The rambler will meet with specimens of the Lichen tribes at every turn, when he has got fairly away from the smoke of towns. He will find them on the tree-trunks or rocks and walls, old posts and palings, on thatch and on the ground. Wherever they are found they may be accepted as certificates of the purity of the air. Formerly considered as a distinct type, they are now held by the advanced school of cryptogamic botanists as commensals, or partnerships formed between a fungus and an alga. They are usually thin crusts, consisting of an upper and a lower epidermis, formed of closely crowded cells, and to the lower layer rootlike filaments are attached. Between these layers are two differing elements; a loose stratum of green cells (gonidia), which are said to be algÆ, and below these a layer of fungoid threads. The contention of the new school is that these algÆ have been captured by a fungus and held in bondage, being forced to elaborate starch by means of their chlorophyll from the inorganic material obtained by the rootlike filaments, which starch the fungus is able to feed upon. Some of the green cells are pushed out from time to time invested with a few wisps of fungus-threads, and so reproduce the partnership. It is but right to add that some good authorities on this branch of botany decline to accept these views, and still regard lichens as independent organisms and not partnerships.

The species are very numerous, but their identification is not easy, and requires serious application. The two figured are exceedingly common in some districts. Various species of Cup-moss (Cladonia) will be met on heaths, sandy hedge-banks, etc. They have a flat crust-like base, from which arise pale grey tubes or cups, bearing at their tips the bright scarlet, pinky-brown, or even black fruits. A more common form in woods and on banks is Cladonia pyxidata, with the tube greatly increasing in width upwards. Cladonia rangiferina is the well-known Reindeer-moss, of inestimable value in extreme Northern latitudes as the food of the useful animal whose name it bears; it may be found in abundance in this country on heaths and hillsides covering the ground beneath the heather.

The other species figured in our plate, the Wall-lichen (Physcia parietina), is also very common, forming the familiar orange stains upon walls and maritime rocks. A closely allied species, the grey Parmelia saxatilis, is common on tree-trunks: it has been used time out of mind in the production of a brownish-red dye for wools. Several others of the same genus are valuable in a similar direction: our own Parmelia perlata, which grows on tree trunks, is largely imported from the Canaries as a dye-weed, and has been sold at as high a rate as £200 per ton.

Lichens are generally of slow growth and long life. Mr. Berkeley kept watch upon a patch of Lecidia geographica for twenty-five years, and found little change in it all that time. The Rev. Hugh Macmillan recounts how he found on the top of Schiehallion a species of lichen encrusting quartz rocks, which exhibited beneath the lichen the marks of glacial action as distinct and unchanged by atmospheric effects as though the glacier had only passed over them yesterday. He suggests that the lichen may reckon its days back very nearly if not quite to the glacial period in Britain!

There are upwards of a thousand British species, and the best list of them will be found in “Crombie’s British Museum Catalogue of Lichens,” of which the first part was published in 1894.