BRITISH FOREST TREES 215 



reached between the fortieth and eightieth year, according 

 to circumstances, and only exceptionally is it allowed to 

 attain so long an existence as a hundred years. 



Liability to Suffer from External Dangers cannot be 

 ascribed to the birch. Against drought and frost it is 

 equally insensitive ; winds, snow, and ice are alike incapable 

 of doing it much damage ; from deer or cattle it has little 

 to suffer, although rubbed stems have only a weak recupera- 

 tive power ; in the insect world it has no enemies of real 

 importance. Stems covered with the tree-louse, Lachnus 

 longirostris, attract swarms of wood-ants, Lasius fuliginosus, 

 which gnaw the cambium, and undermine the bark. 

 Many larvae and weevils feed on the foliage (Anomala, 

 Brachyderes, Cimbex, Nematus, Polydrosus, Phyllobius, 

 Porthesia, Rhynchites, &c.), without in general committing 

 very much havoc or destruction. Somewhat more import- 

 ant, however, is the damage done to timber by the grubs of 

 Hylecatus dermestoides, and the caterpillars of Cossus 

 ligniperda, Sesia culiciformis, and Zeuzera cescidi. 



From fungoid disease it is also comparatively exempt, 

 although red-rot is occasionally caused in the wood by 

 Polyporus belulinus and sulphureus, and whife-rot by 

 jf. lavigatus at places where the stems have sustained 

 injuries. 



Sylviculture Treatment of the Birch. However valuable 

 may be the services of the birch in forming forests in the 

 far north and north-east of Europe, its sylvicultural import- 

 ance is somewhat limited in Britain, particularly in the 

 southern portion, and its chief value is rather as a nurse 

 against frost, or for stimulating to growth in height, with 

 subsequent retention here and there as a subordinate species, 

 than in the formation of pure forests. For with its unequi- 

 vocal demand for light, its branching growth, its early 

 interruption of canopy, and its light foliage, it does not 



