This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
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 cryptogarnic 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 algae, and below these a layer of fungoid threads. The contention of the new school is that these algae 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.
A - Scarlet Cup-moss. B. - Wall-Lichen.
Cladonia cornucopioides. Physcia parietina.
- lichenes. -