142 OLAF GALLOE 
cularly wind-affected coppices of which the tops of the shoots 
are dead. 
The conditions pertaining to propagation in the foliaceous lichens 
do not appear to differ from those in the crustaceous lichens. 
Iceland has the following foliaceous bark-lichens: — 
Cetraria sæpincola (Ehrh.) Arn. Parmelia olivacea L. 
Collema flaccidum L. = physodes L. 
— nigrescens L. == saxatilis L. 
Evernia furfuracea L. Physcia ciliaris L. 
Leptogium plicatile Ach. = obscura (Ehrh.) Nyl. 
Nephroma laevigatum. = stellaris L. 
— tomentosum. Sticta scrobiculata Scop. 
Pannaria triptophylla Ach. Xanthoria lychnea (Ach.) Th. Fr. 
Parmelia ambigua Ach. =: parietina L. 
Fruticose bark-lichens (Usnea, Ramalina, Bryopogon) are 
not found in Iceland, so they will not be discussed more fully 
here. They are described in my treatise of 1913, pp. 19 et seq. 
2. EPIPHYLLOUS LICHENS. 
are not found in Iceland. They require evergreen leaves as a sub- 
stratum. These extremely interesting plants received brief mention 
in my paper of 1913. The chief work on them is Ward’s treatise 
of 1893. 
3. EARTH LICHENS. 
Three types may be distinguished: Crustaceous, Foliaceous 
and Fruticose lichens, all three of which are found in Iceland. 
In all Crustaceous earth-lichens there is a distinct de- 
marcation between that part of the thallus which is buried in the 
ground (subterranean, hypogaean thallus) and that which rests upon 
the surface of the ground (epigaean thallus). The subterranean 
thallus may vary fairly markedly in appearance: it may be com- 
posed of small, more or less loosely connected grains (Lecidea uli- 
ginosa, L. alpestris, L. arctica, Gyalecta geoica), or it may consist of 
a homogeneous crust (Bilimbia sabuletorum, Lecidea Diapensie, etc.), 
or of small, somewhat scale-like parts coherent at the base (Sphy- 
ridium byssoides). The biological importance of these forms has not 
yet been investigated. 
The gonidia occur sometimes evenly distributed in the whole 
of the epigaean thallus, sometimes arranged in a definite layer im- 
mediately beneath the cortex. 
