VEGETATION OF ROCKY PLACES. 6oi 



the rock this stage may continue for ages unless the rock is so 

 situated that the disintegrated plant remains and the weatherings 

 of the rock are not washed away. Umbilicaria, a dark-colored 

 lichen, circular in form, often 10-2 $cm broad, and attached 

 to the smooth rock by a central "umbilicus," is a good example. 

 This is sometimes very common on smooth, perpendicular rocks 

 by lake shores, or on boulders in moist woods or regions. Par- 

 melia is also a common lichen clinging closely to the smooth 

 surface of rocks. So there are here Umbilicaria formations, 

 Parmelia formations, Theloschistes formations, etc. 



1095. 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 

 figures of varied color. Disintegrating rock, debris of plants 

 and animals, join to form the virgin soil. Certain of the blue- 

 green algae, as well as some of the mosses, are able to gain a 

 foothold on rocks and assist in this process of soil formation. 

 A view of rocks thrown down by the melting and retreating edge 

 of a glacier in Greenland is shown in fig. 525. These rocks at 

 the time the photograph was taken had no plant life on them. 

 At other places in the vicinity of this glacier, rocks longer un- 

 covered by ice were being covered by plant life. One of the 

 Greenland rock lichens is shown in fig. 524. 



1095a. Succession of plants as soil is formed. If the rock is 

 creviced, these rudimentary soil ingredients collect in the crevices 

 and afford a place for mosses or the smaller seed plants, especially 

 those which form low, compact, cushion-shaped masses. Examples 

 of crevice plants are, lichens like Cladonia, with several species; 

 mosses like Hedwigia, Andraea, though these also grow on smooth 

 rock; ferns like Polypodium, Dryopteris, Pellaea, some species 

 of Asplenium; herbs like the harebell (Campanula rotundifolia), 



