432 THE ECOLOGY OF PLANTS. 



water-logged soil in which oxidation and the action of 

 oxygen-requiring bacteria is checked so that complete decay 

 cannot occur. In some districts extensive Sphagnum, bogs 

 are formed, and Sphagnum plays an important part in filling 

 up pools and converting them into swamps and thus pre- 

 paring the way for the growth of various bog and marsh 

 plants. 



An interesting point in connection with peat is that in 

 some places it contains trunks of trees, chiefly Birches, Pines, 

 Oaks, and Willows. This occurs chiefly in the valleys, but 

 the distribution of these trees under the peat indicates that 

 at some past time the present moors bore woods, except on 

 the exposed summits, which probably had never, since the 

 glacial periods, been anything better than heath or scrub. 

 The peat is post-glacial in age, and a large proportion of it 

 has probably been formed within the historical period. 



The flowering plants found in bogs have mostly been mentioned 

 already ; they include Ericaceae (Ling, Heather, Bilberry) ; Sedges, 

 Rushes, and Grasses, mostly different from the kinds found in marshes 

 or wet meadows ; Bog Asphodel, Bog Orchids, Bog Myrtle, Grass of 

 Parnassus, Bog Cinquefoil, Sundews, Marsh Gentian (Gentiana pneu- 

 monanthe), Bog-bean, Bog Pimpernel, Butterwort, Bladderwort. 



As already stated, peat is very poor in available plant-food, and this 

 fact largely accounts for (1) the xerophilous character of most bog- 

 plants, (2) the presence of insectivorous plants like Sundew and 

 Butterwort in bogs. 



407. Humus Plants. Some of these simply prefer the 

 humus as a medium on which to grow. Others, however, are 

 saprophytes, and use the humus as a source of food. They 

 are aided in this by the presence of symbiotic fungi 

 (mycorhiza) on their roots, which play the part of root-hairs, 

 and after decomposing the humus hand over all or a portion 

 of it in a suitable form to the plant. Plants of this kind 

 commonly have their foliage-leaves reduced to mere scales, 

 as in the Bird's-nest Orchid. When (as in Ericaceae) the 

 leaves are fully developed, the mycorhiza probably enables 

 the plant to absorb nitrogen from the humus, and not 

 carbon. 



Humus collects in most bogs and swamps, but here it is 

 the water which is of primary importance in influencing 

 distribution. 



