258 



LICHENS 



of a shingle-beach, are due to various Lichens (e.g. Rhizocarpon 

 confervoides, Fig. 141, C), which are here the first colonisers. 



The shape of the thallus is very diverse, and, as a general 

 rule, almost entirely determined by the Fungus. It most com- 

 monly takes the form of flat, lobed expansions which are often 



FIG. 141. Various common Lichens. A, Usnea barbata. B, Parmelia 

 physodes, on twig. C, Rhizocarpon confervoides, on pebble from shingle- 

 beach at Pevensey. D, Xanthoria parietina (showing numerous 

 apothecia). E, Cladonia sp., showing the upgrowths on which the 

 apothecia are borne. 



almost circular (e.g. Parmelia, Xanthoria, Fig. 141, D). Other fre- 

 quent types are those exhibiting repeated branching (Fig. 141, B) ; 

 in such the segments may be upright (e.g. Iceland Moss, Cetraria 

 islandica, Fig. 143) or hanging (e.g. Old Man's Beard, Usnea, 

 Fig. 141, A). In some cases the thallus is highly gelatinous, 

 so that it is much more conspicuous in wet than in dry weather 

 (e.g Collema). 



