Lichens invented the word symbiosis as living partnerships
A lichen is not a plant but a hybrid colony: algal or cyanobacterial partners woven among fungal filaments, often with bacteria in the outer skin. Roughly twenty thousand species tint rock, bark, and desert crusts from sea level to alpine summits, sometimes lasting among Earth’s oldest organisms.
The partnership that first pulled Symbiotismus into biology still shapes ecology. Fungal hyphae form most of the bulk as a thallus; photobionts—green algae or cyanobacteria—feed the mesh by photosynthesis. Growth forms include fruticose tufts, foliose leaf-like lobes, paint-tight crustose sheets, squamulose scales, and filamentous strands. Common names say "moss," yet lichens are unrelated to mosses and lack absorbing roots; when they coat trees they use bark as substrate, not as host to parasitise.
Colour often tracks the photosynthetic partner and pigments such as yellow usnic acid. Wet cortex turns translucent, flashing bright greens; dry crusts grey out. Cyanobacterial partners stay dark when dry. Identification keys use the dry colour. Endolithic species hide between rock grains, showing only fruiting discs outside. Yosemite’s painted cliffs and arctic tundra both host these composites, which also colonise rubber, bone, slag, and fresh landslide rock.
English lichen comes via Latin from Greek leichēn, linked to "to lick." Some lineages lost sexual reproduction yet still diversify. Slow, steady growth lets scientists date surfaces by lichenometry. Reindeer, snails, mites, and springtails graze them; trees and birds gain from their keystone roles. A lichen behaves less like a single species than a miniature ecosystem whose fungal, algal, and microbial cast can evolve together into still stranger composites. From toxic slag heaps to alpine cliffs, that composite body keeps rewriting where life can hold a foothold after ice, fire, or fresh rockfall.
Source: Lichen
