WOODS AND FORESTS. 317 



of water is greatly retarded. In a wood with leaf 

 mould less than one-sixth of the water is evaporated, 

 which would in the same space of time be lost on 

 ordinary soil. 



All water absorbed, of course, eventually passes out 

 by the leaves, but it does so gradually. Thus, within 

 the forest, there is a moist atmosphere, and of course 

 also a shelter from wind, conditions which are excessively 

 favourable to vegetation. Small streams and rivulets 

 are much retarded in their work of erosion, the roots of 

 the trees and other plants binding the soil together, and 

 preventing it from being washed away, whereas bare 

 soil is rapidly carried off by running water. Thus, 

 by hindering the erosion of the land by the streams, 

 and by delaying the evaporation of water, forests are of 

 great importance to the climate of a country and to its 

 agriculture. Whether the actual rainfall in a region 

 can be increased by forests has not yet been certainly 

 determined. 



The annual formation of organic matter in a wood is 

 very great. An average well-grown tree will produce 

 perhaps 700,000 leaves in a year. In the Scotch 

 Lowlands the following table constructed from the 

 observation of Messrs. Ren wick and Mackay shows the 

 average increase in girth per annum of the trees quoted : 



