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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 chrysophthalmus.
Placodium cerinum.
Buellia parasema (disciformis\ B. turgescens.
Calicium parietinum.
Lecanora Hagem, 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 permament
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 rosens appear and disappear within a
year. Thrombium epigaeum develops in half a year; Thelidium mtnutulum
in cultures grew from spore to 2
spore, according to Stahl 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.
3 Stahl 1877.