622 A. HESSELBO 



lycopodioides was usually the only species found and it fornied there 

 extensive mats. On the ground amoiig the blocks and on the sides 

 of these blocks as also on those of broad clefts there grew Brachij- 

 thechim refleæiim, Lesciiræa filamentosa, Eurhijnchium diversifolium 

 (scantily), Polytrichiim alpinum, Timmia aiistriaca, Bartramia ityphylla, 

 Encalypta rhabdocarpa, Bryiim elegans, B. inclimdum, Schistidium 

 apocarpiim, Didymodon riihellus and Plagiochila asplenioides. 



The above is a decidedly xerophilous flora from which not only 

 all the mesophilous and hygrophilous forms from SW. Iceland, but 

 also the lowland or southern forms proper, are entirely absent. 



In several piaces in this lava-field the heat of the substratum 

 makes itself felt in deep caves and clefts. In a warm, damp cave 

 where the temperature was about 25° (outside 4°-5°) there grew 

 Fissidens osmiindoides, Plagiotheciiim denticalatnm, Calypogeia Tri- 

 chomanis, Plagiochila asplenioides and Sphagnum rubellum. 



The development of the Bryophyte vegetation of the lava-fields 

 has been very little investigated. The Bryophyta together with the 

 lichens are the first piants which appear. The surface of the lava 

 is very rough, consisting of small round cavities (lava vesicles) in 

 which the spores find favourable conditions for germination, and 

 form small rounded cushions which adhere very closely to the sub- 

 stratum and, if conditions are favourable, extend over the entire 

 surface of the lava as a continuous carpet. It is, however, onh' in 

 the most exposed parts of the lava-field, that the mosses are con- 

 fmed to the vesicles; this is not the case in the clefts. 



Jonsson (1905, pp. 55 and 56) has given a description of Kra- 

 katindshraun near Hekla, a lava-field about 23 years old. The 

 mosses grew there only in tiny, scattered cushions upon the lava, 

 without any where forming continuous carpets; down in the clefts 

 they grew somewhat more abundantly. There the following species 

 were found: Bartramia ityphylla, Pohlia cruda, P. commntata, Bryum 

 pallens, Bryum spp., Ceratodon purpureus, Dicranoweisia crispula (the 

 commonest species), Rhacomitrium hypnoides, R. canescens, Scapania 

 curta and Lophozia alpestris. The further development is exceedingly 

 slow. On the flåts Rhacomitrium hypnoides will usually grow over 

 all the species and suppress them, but the formation of a continuous 

 Rhacomitrium heath appears to require a very long time, in many 

 cases, centuries, and the further development of the moss-heath into 

 heather moor, birch coppice or other formations, through the decay 

 of the mosses and their forming soil, undoubtedly takes place even 

 more slowly. 



