OSMOTIC CONCENTRATION OF LEAF SAP 269 
head. They found a slightly greater depression of the freezing 
point for sap from the more distantly placed leaves. From deter- 
minations on Ulmus campestris they conclude, however, that the 
resistance of the water tracts is not the controlling factor but that 
variation in osmotic concentration is to be attributed principally 
to the fluctuations in the sugar content due to differences in 
illumination. : 
As a final conclusion they state: 
‘Variation in pressure is not defined by the height of the leaves above the ground, 
nor by the resistance of the conducting tracts supplying the leaves. In each case 
the =. pressure was much greater than the tension of the water-supply could 
have been 
Thus the studies of Ewart leave the fundamental NieeHen of 
the existence of a differentiation in osmotic concentration asso- 
ciated with height quite unanswered, and the careful measurements 
by Dixon and Atkins are not adequately numerous for a final 
answer. 
Conclusions have been drawn quite contrary to those resulting 
from the masses of data to be presented below. Thus Hannig* 
in the introduction to his study of osmotic concentration in root 
and leaf concludes that the work of Ewart and of Dixon and Atkins 
shows that ‘‘die Differenzen in der osmotischen Werten der 
Blattzellen einer Baumes von der Héhe des Blattansatzes unab- 
hangig sind.” 
‘ A proposal to investigate the concentration of the sap of 
leaves inserted at different levels is frequently met by the objec- 
tion that the leaves at various heights are really subject to distinct 
environmental conditions (insolation and air movements) and 
that these external factors will either obscure any relationship, 
or, on the other hand, if a relationship between height and con- 
centration be actually found, serve to fully explain it. 
To us it has seemed that the primary task, the execution of 
which should precede all others, is to determine whether as a 
matter of fact there is a relationship between the level of origin 
of leaves and the properties of their sap. If there is no auch 
* Hannig, E. Untersuchungen iiber die Verteilung des osmotischen Driickes 
in der Pflanze in Hinsicht auf die Wasserleitung. Ber. Deutsch. Bot. Ges. 30: 194- 
204. I912. 
