CHAPTER LIU. 



SOIL FORMATION IN ROCKY REGIONS AND 

 IN MOORS. 



Lichens. 



690. 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. 481). Its pretty, 

 flattened, forking lobes radiate 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 scat- 

 tered over the central area. The thallus clings closely to the rock 

 surface by numerous holdfasts from the under side, which pene- 

 trate 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 disinte- 

 grate the surface ot the rock. These disintegrated particles of 

 the rock, mingled with the lichen debris, add to the soil in those 

 localities. 



691. 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 

 the lichens lay hold on them and invest them with fantastic 



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