618 A. HESSELBO 



higher plants and the Hylocomium mats do not thrive. The snow 

 lies here till far into the summer, and in many cases the tempera- 

 ture hardly ever rises above freezing point. The bottom of these 

 clefts is therefore either quite bare of vegetation, or covered only 

 with mosses. At the bottom of narrow clefts (1520 metres or more 

 in depth) near Thingvellir, where snow and ice were still to be 

 found even at the end of July, Anthelieta occurred on a damp sub- 

 stratum as they do on flats irrigated by melting snow on mountain 

 heights. Polytrichum sexangulare and Pohlia commutata grew abund- 

 antly here as on rocky flats. The plants were considerably higher 

 and more slender than on the rocky flats,, and only slightly tomen- 

 tose. The same was the case with Conostomum boreale and Mniobryum 

 albicans, which likewise occurred in several clefts. The Bryophyte 

 vegetation occurring there is affected both by the low temperature 

 and also the deficient light and total absence of wind, which all 

 combined contribute to produce a stronger longitudinal growth and 

 a slighter development of rhizoids. Several other species, which 

 have their main distribution on mountain heights, and are rare in 

 or quite absent from the lowlands, are widely distributed about the 

 lava-fields. In almost all the lava -fields of South-west Iceland, 

 Pleuroclada albescens var. islandica, Dicranum molle, D. Blyttii and 

 D. Starckei are commonlv found in clefts, not onlv in those that 



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are narrow, but especially in the broad moss-grown clefts, where 

 Dicranum spp. form large tufts on the blocks of lava. 



On the sides of the clefts and of the fallen blocks, a great 

 many mosses grow. Diplophyllum albicans is a character-plant of 

 the lava-fields of SW. and W. Iceland; it is extremely common there, 

 but is very rare and scanty in other localities. On the vertical, dry 

 sides of clefts the mosses usually grow in large rounded cushions. 

 The commonest species are Tortella tortuosa, Anoectangium com- 

 pactum, Amphidium Moiigeottii, Grimmia torquata and G. funalis, but 

 many other of the species occurring on rocks and on the ground 

 are always found both on the walls and also on, and among, the 

 blocks. The Bryophyte vegetation of lava-clefts has the greatest 

 resemblance to that found on the Urd, but some of the most light- 

 loving species are absent, for instance Orthotrichum, several Grimmia 

 and Rhacomitrium spp. and Dicranoweisia crispula, while, on the 

 other hand, others are found which belong to damp clefts or which 

 need more shelter. Mesophilous forms are in the majority, whilst 

 hygrophilous forms are, as a rule, entirely absent. 



