LICHENOLOGY OF ICELAND 221 



the other name, hut for the sake of synonymy it is necessary to 

 have simple and easily definahle conceptions, and this is hest done 

 by naming the association after the dominant plant-growth-form. 



With regard to lichens we will therefore employ as the prin- 

 ciple of main division the grouping indicated above, vi/. that of 

 crustaceous, foliaceous and fruticose lichens, and, as far as possible, 

 follow them on each rock-substratum. 



The Crustaceous-lichen-association is widely distributed 

 on all kinds of basalt. Several types (formations) may be distin- 

 guished, e. g. mixed crustaceous-lichen-formations, Staurothele- 

 formations. C/o/)/aca-formations and Verrucaria-formations. 



Mixed crustaceous-lichen- formations are widely distributed 

 especially on the almost vertical faces of basalt rocks along the 

 fjords. 



The plant-density is often rather slight, in that the individuals 

 are not in contact with each other, i. e. they leave the rock-surface 

 visible between them. In such places, therefore, there is no actual 

 competition between the species, and the community is consequently 

 analogous to the desert-vegetation of loose soil. 



In other places the plants may be closely in contact with each 

 other, and struggle for space. In this case competition arises, where 

 sometimes the one and sometimes the other plant predominates, 

 but all the circumstances concerning this interesting struggle have 

 not been investigated and are not known. 



Many interesting observations could undoubtedly be made as 

 regards the frequency-number and mass-occurrence of the single 

 species under different conditions, but all this requires both a long 

 sojourn and also patient investigations on the spot. I presume, that 

 among other things, we should thereby acquire a closer knowledge 

 of the life-necessities of each species, and that we should be able 

 to sub-divide the "mixed crustaceous-lichen-associalions" into per- 

 haps as many formations as the number of the systematic species. 

 But this the future must decide. 



To this association almost all the crustaceous lichens of Iceland 

 must undoubtedly be referred, i. e. somewhat above 100 species. 

 There are, however, some which occur repeatedly and which ought 

 to be enumerated as characteristic of the association, viz. 



Lecanora cinerea. Lecanora intricata. 



pallescens. frustulosa. 



atra. sordida v. glaucoma. 



