The few areas of marsh and fenland which remain undrained bear their own characteristic vegetation of reeds, rushes, sedges, thickets of alder, birch and willow, and many other plants which grow only in waterlogged soil. This vegetation is essentially natural, though its detailed distribution is often determined by human activity. Thus regular cutting of the reeds and sedges for thatching and leaf litter prevents the trees and bushes colonising the marsh or fen by destroying their seedlings and saplings, while the reeds and sedges spring again from their underground parts.
A very few 'raised bogs'1 of the kind quite numerous in the central Irish Plain still survive in the west and north of Britain, and they possess an extremely distinct and interesting plant population. Most of them have long since been destroyed by draining and peat cutting.
The aquatic vegetation inhabiting rivers, lakes, pools, canals and ponds is again quite distinct and essentially natural, though many of its individual habitats, such as canals and ponds, have actually been provided by man.
Finally there is the vegetation of the sea coast, of the salt marshes, sand dunes, shingle beaches and sea cliffs. They bear quite special, characteristic vegetation, determined by the different kinds of maritime habitat, wherever it has not been destroyed and replaced by artificial constructions such as sea walls.