446 BOTANICAL GAZETTE [DECEMBER 
high pressure on several occasions since April 5. A hard freeze 
the night before and a temperature not exceeding 40°F. during 
the day seem to have been the chief factors controlling the negative 
pressure. 
The chief characteristics of sap pressure in the maple 
On some warm winter days, at least as early as February 1, sap 
will flow in amounts of a few cubic centimeters from tap holes in small 
maples that are exposed to the sun; but the maximum flow and cor- 
responding pressure do not occur till the ground is thawing in the 
spring. At this time pressures in a tree are not distributed with any 
apparent regularity. Portions of the trunk at the same level may give 
very different pressures, and for different heights the pressure may be 
greatest in either the highest or the lowest situation, though usually 
pressure decreases irregularly with height (fig. 1, A and B). 
Pressure in one tap hole may be but little decreased by sap flowing 
freely from another hole a few inches at one side of it, but there may 
be a decided drop in pressure if the flow is from a hole a few feet 
above or below the hole to which the gauge is attached. From these 
facts it may be inferred that the ducts of the maple communicate with 
some freedom along the grain of the wood, but scarcely at all across 
the grain: 
| When pressure begins it may be manifest first either near the ree 
or in the branches, but for any given place in the trunk there 15 @ 
strong tendency toward a daily increase of pressure during the 
morning hours, and a decrease during the afternoon. The decrease 
often goes beyond zero to a considerable suction (fig. 1, B; after 
T:00 P.M.). Size of the tree, situation, and depth of tapping all 
affect the character of the daily pressure variation. Small size, 
€xposure to the sun, and shallow tapping are all favorable to extreme 
and rapid pressure changes. In spite of all the variations already 
discussed, there is a tendency toward parallelism of the pressures 
developed in different parts of the same tree, and in various tee 
during a daily period. The causes of such pressure variations, 4 
related especially to daily periods of temperature change, have beet 
discussed by various writers. 
There is a general agreement that rises and falls of temperature 
