SAP DENSITY AND FREEZING POINTS OF LEAVES. 105 
an ordinary frost by the fact that the major part of the 
slope is north and northwest. However, “the cold spell of 
April 24, 1910, was accompanied by cloudy skies and rain, 
mixed with snow, and the movement of the cold air cur- 
rents was quite different from that which takes place under 
a clear sky, or when there is no rain or snow. It will be 
observed that during cold spells, with more or less cloudi- 
ness or moisture, the temperature on the hills may fall to 
a lower value than the temperature in the valleys. On the 
other hand, had clear weather prevailed * * * the tempera- 
ture conditions would have been reversed, the valleys prob- 
ably being colder.” (Garriott, E. B. Notes on Frost. 
Farmers’ Bull. U. S. Dept. Agr. 104:12. 1910.) More- 
over, the minimum temperature of the 24th was, as reported 
by the Weather Bureau, 25°F., and acted for a time long 
enough to insure freezing of such things as were in condi- 
tion to freeze. As a matter of fact no frost was formed, and 
the results are simply those of direct freezing. This makes 
the later comparisons of natural and artificial freezing much 
more convincing than would be the case had the damage 
to vegetation been due to frost, for we could not be quite 
sure that we had duplicated frost conditions of tempera- 
ture and moisture, while duplicating freezing temperatures 
is a comparatively easy matter. 
The effect of cold upon vegetation in general depends 
very largely upon the rapidity with which the destructive 
change in temperature is brought about, being greater when 
the change takes place within narrow limits of time. 
Temperature changes in March cannot be said to directly 
concern the plants affected by a freeze during the next 
month, yet the same temperatures occurring a few days 
before a heavy freeze, could have serious results. In the 
one case acclimatization results, whereas in the other no 
adaptive change takes place. In how far the process of 
acclimatization can protect plants from extremes of cold we 
do not know, although the principle is applied in using 
cold frames to acclimatize former greenhouse plants. How- 
