INJURIES DUE TO ATMOSPHERIC INFLUENCES 285 



are most abundant on the north-east side of trees, because intense 

 cold usually occurs with a north-east wind. As a rule, frost- 

 cracks are formed only when a great reduction of temperature 

 occurs suddenly, and when the interior of the tree is therefore 

 relatively warm, so that excessive shrinkage is confined to the 

 outer layers of the wood. 



It is a familiar fact that, when such frost-cracks have closed up 

 with the restoration of a higher temperature, they become oc- 

 cluded by the callus that forms along their edges. The reduced 

 pressure of the bark 

 causes the forma- 

 tion of new tissues 

 along both sides of 

 the crack, and these 

 project from the 

 surface as a " frost- 

 rib." On account 



of the thin callus- 

 layers being easily 

 ruptured, it requires 

 but a few degrees 

 of frost in succeed- 

 ing years to re- 

 open the crack. 

 Repeated opening 

 and closing of the 

 wound sometimes 

 induces the forma- 

 tion of strikingly 

 prominent frost-ribs. 



FIG. 157. An oak-stem showing a frost-crack which has 

 been produced in the winter before the wood-ring, 

 a, was formed. Originally the crack extended from 

 a to d. For nine years in succession the crack has 

 been annually reopened, so that the frost- rib, a to b, 

 has been formed, and this has ruptured laterally 

 at c. During the last five years the crack has re- 

 mained closed. One half natural size. 



Should several mild winters occur in 

 succession, a frost-crack may close up entirely, as is seen in 



Fig- 157- 



In the interior of old oaks I have sometimes noticed numerous 



radial and peripheral cracks which did not extend to the outside 

 of the stem, nor had they reached the surface even at the time 

 when they were formed. At present it is uncertain whether 

 these cracks are also to be attributed to the action of frost, and 

 no satisfactory explanation has been given as to the circum- 

 stances under which they originated. 



