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 conglomerate, J. supinus, J. articu- 



