Lichens

Lichens, a large and varied group of plants, mostly dry, dead-looking, slow-growing, but long-lived, that used formerly to be classed as a separate division of the Thallophyta, co-ordinate with Algae and Fungi. The view originally put forth by Schwendener is, however, now generally accepted, according to which lichens are regarded as Algae living in symbiosis (q.v.) with Fungi. The alga is one of the lower, unicellular forms, such as Protococcus or Nostoc, forming a layer of green cells, or gonidia, enclosed by the hyphae of an ascomycetous fungus, belonging either to the Discomycetes or to the Pyrenomycetes, or very rarely by those of a basidiomycetous one. The spores of the lichenfungus are generally produced in apothecia or in pyrenocarps and, on germinating, produce hypha; which enclose gonidial cells or soredia, the two growing into a new thallus. The apothecia and pyrenocarps seem to be fructifications resulting from a sexual act. A branch hypha (procarp) has its basal portion twisted like a corkscrew (the archica.rp or ascogoniunt), whilst its upper portion is a row of cells (trichogyne) reaching to the surface of the thallus. In special cavities in the thallus (spermogonia) numerous male bodies or spermatia are produced on hyphal bases known as sterigmata. The spermatia are conveyed by water to the trichogyne, with which they conjugate, and, as a result, asci grow out from the ascogonium.

Lichens grow on exposed situations on rocks, walls, or trees, in all parts of the globe, extending farther up mountains and towards the poles than other plants, and living far longer than most fungi or algae. Some are nutritious, such as Iceland Moss (q.v.), tripe-de-roche (Umbilicaria) and Reindeer Moss (Cladonia rangiferina); and others afford important dyes, such as orchil and litmus (q.v.), from species of Roccella, and cudbear, from Lecanora tartarea.