NATURAL WOUNDS. 209 



Shot-holes in Cherry, Wahiut, Tobacco, and 

 Plum leaves are due to Phyllosticta, in Cherry 

 leaves also to Clasterosporium, and in Potato leaves 

 to Haltica. 



Frost-cracks. The trunks of trees exposed to 

 the north-east, and occasionally with other aspects, 

 are apt to show longitudinal ridges which realise 

 on a larger scale the features of healed wounds 

 scored with a knife. These wounds are due to 

 the outer layers of wood losing water from their 

 cell-walls as it congeals to ice in their lumina, 

 more rapidly than do the warmer internal parts of 

 the trunk ; as this drying of the wood causes its 

 shrinkage, especially in the tangential direction, 

 the effect of a sudden frost and north-east wind 

 is to rend the wood, which splits longitudinally 

 with a loud report, as may often be heard in 

 severe winters. Since the cortex and bark are 

 ruptured at the same time the total effect 

 resembles that of a deep knife-cut, and the same 

 healing processes result on a larger scale when the 

 wood swells and closes up the wound again in 

 spring. But this recently-closed lesion is evidently 

 a plane of weakness, and if a similarly severe 

 winter follows the wound reopens and again 

 heals, and so on, until after a succession of years 

 a prominent F^-ost-ridge results, which may finally 

 heal completely if milder winters ensue or the tree 

 be eventually protected. 



Strangulations. We are now in a position to 

 understand the so-called strangulations which 

 result when woody climbers, telegraph wires, etc., 



o 



