What is a Lichen?

"Lichens are fungi that have discovered agriculture"- Trevor Goward (lichenologist)

A lichen is a compound organism involving a symbiotic relationship between fungus and either a cyanobacteria or algae. Both organisms benefit from the association, the fungus providing a structure of hyphae that houses the algal cells, and the algae feeding the fungus with carbohydrates via photosynthesis. The fungus does not, however, consume all of the carbohydrates, leaving just enough for the algae to survive. The fungal component of a lichen (the mycobiont) is the sole architect, entirely responsible for the shape of the lichen, while the algal component (the photobiont) can be a mere single layer of cells. The process of the fungus and alga joining is called lichenization.

Image from Buffleskloof Eco Reserve: lichens

There are three kinds of lichen:

• Crustose lichens: found on rocks, often brightly colored

• Foliose lichens: leafy, stringy kind often found on trees

• Fruticose lichens: stalkier, often round, mostly found on trees and decaying log

The body of all kinds of lichen is called the Thallus, which can be divided at a cross section into three main layers: the cortex, algal layer and medulla.

The Thallus

Image from Buffleskloof Eco Reserve

Upper cortex: compact fungal cells

Algal layer: interlaced with hyphae


Medulla: fungal hyphae


Lower cortex: compact fungal cells
Rhizines: attach lichen to substrate



Lichen Reproduction
Lichens can reproduce as lichens, or the individual fungal cells that composes a lichen can reproduce separately solely as fungus. A lichen reproduces as a lichen by asexual means, either producing small propagules (specialized structures) or by fragmentation. For example, when certain lichens are stepped on or broken, the broken pieces may attach elsewhere and become a new, separate lichen.
Other lichens produce propagules which contain a few algal cells and some fungal hyphae, two distinct forms exist, namely: soredia and isidia.

• Soredia are small granules that form in the algal layer in patches. Each soredium is produced by cell division of the fungal and algal cells, containing both elements, when brushed off, they form new lichen. 
• Isidia are thumb-shaped growths of the upper cortex that can occur all along the surface of a lichen. They consist of all the tissues of tiny pieces of all the lichen tissues, so when they are broken off and dispersed, they grow into clones of the original lichen. 

Both soredia and isidia are produced asexually, and essentially constitute a kind of cloning of original lichen. Sexual reproduction can take place only for the fungal component of a lichen, when fungal spores and produced by meiosis. The resultant fungus however has no guarantee of becoming lichenized, a process which has proven nearly impossible to demonstrate in nature.

Sources: (1), (2)

Sources: (1), (2), (3)