A lichen is a unique biological entity that appears to be a single organism but is actually a partnership between a fungus and a photosynthetic partner, typically green algae or cyanobacteria. This dual nature creates a structure, known as a thallus, that possesses properties distinct from either partner growing alone, allowing lichens to colonize environments ranging from deserts to the Arctic. Their varied forms make them a visually fascinating group, with their appearance determined primarily by the fungal component, which constructs the visible body.
Defining the Major Growth Forms
The most immediate way to identify a lichen is by its physical growth structure, or morphology, which is determined by the fungal partner. Lichenologists classify these forms into three main types based on how the thallus is attached to its surface. The crustose form is extremely thin, like a layer of paint or a hard crust firmly adhered to the substrate. It is impossible to remove without destroying the surface underneath. Approximately 75% of all lichens exhibit this morphology, often appearing as a thin stain on rocks or bark.
Foliose lichens appear like flattened, leaf-like lobes that are loosely attached to the substrate. These lobes generally have a distinct upper and lower surface, with the underside often differing in color or texture from the top surface. They are attached by root-like structures called rhizines, which anchor them but allow the edges of the thallus to lift away from the surface.
Fruticose lichens exhibit a three-dimensional, shrubby, or hair-like structure, growing either upright in tufts or hanging down in strands. These forms attach only at a single point at their base, and their branches are typically round in cross-section, lacking a distinct upper and lower side.
Two secondary forms also contribute to the visual diversity of lichens. Squamulose lichens are comprised of small, overlapping scales or leaf-like lobes, which are crustose at the base but have their tips slightly raised and free from the substrate. The leprose form has the simplest structure, appearing as a powdery or dust-like mass of fungal filaments and algal cells. This form lacks any protective outer layer or discernible organization.
Understanding Lichen Coloration
The diverse colors of lichens—which span from bright yellows and oranges to greens, grays, and blacks—are largely a result of pigments produced by the fungal partner. These pigments are often secondary metabolites, commonly called lichen acids, which accumulate on the outer layer of the thallus, known as the cortex. These compounds serve protective functions, acting as sunscreens by absorbing ultraviolet radiation that could damage the internal algae.
The precise hue is also influenced by the photobiont, the photosynthetic organism trapped within the fungal network. When the photobiont is green algae, the lichen often displays shades of gray-green, especially when wet, as moisture allows the algae’s chlorophyll to be more visible through the fungal layer. Lichens containing cyanobacteria tend to appear darker, often presenting as dark gray, brown, or black when dry.
Environmental conditions significantly affect the intensity and shade of a lichen’s color. When dry, a lichen shrivels and its outer surface becomes opaque, causing the colors to appear faded or muted. This state of desiccation protects the internal photobiont. When re-moistened by rain or fog, the thallus expands, and the cortex becomes more transparent, allowing the internal pigments and the photobiont to shine through clearly. The production of certain pigments, such as carotenoids, can also depend on the intensity of sunlight exposure, further altering the observable color.
Substrate and Habitat Appearance
The appearance of a lichen is inherently tied to the material it grows on, referred to as its substrate. Lichens are categorized based on this preference. Corticolous lichens grow on the bark of trees, and lignicolous lichens colonize wood stripped of its bark. The visual texture of a lichen growing on smooth bark will differ greatly from one growing on rough, fissured bark, often appearing denser and more continuous on the latter.
Saxicolous lichens grow on rocks, where their crustose forms can appear fused directly into the stone surface. The nature of the rock, such as its porosity and mineral content, can influence the growth pattern and overall color saturation. Lichens are also found on soil, where they are termed terricolous, often forming extensive, low-lying mats that help stabilize the ground surface.
The general environment also affects the visual density of the lichen population. In areas with high humidity or frequent fog, fruticose forms may hang down in long, dense strands. In exposed, dry environments, the crustose forms that adhere tightly to the substrate tend to dominate. The substrate provides a stable base and a source of nutrients, which influences the distribution and visible abundance of different lichen types.
Distinguishing Lichens from Similar Organisms
Lichens are often confused with mosses and simple algae, but distinct visual characteristics help separate them. Mosses are non-vascular plants with true leaves and stems that are clearly visible, often giving them a soft, spongy texture when wet. Unlike lichens, which can be brittle and hard when dry, mosses retain a more plant-like, pliable structure.
Algae, when found on surfaces like bark or stone, typically appear as a thin, purely green or slimy film without any defined structure or raised body. They rarely exhibit the clear, structured growth forms characteristic of the fungal partnership, such as the leaf-like lobes of a foliose lichen or the three-dimensional branching of a fruticose lichen. Lichens, even the simplest crustose forms, display a more defined, often crusty or leathery texture that sets them apart from the simple films of algae.