244 BRITISH PLANTS 



if tlie soil is marshy. Some of the thickets possibly 

 represent primitive woodland, but in most cases they 

 are plantations. Osier-beds are artificial associations of 

 Salix 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), Ldgustrum vulgare (privet), Ribes (currant), 

 and many other species of willow. The undergrowth 

 consists of ordinary marsh-plants — e.g., Caliha palustris 

 (marsh-marigold), Chrysosplenium oppositifoUum (golden 

 saxifrage), Cardamine pratensis (cuckoo-flower), Valeriana 

 dioica (marsh-valerian), various species of mint, Myosotis 

 palustris (marsh forget-me-not), Spircea Ulmaria (meadow- 

 sweet), Epilobium hirsutum (hairy willowherb), Molinia 

 ccerulea, and shade-loving grasses, such as Aira ccespitosa 

 (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- 



