Lichen Terminology

 Lichen Terminology — A Quick Guide

Basic Structure & Partnership

• Lichen — a stable symbiotic partnership between a fungus and a photosynthetic organism (an alga or cyanobacterium) growing together as a single body

• Mycobiont — the fungal partner; forms the main body and structure of the lichen

• Photobiont — the photosynthetic partner; produces food by photosynthesis — usually a green alga or cyanobacterium (sometimes called a phycobiont)

• Thallus — the main body or vegetative part of the lichen — what you actually see

• Hyphae — the microscopic fungal filaments that make up the thallus and wrap around the photobiont cells

Growth Forms

• Crustose — forms a crust firmly attached to the substrate (rock, bark) — cannot be removed without damaging it

• Foliose — leafy, with distinct upper and lower surfaces, loosely attached by root-like threads

• Fruticose — shrubby or bushy, upright or hanging, round in cross-section

• Squamulose — made up of small, scale-like plates attached at one edge

Reproduction & Structures

Vegetative (asexual) — both partners together

• Soredia — tiny powdery clusters of fungal threads + photobiont cells, produced in powdery patches called soralia; dusted away by wind/rain

• Isidia — tiny finger-like or coral-like outgrowths from the surface; break off and grow into new lichens

Sexual — fungus only

• Apothecia — cup- or disc-shaped fruiting bodies that release fungal spores; the most common sexual structure

• Perithecia — flask-shaped structures embedded in the thallus, opening through a tiny pore at the top

• Pycnidia — tiny, often sunken cavities that produce conidia (asexual fungal spores)

• Spore — the fungal reproductive cell; contains only the mycobiont — must find a suitable photobiont to form a new lichen

Habitat & Chemistry

• Substrate — the surface the lichen grows on: rock (saxicolous), bark (corticolous), soil (terricolous), moss (bryophilous)

• Cortex — the dense outer "skin" layer of fungal cells protecting the photobiont layer beneath

• Medulla — the loose, cottony inner layer of the thallus

• Hypothallus / Prothallus — the sometimes visible edge or "border" where the lichen meets the substrate

• Spot tests — simple chemical tests using drops of potassium hydroxide (K) or bleach (C) to help identify species by colour changes in the thallus.

Life Cycle, Symbionts and Ecological Role

Lichens represent a remarkable symbiotic partnership between a fungus (the mycobiont) and a photosynthetic alga or cyanobacterium (the photobiont), each partner providing what the other cannot: the fungus creates the protective body structure and absorbs moisture and minerals, while the photobiont produces sugars through photosynthesis. They reproduce in two ways — vegetatively, by spreading tiny powdery soredia or finger-like isidia that contain both partners, or sexually, when the fungus alone produces spores that must germinate and find a suitable photobiont to establish a new lichen. Ecologically, lichens are vital pioneer organisms that break down bare rock to create soil, stabilise surfaces, absorb pollutants, and provide food and shelter for a wide range of invertebrates as well as nesting material for birds. They are exceptionally hardy and can survive in some of the harshest environments on Earth — from arctic tundra and high mountain summits to deserts and rainforests — though some are highly sensitive to air quality, making them important natural indicators of environmental health.