HEAT AND DROUGHT. 257 



caused by rapid shrinkage of the bark and sapwood during 

 intense frost, a sudden rupture of the woody -fibrous tissue 

 taking place along the line of least resistance, usually accom- 

 panied by a loud noise. The clefts or frost-shakes, often only 

 about a yard long, sometimes extend all along the bole and 

 go deep into the tree. When the thaw occurs, the woody- 

 fibrous tissue expands and almost closes up the wound, which 

 cicatrises by a ridge of callus tissue. This may be repeated 

 every winter, or the cleft may remain closed during mild 

 winters ; but the frost-shake remains visible as a long swollen 

 ridge, spoils the timber, and enables fungus-spores to enter : 

 rot is therefore frequent near frost-shaken parts. 



Frost - cracks are mostly to be found on trees with large 

 medullary rays (Oak, Elm, Chestnut), but also occur on Beech, 

 Ash, Maple, Sycamore, Lime, Poplar, and Willow. Conifers 

 are seldom split by frost, though cracks are sometimes to be 

 seen on Spruce and Silver Fir. 



Heat causes sun-burn or bark-scorching by direct insolation, 

 whereas Drought impoverishes a dry soil by exhausting the 

 soil-moisture through evaporation, though warmth stimulates 

 the activity of vegetation so long as there is a sufficiency of 

 soil-moisture obtainable from percolations, capillarity, or rain- 

 fall. But when transpiration from foliage is increased by 

 warmth and dry winds, without ample water - supply being 

 obtained from the soil, the natural balance between imbibation 

 and transpiration is disturbed, leaves droop and wither, and 

 even in the usually damp climate of the Western Highlands 

 many deaths are thus caused during a warm, dry, windy May 

 and June, especially when there is no growth of heather, &c., 

 to protect the young plants in new plantations until they have 

 established themselves and got their roots down well into the 

 lower soil where the supply of soil-moisture is more constant 

 than nearer the surface. And, of course, in sandy districts 

 the danger of fire is always great when July and August 



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