Lichens are the complex organisms that show a mutualistic relationship between fungi and algae by possessing characteristics of both. These show polymorphism, as it exists in diversified forms (sometimes plant-like) with different colours, sizes, texture etc. Lichens are not plants, but sometimes these appear as tiny leafless branches or as flat leaf-like structures etc. In addition to this, they also form organic food by their own through photosynthesis, as seen in plants.
There are two kinds of lichens, namely macro and micro-lichens depending upon their various forms. Unlike plants, lichens lack a root system for the water and nutrient absorption. It constitutes about 6% of Earth’s land surface. It is estimated that the lichen species accounts nearly 17,000 to 20,000, which are known so far.
Meaning of Lichen
Lichens can define as the bizarre organisms that show mutualism in fungi (mycobionts) and algae (phycobionts). Ascomycetes and Basidiomycetes fungi, green algae or cyanobacteria (blue-green algae) associates with lichen. A term lichen was introduced by a scientist named Theophrastus.
Lichens are ubiquitous, found in a variety of habitats. The examination of macroscopic and microscopic characters (such as reproductive structures, spores and cellular features) of lichen can be studied via chemical tests and chromatographic methods.
Habitat: Lichen generally lives in humid and exposed conditions, and are also capable to thrive in extreme desiccation. These cannot remain alive in extreme air pollution, because of sulphur dioxide presence. The growth pattern of lichen is slow but perennial (some may remain alive for thousands of year).
Substrate: Lichens develop on different substrates like:
- Bark of tree
- Leaves of higher plants
- Living as epiphytes in temperate woodland.
- Solid surfaces like rock outcrops, walls, roofs etc.
- In the soil as part of a biological soil crust.
The anatomy of lichen reveals that the internal morphology of lichens is almost same in all the species.
The outer surface of lichen contains filaments of fungi that are rigorously packed together to form the cortex, which protects the algal cells from heat.
Below the dense layer of fungal filaments, a layer of algal cells is found. The algal cells do not pack firmly or not so dense to permit the gaseous exchange between the cells and the environment. It also participates in the photosynthetic activity, which produces organic food for the lichen.
The medulla is found below the mass of algal cells. The medulla of crustose and squamulose lichens directly affixes to the suitable substratum. In contrast, the medulla in foliose lichens is followed by a second cortical layer.
There are four distinct kinds of lichen.
Crustose: It appears as a thin crust firmly appressed over the substrate, and markedly two dimensional. It includes Graphis, Caloplaca, Rhizocarpon, Haematoma etc.
Foliose: In this type, the lichen appears as flat leaf-like, branched. Unlike crustose, the lichen body does not attach firmly to the substrate. It includes Parmelia, Peltigera etc.
Squamulose: It appears as numerous small rounded lobes that are tightly clustered and slightly flattened pebble-like units. A squamulose comprises some scaly patterns called squamules. It resembles foliose lichen when its body rises from the substrate and appears leaf-like, but devoid of the cortex.
Squamulose lichens resemble with crustose lichens, as its body is tightly clustered. Therefore, it forms a connecting link between crustose and foliose lichens. It includes Vahliella leucophaea, Cladonia subcervicornis etc.
Fruticose: It appears as erect branching tubes attached to the substratum via disc, and markedly three-dimensional. Fungi (mycobionts) dominantly constitutes the lichen body, whereas algae (phycobionts) constitutes only 5%. Its structure generally includes the surface, medulla and rhizines. It includes Usnea, Evernia etc.
Reproduction of Lichens
Majority of lichens reproduce through vegetative reproduction, but few species reproduce sexually.
Lichens multiply vegetatively by the following ways:
Fragmentation: It is a vegetative method, in which the lichen body can separate into fragments via mechanical injury, trampling etc. and each fragment can give rise to the new vegetative structure.
Reproduction by specialized structures
- Isidia are the columnar protrusions arising from the thallus that enhance the photosynthetic activity and carry both fungal hyphae and algal cells. During unfavourable conditions, it breaks off that can give rise to new lichen.
- Soredia are microscopic or powdery reproductive structures. It produces in the large number and comprises a hyphal network around the green alga or cyanobacterial cell. Soredia are the propagules found inside the sori or pustules that can form new vegetative body when it falls suitable substratum via wind transmission.
Some lichens may also reproduce sexually by means of different spores or fruiting bodies as in fungi.
As we know, the majority of the members belonging to the family of Ascomycetes (Ascolichens) and Basidiomycetes (Basidiolichens) associate with lichen. Apothecia or perithecia are the reproductive structures that develop fruiting bodies or ascospores inside a sac called “Asci”.
- Apothecia appears as small cup-like, open structure that comprises ascospores in the terminal layer or hymenium region, above the thallus. It is produced by the members beloging to the Discomycetes group. Its structure generally possesses an upper (hymenium), middle hypothecium and a terminal (excipulum) regions.
- Perithecia appear as a partially closed, globose and funnel-like structure that contains asci embedded inside the thallus. It is produced by the members beloging to the Pyrenomycetes group. It is opened by a small pore (ostiole). In perithecium, asci develop from the base and face the ostiole.
- Cleistothecia appears completely enclosed, inside which the asci (club-shaped to spherical) are dispersed. It is produced by the members beloging to the Plectomycetes group. At maturity, the cliestothecial wall lyses, from which the ascospores escape.
Basidiolichens comprise a fruiting body (Basidia) that form reproductive structures known as basidiospores. The basidia appears palisade-like. A basidium carries four basidiospores at the tips of sterigmata that detaches during the unfortunate conditions.
Therefore, the spores dispersed in the environment falls on suitable alga, and on the favourable condition, it forms a germ tube. The germination tube grows in length and fuses with the wall of alga, which later grows into a vegetative body.
Economic Importance of Lichens
Lichens have many uses.
Role in Environmental Science: Lichens are the macroscopic creatures, sensitive to the air pollutants like SO2. Therefore it is used as a pollution indicator that estimates the concentration of contaminants in the atmosphere.
Role in Medical Science: It also plays a significant role in medicinal preparations. Lichen releases certain secondary metabolites that are resistant in counter to bacterial, viral and parasitic infections. Hence, few species of lichen are used in anti-viral and anti-bacterial medications.
Importance in Tanning and Dyeing: Traditionally, the secondary metabolites released by the lichen were used to dye woollen cloth, before the discovery of synthetic dyes. It is produced commercially in litmus paper preparation to check the acidity or basicity of the solutions.
Role in the Cosmetic Industry: Lichens like Evernia, Ramalina etc. produces certain aromatic substances that are used in the production of essential oil, cosmetic products, perfumes etc.