1909] MERWIN AND LYON—SAP PRESSURE IN THE BIRCH 449 
content of the vessels as about 1 per cent. of the total volume of the 
vessels. This fact easily explains the state of hydrostatic equilibrium 
which observation shows to exist in the birch stem when pressures 
are high. Increasing pressure must diminish the size of gas bubbles 
to a considerable extent by increasing the solubility of the gas in the 
sap; decreasing pressure would have an opposite effect. 
There is, however, until rather late in the sap season, a good deal 
of gas in the closed cavities of the wood fibers. This is shown by 
specific gravity tests. A density of 1 is not attained in the wood 
of the branches until the buds are about one-third longer than in their 
winter condition. The maximum density of 1.14 to 1.17 is reached 
when the first leaves are about 8™™ long, near the end of the sap 
season. About a month is required for an increase of 25 per cent. in 
density. Therefore, the aerial parts of the tree considered in exp. 1, 
with an estimated volume of 57,000°°, would require about 500°° of 
water as a daily supply from the roots to bring about this increase 
in density. | 
To get an idea of the amount of water required to maintain evapora- 
tion from a tree at the middle of the sap season, two twigs, weighing 
T.94°™ and 2.3556%™ respectively, after the cut ends had been 
sealed with balsam were exposed for 3 hours, the first to a temperature 
of 70° F. in the laboratory, the second to about 42° F. in a breeze. 
The first twig lost o.03812™, and the second 0.0256". An average 
evaporation of o.o1®™ per hour for a twig weighing 2®™ may be taken, 
therefore, as an approximate measure of evaporation from the birch 
for the middle of the sap season. The tree of the experiment bore 
about 2000 such twigs, from which the evaporation at this rate would 
be 480°° of water per day. 
This water evaporated from the tree and that absorbed by the 
wood fibers during the increase of density of the tree may be taken as 
the approximate amount of water supplied to the tree daily by root 
absorption. 
Inasmuch as pressures are freely transmitted throughout the 
birch stem, it is evident that variations in the rate of evaporation and 
‘™filiration and of root absorption> will cause variations in pressure. 
3 LivIncston (The réle of diffusion and osmotic pressure in plants. 1903) has 
the factors concerned in the control of absorption and pressure in roots. 
