UCHKNOLOOY <)!' ICKLANI) 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, 

 hypophlceodal and epiphlceodal. 



The hy pophloeodal crustaceous-lichens (numerous Graphidece, 

 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 hyphse 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 hypophlojodal 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 fe\v (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. 



