134 THE SCOTTISH BOTANICAL REVIEW 



an abundant water supply and the absence of disturbing 

 factors to phototaxis. This is specially noticeable in species 

 of the subdivision Calliei-gon of the genus Hypnuin in some 

 other mosses and hepatics, and in Montia fontana and Stellaria 

 uliginosa. 



Shallow channels leading from springs and others occupied 

 by the surface run-off of water in wet weather may be 

 entirely carpeted with vegetation, from the velocity of the 

 current and mobility of sand or stones being insufficient to 

 prevent a closed plant carpet. Such channels have been 

 termed " Flushes " by the writer, who has made a preliminary 

 attempt at classifying those encountered in the moorland, as 

 permanently w^et (wet flushes), or periodically wet and dry 

 (dry flushes), and also as acid, and calcareous flushes, 

 according to the conditions of the waters (26). 



Further field examination of plant habitats that have 

 ecological relations to flushing by water reveals the fact that 

 flushed surfaces should be primarily subdivided into two 

 groups : — (i) hard rock surfaces, and (2) surfaces capable of 

 easy erosion by running water. The former may be com- 

 pletely clothed with vegetation, though subject to torrential 

 action, as may often be seen in waterfalls, where the whole 

 rock face may be clothed with a special vegetation of mosses 

 and algae. Rock surfaces with a comparatively constant but 

 gentle trickling stream of water, and especially if vertical or 

 nearly so, usually support an algal vegetation only, while 

 others, only periodically flushed, form the stations of very 

 defined associations of bryophytes and algae, varying from 

 open to closed formations, and these again to open formations, 

 with the results of progression and retrogression or reinitiation 

 of plant succession induced by an increasing erosive power 

 of the water as soil accumulates, and its tendency to migrate 

 from point to point, deserting one part of the rock surface 

 for another. The various stages of migration of a "rock 

 flush " are marked by different plant association, but the 

 presence of certain algai and bryophytes, and the absence 

 of lichens, except species of Verrucaria, etc. , appears to 

 specially characterise rock surfaces subject to flushing, as 

 distinct from others only receiving atmospheric moisture. 



We must therefore distinguish "rock flushes" as a group 



