LICHENOLOGY OF ICELAND 215 



The Growth-forms of the Heath-lichens. 



Of these numbers the Danish will scarcely be altered to any 

 extent, whilst the Icelandic, through more numerous and more de- 

 tailed investigations, will probably undergo a very radical alteration 

 in favour of the crustaceous lichens. On riding across the heaths 

 in Iceland it strikes one that the crustaceous lichens are more do- 

 minant there than in Denmark. This agrees closely with the fact 

 that the chamsephytes are, as a rule, obviously poorer and less well- 

 developed there than on Danish heaths, and are consequently more 

 hospitable towards lichens, than are the taller, well-grown species. 



With respect to the frequency-number of the lichens, none are 

 to hand from Denmark. Those from Iceland are given above. 

 Judging from what I remember, and compared with what I wrote 

 on the subject in "Danske Licheners 0kologi" (p. 301 et seq.), I am, 

 however, inclined to believe that in Denmark all possible frequency- 

 numbers occur, from to 100, as in Iceland, in that we have in 

 Denmark Ca//H/?a-heaths, which are sometimes very rich in lichens 

 and sometimes almost devoid of them. In this respect there is 

 scarcely any difference worth mentioning between the Danish and 

 the Icelandic heaths. 



It is as difficult or, properly speaking, still more difficult to state 

 anything about the mass-occurrence of the lichens in Denmark com- 

 pared with Iceland. I must, however, enter somewhat more closely 

 into this question, as it is, in addition, of more far-reaching eco- 

 logical importance. 



If we are briefly to compare Iceland and Denmark as regards 

 their lichen-vegetation on heaths, as far as this can be done on the 

 basis of the investigations hitherto made, which in a high degree 

 require to be more detailed as regards both countries, especially 

 with reference to the mass-occurrence of the species, it may be 

 stated that: 



