THE ECOLOGY OF PLANTS. 425 



and the peat-layer at the surface there is usually a more or 

 less impermeable intermediate layer of decaying granite or 

 shale, or of glacial clay. The soil is poor and either deficient 

 in water owing to the subsoil being too porous, or saturated 

 with water owing to an impervious subsoil or to excessive 

 rain and mist, or to both causes. On high moors the strong 

 winds not only increase transpiration but also prevent the 

 growth of trees in exposed places, so that the entire vegeta- 

 tion is dwarfed and stunted, whereas on low moors (heaths) 

 there flourish such trees as Pines and Birches, with Alder, 

 Willows, and Bog Myrtle beside streams and in wet places. 



Ling, Heaths, Cotton Sedge, and several of the wiry 

 Grasses making up the chief elements of the "heather 

 association " grow at all levels so long as the soil conditions 

 are similar. Moreover, wherever water collects owing to lack 

 of drainage and the soil becomes peaty owing to the accumu- 

 lation of plant- remains, bogs are formed, so that the difference 

 between moors and heaths, with their bog vegetation, is 

 largely a difference of degree only. 



On high moors the rainfall is usually very great over a 

 hundred inches a year, for instance, on the central plateau or 

 basin of Dartmoor, which I have explored with some thorough- 

 ness and the frequent dense mists help to keep the moor 

 extremely moist. The summit of Dartmoor is a large boggy 

 plateau, or basin, surrounded by the characteristic " tor "- 

 capped hills and riven in all directions by crevasses which 

 form an intricate network of peaty trenches and which 

 gradually give rise to the streams the chief rivers of Devon 

 arise from this desolate region. The aspect of the vegetation 

 is dreary and monotonous, though livened in early summer 

 by the white tassel-like fruit-tufts of the Cotton Sedge 

 (JEriophorum) , which often form large snow-like patches. 



The chief plants found in the boggy and crevassed parts are 

 Rushes, Sedges, Cotton Sedge, Bell and Cross-leaved Heaths, 

 great cushions of Hair-moss (Polytrichum), and Woolly 

 Fringe-moss (Rhacomitrium) , with Bog- mosses (Sphagnum) 

 in the wetter places and tufted narrow-leaved Grasses on the 

 hill-sides. Farther down and on the wind-swept outlying 

 ridges the dominant plant is usually Bilberry in the drier 

 parts, with lichens, and Rushes, Sedges, Bog-mosses, etc., in 

 the swamps and along the streams. 



