CHAPTER L. 



SOIL FORMATION IN ROCKY REGIONS AND 

 IN MOORS. 



Lichens. 



477. The lichen, parmelia. — Many of the lichens are small 

 and inconspicuous. They often appear only as bits of color 

 on tree trunk or rock. One of the conspicuous ones on stones 

 lying on the ground is the grayish-green thallus of Parmelia 

 contigua (fig. 260). Its pretty, flattened, forking lobes ra- 

 diate in all directions, advancing at the margin, and covering 

 year by year more and more of the stone surface. Numerous 

 cup-shaped fruit bodies (apothecia) are scattered over the 

 central area. The thallus clings closely to the rock surface by 

 numerous holdfasts from the under side, which penetrate minute 

 crevices of the rock. The lichen derives its food from the air 

 and water. By its closely fitting habit it retains in contact with 

 the rock certain acids formed by the plant in growth, or in the 

 decay of the older parts, which slowly disintegrate the surface 

 of the rock. These disintegrated particles of the rock, mingled 

 with the lichen debris, add to the soil in those localities. 



478. Lichens are among the pioneers in soil making. — The 

 habit which many lichens have of flourishing on the bare rocks 

 fits them to be among the pioneers in the formation of soil in 

 rocky regions which have recently become bared of ice or snow. 

 The retreat of glaciers from peaks long scoured by ice, or the 

 unloading of broken rocks along its melting edge, exposes the 

 rocks to the weathering action of the different elements. Now 



3" 



