Hill: Penobscot Vegetation 335 



region affords an extremely good opportunity for a study 

 of the succession of vegetation in such a habitat. 



Rock surface association-type 

 The first forms to appear on the bare surface of the rocks 

 are the crustose lichens, which are adapted both by their 

 habit and requirements to the severest and most unfavorable 

 of habitats, such forms as Rhizocarpon (Buellia) geograph- 

 ica, R. confervoides, Lecidia platycarpa, Lecanora poly- 

 tropa and L. Hageni being common. As a result of the 

 activities of these plants the rock surface is etched away 

 and a little soil formed in which a second group of lichens, 

 the foliose species, can find a foothold. These are much 

 more conspicuous and readily attract attention. They 

 comprise various of the gray-green parmelias, including 

 such forms as Parmelia saxatilis, P. cons per sa and P. sul- 

 cata, which often form irregular patches several feet in di- 

 ameter ; and the striking yellow Candelariella (Placodium) 

 vitellina and Xanthoria (Theloschistes) parietina, together 

 with several species of Umbilicaria and Gyrophora. With 

 these lichens are associated such xerophytic mosses as Hed- 

 wigia ciliata and Grimmia apocarpa. As soon as sufficient 

 soil has accumulated the fruticose lichens come in. These 

 consist of numerous species of Cladonia, chiefly C. rangi- 

 ferina, C. pyxidata and C. f areata; Stereocaulon coral- 

 loides, and Cetraria islandica. The gradual growth of these 

 forms soon result in a mat of lichens which eventually covers 

 the surface of the rock, provided the slope is not too steep. 



Crevice association-type 

 The crevices in rock outcrops, which are of common oc- 

 currence, support a type of vegetation which is more ad- 

 vanced from the very beginning, owing to the presence of 

 particles of soil and the more sheltered environment, and, in 

 addition to cladonias and other fruticose lichens, various 

 herbaceous and shrubby plants early appear. Chief among 



