STABLE AND MIGRATORY PLANT FORMATIONS 79 



country and the plant - formations chiefly or entirely 

 migratory. 



2. Moorland Region — the stronghold of plant-formations 

 isolated through the influence of an acid-humus barrier, due 

 to the maximum combined effects of the late glaciation and 

 the range of the migrations of the moorland climatic province. 



3. Mesophytic Forest Region — occupying the area where 

 the above barrier is ineffective or wanting within the meso- 

 phytic forest province. 



4. Steppe Province — absent in Britain. 



We may tentatively exemplify topographic stable plant- 

 formations in the following : — 



1. TheS.E. Heath Formation, which is invasional in the 

 Mesophytic Forest Region through leaching of the surface. 



2. The N.W. Heath Formation (possibly including the 

 extinct pine-forest in the peat), which is invasional in the 

 Moorland Region through climatic and topographic retro- 

 gressive changes in the moorland. 



3. The Chalk-grassland Formation — invasional in the 

 Mesophytic Forest Region through the physiographic and 

 edaphic nature of the substratum, perhaps comparable with 

 the " Sunny Pontic Hill " formation of Graebner (2) and 

 chersophyte formations of Warming (13). 



It is probable that both the Moorland and Mesophytic 

 Forest will eventually be split up into plant-formations based 

 chiefly upon differences in the physiography of the habitat. 



( To be continued.) 



REFERENCES. 



(22) Hall, A. D. — The Soil. 2nd ed., 1909. 



(23) Weber, C. A. — Jahresber. der Manner vom Morgenstern, rgoo. 



Heimatbund an Elbund Wesermiindung. 

 Graebner, P. — " Studien iiber die norddeutsche Heide." 



Englers Jahrb., 1895. 

 Transeau, E. N. — "The Bogs and Bog Flora of the Huron 



River Valley." Bot. Gazette, 1905-6. 



(24) Fruh, J., and Schroter, C. — Die Moore der Schweiz. 1904. 

 Warming, E. — CEcology of Plants. Eng. trans. Oxford, 1909. 



(25) Yapp, R. H.— "Sketches of Vegetation at Home and Abroad: 



Wicken Fen.'" The New Phytologist, 1908. 



