OSMOTIC CONCENTRATION OF LEAF SAP 279 
: P = Po = ys 
where 
H = (L — Ly)/34, 
where except for differences due to the elevation above sea level 
at which collections are made, H is invariable. 
The resistance of the conducting tracts may be most conveni- 
ently expressed in.terms of the ratio between (1) height of a 
column of water, R’, which is required to force water through a 
horizontal stem of length L — Lo at the rate of the transpiration 
stream, and (2) the height of a column of water which is the 
equivalent of one atmosphere pressure, i. e., R = R’/34. 
Possibly R, the constant for the resistance of the conducting 
tracts, may differ from species to species. However this may be, 
workers who have attempted to determine the value of this 
constant have come to widely divergent conclusions. 
Dixon devotes Chapter VI of his book on Transpiration and 
the Ascent of Sap to the criticism of the experimental data on the 
resistance offered by the wood to the movement of the transpira- 
tion stream. He concludes: 
“Tt appears that water may be moved through a stem ina horizontal position 
with the velocity of the transpiration current if urged by a head equal to the lengt 
of the stem. To raise water in a vertical stem at the same velocity, evidently twice 
the head will be required. Consequently when the force is applied as tension at the 
upper end, the greatest stress the water need be subjected to is double the height 
of the moving column.’ 
Now while we may assume that 
P—P=H+R 
represents approximately the twe most important factors deter- 
mining any adjustment (if such adjustment really exists) of the 
cells to increased load, it is quite clear that the equation may 
not be so simple. Other factors are probably also involved. 
For example capillarity may diminish the downward pull due to 
gravity. Possibly the power of imbibition of the leaf colloids may 
also play a réle similar to that of the osmotic concentration of 
the solutes which contribute to the freezing-point lowering. 
It is clear that the testing of the experimental results secur ed 
i our height studies against this formula is rendered highly 
questionable by reason of the fact that nothing is definitely known 
