LICHENOLOGY OF ICELAND 141 
Microthelia micula Flot. Pertusaria xanthostoma (Sm.) Fr. 
Pertusaria communis D C. Rinodina sophodes Ach. 
The Foliaceous bark-lichens appear to be far richer and 
more varied in structure, and probably comprise very different types, 
which have not yet, however, been investigated from a biological 
point of view. With regard to this point, it will suffice for me to 
draw attention to the striking difference between such species as are 
adpressed to the substratum (Physcia pulverulenta), the surface of 
which the lichen follows along all its irregularities; and on the 
other hand Parmelia physodes, the greater part of which rises into 
the air, and lastly Evernia Prunastri, which hangs down in tufts 
from trunks and branches. 
The thallus of the foliaceous lichens is dorsiventral, is covered 
by a cortex, and has rhizines on its under surface. The rhizines 
attach the lichen to the substratum in the way described by Lin- 
dau, for, on coming into contact with the bark, they spread out 
flat over the substratum and, when the bark is well decomposed, 
send hyphæ down into its cracks for further attachment. The rhi- 
zines are unable to dissolve the cellulose, but it may be presumed 
that they absorb the salts set free by the decomposition of the bark. 
The gonidia-containing thallus itself is, as is well-known, in- 
dented in various ways, and grows centrifugally over the substratum, 
for which reason it often dies away in the centre, a fact commonly 
observed, especially in Parmelia saxatilis and Sticta pulmonacea. The 
edge of the thallus gradually forms new rhizines on the side turned 
downwards. The gonidia are situated just below the cortical layer 
of the morphological upper-surface. 
Propagation takes place by means of spores, possibly by 
pycnoconidia and soredia, which are extremely common in several 
species (Parmelia and Evernia spp.). 
In their competitive capacity the foliaceous lichens stand, in 
many habitats, far above the crustaceous lichens. They generally 
require more thoroughly decomposed bark than do the latter, there- 
fore (with a few exceptions) they do not live on young branches. 
In addition, they generally demand more light. Consequently, where 
abundant light and well-decomposed bark are found, the vegetation 
of the bark of the tree consists of foliaceous lichens, which easily 
grow over and exterminate the original vegetation of crustaceous 
lichens. In the birch coppices of Iceland this may be observed 
here and there on older trunks and branches, especially in parti- 
