444 BOTANICAL GAZETTE [DECEMBER 
times a very regular distribution of pressure in the birch trunk, taps 
at the same height giving like pressures, and taps at different heights 
showing most pressure in the lowest. The difference between the 
pressure in the lowest tap hole and that in the highest is usually 
slightly greater than the hydrostatic difference in level between the 
holes (fig. 1). 
Pressure in one hole is always immediately and markedly lowered 
by sap flowing freely from another hole, even though the holes are 
on opposite sides of the tree and many feet apart vertically. This 
fact, of course, indicates a free intercommunication among the ducts 
of the birch wood. ; 
In all of the cases we have observed, pressure began to be evident 
at the base of the tree first, and as pressure increased there it showed 
itself higher and higher up. 
Daily fluctuations of pressure in the birch were reported by CLARK.’ 
The general character of these fluctuations was brought out by sen) 
observations in April 1904. A rapid rise of pressure beginning In 
the morning is followed by a slow decline till near sunset, then a 
gradual rise is kept up during the night. The nightly rise of pressure 
is checked if the temperature of the tree falls below freezing. Changes 
of pressure are only slight the next day after a freezing night, unless 
the air temperature reaches 40° to 45° F.ormore. (These oscillations 
of pressure occupying a period of a day are graphically shown in jig. 3 
of the second part of this paper.) ey 
The most striking phenomenon of the birch sap pressure is its 
variability during those rapid changes of sunshine that take place on 
days when cumulus clouds occasionally drift before the sun. ya 
have seen the mercury column in a gauge move more than 2-5 
vertically in a minute in response to a change of less than 1°C. as 
registered by a blackened-bulb thermometer exposed to the sun- 
Furthermore, pressure changes of this rapidity have been kept up. 
for nearly ten minutes at a time. Fig. 2 is a record for part of an 
afternoon in which bright sunshine and dense cloud-shadow - 
nated. A drop in pressure of 37-5°™ of mercury during a period © 
cloud-shadow, and a subsequent rise of 30°" when the sun appeared, 
took place between 2:30 and 3:20 p.m. A comparison of the bir 
? CLarK, W. S., The circulation of sap in plants. 1874. 
