64 A Study of the Vegetation of 



Composition of the Rhizocarpon-Lecidea Associes 



Dominant Species 

 Rhizocarpon geographicum Lecidea lapicida 



Principal Species 



Acarospora chlorophana Lecidea fuscocinerea 



Blastenia ferruginea Rinodina oreina 



Lecanora calcarea 



Rhizocarpon geographicum is by far the most abundant and 

 widely distributed species. It occupies the most exposed situa- 

 tions and forms a well defined consocies. The greenish-yellow 

 color of the rocks is due to its presence. The characteristic 

 checking of the dry thallus undoubtedly aids greatly in the ab- 

 sorption of water. 



Lecidea lapicida often covers areas of several square inches ex- 

 clusively. Like Rhizocarpon it is of sufficient abundance and im- 

 portance to be ranked as an associes. On the quartzite rocks 

 Rinodina oreina often replaces Rhizocarpon, dominating large 

 areas with its orange-colored thallus. It is often accompanied 

 by the cinnamon-brown Lecidea fuscocinerea. Further investiga- 

 tion will probably show these to be dominants of a distinct as- 

 socies. 



Often large areas of vertical cliffs of basalt appear as if painted 

 lemon-color because of the presence of the crustose Acarospora 

 chlorophana. 



Crustose lichens not only exert an influence at the contact of 

 thallus and rock, but the corroding effect of carbon dioxide and 

 other secretions extends beyond the thallus margins during moist 

 weather. This permits slow extension of the thalli or furnishes 

 the starting place for new ones. Thus lichens corrode and de- 

 compose the rock and by mixing its particles with their own re- 

 mains give opportunity for the growth of other vegetation. Cen- 

 ters of crustose thalli break up and folious lichens get in. Espe- 

 cially on the more weathered rocks and in slightly less exposed 

 situations a number of foliose lichens replace crustose forms. 



