HAIL AND ICE. 259 



Hail-storms "beat down, injure, and often kill young plants 

 in nurseries, besides making surface-wounds on the young shoots 

 of older trees and thus enabling the spores of fungus diseases to 

 effect an entrance. Oak-bark will not strip at damaged spots, 

 and much damage is done to Osier-beds in the fen districts, as 

 the withes do not peel freely, and break at the injured parts. 

 The only remedy is to cut back badly-damaged broad-leaved 

 saplings, and to fill blanks in older crops with stout plants 

 of any suitable kind. 



Ice and Hoar-frost do damage in mountain-tracts to brittle- 

 branched trees like Scots Pine and Alder, and greatly increase 

 the danger of serious damage should snow fall while the twigs 

 are frozen, and especially in evergreen Conifer crops. Mixed 

 woods suffer less than pure crops of Scots Pine. 



Lightning does practically no damage in woodlands, though 

 here and there a tree may be struck and badly damaged or 

 killed ; but park-, field-, hedgerow-, and avenue-trees are more 

 often struck and blasted. 



Among atmospheric impurities the particles of carbon in 

 city smoke choke the pores of the leaves, while poisonous 

 gases from factories and smelting-works, and even from railway 

 trains running frequently through wooded valleys, are always 

 more or less injurious to trees and woodlands, the cause of 

 damage being mainly the sulphurous acid contained in the 

 smoke, which changes the natural colour of the leaves and 

 kills off many poles and trees. But nitrous, hydrochloric, and 

 arsenious gases cause similar damage, though to a less extent ; 

 and the damage is always greatest in damp localities. When 

 dew or rain falls on a leaf-surface, in a smoky locality, the 

 sulphurous acid combines with the water, oxidises into sulphuric 

 acid (S0 2 + H 2 + = H 2 S0 4 ), and acts very injuriously on 

 the leaf -tissue, especially of evergreen Conifers. In broad- 

 leaved trees the leaves become mottled with damaged patches 

 (sulphuric acid) or discoloured at the edge (nitric and hydro- 



