Il] ACTION ON SOILS, ETC. 85 



sand-dunes with species like Psamma arenaria, Elymus 

 arenarius, Agropyrum junceum, &c. (together with sand- 

 binding species of sedges) and so not only fixing the sand, 

 but preparing it for gradual afforestation with bushes 

 and eventually trees, and so saving enormous tracts of 

 land and sums of money, as has been done on the West 

 coasts of France. 



Moreover, the action of ruderal plants — including | 

 grasses — is to completely alter the nature of the poor soil I 

 and gradually fit it for other plants. Coverings of grass 

 greatly affect the actions of heat and sunshine on the 

 surface soil, and modify the effects of radiation and 

 evaporation, to say nothing of the penetrating and other 

 effects, of the roots. 



Rhizomes and stolons break up stiff soils ; and every ' 

 engineer and forester knows how useful certain grasses 

 are in keeping the surface-soil from being washed down by f 

 heavy rains on steep hill-sides or embankments. 



On the other hand, luxuriant growths of tall grasses 

 may do harm to young plants, by their action as weeds 

 and especially as shade-plants ; though foresters can em- 

 ploy them in the latter capacity, under restrictions, to 

 shelter young trees from the sun. Again, too much dry 

 grass near a forest offers dangers from fire ; and it is a 

 well known fact that certain injurious animals, e.g. mice f 

 and other vermin, are favoured by a covering of grass. 



Graminacese are for the most part chalk-fleeing plants, i 

 in spite of the fact that certain species can grow in very 

 thin layers of soil on chalk downs. They must be re- 

 garded as requiring moderate supplies of humus as a 



3—2 



