246 BRITISH PLANTS 
be converted by proper drainage into good pasture-land, 
the typical marsh-plants disappearing, and the finer 
grasses taking their place. In other cases shrubs and 
trees appear on the drying marsh, and in the course of. 
time a woodland-association is developed. 
4. Bog-Associations. 
The bog is related ecologically to both the marsh and 
the moorland. The water-level is at or above the sur- 
face, as in the marsh, but the amount of humous acid 
present is great, and floristically the bog very closely 
resembles a moor. For this latter reason, most of the 
associations of bog-plants are better dealt with in con- 
nection with the moorland (Chapter XXV.). 
The bog is best developed on the flat margins of high- 
land lakes, and in places where the soil-water is derived 
almost solely from atmospheric precipitations—rain and 
dew—as on moors. In either case the water is deficient 
in mineral salts, especially lime, and this, together with 
the amount of humous acid present, has a great influence 
on the vegetation. Carnivorous plants, which are quite 
absent from the marsh, are here abundant ; mycorhiza 
(p. 124) occurs on many of the plants, and Sphagnum 
(bog-moss), which can only exist in water containing a 
very low percentage of salts in solution, is common. 
Naturally there can be no sharp line of distinction 
between the bog and the marsh, for conditions exist 
intermediate between those necessary for the typical 
development of either. Owing to the gradual accumula- 
tion of peat in a marsh, the water becomes more and 
more sour, whilst lime becomes less abundant ; bog-plants 
then begin to appear, and in time a typical bog may be 
built up on top of the original marsh. 
The bog is sometimes spoken of as a high-moor, and 
the marsh as a low-moor. These terms do not refer to 
altitude, as is commonly supposed, but to the relative 
abundance of humous acids. A high-moor may develop 
in the lowlands if the conditions are favourable, and 
a low-moor at high levels. 
Many associations of bog-plants may be distinguished 
Those which come nearest to the marsh are dominated 
by rushes (Juncus conglomeratus, J. supinus, J. articu- 
