ae 
t as 
: 4, 
SAP DENSITY AND FREEZING POINTS OF LEAVES. 1381 
the same conditions of light, exposure, etc. It is quite 
probable that the differences noted above may have been 
caused by the failure, in either of the two seasons, to take 
account of the relative protective positions of the trees from 
which specimens for sap determination were taken. 
3. CONCLUSIONS. 
Our conclusions are based upon two series of investi- 
gations: one, the observations of the effect of a freeze on 
many trees and shrubs; the other, the artificial freezing of 
the leaf saps of the above trees and shrubs. The first is more 
or less arbitrary, being based mostly on external appear- 
ances; the second is hypothetically accurate. From a 
comparison of these two sets of data we are led to the 
following conclusions :— 
1. That extreme differences in sap density, in general, 
are accompanied by a corresponding difference in their 
resistance to freezing. 
2. That exceptions to this general rule are probably 
due to differences of cell structure, and other causes that 
may enter in, as protective location, etc. 
8. That where cell structure is the same, the densities 
of the cell saps indicate their relative hardiness, as in the 
magnolias. 3 
4. That in plants of the same genus, or varieties of the 
same species, differences in sap density correspond to differ- 
ences in their resistance to freezing. 
