Plants as Affected by Cold. 117 
194, Tree Trunks are sometimes Split Open by Se= 
vere Freezing, the split remaining open until the return of 
mild weather. This most often occurs in hard-wooded, deep- 
rooted, deciduous trees as the oak, and appears to result 
from the more rapid contraction of the outer layers of the 
wood in a sudden fall of temperature. The rents are usu- 
ally overgrown by the next annual wood layer (71). 
The splitting down of the main branches of certain varie- 
ties of the apple tree appears to be favored by the expan- 
sive force of ice in narrow crotches, which retain more or 
less of snow and water. Varieties of which the branches 
leave the trunk at a wide angle are not subject to this trouble. 
195. Bark-Bursting on the trunks of young apple 
trees often occurs when freezing weather overtakes late- 
growing and hence poorly-matured wood. In severe cases, 
the bark splits longitudinally clear through the cambium 
layer, and from the ground to the lower branches; and the 
bark is loosened from the wood nearly or quite around the 
trunk. Such trees are practically ruined, but trees slightly 
injured by bark-bursting may fully recover. 
Bark-bursting is usually most severe on deep, rich, moist 
soil and in seasons that favor late growth, or in which 
freezing weather occurs unusually early. Late-growing 
varieties are most subject to it. Its occurrence is lessened 
by treatment that favors early maturity of the wood (200, 
201). 
196. Root-Killing of trees. When a very dry autumn 
passes to winter without rain or snow, the surface layers of 
the soil sometimes freeze so severely as to destroy the roots 
of young trees. Root-killing is usually most serious on 
light soils, and on one-year-old, root-grafted (391), nursery 
