142 OLAF C.ALL0K 



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 ssepincola (Ehrh.) Am. Parmelia olivacea L. 

 r.ollema flaccidum L. physodes L. 



nigrescens L. saxatilis L. 



Kvernia furfuracea L. Physcia ciliaris L. 

 Leptogium plicatile Ach. obscura (Ehrh.) Nyl. 



Ncphroma laevigatum. stcllaris L. 



tomentosum. Sticta scrobiculata Scop. 



Pannaria triptophylla Ach. Xanthoria lychnea (Ach.) Th. Fr. 

 Parmelia ambigua Ach. parietina L. 



Fruticose bark- lichens (Usnea, Ranuilina, 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 nli- 

 yiiiosa, L. dlpestris, L. arctica, Gijalecta (jeoica), or it may consist of 

 a homogeneous crust (Bilimbia sabnletorum. Lecidea Diapensicr, etc.), 

 or of small, somewhat scale-like parts coherent at the base (Splnj- 

 ridiuin bijssoides). 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. 



