DAMAGE CAUSED BY LOW OR HIGH TEMPERATURE. 13 



re-open, the stem may assume the external appearance of being 

 healed, and may remain permanently closed. 



Along the edges of the cleft the formation of the new annual 

 zone is always somewhat thicker than on the rest of the circum- 

 ference of the stem ; and as these two cicatrised edges are close 

 together, they often gradually form ridges or frost scars standing 

 out about a hand-breadth from the normal outline of the stem. 



Hardwoods with strongly developed medullary rays, like Oak, 

 Elm, and Sweet Chestnut, are chiefly exposed to the danger of 

 forming frost-shakes ; they are also, but less frequently, formed in 

 Beech, a few softwoods (Lime, Poplar, Willow), and Conifers. They 

 are especially to be found on old isolated stems, or on standards in 

 copse, or in high forest, and usually occur on the east and north- 

 east sides of the stems, as the harder frosts only set in when the 

 wind comes from these directions, and consequently work most 

 powerfully on these sides. Low-lying tracts and localities with 

 fresh or moist soil are more exposed than others to this particular 

 danger. 



Preventive measures can hardly be applied to mitigate this 

 evil, but the early utilisation of trees with bad frost-shakes is 

 recommendable in view of the liability to which they become 

 exposed to fungoid disease, and consequent depreciation in value. 



9. Damage done ly lifting of the Soil from Frost (Ground- 

 frost) ; Protective Measures. 



Ground- frost consists in the raising of the soil in consequence 

 of frost in winter, the loose, naked soil, saturated with moisture, 

 being lifted up through the crystallisation of the water. When 

 young plants are in the soil, as in nurseries and young sowings or 

 plantations, these are also raised up, and when the soil sets again 

 after thawing, they do not sink along with it, but remain with 

 the roots more or less exposed, and very frequently perish. 

 Thus, whilst the soil on being frozen first rises and then sets, 

 the plants are gradually lifted by the frost. 



This occurrence only takes place in light or loosened, moist, 

 and especially in humose soil, and is most frequent at the early 

 part of the year, about February or March, when frosts at night 

 are apt to be followed by thaw during the day. The soil below 

 the surface being still frozen, the percolation of the surface- 



