BI ZTHE BIOLOGY OF THE LICHENS OF ICELAND. 
ICHENS may be divided into the following biological types: — 
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 Undersøgelser til en almindelig Lichenekologi” (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 
