PRECIPITATION AND VEGETATION 33 



their lawns in dry weather, and so keep them permanently green 

 and fresh. If they are not watered and the summer is a dry 

 one, the grass soon gets brown and withered. As it withers 

 the earth bakes hard or becomes converted into dust. Watch 

 such a lawn, especially if placed on a slope, during a thunder- 

 storm. The rain may not penetrate the baked earth at all, 

 but simply run off its surface, leaving great furrows as it runs. 

 At the same time note the contrast between the effect of heavy 

 rain on a surface covered by vegetation and one which is from 

 any cause devoid of a plant covering. The latter is worn into 

 channels and gutters much more readily, the soil is quickly washed 

 away. On the other hand, soil completely covered with vegetation 

 is not nearly so readily washed away. We thus see that the plant- 

 covering has a protective effect. 



During dry weather also, observations should be made in the 

 school garden or elsewhere, to show the depth to which the drying 

 of the soil extends. That the surface layers of the soil may be 

 quite dry while the deeper layers are moist ; that shallow-rooted 

 plants flag more quickly in drought than deep-rooted ones ; that 

 a number of plants growing together, as plants usually do in 

 nature, suffer less from drought than solitary plants in a garden 

 border such facts as these may be obtained from actual observa- 

 tion, and have an important bearing on the question of drought. 

 Similarly, the fact that while we may let our window-boxes get 

 very dry in cold or frosty weather without harming the plants, yet 

 they flag at once if we allow the soil to get very dry in summer, 

 is of importance in considering the connection between rainfall 

 and vegetation. 



Once again, the teacher should try to find local examples 

 showing how greatly soils differ in their power of holding water. 

 Over a great part of our country the frequency of boulder clay 

 as the sub-soil enables us to show how a clay soil holds water. 

 With this should be contrasted the lighter, sandier soils, which 

 dry with great rapidity. If the school is near the seashore, 

 and this shore affords examples of sand dunes, much may be learnt 

 in regard to desert conditions from a well-planned excursion. 

 Just as in studying the distribution of temperature over the 

 surface of the globe much additional interest may be roused by 



VOL. VI. 3 



