Lichens

Overview

Lichens are composite organisms composed of a fungus and a photosynthetic partner.

The photosynthetic partner (photobiont) in 90% of all lichens is green algae (phylum Chlorophyta). In the remaining 10% the photobiont is cyanobacteria (phylum Cyanobacteria). Some lichens have both green algae and cyanobacteria as photobionts. Some lichens have a third partner, yeast. It has been suggested that many or even the majority of lichens have a symbiotic relationship with yeast. That would explain why it is difficult to propagate lichens in a laboratory.

Lichens were formerly placed by some taxonomists in the division Mycophytophyta. However, that division is no longer recognized. Lichens are found in many separate lineages. They do not form a single valid taxonomic group. The lowest taxonomic rank that contains all lichens is the fungi subkingdom Dikarya.

Description

Lichens are highly diverse. They are commonly grouped in field guides by morphological type (growth form) and by the surface that they grow on (substrate).

Growth form

Lichens are commonly grouped by growth form. The most common growth forms used in field guides are crusticose, foliose, and fruticose.

Crusticose lichens are crust-like. They adhere tightly to and blend directly into a substrate. They look like they were spray-painted on a substrate.

Foliose lichens appear leaf-like and are divided into lobes. The lower surface is often a different color than the upper surface.

Fruticose lichens look like a leafless shrub or bush. They may grow upright or hang down. The branches may be round or flattened. They may be few, thick, and erect, or many, thin, and draping down.

Other growth forms include scale-like (squamulose), powdery (leprose), jelly-like (gelatinous), stringy (filamentous) and wispy (byssoid).

Substrate

Lichens are also grouped by substrate. They grow on a wide variety of substrates, but in field guides these are commonly limited to ground, rocks, and trees. They also grow on roofs, walls, gravestones, and statues. Some lichens, called vagrant lichens, are not attached or become detached from a substrate and blow about in the wind. There are fewer than 100 species of vagrant lichens, but they occur all over the world, in all kinds of temperature zones.