IT] ACTION ON SOILS, ETC. 35 
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 
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 10 keeping the surface-soil from being washed down by 
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 
and other vermin, are favoured by a covering of grass. 
Graminacez are for the most part chalk-fleeing plants, 
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 
