1909] MERWIN AND LYON—SAP PRESSURE IN THE BIRCH 445 
with the maple, in respect to pressure and the passing of clouds, may 
be made from jig. r. Even the most decided ups and downs in the 
_ curve of the birch pressure are scarcely more than suggested by the 
slight steepenings or flattenings in the long slopes of the maple curve. 
e have always found the maximum pressures in large birches 
higher than in small ones. The highest pressures observed in both 
large and small trees were on May 2, 1904. A 7.5°™ black birch then 
gave a record of g1°™ (1.2 atmospheres), a 17.5°™ black birch of 
146°™ (1.9 atmos-  pmrzo 2:00 2:40 320 4:00 4:40 
Pheres), and a 35°™ = 
black birch, about 20™ — Z 
high, the astonishing 
pressure of 204° V/\ hy ae 
(2.68 atmospheres). x 
The last pressure 
would doubtless have 70 
been even greater if it 
had been taken two 
hours earlier, for both Pr = 
he 7. 5°". and « the 
17.5°" trees had ) rt Adi ‘Al 
already declined more : 
than ro per cent. from EY 
maximum when the : 
large tree was tap ped. Fic. 2.—The intensity of the sunshine as measured 
As it is, this pressure by a blackened-bulb thermometer and the concurrent 
is equal to 200°™ of pressure, measured in inches of mercury, at the base of 
mercury or 27™ of a 35°™ black birch, May 2, 1904. 
water, being 1.8™ of water higher than any previously recorded 
sap pressure (CLARK, /.c.) Such pressure would support a column 
of water 7.8™ higher than the tree. The highest point on the pres- 
sure curve of jig. 2 represents this pressure. 
Negative tensions occur frequently in the higher parts of the trunk 
of the birch, and less frequently near the base. In the latter position 
it is only in the early part of the season that suction is kept up for 
more than a few minutes at a time. On April 22, 1904, suction pre- 
vailed all day in yellow and black birches that had been in states of 
