270 HARRIS, GORTNER, AND LAWRENCE: 
interdependence, it is idle to advance theories in explanation of 
it, as some botanists seem inclined to do. If, however, careful 
sets of determinations show that a consistent relationship between 
height at origin and the osmotic concentration of the leaf sap does 
obtain, then, and only then, will it be worth while to inquire 
whether this relationship is of the magnitude necessary to over- 
come the weight due to the hydrostatic head and the resistance 
due to the conducting tracts, to measure the various environ- 
mental factors with a view to correcting for their influence upon 
sap properties, and to consider what is the most suitable biological 
interpretation of the observed relationship. 
II. MATERIALS AND METHODS 
We took our samples* from trees growing both in the open and 
in the woods. In collecting the samples we tried to secure them 
from branches exposed, as nearly as possible, to comparable 
atmospheric conditions. Such trees are not easy to find, however, 
and until actual instrumentation for light intensity and for the 
evaporating power of the air is carried out we shall make no claims 
whatever for freedom from such influences. In a few cases, 
indeed, we included trees which for some reasons were desirable, 
but suffered from. lack of uniformity of exposure to light and air. 
Thus the tree of Quercus Prinus was higher than the surrounding 
vegetation, and the lower branches were somewhat more shaded 
than the upper ones. The leaves of Betula lutea collected July 23 
from branches 66 feet above the ground were certainly more 
intensely illuminated than those at 52, 39, 25 or I1 feet. 
In collecting samples of leaves, branches were lopped off at 
the respective heights and samples of the leaves were picked off 
as quickly as possible and enclosed in large test tubes which were 
*In undertaking these studies, we were concerned primarily with securing 
samples of leaves originating at different levels above the ground. We neglected, 
therefore, any tof the length of branch through which the sap would 
have to pass after leaving the trunk of the tree. Thus it is not possible to discuss 
in any exact way the possible influence of the resistance of conducting tracts on the 
magnitude of the concentration of the sap. Any errors due to this source may 
probably be disregarded in a preliminary study like the present. In a few cases 
we secured determinations from leaves originating on horizontal branches at dif- 
ferent distances from the main trunk, but almost without exception these materials 
are open to some objection, and we have excluded them. 
