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 Frost-ridge results, which may finally heal completely if milder winters ensue or the tree be eventually protected.