1909] MERWIN AND LYON—SAP PRESSURE IN THE BIRCH 455 
of gas. That there was a small amount of gas present was shown 
by pouring mercury into the free arm of the lower gauge and thereby 
forcing back into-the tree 0.45°° of sap. This procedure caused the 
pressure within the tree to go up from 54 to 55.3°™. A simple calcu- 
lation shows that there was a contraction of nearly x per cent. in the 
gas in the vessels. The total volume of gas contracting was then 45°°. 
To get an estimate of the amount of duct space in the tree, I 
examined several cross-sections of branches about 2.5°™ in diameter. 
An area of 1. 25° ™™ contained an average of 74 ducts, with an average 
cross-section of o.002°™™, The ducts, therefore, occupy about — 
12 per cent. of the volume of the tree. The volume of the tree (esti- 
mating the upper 3™ of the stem and branches as equal to the lower 
3™ of the trunk) was 57,000°°. The duct space, then, amounted to 
about 6800°°. Finally, 45°° (the gas content of the ducts) is nearly 
0.7 per cent. of the volume of the ducts. A second addition of mer- 
cury gave the gas content as 1.3 per cent. The ducts, therefore, at 
this time of the year were almost entirely filled with water. In a 
larger tree, in which there is a good deal of heart wood, there might 
be considerably more gas in the ducts, but many of the ducts in the 
heart wood are probably not freely communicating with the ducts in 
the sap wood. ; 
It was noticed in connection with adding the mercury to the gauge, 
that after the rise of pressure thus produced had taken place, the 
pressure remained stationary for 15 minutes the first time, and for 30 
minutes the second time, and then began rising at its previous rate. 
Furthermore, the length of time that pressure was stationary was 
such that during that time pressure would have increased naturally 
the same amount that it was artificially raised. We may conclude 
from this that the intensity of root pressure was increasing during the 
night. This is in accord with the idea that the concentration of the 
sap in the roots—and the corresponding osmotic pressure—becomes 
greater when evaporation from the branches lessens. 
EXPERIMENT 2.—Before sunrise April 16, 1906, when pressure was 
high and slowly rising, three holes were bored in a small birch trunk, 
about 1.5™ apart vertically. Gauges were attached to the lower 
holes and the upper one was plugged. Equilibrium was soon estab- 
lished between the gauges, the upper one reading 61.9°™ and the 
