GROUND LICHEN 135 



The common Ground Lichen (Peltigera canina) has a large 

 green thallus with broad spreading lobes, resembling a large 

 Pellia. When growing on shaded banks among damp mosses 

 its favourite habitat the upper side of the plant is deep green, 

 but when dry or growing in more exposed places it is grey or 

 brown. The under side, however, is always white, with pale 

 brown veins from which arise, here and there, rooting processes 

 (rhizines) firmly attached to the substratum (mosses or soil). 

 This handsome plant is the best species in which to study, by 

 means of sections, the inner structure of a typical " leafy " lichen. 

 The spore fruits, chestnut-brown or brick-red in colour, are curved 

 around the lobes, on which they are carried at the edge of the 

 thallus. In some of the leafy lichens the thallus spreads out 

 as a flat and more or less circular rosette, with rhizines all over 

 the under side except at the edges. The commonest kinds belong 

 to the genus Parmelia, of which several species are very abundant. 

 P. physodes, perhaps the commonest British lichen, grows chiefly 

 on trees, forming large and often circular patches several inches 

 (sometimes a foot) across, and is whitish or light bluish-green 

 above, glossy and brown below, and quite smooth, with rounded 

 and turned-up lobes. P. caperata, nearly as common, is yellowish- 

 grey above, rough and blackish below ; it also grows mostly on 

 tree trunks. P. saxatilis is the lichen which so often forms 

 hundreds of large flat grey patches on walls and rocks, though 

 sometimes also found on trees. The thallus lobes branch by 

 repeated forking ; the under side is blackish, but brown near 

 the tips of the lobes. It is rarely fertile except in mountain 

 regions, where the thallus is often brown like the fruit cups. The 

 next two species are distinguished by having very narrow lobes 

 and by practically always bearing abundant fruits. P. pulveru- 

 lenta, on trees chiefly, is olive-green when moist but whitish 

 when dry, with brown or nearly black under side and fruit cups. 

 It is often covered with a frosty greyish-green powder consisting 

 of minute buds or soredes, which on being blown away give rise 

 to new plants. Finally, P. parietina is the most conspicuous 

 of our lichens, growing on roofs, walls, rocks (especially near 

 the sea), palings, and roadside trees ; the thallus is yellow or 

 orange above, and whitish below, while the abundant bright 



