LICHENS AS PIONEERS 393 



action of hyphae and rhizinae, the component particles of rocks such as 

 granite are gradual!}/ dissolved and broken up. Rocks exposed to weather 

 alone are unchanged, while those covered with lichens have their surface 

 disintegrated and destroyed. 



The decaying parts of the lichen thallus add to the soil material as 

 observed by Linnaeus, and in time mosses follow, and, later, phanerogams. 

 Goeppert' has pointed out the succession observed on roofs of houses as : 

 "first some lichen such as Lecanora saxicola, then the moss Griininia pulvi- 

 nata, which forms compact cushions on which later grow Poa coinpressa, 

 small crucifers, etc." 



Goeppert^ has noted as special rock-destroyers some foliaceous species, 

 Parmelia saxatilis, P. stygia and P encausta, the underlying rock being 

 roughened and broken up by their rhizoids. Species of Gyrophora and 

 Sphaerophorus have the same disintegrating effect, so that the surface of the 

 rock may in time lose its coherence to a depth of 2 to 4 inches. Crustaceous 

 species such as Lecanoi^a polytropa, Candelariella vitellina, etc., exercise an 

 equally powerful solvent action, while underneath closely appressed growers 

 like Lecanora atra and Acarospora smaragdula the stone is converted to 

 a friable substance that can be sliced away with a knife. 



Salter'' concluded that oxalic acid was the principal agent in disintegra- 

 tion. He found that it acted more or less rapidly on minerals and almost 

 any class of saline compounds; it even attacked glass finely powdered, 

 though silica remained unchanged. 



Bachmann^ found that granite was reduced by lichens to a clay-like 

 granular yellow mass in a comparatively short time, the lichen seizing on 

 the particles of mica first; but the spread of the lichen over the rock, he 

 observes, is largely directed by the amount of humidity and by the chance 

 of gaining a foothold. In the case of calcareous rocks he* tested the relative 

 dampness of those containing lichens and those that were lichen-free. In 

 the former case water was absorbed more freely and retained much longer 

 than in the barren rock, thus encouraging further vegetation. 



Lucy E. Braun" has described the successive colonization of limestone 

 conglomerate in Cincinnati. The rock is somewhat resistant to erosion 

 and stands out in irregular outcrops on the hillsides of the region. The 

 first plants to gain a footing are certain crustaceous lichens, Lecidea sp., 

 Pertusaria connnunis, Staurothele mnbrina, Verrucaria vmralis and Placo- 

 diuin citrinum which occur as patches on the smoother and more exposed 

 rock faces. With these were associated small quantities of a moss, Griiuvna 

 apocarpa. In the second stage of growth Dermatocarpon miniatiim, and, to 

 a lesser degree, a gelatinous Omphalaria sp. were the most prominent plants, 



^ Goeppert i860. ^ Salter 1856. •' Bachmann 191 r. 



■• Bachmann 1913. ^ Braun 1917. 



