596 A. HESSELBO 



Ihe Bryophytes ol" the clefts and Urds live in a comparatively hu- 

 mid atmosphere and are protected by the snow-covering during the 

 winter. The conditions pertaining lo light are also ol" importance 

 with regard lo the composilion ol" the Bryophyte vegetation. On 

 the ground between the stones of the talus, grow other species than 

 those to be found upon the stones. Gaves and dark clefts contain 

 species which belong specially to these localilies, and the vegeta- 

 tion on vertical surlaces or slopes with a southern exposure is 

 difterent lo that found on the same kind of ground with a norlhern 

 exposure. 



Owing to the extremely dilYerent conditions to which piants 

 are exposed when growing on rocks, all kinds of associations are 

 mel with there, from decidedly xerophilous to hygrophilous, light- 

 forms, and species which thrive best in dark caves. The rocky 

 substratum may be compared to a mosaic pavemenl, in which 

 each parlicular piece of homogeneous conditions is exceedingly 

 small. 



The mosses grow partly on the rock ilself (lilhophytes) and 

 partl}^ in soil which has accumulated in lissures and depressions, 

 or on ledges (chomophytes). The boundary line between these two 

 groups has, however, been elTaced to a great extent as many of 

 the lilhophytes proper also occasionally grow on gravel or even 

 on soil, and il is not ahvays possible lo decide whelher a moss- 

 cushion has originally developed on humus-covered rock, or has 

 itself formed humus by a process of decay. Especially on mountain 

 heights the two groups merge one into the other, as species such 

 as Andreæa petrophila, Dicranoweisia crispiila, Rhacomitrinm fas- 

 ciciilare and R. siideticiini, which in low-lying regions are decidedly 

 lilhophytes, descend lo the ground on mountain heights and often 

 form extensive growlhs on rocky flåts. The following species may 

 be referred to the lilhophytes proper (those marked * are common): 



Species belo n gin g to Coastal Bocks. 

 ■' Ulota phyllantha *Schistidium maritimum 



Water Lilhophytes. 



*Schistidium rivulare *Fontinalis longifolia 



*Fontinalis antipyrelica *Rhynchostegium rusciforme 



— androgyna *Hypnuni ochraceiim 



— thulensis — alpinum 



— islandica — alpeslre 



