I l6 PLANT LIFE. 



effect on the climate, and for a similar reason. The 

 springs and little streams are hindered in their escape, 

 and shaded from evaporation. Even in our own coun- 

 try, draining the marshes of the hill pastures and 

 moorland has had a distinct effect on the rivers. 

 Periods of excessive spate, soon after rain, alternate 

 with droughts, when the river usually falls very 

 low. 



The manner in which any kind of soil permits water 

 to trickle through it, and the depth at which a layer of 

 clay or other impervious soil exists, are, therefore, very 

 important factors in determining the vegetation of the 

 locality. The following experiment by Professor 

 Wollny shows this influence of vegetation on erosion : 

 one square metre of bare soil at a slope of 30° lost 

 3104 grammes, whereas grass-covered soil at the same 

 slope only lost 50.8 grammes. 



Soil may be roughly described as a skeleton or 

 framework of stones or hard particles, generally more 

 than three millimetres in diameter, between which there 

 lies the finer earth. This latter consists chiefly of clay 

 (alumina and silica), and fragments of decayed animal 

 and vegetable matter. The clay is formed of the 

 original rocks, because water, acidulated by carbonic 

 and nitric acid, has dissolved and removed the other 

 materials. Even the subsoil has been, in most cases, 

 greatly altered by previous vegetations. The past 

 history of sandstones, shales, alluvium, and especially of 

 boulder clay, involves very great alteration by physical 

 and biological factors ; and their present constitution 

 and value for plant life depends upon this. 



As regards water, soils behave quite differently. 

 Schwarz gives the following example : 



A layer of soil 10 centimetres deep will allow the 



