LICHEN COMMUNITIES 367 



gained a footing, though the larger and more vigorous lichens tend to crowd 

 out the others. Bruce Fink 1 has recorded associations in Minnesota: 



On wood : 



Teloschistes chrysophthalmits. Buellia parasema (disciformis), B. turgescens. 



Placodium cerinum. Calicium parietinum. 



Lecanora Hageni, L. varia. Thelocarpon prasinellum. 

 Rinodina sophodes, R. exigua. 



On rotten stumps and prostrate logs : Peltigera canina. Cladonia fim- 

 briata var. tubaeformis, Cl. gracilis, Cl. verticillata. Cl. symphicarpia, Cl. 

 macilenta, Cl. cristatella. 



Except for one or two species such as Buellia turgescens, Cladonia sym- 

 phicarpia, etc., the associations could be easy paralleled in our own country, 

 though with us Peltigera canina, Cladonia gracilis and CL verticillata are 

 ground forms. 



2. TERRICOLOUS 



In this community other vegetation is dominant, lichens are subsidiary. 

 In certain conditions, as on heaths, they gain a permanent footing, in others 

 they are temporary denizens and are easily crowded out. As they are 

 generally in close contact with the ground they are peculiarly dependent 

 on the nature of the soil and the water content. There are several distinct 

 substrata to be considered each with its characteristic flora. Cultivated soil 

 and grass lands need scarcely be included, as in the former the processes of 

 cultivation are too harassing for lichen growth, and only on the more perma- 

 ment somewhat damp mossy meadows do we get such a species as Peltigera 

 canina in abundance. Some of the earth-lichens are among the quickest 

 growers : the apothecia of Baeomyces roseits appear and disappear within a 

 year. Thrombium epigaeum develops in half a year; Thelidium minutulum 

 in cultures grew from spore to spore, according to Stahl 2 , in three months. 



There are three principal types of soil composition: (i) that in which 

 there is more or less of lime; (2) soils in which silica in some form or other 

 predominates, and (3) soils which contain an appreciable amount of humus. 



Communities restricted to certain soils such as sand-dunes, etc., are 

 treated separately. 



a. ON CALCAREOUS SOIL. Any admixture of lime in the soil, either as 

 chalk, limy clay or shell sand is at once reflected in the character of the 

 lichen flora. On calcareous soil we may look for any of the squamulose 

 Lecanorae or Lecideae that are terricolous species, such as Lecanora crassa, 

 L. lentigera, Placodium fulgens, Lecidea lurida and L. decipiens. There are 

 also the many lichens that grow on mortar or on the accumulated debris 

 mixed with lime in the crevices of walls, such as Biatorina coeruleonigricans , 

 species of Placodium, several species of Collema and of Verrucariaceae. 

 1 Fink 1896, etc. a Stahl 1877. 



