surface of 
the tree most abundantly; then the temperature of the 
latter rises most speedily and acquires the greatest eleva- 
tion—even surpasses that of the atmosphere by several 
degrees; then, too, the yield of sap is most copious. On 
clear nights, cooling of the tree takes place with corre- 
sponding rapidity ; then the snow or surface of the ground 
is frozen, and the flow of sap is checked altogether. 
From trees that have a sunny exposure, sap runs earlier 
and faster than from those having a cold northern aspect. 
Sap starts sooner from the spiles on the south side of a 
tree than from those towards the north. 
Duchartre, (Comptes Rendus, IX, '754,) passed a vine 
situated in a grapery, out of doors, and back again, 
through holes, so that a middle portion of the stem was 
exposed to a steady winter temperature ranging from 18 
to 10° F., while the remainder of the vine, in the house, 
was surrounded by an atmosphere of 70° F. Under 
_ these circumstances the buds within developed vigorously, 
but those without remained dormant and opened not a 
day sooner than buds upon an adjacent vine whose stem 
was all out of doors. That sap passed through the cold 
part of the stem was shown by the fact that the interior 
shoots sometimes wilted, but again recovered their turgor, 
which could only happen from the partial suppression and 
renewal of a supply of water through the stem. Payen 
examined the wood of the vine at the conclusion of the 
experiment, and found the starch which it originally con- 
tained to have been equally removed from the warm and 
the exposed parts. 
That the rate at which sap passed through the stem was 
