III. THE BIOLOGY OF THE LICHENS OF ICELAND. 



TICHENS may be divided into the following biological types : 

 -Lj Bark, (Epiphyllous), Earth, Rock, (Parasitic) and (Sa- 

 prophytic) lichens. The three enclosed in brackets are wanting 

 in Iceland, (but possibly one or other of the last two groups may be 

 found there), and therefore will not be discussed here. With regard 

 to these it will suffice to refer the reader to my treatise "For- 

 beredende Unders0gelser til en almindelig Lichenokologi" (1913). 



1. BARK LICHENS. 



To this group I refer not only those which grow on the bark 

 of trees, but also such as grow on bare wood (telegraph poles, sur- 

 faces of wooden houses, etc.). These substrata have practically not 

 been investigated as regards lichen-biology, whilst their anatomy has 

 been investigated long ago. 



The chemical properties of the bark and their importance to 

 lichens, are as yet very superficially known. The bark always con- 

 tains organic substances (suberin, cellulose, tannin, resin, etc.), in- 

 organic salts, etc. Besides, it may be taken for granted, that the 

 outside layer of bark is generally more decomposed than are the 

 inner ones. That the bark differs distinctly as a substratum ac- 

 cording to whether it is young or old, is evident from the investi- 

 gations which Lotsy (1890) and I myself have made, regarding the 

 immigration-history of the lichens on bark. These investigations 

 have shown that the pioneer vegetation always consists of certain 

 crustaceous lichens, and is not replaced until later by the permanent 

 vegetation. I have no certain knowledge of this immigration-history 

 as regards Iceland, but I have reason to believe that the rule men- 

 tioned above also holds good there. 



Judging from what is known, the reason for this vegetation 

 order is a fairly similar process of decomposition in the different 

 kinds of bark, for barks even very different physically (smooth and 



