510 PROTECTION AGAINST FROST. 



SECTION III. FROST-CANKER. 



Cankers may be caused by frost among young broadleaved 

 species, such as oaks, ash, maples, beech, fruit-trees, etc., 

 which have not yet grown above the local frost-level. Thus, 

 at the base of a young s''oot, which has been repeatedly frozen 

 down to the main stem, the living bark separates from the dead 

 wood. A callus forms round the wood in the growing season, 

 but is frozen on the recurrence of severe frost, and as, in 

 frost-hollows, this may happen annually, a canker is thus 

 formed, and the wood may be killed down to the pith, on the 

 side from which the branch arose. These cankers are formed 

 near the root-stock of oaks and ash growing in depressions on 

 stiff clay soil ; they may be distinguished from others caused 

 by fungi, as they increase in size only after severe frost. 



Frost-cankers on Shorea robusta are very common in frosty 

 depressions in Northern India, the shoots being killed down 

 to the ground annually until an abnormally large flattened 

 stool is formed. Coppice-shoots of a variety of sweet chestnut 

 from the south of France are also similarly frozen down in 

 Alsace, while the common variety of the tree produces splendid 

 coppice-poles. 



SECTION IV. UPROOTING OF SEEDLINGS BY FROST. 

 1. General Account. 



During February and March, when night-frosts alternate with 

 thaws in the day-time, it is often found that young seedlings 

 are raised with the soil, and in the subsequent thaw, when the 

 soil sinks back again, their roots lose their held on the ground, 

 and the plants fall over and die. This action is termed frost- 

 lifting, the seedlings being lifted by the alternate frost and thaw. 



In such cases the surface-soil is raised by the conversion of 

 the water in it to ice-crystals, and the little seedlings are thus 

 lifted above their original position. When the thaw sets in, 

 and the soil gradually softens and returns to its original level, 

 the plants cannot do so, as their roots are in the deeper and 

 still frozen soil, while the surface-soil is thawing ; when, there- 

 fore, the soil has completely thawed, the plants lose their 

 root-hold and fall over, as shown in Fig. 240. Even when the 



