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129 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.

Another important tribe of flowerless plants, to which we must be content with merely giving the general characters, for in a volume primarily intended as a guide to wild-flowers we must not occupy too much space with plants that do not produce flowers. At the same time, we believe the non-botanical among our readers will be glad to have a slight introduction, upon the strength of which they may cultivate the closer acquaintance of a most beautiful and interesting group of plants.