LICHENS, lī'kĕnz (Lat. lichen, from Gk. λειχήν, leichēn, λιχήν, lichēn, lichen). A very large group of plants, mostly northern and arctic in their distribution, some of the forms living at the very limits of vegetation. They have generally flat, membranous, or crustaceous bodies, but some forms have branching stalks. Lichens are exceedingly interesting in their structure and life activities, for they are known to be composed of two plants living together (smybiosis, q.v.). Every lichen is made up of two elements: (1) colorless threads (filaments); and (2) cells (gonidia) containing green or blue-green pigment. These latter are blue-green (Cyanophyceæ) or green algæ (Chlorophyceæ), and the colorless thready portion is a fungus. The fruit of a lichen is that of an ascomycete fungus, generally a flattened or cup-shaped structure with variously colored lining (black, brown, red, etc.). The colored lining of the cup or disk consists chiefly of the little sacs (asci) which contain the spores. There is a small group of tropical lichens whose fungus constituent is related to the mushrooms (Basidiomycetes). Lichens generally live in exposed situations, where their only source of water is that from rain and fog. This water is used by the algal cells which manufacture organic food in the manner characteristic of all green plants (photosynthesis, q.v.). When the lichen dries up in the sunshine all vegetative activities cease for the time, to be resumed when the plants are again moistened. However, lichens are especially adapted to retain moisture by the gelatinous character of the thallus. There are also present thread-like outgrowths (rhizines) from the fungal part of the plant, which may draw up some water from the substratum, but probably serve chiefly as organs of attachment. The algal portion of a lichen manufactures the organic food that is absorbed by the fungal elements, which bear the same relation to the algal cells as does any parasitic fungus to its host. The fungal portion makes no return at all commensurate with the benefits it receives; indeed, it is well known that the fungus may injure and even destroy the alga. The algæ are really slaves of the fungi, and under such conditions the latter are absolutely dependent upon the former. The algæ are of course perfectly able to take care of themselves, and do actually grow wild on many of the surfaces frequented by lichens. Some of the lichens are conspicuous in the landscape, e.g. many forms in barren highlands among the mountains, and on the rocky shores in the Arctic regions. The lichen called ‘reindeer moss’ (Cladonia rangiferino) supports imniense herds of reindeer and cattle in the highlands of Norway, and the ‘Iceland moss’ (Cetraria islandica) is eaten by man. Some typical lichens are shown on the accompanying plate.
A lichen-spore can produce a new plant only if it germinates among the cells of a suitable alga. It then proceeds to inclose the algal cells in a web of threads (mycelium) and holds them ever afterwards in slavery. This hehavior has been well established by a number of investigators who have produced artificial lichens by bringing the spores in contact with the proper wild algæ. Groups of algal cells with a portion of the thready body (mycelium) may form scales (‘soredia’) on the surface of the lichen body, and these are capable of reproduction.
For American species, consult Tuckerman, A Synopsis of North American Lichens. SeeAscomycetes and Colored Plate of Mosses and Lichens with Musci.
SECTION OF STICTA, showing algal cells surrounded by fungal filaments.
AN ALGAL CELL with investing fungal filaments, from Cladonia.