SAP DENSITY AND.FREEZING POINTS OF LEAVES. 119 
are among the first to open and develop their leaves in the 
spring, as are those only partly affected; and those which 
have been the hardest hit of all the trees and shrubs, were 
among the last of all to open and expand their leaves. If 
it were possible to tabulate the ages of leaves and the order 
of their opening, we might also get an answer to our ques- 
tion, in that the older the leaf is, the more resistant it 
becomes. The time consumed in the experiments on freez- 
ing point lowering left no opportunity for complete work 
on this subject, and, except for the general confirmation 
of this supposition, nothing further has been done on the 
subject. (c) If, then, the ages of the leaves in general 
become a criterion of their resistance to freezing, then the 
difference in the ages of the parts of the leaves may 
account for the partial freezing of maples, ete., noticed in 
Section II. (d) Since the minimum of temperature 
recorded occurred during the night, we may also assume 
another cause. In the transformation of starch into sugar, 
which goes on in the leaf at night (as well as in the day- 
time), the plant is acquiring for itself a medium with a 
lower freezing point, for the insoluble starch grains have 
been changed into sugar, which, in solution, is gradually 
removed from the leaf. Such being the case the sugar 
solution would leave the tips of the leaves first and the 
more concentrated part would be near the center of the 
leaf. In other words the freezing point is lower at night, 
or at any time that respiration is greater than photo- 
synthesis, in the center of the leaf than at the edge. It is 
probable that we have here a possible explanation of the 
partial freezing noticed in the leaves of Acer and Aesculus. 
2. THE FREEZING POINTS OF LEAF SAPS, DETERMINED 
ARTIFICIALLY, AND NOTES IN CONNECTION THEREWITH. 
At the time when the writer was expecting to make use 
of the rather cumbersome method of Beckman for determin- 
ing the freezing point lowering of the cell saps, a paper 
appeared by Messrs. Dixon and Atkins, (Osmotic Pressure 
in Plants. Notes from the Botanical School of Trinity Col- 
ae 
