lichen

The word 'lichen' entered English in the early seventeenth century from Latin, which had borrowed it directly from Greek 'leichḗn' (λειχήν).‍​‌​‍​‌​‍​‍​‌​‌​‌​‌​‍​‌​‍​‍​‌​‌​‌​‍​‍​‌​‍​‍​‌​‌​‍​‌​‌​‍​‌​‌​‌​‌ In Greek, the word had two related meanings: it referred to tree-moss (what we now call lichen) and to certain skin diseases characterized by spreading, scaly eruptions. Both meanings derive from the Greek verb 'leíchein' (λείχειν, to lick), from PIE *leyǵh- (to lick). The metaphor is vivid: lichen appears to 'lick' the surface of a rock or tree bark, spreading across it in a thin, intimate layer. The skin disease sense — a rash that 'licks' across the body — uses the same image.

The PIE root *leyǵh- (to lick) produced a wide family of words. In Germanic: English 'lick,' German 'lecken,' Dutch 'likken,' Old Norse 'sleikja.' In Latin: 'lingere' (to lick), which gave English 'electuary' (a medicinal paste to be licked up). In Greek: 'leíchein.' The English word 'lecherous' may also be distantly related, through Old French 'lechier' (to lick, to live in debauchery), from a Frankish cognate of 'lick.' If so, 'lichen' and 'lecherous' are etymological cousins — both descendants of the act of licking.

The nature of lichen as a composite organism was not understood until the nineteenth century. In 1867, the Swiss botanist Simon Schwendener proposed that lichens were not single organisms but symbiotic associations between fungi and algae. The idea was revolutionary and initially controversial — many botanists rejected the notion that two organisms could so thoroughly merge their identities. The lichenologist and clergyman James Crombie called the theory 'unnatural' and compared the fungus to a parasitic enslaver of the alga. The discovery that lichens were dual organisms (now known to sometimes include three or more partners, including cyanobacteria and yeasts) made them a foundational example in the study of symbiosis.

Lichens are extraordinarily hardy organisms. They colonize some of the most inhospitable environments on Earth: bare rock faces, desert surfaces, Antarctic ice-free zones, and high-altitude alpine regions. Some lichen species are estimated to be thousands of years old — the Arctic species Rhizocarpon geographicum grows so slowly and predictably (about 0.5 millimeters per year) that its growth rings can be used for lichenometric dating of rock surfaces, a technique employed by geologists to date glacial moraines, landslides, and other geological events.

Lichens are extremely sensitive to air pollution, particularly sulfur dioxide. The presence or absence of certain lichen species is used as a bioindicator of air quality — a technique called lichenometric monitoring. During the Industrial Revolution, lichens disappeared from the most polluted areas of Britain and Europe. Their gradual return to cities in the late twentieth century signaled improving air quality after the Clean Air Acts.

The pronunciation of 'lichen' varies: the traditional British pronunciation is /ˈlaɪkən/ (rhyming with 'liken'), while some speakers, particularly in North America, say /ˈlɪtʃən/ (rhyming with 'kitchen'). The /ˈlaɪkən/ pronunciation preserves the Greek original more faithfully, since the 'ch' represents Greek chi (χ), a voiceless velar fricative, not an English 'ch' sound.

Lichens have practical uses as well. 'Litmus' — the pH indicator — is derived from lichen dyes (the word 'litmus' comes from Old Norse 'litmosi,' meaning 'dye-moss'). The purple dye orchil (orcein) was extracted from lichens for centuries. In Scandinavian and northern Asian cultures, reindeer lichen (Cladonia rangiferina) is a crucial food source for reindeer and caribou, and by extension for the human communities that depend on these animals.