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 
Blastema 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. 
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