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 roseus 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[1144], in three months.

There are three principal types of soil composition: (1) 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.

Bruce Fink[1145] found in N.W. Minnesota an association on exposed calcareous earth as follows:

This particular association occupied the slope of a hill that was washed by lime-impregnated water. It was normally a dry habitat and the lichens were distinguished by small closely adnate thalli.

There are more lichens confined to limy than to sandy soil. Arnold[1146] gives a list of those he observed near Munich on the former habitat: