258 



LICHENS 



of a shingic-bcach, are due to various Lichens {e.g. Rhizocarpon 

 confervoides, Fig. 141, C), which are here the iirst 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 fiat, lobed expansions which are often 



Fig. 141. — Various common Lichens. A, Usnea barbata. B, Partnelia 

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

 beach at Pevensey. D, Xantlioria parietina (showing numerous 

 apothccia). E, Cladonia sp., showing the upgrowths on whiclr the 

 apfjthccia arc borne. 



almost circular [e.g. Parmdia, 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, Cctraria 

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

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

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

 {eg Collema). 



