42 TRANSACTIONS OF THE [Sess. lxxvi. 



alpina also occur on snow-flushes, while Poa alpina, Des- 

 champsia ccespitosa (arctic-alpine form), and Polygonum 

 viviparum are widely distributed over other arctic-alpine 

 associations. 



The various authors point out that the conspicuous 

 flowering species of the snow-flush vary from place to 

 place. Here Polytrichum may form a carpet, there Salix 

 herbacea a mat, or Gnaphalium in white tufts. Riibel 

 gives prominence as tone-imparting species to Polytrichum, 

 Salix herbacea, Sibbaldia, Alchemilla, Gnaphalium, and 

 Ligusticum Mutellina, and he demonstrates that the 

 association presents sub-types according to the dominant 

 plant; thus Arenaria biflora is given as much more 

 abundant when Polytrichum is the conspicuous plant ; 

 on the other hand, Gnaphalium is fairly constant in all 

 the sub-types of snow-flush. This patchy occurrence of 

 various plants in separate stations or within the same 

 snow-flush leads Brockmann to the conclusion that accident 

 or chance plays an important part in the constitution of 

 the plant-covering. In other words, the snow-flush is an 

 open association into which species from neighbouring 

 plant communities migrate. The existence of compara- 

 tively large pure patches of a single species, frequently 

 observed in the snow-flush, is attributed to the capacity 

 of the plants for vegetative propagation. It seems to be 

 essential for existence in the snow-flush habitat that the 

 species should form low matted tufts sufficiently compact 

 to resist periodic flooding, and along with this the power 

 to extend laterally by short horizontal shoots. Gnapha- 

 lium supinum forms mats from which the short flowering 

 stems arise, and short horizontal shoots extend along the 

 ground to give off" new leafy shoots which flower in some 

 future year. Cerastium, Sibbaldia, Alchemilla, and 

 Veronica alpina extend by lateral branches rooting in 

 the underlying carpet of Hepaticse or Moss. Salix 

 herbacea forms a loose mat extending comparatively rapidly 

 by underground horizontal branches which root and may 

 form new plants. That this habit of growth leads to con- 

 siderable stability may be experienced when one collects 

 specimens ; either one brings away much soil attached, or 

 the shoots break off short, leaving most of the plant behind. 



