604 A. HESSELBO 



cially Rhacomitrinm hypnoides which grew abundantly on debris pro- 

 duced by disintegration. On the rocks there grew Hypnum cnpressi- 

 forme, H. revolution, H. imponens, Pterigynandrnm filiforme, Leskea ner- 

 uosa, Rhacomitrinm fasciculare^R. heterostichum, Grimmia funalis, G.Do- 

 niana, Schistidium apocarpum, S. confertum , Dicranoweisia crispula, 

 Andrecea petrophila, Orthotrichum rupestre, O. Sturmii, Gymnomitrium 

 coralhides, Frullania Tamarisci, Radula complanata, Metzgeria furcata 

 and at the base of the rock, Madotheca Cordccana. A southern rock- 

 face was covered with Leiicodon scinroides var. morensis. In clefts 

 and crevices and on soil-covered ledges there grew Mnium orthor- 

 rhynchum, M.stellare, Pohlia cruda, Encalypta ciliata, E. rhabdo- 

 carpa, Bartramia ityphylla, Dicranum Andersonii, Distichinm monta- 

 nnm, Ditrichnm flexicaule, Didymodon rubellns, Amphidium tapponi- 

 cum, Plagiochila asplenioides , Marsupella Fnnckii and Lophozia al- 

 pestris. 



The locality just described comes nearest to that know r n in 

 Iceland as Ck Holt", z. e. low, stony ridges or protruding masses of 

 rock in the low land. The vegetation upon these "Holts" is gene- 

 rally xerophilous in character and is, as a rule, not so rich in 

 species as is the vegetation mentioned above. The ridges are usu- 

 ally more or less covered with soil which bears heath-vegetation, or 

 are occupied by gravelly flats, therefore the Bryophyte vegetation 

 consists chiefly of species belonging to heathland or grassland (Hy- 

 locominm spp., Polylrichnm alpinnm, P. juniperinum, P. pilosum, Hyp- 

 num nncinatiim, Frnllania Tamarisci, Rhacomitrinm spp., etc.), while 

 directly upon the rocks there grow species such as Hypnum cu- 

 pressiforme, H. hamulosum, H. revolutum, Orthotrichum and Grimmia 

 spp., and in the crevices Bartramia, Pohlia cruda, Amphidium tap- 

 ponicum, Mnium orthorrhynchum, Myurella, Distichium montanum 

 and other of the species found in rock-clefts in greater or smaller 

 quantities. 



The Bryophyte Vegetation of the Tuff Rocks. 



The reason why the Bryophyte vegetation of the tuff rocks is 

 here treated in a separate section is that the life-conditions of the 

 plants are essentially different on a basalt and on a tuff substratum. 

 Tuff consists of consolidated fine-grained material (volcanic ash and 

 dust) through which are scattered larger and smaller blocks of 

 rock. While the basalt has a smooth surface, with fissures and de- 

 pressions only here and there, the surface of the tuff is uneven and 



