454 BOTANICAL GAZETTE [DECEMBER 
for a case in which the resistance to flow of sap is twice as great radially 
as longitudinally.) 
Now let the rate of flow for the first ten minutes after tapping be 
represented by jig. 4, D, curve N, and let B show the distribution of 
pressure at the end of two minutes, and C at the end of the 10 minutes. 
Let the pressure be measured at the three points x, y, z. The pres- 
sure in x before tapping is 40.5°™, at the end of 2 minutes it is 35°", 
and at the end of ro minutes it is 25°". These values are plotted in 
fig. 4, D, curve X. The values for the pressure in holes y and z are 
likewise plotted in fig. 4, D, curves Y and Z. Inspection of these 
0 
88 
Ke] 
SE | 0 40 Na 
—___ md 
: og 
40 eae | X 
hea Ss 
Bs 
Fic. 4.—For explanation see text. 
20 
NP 
N 
~ 
a So 
T 
{ 
! 
! 
oO 1 
' 
/ 
é 
Ss 
+ —~~_ J 
N Pp ; 
A B 
curves of pressure and the curve of flow shows that there is no 
definite general relation between pressure and flow. 
Experiments 
EXPERIMENT 1.—During the evening of April 14, 1906, while the 
normal evening rise of pressure was in progress, a white birch (Betula 
populijolia) 11°™ in diameter and 6™ high was tapped for two gauge 
157°" vertically apart. The pressure from the first was more bea 
enough to sustain a column of water as high as the tree. The differ- 
ence in pressure of the two gauges was 12 to 12.3°™ of mercury; the 
hydrostatic pressure corresponding to this difference in the height of 
the two gauges being 11.6°™, This hydrostatic equilibrium may be 
explained on the supposition that a practically continuous column 
of water could have been traced through at least some of the ducts 
between the tap holes. Other ducts might have contained bubbles 
mn.2 4 6 8 10 
