50 TREES AND TREE-PLANTING. 
cold and uniformly warm and foggy weather are the 
most unfavorable, while the best sap days are bright and 
warm, preceded by freezing nights. 
The variations of temperature which affect the flow of 
maple sap are most likely to occur when the ground is 
covered with snow, because the heat of the sun during 
the day cannot then overcome the cooling influence of 
night. The most probable explanation of the effect of 
these alternations appears to be that the contracting 
influence of the cold drives sap from the outer tissue of 
the tree into the heart-wood of the higher parts of the 
trunk. Meanwhile absorption goes on as usual under- 
ground, and thus, when relief is afforded by the expan- 
sive influence of the sun, the sap rushes again to the 
surface and flows abundantly. This explanation is con- 
firmed by the observations of Biot, in France, as to the 
poplar; and by Nevins, in Ireland, as to the elm. To 
determine whether sap would flow from the heart-wood 
of a sugar-tree a piece of gas-pipe was driven to a depth 
of six inches. The flow was regular and long-continued, 
but not abundant. From another tree a piece of bark 
five inches wide and three inches high was removed, and 
a piece of sheet-iron driven into the bark below to catch 
the sap which flowed very profusely but stopped very 
early. From the first tree the sap flowed eleven days 
longer than from the last, but the latter yielded twelve 
pounds more of the fluid. 
In case of a tree tapped on both the north and south 
sides at the same level, it was found that the north spout 
yielded daily about twice as much sap as the south, and 
continued to flow nearly two weeks longer. To discover 
whether the sweetness of the sap was the same in all 
parts of the same tree, spouts were inserted in a tree 
which had never previously been tapped—one at the 
usual height, one fifty feet higher, where the trunk was 
about five inches in diameter, and a limb thirty-five feet 
from the ground was cut. In several hours the lower 
