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expansion on being converted into ice. During the 

 formation of ice, water is abstracted from the cell 

 walls, and this results in the drying and shrinkage of 

 the wood, just as happens when cut timber is dried 

 in the air. During severe cold, especially if the 

 temperature falls very rapidly, the outer wood layers 

 shrink more rapidly than the inner warmer layers, 

 and longitudinal cracks and fissures are caused 

 in the stem. Such cracks may become occluded in 

 the usual way, but subsequent frost frequently 

 re-opens the wounds, and if this is repeated several 

 times prominent ridges, or frost-ribs, result. 



(4) The formation of cankers. Frost cankers are usually 



found at the base of a young branch which has 

 been killed by frost down to the main stem. The 

 callus forming at the base of the dead shoot is 

 again killed by subsequent frosts, and with recur- 

 ring frosts the cancerous area spreads. The growth 

 of the callus is also interfered with by the pressure 

 of the dead tissues under which it forms, and 

 such diseased areas usually heal slowly, if at all, 

 and aiford a favourable point of attack for wound 

 fungi. Cankers due to frost may be distinguished 

 from those due to fungi as they only increase in size 

 in frosty years. 



(5) The uprooting of seedlings. When the soil freezes, 



the expansion which the water in its interstices 

 undergoes, on being converted into ice, raises the 

 surface of the soil and pushes up the roots above 

 their original position. When the ice melts with 

 the thaw the particles of soil fall away from the roots 

 which become exposed and the plant falls over and 

 dies. 



(6) The mechanical bending and breaking of stems or 



branches, owing to the accumulation of snow or ice 

 upon them. On hill sides the pressure of snow 

 against the upper side of young stems often causes 

 the latter to curve outwards at the base, and this 

 curve, being retained as the plant grows older, is 

 often visible in mature stems. This bending over 

 of young stems often also ruptures the tissues on the 

 uphill side and causes wounds which may give 

 access to injurious fungi. 



