596 A. HESSELBO 



the Bryophytes of the clefts and Urds live in a comparatively hu- 

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

 winter. The conditions pertaining to light are also of importance 

 with regard to the composition of the Bryophyte vegetation. On 

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

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

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

 tion on vertical surfaces or slopes with a southern exposure is 

 different to that found on the same kind of ground with a northern 

 exposure. 



Owing to the extremely different conditions to which plants 

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

 met 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 pavement, in which 

 each particular piece of homogeneous conditions is exceedingly 

 small. 



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

 partly in soil which has accumulated in fissures and depressions, 

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

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

 the lithophytes proper also occasionally grow on gravel or even 

 on soil, and it is not always possible io decide whether 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 Andrecea petrophila, Dicranoweisia crispula, Rhacomitrium fas- 

 ciculare and R. sudeticum, which in low-lying regions are decidedly 

 lithophyles, descend to the ground on mountain heights and often 

 form extensive growths on rocky flats. The following species may 



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



/ 



Species belonging to Coastal Rocks. 

 *Ulota phyllantha *Schistidium marilimum 



Water Lithophyles. 



*Schistidium rivulare *Fontinalis longil'olia 



*Fontinalis antipyretica *Rhynchostegium rusciforme 



androgyna *Hypnum ochraccuni 

 thulcnsis alpinum 



islandica alpestre 



