280 HARRIS, GORTNER, AND LAWRENCE: 
concerning the value of R in the species of trees with which we 
have worked. 
The average ratio of the differences in height of all the possible 
pairs of samples from the same individual tree to the water- 
column equivalent of one atmosphere have been computed and 
found to be i ; 
H = (L — Ly)/34 = 0.646. 
Thus if differences in hydrostatic head stood in a causal relation 
to differences in the osmotic concentration of the leaf sap, and 
were the only factor of significance, one would expect an average 
difference of 0.646 atmospheres in the series of differences deter- 
mined from our observations. 
Naturally the resistance of the conducting tracts cannot be 
neglected. Dixon estimates that at most this is not greater than 
the hydrostatic head. If twice this value be taken for the sake of 
obtaining concrete figures the theoretical mean value of the 
difference in osmotic concentration at two levels would be 1297; 
The observed mean difference in osmotic concentration, P, 
is almost exactly midway between the two values 0.646 and 
2 X 0.646, i. €., 0.978 as compared with the mid value 0.0968. 
The closeness of agreement to be expected between observa- 
tion and theory must depend entirely upon the value assigned to 
R. Taking R = H the agreement is not good, for the mean 
value of P — Py is about 1.00 as compared with the expected 1.30. 
The value of R which would harmonize hypothesis and observa- 
tion would be .333 as compared with .646, or about half the 
hydrostatic head instead of the equivalent of the hydrostatic head 
considered the maximum value by Dixon. Since determinations 
of the resistance of the conducting tissues have for the most part 
been made on cut branches and with methods in which errors 
are unavoidable, it seems quite possible that half rather than the 
equivalent of the hydrostatic head may be the true value of the 
force required to overcome the resistance encountered by water 
in moving through the conducting tracts of these species at the 
rate of the transpiration stream.* 
apillary rise in the stem and imbibition pressure in the leaf cells would both 
tend to acl R. Neither of these forces would be included, at least in their entirety, 
in the direct measurements of R which have been made. 
