LICHENOLOGY OF ICELAND 139 
elliptic in shape, with their main axis parallel with the longitudinal 
axis of the tree. 
In Iceland there occurs rather a common wood-substratum, 
namely the old decomposed walls of the wooden houses. On such 
walls I found the following common species: — 
Buellia myriocarpa. Caloplaca vitellina. 
Lecanora Hageni. = pyracea. 
= varia. Physcia obscura. 
— subfusca. 
Bark-lichens may be divided into Crustaceous, Foliaceous 
and Fruticose lichens. 
Of Crustaceous bark-lichens there are two different types, 
hypophlæodal and epiphlæodal. 
The hypophlceodal crustaceous-lichens (numerous Graphideæ, 
etc.) have, as regards their attachment to the substratum, been long 
ago investigated very thoroughly by Lindau (1895), to whose treatise 
I refer the reader. 
Their thallus lives in the interior of the bark of trees, covered 
by its cells, which afford the lichen protection against evaporation. 
According to Lindau their hyphæ appear to be quite unable to 
dissolve the cellulose of the bark, so they probably live on its de- 
composition-products. They themselves, however, contribute towards 
decomposition by bursting asunder the cells by the tension of their 
growth, whereby air and water gain access to the bark. The thallus 
is otherwise homoiomerous in structure in several of the species, 
in others distinctly heteromerous; consequently, on the whole, very 
primitive, and only slightly removed from the purely mycelial 
fungal prototypes. 
The hypophlæodal crustaceous-lichens stand extremely low both 
in respect to morphology and anatomy, and as regards their capacity 
for competition with other plants. They live exclusively on the bark 
of trees and have no analogues among the earth-lichens and only 
a few (and these even very disputable,) among the rock-lichens 
with endolithic thallus. Only where other bark-lichens are absent 
for various reasons, may these occur, but if the conditions are 
favourable to fruticose and foliaceous lichens, they are imme- 
diately expelled by these. They are most frequent on smooth bark 
— the numerous smooth-barked trees of the tropics house an abun- 
dance of them — and they remain there so long as the bark is 
not decomposed enough to house other, more pretentious types. 
