THE BRYOPHYTA OF ICKLAND 595 



brownish-bhick carpet. Conostomiim boreale also — usually inler- 

 woven willi liverworts such as Gymnomitriiim concinndtiim, Lopho- 

 :i(i neiitricosa, Pleiiroclada albcscens and species of Cephalozia — 

 lorins exceedingly conipact ciishions on dry, stony ground. Botli 

 these species grow in particulaily cxposed iocalities, wliere Ihe 

 snoNV-covering is abscnt during winler, bnt Iheir compact tufts, 

 densely covered wilh lomentuni to an unusual degree, prolect Ibein 

 (Vom being destroyed by strong winds. Diphijscinm forms quile 

 low (only one cm. higb), but ralber wide tufts, wbile Conostomiim 

 forms lufts a few cm. in diameter, but oflen more tban 10 cm. in 

 beigbl. In tbe latter case tbe basal portion of the tuft is securely 

 wedged inlo the gravelly substratum and attached to the stones 

 with numerous rhizoids. Diphysciiim is found al all levels, while 

 Conostomiim is most widely distributed on the rocky flåts, but in 

 the low land occurs only somewhat scantily in the hcath formation. 



The Bryophyte Vegetation of the Rocks. 



The rock vegetation includes formations of very various com- 

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

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

 stance rock-faces, flåts 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 

 (juite dry part with decidedly xerophilous species may be met with 

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

 condilions 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 tuir and breccia the vegetation is far more luxuriant than on 

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

 face of the tuff alTords better habitats for the piants, partly because 

 the water penetrates into the tufl" comparativcly casily, 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 jiresent is also of importance for the fre- 

 (|uency of many species. The direction of the exposure and the 

 conditions pertaining to shelter play a great part with regard to 

 the Bryophyte vegetation. Species growing on exposed rock-faces 

 and flåts must be able to cndure complcte desiccation by the sun 

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



