Lichens are now considered to belong to the Mycota (fungi) kingdom and all British species are related to a group of fungi called the Ascomycetes. True Ascomycete fungi include the Morels, False Morels, True Truffels, Disc Fungi and Flask Fungi such as Candle Snuff and Dead Man's Fingers. However they are not usually studied my Mycologists (fungi specialists). Lichen specialists are a different 'species' and are known as Lichenologists. Very few people study both - I am a rare exception to this.
Lichens are visible as all times of the year and most are fairly constant in appearance but they can shrivel somewhat or change colour in dry periods. They are mostly studied in the winter months, as there is not much other botanical interest at this time.
Within lichen there are two types of organisms living symbiotically. In other words they live together to their mutual benefit. These organisms are a mycobiont and a photobiont. The mycobiont is the dominant fungal partner and it is an Ascomycete fungus (see above). It provides protection to the other partner from extremes of moisture, temperature and light levels and also provides minerals from the substrate. The photobiont partner is either a green alga, a cyanobacteria or, in rare cases, both together. This partner provides sugars to feed the fungal partner by means of the same photosynthetic process familiar in higher plants. Also without the presence of the photobiont the lichen would be just a shapeless mass.
The fungal partner (mycobiont) makes up about 90 to 95% of the mass of the lichen and each lichen species contains a different fungal species. This is the reason why different, but closely related lichens, occur on different types of tree bark or rock. However the algal partner or cyanobacteria (photobiont) can occur in several different species of lichen. Each genus of lichens uses a particular genus of alga or cyanobacteria as a photobiont. The microscopic structure of lichens containing algae tends to be something like spaghetti bolognaise or even lasagne - with either algal cells forming discreet thin layers or embedded as clusters of cells embedded amongst the spaghetti like fungal strands (hyphae). If the lichen contains cyanobacteria then these cells are distributed evenly throughout the body of the lichen in a gelatinous matrix.
In British lichens the photobiont is usually either of two types of green algae known as Trebouxia or Trentepohlia. However about 20% of British species contain a cyanobacteria, usually something called Nostoc. It is interesting to note that Trentepohlia can live independently when it consists of very many of tiny fluffy orange balls often turning the trunks of trees, such as ash, a bright orange colour. Similarly Nostoc can occur independently as dark green irregular jelly like masses about an inch or so across on bare, usually chalky, ground in wet weather. In contrast the fungal partner of lichens cannot live independently.
Lichens reproduce sexually via spore production or in an asexual vegetative way. Lichen produce spores over a long period of time via fruit bodies that are either stalked potedia or disc-like apothecia. However some species rarely produce these bodies and rely on asexual reproduction. Vegetative reproduction usually depends on the production of structures consisting of a matrix of algal cells and fungal hyphae - these being known as either soralia or isidia.