L1CHENOLOGY OF ICELAND 141 



Microthelia micula Plot. 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 pulveralenta), 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 hyphae 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 pnlmonacea. 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- 



