244 BRITISH PLANTS 
if the soil is marshy. Some of the thickets possibly 
represent primitive woodland, but in most cases they 
are plantations. Osier-beds are artificial associations of 
Salia viminalis. . 
The dominant trees are: Alnus glutinosa (alder), Salix 
Caprea (goat-sallow), S. cinerea (grey sallow), and S. 
fragilis (crack-willow). Many other shrubs and trees are 
found in small numbers—e.g.. ash, oak, Rhamnus Frangula 
(alder-buckthorn), R. catharticus (common buckthorn), 
Viburnum Lantana (wayfaring-tree)—the latter two only 
when the water is rich in lime—Viburnum Opulus (wild 
guelder-rose), Ligustrum vulgare (privet), Ribes (currant), 
and many other species of willow. The undergrowth 
consists of ordinary marsh-plants—e.g., Caltha palustris 
(marsh-marigold), Chrysosplenium oppositifolium (golden 
saxifrage), Cardamine pratensis (cuckoo-flower), Valeriana 
dioica (marsh-valerian), various species of mint, Myosotis 
palustris (marsh forget-me-not), Spirea Ulmaria (meadow- 
sweet), Epilobium hirsutum (hairy willowherb), Molinia 
cerulea, and shade-loving grasses, such as Azra cespitosa 
(tufted hair-grass), the rare Calamagrostis Epigeios, and 
the wood club-rush (Scirpus sylvaticus). 
3. True Marsh. 
Wherever comparatively fresh water accumulates in 
the soil to such an extent that the water-level is at or 
just above the surface—e.g., on the landward side of 
reed-swamps, in the neighbourhood of springs, or low- 
lying ground—a marsh flora springs up. The marsh 
differs from the reed-swamp chiefly in the level of the 
water, which has an important effect on the vegetation. 
No parts of the assimilating organs of the marsh-plants 
are under water, and the vegetation more nearly 
approaches that of dry land. 
Peat tends to accumulate in the water of the marsh, 
for the amount of oxygen present is insufficient to enable 
bacteria to decompose completely the plant-remains. 
But disintegration takes place to such a degree that the 
peat is black and amorphous, very close in texture, and 
quite different from that of the bog (see p. 248). The 
partial decomposition of the vegetable remains results 
in the formation of humous acids, but the amount pro- 
