THE BRYOPHYTA OF ICKLAND 595 



brownish-black carpet. Conostonuini boreale also usually inter- 

 woven with liverworts such as Gymnomitrium concinnatnm, Lopho- 

 iid ventricosa, Plenroclada albcscens and species of Cephalozia 

 forms exceedingly compact cushions on dry, stony ground. Both 

 these species grow in particularly exposed localities, where the 

 snow-covering is absent during winter, but their compact tufts, 

 densely covered with tomentum to an unusual degree, protect them 

 from being destroyed by strong winds. Diphyscium forms qui It- 

 low (only one cm. high), but rather wide tufts, while Conostonuini 

 forms tufts a few cm. in diameter, but often more than 10 cm. in 

 height. In the latter case the basal portion of the tuft is securely 

 wedged into the gravelly substratum and attached to the stones 

 with numerous rhizoids. Diphyscium is found at all levels, while 

 Conostonuini is most widely distributed on the rocky flats, but in 

 Ihe low' land occurs only somewhat scantily in the heath formation. 



The Bryophyte Vegetation of the Rocks. 



The rock vegetation includes formations of very various com- 

 position, with extremely different demands as regards light, mois- 

 ture, etc. The substratum may consist of solid rock, for in- 

 stance rock-faces, flats or large blocks, or it may consist of de- 

 tached stones of various sizes, as on gravelly and stony slopes and on 

 taluses. The distribution of the water is very unequal, since a 

 quite dry part with decidedly xerophilous species may be met with 

 next to a part where the water flows over the rock, and provides 

 conditions for the growth of hygrophilous species. 



The chemical and physical nature of the substratum exercises 

 great influence upon the composition of the Bryophyte covering. 

 On tuff and breccia the vegetation is far more luxuriant than on 

 basalt, and the reason for this is partly because the uneven sur- 

 face of the tuff affords better habitats for the plants, partly because 

 the water penetrates into the tuff comparatively easily, and after- 

 wards reappears through the fissures and porose parts of the rock- 

 surface so that these are, as a rule, rather damp. The greater or 

 smaller amount of lime present is also of importance for the fre- 

 quency of many species. The direction of the exposure and the 

 conditions pertaining to shelter play a great part with regard to 

 the Bryophyle vegetation. Species growing on exposed rock-faces 

 and flats must be able to endure complete desiccation by the sun 

 during the summer, and by strong winds during the winter, while 



