OSMOTIC CONCENTRATION OF LEAF SAP 285 
Our own feeling in regard to the matter is that conclusions 
concerning this question should be deferred until actual measure- 
ments of the suggested environmental factors are available. 
IV. RECAPITULATION 
In the preceding pages we have considered the problem of the 
relationship between level of insertion and the physico-chemical 
properties of leaf sap in trees. 
Measurements on 26 trees belonging to 12 species show that: 
(a) The osmotic concentration of the leaf sap as determined 
by the freezing-point lowering method increases from lower to 
higher levels. This is true almost without exception. 
(b) Specific electrical conductivity, x, shows a tendency to 
decrease from lower to higher levels. The results are, however, 
by no means so regular as those for the depression of the freezing 
point, A. 
(c) The ratio of specific electrical conductivity to freezing-point 
lowering, x/A, decreases from lower to higher levels. This is true, 
almost without exception. 
The relationship of physico-chemical properties to level of leaf 
insertion must be due to either internal or environmental factors. 
The suggestion is made as a basis for further investigation 
that if the increase in osmotic concentration with increase in 
level of insertion be an adjustment on the part of the leaf cell to 
increased load, the increment in osmotic concentration should be 
approximately given by 
P-P=H+R, 
where P is the osmotic concentration at a higher and Pp that at a 
lower level, H is the weight in terms of atmospheres of a column 
of water L — Lo in height, and R is the resistance, expressed in 
atmospheres, to the passage of water moving at the rate of the 
transpiration stream opposed by conducting tracts of the length 
L — Lo, where L and Ly are the heights of the two levels of leaf 
insertion. 
The ratio 
nee Fs 
(L — Lo)/34’ 
