366 ECOLOGY 



Kamalina calicaris, R. fraxinea with var. ampUata, R. fastigiata, R. farinacea and 



R. pollinaria. 

 Parmelia saxatilis and f. furfuracea, P. caperata, P. physodes. 

 Physcia pulverulenta. Ph. tenella ihispidd). 

 Lecanora subfusca, L. rugosa. 



Pertusaria amara, P. globulifera, P. communis, P. Wulfenii. 

 Lecidea {Buellid) canescens. 

 Graphis scripta. 



And on the soil of these woods : 



Cladonia pyxidata, CI. pungens, CI. macilenia, CI. pityrea, CI. squamosa and CI. 

 sylvatica. 



Paulson^ from his observations of lichens in Hertfordshire, has concluded 

 that the presence or absence of lichens on trees is influenced to a consider- 

 able degree by the nature of the soil. They were more abundant in woods 

 on light well-drained soils than on similar communities of trees on heavier soils, 

 though the shade in the former was slightly more dense and therefore less 

 favourable to their development; the cause of this connection is not known. 



c. LiGNICOLOUS. Lichens frequenting the branches of trees do not long 

 continue when these have fallen to the ground. This may be due to the 

 lack of light and air, but Bouly de Lesdain^ has suggested that the chemical 

 reactions produced by the decomposition of the bast fibres are fatal to them, 

 Lecidea parasema alone continuing to grow and even existing for some time 

 on the detached shreds of bark. 



On worked wood, such as old doors or old palings, light and air are well 

 provided and there is often an abundant growth of lichens, many of which 

 seem to prefer that substratum : the fibres of the wood loosened by weathering 

 retain moisture and yield some nutriment to the lichen hyphae which burrow 

 among them. Though a number of lichens grow willingly on dead wood, 

 there are probably none that are wholly restricted to such a habitat. A few, 

 such as the species of Coniocybe, are generally to be found on dead roots of 

 trees or creeping loosely over dead twigs. They are shade lichens and fond 

 of moisture. 



The species on palings — or " dead wood communities " — most familiar 

 to us in our country are : 



Usnea hirta. Rinodina exigua. 



Cetraria diffusa. Lecanora Hageni, L. varia and its allies, 



Evernia furfuracea. Lecidea osireata, L. parasema. 



Parmelia scoriia, P. physodes. Buellia myriocarpa. 



Xanthoria parietina. Cladoniaceae and Caliciaceae (several species). 



Placodium cerinum. 



These may be found in very varying association. It has indeed been 

 remarked that the dominant plant may be simply -the one that has first 



1 Paulson 1919. 2 Lgsdain 1912. 



