In the plant kingdom lichens are unique, consisting of two totally unrelated life forms one an alga and the other a fungus, living together in a complex but balanced state of symbiosis. A symbiosis is a life form where both the partners benefit from each other. In the case of the lichen, the fungus delivers water, minerals and gases from the environment to the algae, while the algae, which contains the green pigment chlorophyll, photosynthesises to supply the fungi with sugars and carbohydrates. The algal cells are either green or blue-green, while the fungal threads or hyphae are colourless.
The fungal partner forms the body of the plant within which the algal cells are arranged, in many species, in a well defined layer 10-30mï¿½ thick just below the surface, but in others scattered between the fungal threads. The fungus also plays a role in the structure and the protection of the lichen.
The algal cells, through the process of photosynthesis provide the lichen with its organic nutrition. Lichens are photoautotrophs, so they obtain their organic nutrients using energy from sunlight.
1. The Cortex
2. The Algae Layer
3. The Pith
4. Under Cortex
5. The Rhizines
The body of a lichen is termed the thallus, and its general shape enables us to group lichens into four broad categories.
Crustose lichens form a crust on the surface of the substrate on which they are growing. Crustose lichens tend to grow out from their edges and have their fruiting bodies in their centre. Crustose lichens are very difficult to remove from their substrates. An example of a crustose lichen is Pertusaria Multipuncta.
Some lichens have a portion of their thallus removed from the substrate to form squamules. They have an upper cortex but no lower cortex. Cladonia cenotea is an example of a squamulose lichen.
These have an upper and lower cortex. They are generally raised above the substrate but are connected to it by rhizines. Melanelia albertana is an example of a foliose lichen.
Fruticose lichens are shrubby lichens. They are attached to their substrate by a single point, like a stem. An example of a fruticose lichen is Usnea glabrata.
The substances lichens require for nutrition is contained in rainwater and atmospheric moisture and are absorbed over the whole surface of the plant. The carbon dioxide, water and minerals held in solution and the oxygen necessary for respiration are absorbed by the fungal tissues and are passed to the algal cells in suitably modified for. The algal cells, by photosynthesis, convert these substances into the carbohydrates necessary for the nutrition of both components. Some species have blue-green cells capable of fixing nitrogen directly from the atmosphere and presenting it in a form that can be utilised by the whole lichen.
Lichens grow at a very slow rate, generally about 1cm a year for larger foliose lichens and 1mm a year for some of the crustose species. The growth rate will vary according to species, locality and season.
Before the discovery of coal-tar dyes, lichens were of considerable economic importance for the commercial dying of wool. Lichens have also been used to date substrates of unknown age. Also lichens have also been used to date archaeological remains and even to establish the frequency of earthquakes by dating rock falls.
Lichens reproduce either vegetatively, by shredding a tiny part of the parent body, or sexually by the production of microscopic fungal spores in a fruiting structure on the surface of the thallus. In many species both forms of reproduction are encountered and it is interesting that these structures are not known in other fungi.