Lichens are unique organisms, presenting diverse forms, colors, and textures. They are not individual plants, but a complex partnership that allows them to thrive globally. Their appearance is shaped by their biological structure.
The Symbiotic Nature of Lichens
A lichen is a composite organism formed from a symbiotic association between a fungus and a photosynthetic partner, typically a green alga or a cyanobacterium. The fungal component (mycobiont) forms the bulk of the lichen’s structure, providing protection and absorbing water and minerals from the environment. The photosynthetic partner (photobiont) produces carbohydrates through photosynthesis, supplying food. This mutually beneficial relationship allows lichens to colonize diverse environments where neither partner could survive alone. The lichen’s visible form and characteristics are determined by the dominant fungal partner, shaping its body (thallus).
Defining Growth Forms
Lichens are categorized into three main growth forms: crustose, foliose, and fruticose.
Crustose lichens appear as a flattened crust tightly adhering to their substrate, much like a coat of paint, and are often difficult to remove without damaging the surface. They can look like flaky, dry layers on rocks or old walls.
Foliose lichens have a leaf-like or lobed appearance, often with ruffled edges. They are typically attached to the substrate at multiple points or by root-like structures called rhizines, and can be peeled off. They possess distinct upper and lower surfaces, which can differ in color.
Fruticose lichens have shrubby, hair-like, or branched structures that grow upright or hang downwards. Unlike foliose forms, they are usually attached to their substrate at only a single or few points, allowing most of their body to be free. Their branches can be round or flattened, resembling miniature shrubs or coral.
Colors and Textures
Lichens exhibit a wide spectrum of colors, including various shades of green, gray, yellow, orange, black, and red. Colors are influenced by the photobiont type, fungal pigments, and environmental conditions like moisture and light. For example, green algae lichens often appear bright green when wet, while cyanobacteria lichens might look black and jelly-like. Pigments like usnic acid can give lichens a pale yellowish-green hue.
Lichens display a range of textures, from smooth and powdery to granular, bumpy, or wrinkled. Some lichens have specialized structures that contribute to texture and aid reproduction. Soredia are powdery bundles of fungal hyphae and algal cells, not covered by an outer layer, and easily dispersed by wind. Isidia are tiny, finger-like or wart-like outgrowths on the lichen surface, covered by a cortex, and are typically larger and more complex than soredia.
Typical Habitats
Lichens are highly adaptable organisms found in diverse environments, from sea level to alpine elevations and from deserts to tundras. They colonize almost any stable surface, known as a substrate. Common natural substrates include rocks, tree bark, and soil, where they aid nutrient cycling and soil stabilization.
Lichens also frequently grow on man-made structures such as gravestones, walls, roofs, and old farm equipment. A lichen’s appearance can vary depending on its habitat; some thrive in shaded, moist environments, while others with brighter pigments adapt to exposed, sunnier locations. The presence of lichens indicates clean air quality, as many species are sensitive to pollution.