138 OLAF GALLOE 
scaly) show quite analogous features in the development-history of 
the vegetation. 
I am not aware of the existence of any thorough chemical in- 
vestigation of the different kinds of bark, nor do I believe that such 
exists, except perhaps as regards the officinal barks. 
Neither have, as yet, the physical conditions of bark been investi- 
gated. We can, upon a superficial survey, immediately distinguish 
between the two, well-known groups, smooth bark and scaly bark. 
They are easily distinguished from each other. 
I have known, as a general fact, that the systematic species of 
the tree, is of no importance to the biological types which settle 
down on its bark, as crustaceous, foliaceous and fruticose lichens 
may be found on all of them. Which of these types is to dominate 
the vegetation when it is fully developed, depends on the degree of 
light to which the tree is exposed, and other meteorological cir- 
cumstances, as I have shown in my work on “Danske Licheners 
Økologi” (1908). 
On the other hand, the floristic composition of the vegetation 
varies essentially, according to the systematic species of the tree. 
Experience shows that certain lichens occur by preference on cer- 
tain species of trees, (Usnea spp. on coniferous trees, etc.). It is 
possible that, by more thorough investigations, we shall also be able 
to find fixed rules for this association, but as yet nothing is known 
regarding this point. At present we must be content with the lists 
of lichens compiled for each species of tree, as has been done in 
“Danske Likeners Økologi,” and as regards Iceland, when discussing 
the lichens of the Birch later on in this paper. 
Wood. Many species which occur most frequently on bark, 
may occasionally be found on bare wood. Wood is chemically 
closely related to bark, and the lichens which occur on it can, as 
a matter of course, be classified among the bark lichens. It must, 
however, be mentioned with respect to the growth-tensions which 
occur in the bark during the growth of the tree, and which are 
inclined to stretch the crust-shaped lichens into elliptical or oval 
crusts, with the main axis of the ellipse at right-angles to the longi- 
tudinal axis of the tree, that these, of course, will not be fonnd in 
dead wood. There, on the contrary, the crustaceous lichens grow 
parallel with the “fibres,” i.e. parallel with the longitudinal axis 
of the stem, hence the reason why lichen-crusts which grow un- 
influenced by neighbours and competitors, are very often oval or 
