PROTECTION 163 
loud report. This is probably due to the fact that the 
water in the walls of the wood pores is withdrawn 
when it freezes into little crystals on the interior of 
the walls of the pores of the wood. The withdrawal 
of this water causes shrinkage, which is greatest at 
the periphery and nil at the center. A wedge-shaped 
erack, often twenty feet in length, is thus formed. 
Frost cracks are not unlike season checks in logs. 
The sap escapes from the wound, ferments, and black- 
ens that part of the trunk. This is especially notice- 
able in the case of sugar-maples. This sap often 
freezes cold nights in spring, and this tends to pry 
open the split. The tree endeavors to heal over this 
wound year after year, and there results in conse- 
quence a long bulge or rib on the trunk along the line 
of the crack. Large sugar-maples in our northern 
woods which have been injured in this way are very 
common. 
The uprooting of seedlings by ground frost in 
the spring is often a serious difficulty in nursery 
work. The water in the soil forms into ice crystals 
which lift the topsoil and heave the seedlings out so 
that when the crystals melt and the soil falls back 
into place, the little plant is left upon the surface 
uprooted and exposed. In places where the drainage 
is good this is less apt to occur. 
Extreme heat and drought are, of course, injuri- 
