THE BKYOPHYTA OF ICELAND 599 



Talus of fallen blocks and debris (Urd). The con- 

 ditions for the development of plants is far mr>re favourable here 

 than on the stony slope, which is constantly exposed to disturbances 

 through the down-sliding of stones and earth. The most xerophi- 

 lous species, viz. species of Rhacomitrium and Grimmia, Dicrauo- 

 weisia crispula, Orthotrichiim rupestre and Hypnuin revoliilnm grow 

 on the stones, while on the ground between the stones there are 

 favourable conditions for a number of species which have greater 

 requirements with regard to shelter, moisture and shade. The Urd 

 is therefore very rich in species, since lithophilous species are 

 found there on the blocks, and species, which grow on the ground, 

 occur between the stones, and Bryophytes belonging to heathland 

 and grassland, are found on the soil-covered stones and ledges. 



In order to show how heterogeneous the composition of the 

 Bryophyte vegetation may be, the Urd in Heljusdalur on Vest- 

 mannaey may serve as an example. The valley is almost semicir- 

 cular in shape, and open towards the south, and in consequence of 

 this favourable situation combined with the comparatively mild, 

 damp climate of the island , a considerable number of southern 

 species are found there, while species such as Ulota maritima and 

 Schistidium maritimum betoken the proximity of the sea. The rocks 

 around the valley are inhabited by numerous sea-fowls, and they 

 often rest on the blocks of the Urd, and on these spots, manured 

 by the birds, it is species of Bryum (B. argenteum and B. capillare) 

 which particularly make their appearance. 



The fallen blocks are everywhere covered with cushions of 



/ 



mosses and liverworts. The commonest species are Schistidium 

 apocarpum, Ulola maritima, Hypnum cupressiforme, H. uncinatum, 

 Orthotrichiim rupestre and Ceratodon purpnreus. The following 

 species were also observed on the blocks: Radula complanata, Ma- 

 dotlieca Cordceana, Plagiochila asplenioidcs, Cephaloziella Hampcana, 

 Didymodon rubellus, *Barbula cylindrica, Tortella tortuosa, T. fragilis, 

 Tortilla subiilata , *T. miicronifolia , Distichium montanum, * Orthotri- 

 chiim a n om a In m, Encatypta ciliata, E. rhabdocarpa , Pohlia cruda, 

 Bryum capillare, B. elegans, B. inclinatum, B.a'ueum, Mnium serra- 

 tum, M.stellare, M. orthorrhynchum, Myurella julacea, Orthothecium 

 intricatum, *Isothecium myiirum, Homatothecium sericeum, Ambtyste- 

 gium Sprucei, *A. serpens, Hypuum hamutosum and H. revolution. In 

 the caves formed among the stones, into which the light penetrated 

 but scantily, the walls were covered with light-green shining mats 



