SOIL, 23 



the constituents of the atmosphere. Without them, the land 

 would soon lose all moisture. The motion of the atmosphere 

 ensures a proper distribution of moisture, carbon dioxide, 

 oxygen and nitrogen over the earth. 



Air currents affect forest trees injuriously principally in two 

 ways : — 



(a.) By unfavourably modifying the temperature and 

 moisture of a locality. 



(b.) By injuring, breaking, bending, or overturning them. 



Dry winds frequently reduce the moisture of a locality to a 

 dangerous degree ; moist and cold winds may reduce the 

 temperature, and thus interfere with the healthy growth of 

 the trees. Strong winds may break the leading shoots or 

 side branches, cause trees to assume a curved shape, or even 

 throw single trees and whole woods to the ground. 



The damage done to trees by strong winds differs very 

 considerably according to species. Shallow rooted trees, like 

 the spruce, suffer most, while deep rooted trees, like oak or 

 Scotch pine, are much less affected. The most dangerous 

 winds in Western Europe are those which blow from south- 

 west, west, and north-west. Occasionally north-east winds 

 are also dangerous. 



Section III. — Soil. 



It has been already stated that plants, and more especially 

 trees, require a certain layer of soil, into which their roots 

 penetrate, and which provides them with nourishment and the 

 means of stability. Wherever this layer of soil is deep enough 

 to meet the above requirements, the subsoil is only of indirect 

 importance, but in the case of shallow soils the subsoil has, as 

 a rule, to undertake part of the ordinary functions of the soil. 

 Under any circumstances, the subsoil furnishes the materials 

 from which the mineral parts of the soil are principally 

 derived. Hence, in speaking of the soil, the subsoil is more 

 or less included. 



