THE ECOLOGY OF PLANTS. 431 



other. The typical marsh occurs on low ground, its water is 

 rich in mineral substances, especially lime, and its plants 

 grow rapidly, either becoming tall or remaining low, but 

 producing numerous leaves and branches each year. In the 

 typical bog the water is poor in lime and other salts, and the 

 plants are mostly slow-growing and of low stature. 



Peat-bogs, like the heaths or moors on which they occur, 

 are found at all heights, from a few feet above sea-level up to 

 three thousand. The peaty soil in which bog-plants grow is 

 not sufficiently aerated for the proper formation of nitrates, 

 so that although such soils (bog, peat, morass) often appear 

 on analysis as though they should be extremely fertile, very 

 little of the materials they contain is directly available to 

 serve as plant-food. Some heath and moorland plants are 

 able to make use of peat by being partial saprophytes 

 (Art. 239) ; Ling, Heaths, and moor Grasses have a covering 

 of fungus-threads on or in their roots, forming a mycorhiza 

 and enabling them to absorb the decaying vegetable matter. 



Most bog-plants have more or less well- developed xerophi- 

 lous characters, due largely to the excess of peaty acids, which 

 makes water-absorption difficult. Mosses, especially the Bog- 

 mosses (Sphagnum), of which many species and varieties have 

 been distinguished, play an important part in the formation 

 of bogs. The Bog-mosses, whose appearance and remarkably 

 spongy character are well known to all who have walked over 

 our moors, are specially adapted for storing water. The leaf 

 of a Bog-moss consists of a single layer of cells, which are of 

 two kinds: (1) large empty cells with pores on the walls, 

 (2) small green cells. The green cells are long and narrow, 

 forming a network, each mesh of which is occupied by one of 

 the empty cells ; the green cells carry on the living functions, 

 while the large clear cells store up water. Each plant 

 branches and grows upwards, while the lower p-'.rts die and 

 lose their green colour, but are preserved from decay by the 

 absence of oxygen and the presence of the peaty acids. 



In this way masses of peat may be formed, but as a rule 

 Sphagnum only occurs in comparatively small patches on 

 our moors, and its remains are not abundant in the peat 

 itself, which is chiefly composed of the remains of the Cotton 

 Sedge. Peat is formed wherever there is a high rainfall and 

 an impermeable substratum, resulting in the production of 



