232 BRITISH PLANTS 



V. Plant-formation of Calcareous Soils. — The abun- 

 dance of lime in the surface soil or just below the surface 

 is the determining factor. Thi'ee sub-formations are 

 recognized: (a) that of Older Limestones, (6) that of chalk, 

 and (c) that of Marl and Calcareous Sandstones. Asso- 

 ciations : ashwood (a and 6), beechwood (b), ash-oakwood 

 (c), scrub (a, b, and c), grassland (a, b, c), limestone heath 

 (a), and limestone pavement (a). 



VI, — Fresh-water Aquatic Formation. — ^Includes ioxa 

 sub-formations: (a) of foul water, (6) and (c) of waters 

 rich and poor, respectively, in mineral salts, and (d) of 

 quickly-flowing streams. 



VII. Marsh Formation. — Soil saturated, but little or 

 no peat developed. Associations: (1) alder -wiUow; 

 (2) herbaceous marsh. 



VIII. Fen Formation. — Wet, peaty soil, chiefly in 

 East Anglia; ground water mainly telluric, rich in mineral 

 salts and alkaline in reaction. Associations: (1) carr 

 (woodland); (2) fen (herbaceous). 



IX. Moor Formation. — On wet peat in both lowlands and 

 highlands ; ground water mainly or entirely aerial, poor in 

 mineral salts and acid in reaction. Associations : (1) bog- 

 moss; (2) Bhynchosporum (lowland only); (3) cotton- 

 grass; (4) Scirjms lacustris (upland only), (5) Molinia; 

 (6) Nardus; (7) bilberry; (8) heather moor; (9) birch- 

 wood (lowland only); (10) pinewood (lowland only). 



X. Arctic-Alpine Grassland Formation. — Develops only 

 at high altitudes. Not yet sufficiently studied to admit 

 of the recognition of different associations. 



XL Formation of Mountain-top Detritus. — Associations: 

 (1) moss-lichen; (2) Rhacomitrium heath; (3) Rhacomi- 

 trium moor. 



XII. Arctic-Alpine Chomophyte Formation. — Develops 

 on alpine crags and corries. Associations: (!) open 

 communities on exposed rock-faces; (2) assoc. of shel- 

 tered ledges; (3) assoc. of shade chomophytes; (4) assoc. 

 of hydrophilous chomophytes. 



XIII. Salt-Marsh Formation. — Associations: (!) glass- 

 wort; (la) cord-grass; (2) general salt-marsh; (3) Glyceria; 

 (4) sea-rush; (5) assoc. of spray- washed cliffs and rocks. 



XIV. Sand-Dune Formation. — Associations: (1) strand 

 plants; (2) sea couch-grass; (3) marram grass; (4) dune 

 scrub; (6) dune grassland; (6) dune marsh. 



