604 A. HESSELBO 



cially Rhacomitrium hypnoides which grew abiindantly on débris pro- 

 duced by disintegration. On the rocks there grew Hypniim cupressi- 

 forme, H. reuohitiiin, H. imponens, Pierig ynandrnm filiforme, Leskea ner- 

 vosa, Rhacomitrium fasciciilare,R. heterostichiim, Grimmia fiinalis, G. Do- 

 niana, Schistidiiim apocarpiim , S. conferlnm , Dicranoweisia crispida, 

 Andreæa petrophila, Orthotrichiim riipestre, O. Stiirmii, Gymnomitriiim 

 coralloides, Frnllania Tamarisci, Radiila complanata, Metzgeria furcata 

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

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

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

 rhynchiim, M stellare ^ Pohlia cnida, Encalypta ciliata, E. rhabdo- 

 carpa, Bartramia ityphylla, Dicrauum Andersonii, Distichinm monta- 

 niun, Ditrichum flexicaide, Didymodon rnbellns, Amphidiiim lapponi- 

 ciim, Plagiochila asplenioides , Marsupella Fiinckii and Lophozia al- 

 pestris. 



The locality just described comes nearest to that known in 

 Iceland as "Holt", /. e. low, ston}-^ 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 ricli 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 l)y gravelly flåts, therefore the Bryophyte vegetation 

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

 locomiiim spp., Polylrichiim alpimim, P.juniperimim, P. pilosiim, Hyp- 

 mim iincinatum, Frnllania Tamarisci, Rhacomitrinm spp., etc), while 

 directly ui)on the rocks there grow species such as Hypnnm cn- 

 pressiforme, H. hamnlosnm, H. revolntnm, Orlhotrichnm and Grimmia 

 spp., and in the crevices Bartramia, Pohlia crnda , Amphidinm lap- 

 ponicnm, Mninm orthorrhynchnm, Mynrella, Distichinm montannm 

 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 seclion is that the life-conditions of the 

 piants 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 



