Floristik, Geographie, Systematik etc. 113 



alternating, migratory, replacing one another in aecordance with 

 degree of rock disintegration and exposure to wind: 1) Rhacomi- 

 triutn carpet closed association; 2) Calluna-lichcn mat, semi-open; 

 3) Arctostaphylos mat. 



2. Plant formation of the Alpine crags: Habitat influenced by 

 exposure, recent in origin, subjeet to destruetion and renewal by 

 i'rost. All associations migratory: 1) Rock-surface associations (Bryo- 

 phyte associations of damp crags and Lichen-bryophyte associations 

 of dry crags); 2) Rock-crevice associations. 



3" Plant formations of the Alpine scree-slopes. Habitat influenced 

 by slope, recent, unstable, subjeet to change and to destruetion by 

 landslip. Plant associations more or less open, migratory, and arran- 

 ged zonally to the protective influence of the crags, and to areas 

 of maximum and minimum stability. Mosses and dwarf-shrub Erica- 

 ceae play here an important part. Subalpine crags which have 

 become exposed, through glacial action, and the steeper slopes of 

 glacial drift are also dealt with briefiy. 



II. The moorland plant formation (Chap. IV). The dominant 

 stable formation in Caithness, oecurring on a habitat glacial and 

 climatic in origin, fairly stable and continuous on the peat of the 

 plateau and piain. Plant associations alternating in aecordance with 

 surface relief, and with transitions zonal to the stream belt and the 

 coastal belt. The formation is at present in a retrogressive stage 

 with respect to certain of its associations; Sphagmim once dominant 

 has given place to retrogressive phases in which Eriophoriim, 

 Scivpus caespitosus, Calluna, Vaccinium, Empetrum, and Rhacomi- 

 trium are locally dominant. Certain plant associations are differen- 

 tiated where water passes over the surface, either continuously 

 fwet flushes) or intermittently (dry flushes); as these oeeur in every 

 moorland and frequently give rise to grassland, Junceta, or other 

 prominent associations, the aecount of their development is a note- 

 worthy feature of this memoir. 



III. Plant formations zonal to the drainage System (Chap. V). 

 Habitats post-glacial in origin, and mainly dependent on such 

 geological factors as the character of the material in the waters, 

 the water-level, and the marginal deposits. Lacustrine plant forma- 

 tions are divided into those of the sandy lakes with peaty waters, 

 and those of marly lakes with limy waters. The plant formations of 

 the stream belt are dealt with in the following groups: A. Forma- 

 tions on river flood gravels and alluvial terraces, one series depo- 

 sited by waters rhiefly eroding non-calcareous glacial drift and 

 rocks, another series deposited by waters mainly from calcareous 

 substrata; B. Formations consequent on recent erosion of the 

 streams through rock; C. Formations of deserted banks limiting 

 post-glacial erosion. These last include the natural woodlands of 

 Caithness: Betula and heath on banks formed from sandy boulder 

 clay and metamorphic rocks; Corylus- Betula woods on banks of 

 calcareous shelly boulder clay and flagstones. 



IV. Plant formations of the coastal belt (Chap. VI). These 

 include the associations on marine littoral deposits (sea-beaches, 

 coastal sands, salt marsh). Another group of formations oecurs in 

 habitats directly eonsequent on marine erosion mainly rock-surface, 

 crevice, and ledge associations on rocky sea-cliffs. Formations ot 

 the seaward contour due indirectly to sea erosion are also dealt 

 with, including those on the maritime boulder clay drift-slopes, and 

 those of pre-glacial rock slopes. 



Botan. Centralblatt. Band 119. 1912. 8 



