LICHENOLOGY OF ICELAND 241 



Rock-lichens. With regard to these it can be stated that 

 tropical Africa has 182 species (24 %), Italy 729 (46 %), Denmark 

 169 (3940 %) and Iceland 157 (47 %>). The figures indicate that 

 the sub-tropics are rich, and the purely tropical regions poor, in 

 species; whilst the temperate and Arctic regions are less rich in 

 species than are the sub-tropics. 



On comparing more particularly Denmark with Iceland, we find 

 that the number of species is greatest in Denmark, although Iceland 

 is much larger in area. Remembering, moreover, that Iceland has 

 bare rock-substrata, the superficial extent of which is so great that 

 in Denmark we can form no conception of it, whilst our Danish 

 species are limited to the very modest granite-surfaces on Bornholm, 

 and to the loose stones found scattered about in fields and in fences, 

 the small number connected with Iceland appears extremely eluci- 

 datory. It is impossible to explain this as anything else than a 

 direct result of the climate, because Iceland has so many kinds of 

 rock-substrata, that there would be plenty for the lichens to choose 

 among, if the climate had otherwise been favourable to them. 



We can consequently briefly sum up the above in the following 

 few sentences: 



(1) Iceland (as also the Arctic countries) has on the whole a 

 lichen-vegetation poor in species in proportion to its area, poorer 

 than have the temperate and sub-tropical countries. 



(2) The Bark-lichens meet with the most favourable condi- 

 tions in the tropics - - that is to say, they are rich in species there 



in the Sub-tropics and in the Temperate regions they are poorer, 

 whilst in the Arctic countries and in Iceland they are poorest of 

 all; this should probably be correlated w r ith the abundance of sub- 

 strata present. 



(3) The Epiphyllous lichens find the most favourable con- 

 ditions in the Tropics, less favourable in the Sub-tropics, and least 

 favourable of all in the Temperate regions; in the Arctic countries 

 and in Iceland they are entirely wanting. 



(4) The Earth-lichens meet with very unfavourable conditions 

 in the Tropics, better in the Sub-tropics, better still (probably) in 

 the Temperate regions, and best of all in the Arctic regions as 

 regards conditions concerning competition. The climate, on the other 

 hand, appears to be directly unfavourable to them in the Arctic 

 regions and in Iceland. 



(5) The Rock- lichens meet with very unfavourable conditions 



The Botany of Iceland. Vol. II. 16 



