THE LICHENS 



375 



those which are formed among higher plants. The character of these 

 associations is largely determined by the nature of the substratum. Thus 

 limestone rocks produce a different 

 set of species from igneous rocks, and 

 the manuring of certain rocks by birds 



Fig. 360. — Lecanora esculenta. The 

 Manna Lichen. 



(From a specimen lent by the British Museum.) 



Fig. 36 1 . — Cladonia rangiferitia var. 

 alpestris showing the fruticose 

 thallus. 



Stimulates the appearance of yet another association of nitrophilous 

 species. 



On limestones some species may burrow into the substratum of the rock 

 by means of excreted carbonic acid, in such a fashion that the vegetative 

 parts are invisible and only the apothecia, sunk in tiny craters, appear at 

 the surface. 



Cliffs facing the sea usually show a marked zonation of Lichens parallel 

 to high-w^ater mark (Fig. 362). At the base of the cliff, where frequently 

 submerged, the rocks are dappled with dark, crustaceous species of 

 Verrucaria, especially V. ?naura. Above this there may be a zone of the 

 black fruticose, Lkhina vulgaris. Higher still, in the zone only reached 

 by occasional spray, is the brilliant orange belt of Xanthoria pan'etina, and 

 finally, on the western coasts of Britain, comes a broad stretch of the grey 

 fruticose Rama Una scopidormn. 



Lichens are able to withstand great desiccation without injury. For 

 example, Peltigera in dr\' weather may lose all but 5 per cent, of the water 

 which it holds when saturated. Most Lichens can absorb water directly 

 from damp air, but they do not attain in this manner more than 70 per cent, 

 of their saturation content. 



When one compares the biological characteristics of Lichens and of the 

 Fungi, it is surprising how opposite they are, considering that the fungal 



