36 THE BOOK OF NATURE STUDY 



The effect of the destruction of the forests in promoting 

 erosion is not easily illustrated in this country, where the forests 

 have been so thoroughly destroyed. The teacher who travels 

 to the Alps in the summer may often see beautiful examples 

 of the destruction of mountain pasturages by the cutting down 

 of the trees, though the danger is there being increasingly 

 recognised. 



In the United States the matter is also attracting attention, 

 and we give an illustration showing the extraordinarily rapid 

 erosion which follows the cutting down of trees on a mountain 

 slope. The point to be illustrated is that an adequate supply 

 of rain is necessary for the establishment of a continuous 

 covering of vegetation over any area. Once established, this 

 vegetation increases the fertility of the soil in which it grows by 

 the addition to the original sand or rock debris of vegetable mould. 

 If from any cause, however, natural or artificial, this covering 

 of vegetation be destroyed, then the very rainfall which per- 

 mitted the establishment of the vegetation may destroy in a 

 limited period of time the work of ages. If the rainfall is not 

 of the type which permits of a complete covering of vegetation, 

 that is, if it is absolutely deficient in amount, if it comes at the 

 wrong season, or if it comes in torrential downpours of brief 

 duration, then the waste of the surface takes place with a rapidity 

 which is unknown in lands where the rainfall is of a different 

 type. 



The significance of the various types of rainfall in relation 

 both to vegetation and to human activities may, however, be 

 illustrated by other methods. For instance, if the school course 

 includes the study of living plants, and if the school excursions 

 include a study of plant formations as they occur in nature, it is 

 easy to show that our native plants, with few exceptions, are 

 not of the water-economising type (xerophytes) . In the summer- 

 time we may also show that their foliage is such as to allow of 

 the escape into the air of a great amount of water vapour, 

 and as indications of drought on any considerable scale among 

 plants growing under natural conditions are rare, it is obvious 

 that water is being abundantly supplied to their roots. Again, 

 we can show that the fall of the leaves in autumn no less than 



