LICHENOLOGY OF ICELAND 183 



known are the fell- fields of the plateaux; but others exist also, 

 as for instance vast sandy tracts with drifting sand, both in the 

 highlands and in the lowlands. We shall now consider these deserts 

 more closely, leaving out those with a rock-substratum, which will 

 be discussed elsewhere. 



According to the substratum we can divide the deserts into 

 stony, gravelly, sandy and clayey deserts. A division according to 

 the principles of plant-ecology, cannot be undertaken, as the vege- 

 tation has not been sufficiently investigated, from a statistical point 

 of view. 



Stony Deserts are the stone-covered ridges (holt) of the 

 lowlands, and the talus of fallen blocks and debris (Urd) of 

 the highlands. The lichens growing directly on the stone-substratum, 

 do not concern us here, but between the stones on the ridges there 

 grow as chasmophytes, Dryas octopetala, Thymus Serpyllnm, Silene 

 acaulis, Potentilla verna, Cerastium alpinum, Arabis petrcca, Saxifraga 

 ccespitosa, Juncns trifidns, Luznla spicata, Achimilla alpina, Poa glauca, 

 Elyna Bellardi, and a number of less frequent species (according to 

 Jonsson). Interspersed in the moss- car pets occur (according to 

 Jonsson's list in "Vegetationen paa Snaefellsnaes," p. 41) the fol- 

 lowing species: 



Cladonia rangiferina (podetia- wandering fruticose lichen). 



Thamnolia vermicularis 



Cladonia uncialis 



Sphserophorus coralloides 



Cetraria aculeata 



Sphaerophorus fragilis 



Cladonia pyxidata (hypothallus-wandering fruticose lichen). 



cornucopioides 



Cetraria islandica (erect foliaceous lichen). 

 Peltigera canina (horizontal foliaceous lichen). 



rufescens 



aphtosa 



Jonsson does not mention having found any crustaceous lichens, 

 therefore these will hardly occur in conspicuous abundance, whilst 

 it may be expected that, on future investigations, some or other of 

 the small, inconspicuous species may be found, at any rate on 

 decaying moss. 



Jonsson also mentions the fact that here and there Cladonia 

 and Spharophorns spp. may occur as dominants in a sub-vegetation 

 of mosses, in addition to more sparsely occurring Graminece. 



The taluses of fallen blocks and debris (Urd) in the 



