LICHENOLOGY OF ICELAND 239 



If we regard the bark-lichens in the various belts, it is seen 

 that there are many in the Tropics, that for instance tropical Africa 

 has almost 500 bark-lichens, already known, which form 65 % of 

 all its lichens, Italy 508 (about 32 %), Denmark 165 (about 39 %>) 

 and Iceland 59 (about 15 %>, inasmuch as Iceland's 285 systematic 

 species constitute 337 biological forms, as several of the species 

 occur sometimes as earth- and sometimes as rock-lichens, etc.). Now 

 the areas which have here been compared with one another, are 

 far from being all equally large and therefore do not give any figures, 

 which are useful for purposes of direct comparison. But, at any 

 rate, they give an indication of the fact that bark-lichens are com- 

 paratively more numerous in countries rich in trees than in Iceland ; 

 and they give the very important-information that Iceland, although 

 it is much larger than Denmark, has only 59 species, whilst Den- 

 mark has 165! Whether this circumstance is solely due to want of 

 necessary tree-substratum is not easy to decide. For instance, whether 

 the bark-lichens of Denmark would be able to thrive in Iceland, 

 if, by way of experiment, we removed them thither, together with 

 the stems upon which they occurred, or whether the climate alone 

 would kill them, we do not know. But that the paucity of species 

 is due to the climate - - directly or indirectly - is evident enough. 



Epiphyllous lichens. These occur, as is well-known, on ever- 

 green leaves only. 24 species are known to occur in tropical Africa 

 and 3 in Italy. From the climate-belts north of Italy they are prac- 

 tically absent, and in Iceland, with its deciduous birches and wil- 

 lows, they are totally wanting. The same consideration which applies 

 to the bark-lichens may be extended to the epiphyllous lichens, viz., 

 that the climate is, directly or indirectly, a hindrance to their 

 growth in Iceland. 



Earth- lichens. We must expect a priori, that regions with a 

 luxuriant vegetation of phanerogams and other good-sized plants are 

 not favourable to earth-lichens. From the whole of that immense, 

 tropical Africa (outside its alpine regions) only some 50 lichens are 

 known! (about 5 6 %), from Italy 275 (about 17 /o), from Denmark 

 86 (about 20 %) and from Iceland 121 (about 36 %>). As may be 

 seen, the percentage of the earth-lichens becomes greater and greater, 

 the farther we proceed northwards to the cold regions. This is 

 without doubt correlated with the fact, that the number of the com- 

 petitors of the lichens decreases towards the north, the ground be- 

 coming more destitute of other plant-growth. 



