LICHENS, (Lat. lichen, from Gk.
Xecxiiv, NIA*, 11011//, lichen). A very large group it plants, mostly northern and arctic in their distribution, some of the forms living at the very limits of vegetation. They have gen erally flat, membranous, or •ruistaveons bodies, but some forms have branching stalks. Lichens are exceedingly interesting in their structure and life activities, for they are known to be com posed of two plants living together (sinyldosis, q.v.). Every lichen is made up of two elements: (I) colorless threads I filaments) ; and (2) cells (gonidia ) containing green or blue-green pig ment. These latter are blue-green (Cyanophyee:e) or green alga. (Chlorophyceie), and the colorless thready portion is a fungus. The fruit of a lichen is tbat of an ascomvcete 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 (asei) which contain the spores. There is a small group of tropical lichens whose fungus constituent is related to the mushrooms (Basidiom•cetes). Lichens generally live in ex posed 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 espe cially adapted to retain moisture by the gelat inous character of the thallus. Thene are also present thread-like outgrowths (rhizines) from the fungal part of the plant. which may draw up some water from the substratum. hut probably serve chiefly as organs of at Mehment. The algal portion of a lichen manufaetures the organic food that is ahsorbed by the fungal elements, which hear the same relation to the algal eells as does any parasitic fungus to its host. The
fungal portion makes no return at all commen surate with the benefits it receives; indeed. it is well known that the fungus may injure and even destroy the alga. The are really slaves of the fungi, and under suet) conditions the latter arc absolutely dependent upon the former. The are of course perfectly able to take care of themselves, and do actually grow wild on many of the surfaees frequented by liehens. Some of the lichens are conspieuous in the landscape. many forms in barren highlands among the moun tains, and on the rocky shores in the Arctic re );ions. The lichen called 'reindeer moss' (Cla donia ranyif•rino) supports immense herds of reindeer and cattle in the highlands of Norway, and the •leeland moss' (('ct•ariu islandica) is eaten by num. 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 behavior has been well established by a number of investigators who have produced artificial lichens by bringing the spores in contact with the proper wild algae. Groups of algal cells with a portion of the thready body (mycelium) may form scales ('sore dia') on the surface of the lichen body, mid these are capable of reproduction.
For American species. consult Tuekerman, A 8ynopsis of North American Lichens. See and Colored Plate of AlossEs AND LICIIENS with .Thuscf.