392 ECOLOGY 



though in dry weather the lichens may be exposed, and can withstand 

 desiccation for a considerable time : 



Pterygium Kenmorensis. Lecidea conttgua. 



Collemafluviatile. Lecidea albocoerulescens. 



Lecanora lacustris. Dermatocarpon miniatum var. complicatum . 



Lecanora epulotica. Dermatocarpon aquaticum. 



Bacidia inundata. Verrucaria laevata. 



Rhizocarpum obscuratiim. Verrucaria aethiobola. 



Rhizocarpum petraeum. Verrucaria margacea. 



The second group of species usually inhabits damp, shaded rocks of 

 ravines or large boulders by streams or near waterfalls. It includes species 

 of Collenia, Sticta, Peltigcra, Solorina, Pannaria, etc., with Opegrapha zonata, 

 Porina lectissima and Verrucaria nigrcscens. 



The last-mentioned lichen grows by preference on limestone, but in 

 excessive moisture 1 , as by the sea-side, the substratum seems to be of minor 

 importance. 



D. LICHENS AS PIONEERS 



a. SOIL-FORMERS. The part played by lichens in the "Economy of 

 Nature" is of very real importance: to them is allotted the pioneer work 

 of breaking down the hard rock surfaces and preparing a soil on which 

 more highly developed plants can grow. This was pointed out by Linnaeus 2 

 who thus describes the succession of plants : "Crustaceous lichens," he 

 writes, "are the first foundation of vegetation. Though hitherto we have 

 considered theirs a trifling place among plants, nevertheless they are of 

 great importance at that first stage in the economy of nature. When the 

 rocks emerge from the seas, they are so polished by the force of the waves, 

 that scarcely any kind of plant could settle on them, seen more especially 

 near the sea. But very soon, in truth, the smallest crustaceous lichens begin 

 to cover those arid rocks, and are sustained by minute quantities of soil and 

 by imperceptible particles brought to them by rain and by the atmosphere. 

 These lichens in time become converted by decay into a thin layer of 

 humus, so that at length imbricate lichens are able to thrust their rhizoids 

 into it. As these in turn change to humus by natural decay, various mosses 

 such as Hypnum, Bryum and Polytrichum follow, and find suitable place 

 and nourishment. In time there is produced by the dying down of the 

 mosses such a quantity of soil that herbs and shrubs are able to establish 

 themselves and maintain their existence." 



Similar observations have been made since Linnaeus's day, among others 

 by Guembel 3 in his account of Lecanora ventosa. Either by the excretion of 

 carbon dioxide which acidifies the surrounding moisture, or by the mechanical 

 1 Wbeldon and Wilson 1913. 2 Linnaeus 1762. 3 Guembel 1856. 



