WINTER INJURY 299 



Wind may, as has been suggested, ^2 be associated with this type of 

 injury under certain circumstances but there can be no doubt whatever 

 that sphtting occurs on absolutely still nights, the sharp, rifle-like 

 report accompanying the fissure being very noticeable under such con- 

 ditions. Fisher^^ discusses the subject at some length. He reports that 

 most frost cracks occur in cold weather between midnight and morning 

 and may close again with rising temperature; further, that sometimes an 

 internal frost crack occurs, the sap-wood rending while the bark holds 

 intact. Hardwoods with large medullary rays are most liable to this 

 injury, oak, beech, walnut, elm, ash and sweet chestnut being mentioned 

 as specially susceptible in Europe. 



The cracks are said to occur most frequently in the lower part of the 

 trunk, especially where growth is uneven, as near roots, at knots or where 

 the stem is eccentric. The south side, the region of most vigorous cir- 

 cumferential growth, suffers most, according to Fisher. Large old trees 

 suffer more than young because under conditions inducing this injury 

 there is in the old trees a greater difference in temperature between center 

 and periphery. Late winter, when the sap has begun to flow, is said to be 

 the most favorable time for developments of this kind. Under normal 

 conditions these cracks close with a rise in temperature and the tissues 

 in time grow together; this spot is weaker however and subject to a 

 recurrence of the injury. Repeated splitting and healing may give rise 

 to a lipped callus. 



Observations in America agree generally with Fisher's, adding 

 the maple to the list of subject trees and finding perhaps more crack- 

 ing on long, straight-grained, clear boles. Indeed it seems that the two 

 chief reasons for the comparative resistance of fruit trees lie in their 

 being low headed, with short areas of trunk free from branches and in 

 their smaller trunks. Under cultivation they probably do not mature as 

 early as forest trees and the sappy growth of young trees may be injured 

 in early winter in contrast with late winter for forest trees. It is stated 

 that fruit trees growing late and entering the winter with wood not 

 thoroughly ripened are most subject to frost cracks in Colorado. ^^ 



On apple limbs an injury similiar in appearance and likely to be 

 confused with this type, sometimes occurs when there is one sided develop- 

 ment of the limb so that a heavy load of fruit is borne on one side un- 

 balanced by any considerable load on the other side, resulting in a fracture 

 in a vertical plane. Occasionally after a nearly horizontal limb is headed 

 back to a large branch ascending at about 45° a heavy load on the ascend- 

 ing branch will cause a splitting of the upper part of the limb from the 

 lower, the fracture being in this case horizontal. These injuries obvi- 

 ously occur near harvest and should be differentiated from the true 

 "frost cracks" without difficulty. 



The reverse of the conditions described in connection with radial 



