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What are Lichens?

A lichen is a symbiosis of different organisms such as fungus and a cyanobacterium or algae. Cyanobacteria are also known as blue-green algae besides being distinct from algae. The non-fungal part is called a photobiont that consists of Chlorophyll. Many lichen partners including one photobiont and one mycobiont are not universal and lichens with more than one photobiont partner are also present in certain cases. The fungal partner is composed of filamentous cells, where every filament is called a hypha. The most common fungi in lichens are species of Ascomycetes or Basidiomycetes. The green algae Chlorophyta or Cyanophyceae family of blue-green bacteria are the most common algal partners. The fungal partners cannot survive without their phycobiont, whereas algae can survive alone in water or moist soil. The color of lichens ranges from yellow to greens and black hues.


Most of the lichens grow at a slower rate. The phycobiont is a blue-green bacterium that converts nitrogen gas into ammonia. The lichens that live in stressful environments such as the arctic tundra or alpine are said to reach the age of many centuries.


Lichens are Classified Into Three Types

  • Crustose.

  • Foliose.

  • Fruticose.


Crustose

Crustose is a lichen that sticks tight to the substance on which it is growing on. As the name suggests, it is crusty and even forms a crust on the substrate it is growing (such as rocks, trees, or dirt). This crust is difficult to remove.


Fruticose

Fruticose looks like a little branching shrub such as a miniature leafless tree. It looks similar to a bushy coral. It grows on rocks, trees, and soils. The word fruticose has a Latin origin which means a bush or shrub


Foliose

Foliose consists of a leafy shape structure. It can be easily removed from the surface on which it grows. Foliose gets its name as they look similar to foliage which is a plant leaf.


Classification Based on Their Habitat:

  1. Lignicolous is a term used to describe a person who has these lichens can be found in the woods.

  2. Corticolous lichens are those that live on the bark of trees.

  3. Saxicolous lichens are those that live on stones or rocks.

  4. The sea. These lichens can be found growing on siliceous rocks near the sea's edge.

  5. Water that is not salty. These lichens are found growing on hard siliceous rocks, particularly near freshwater.

  6. Terricolous- These lichens grow on the soil and are thus referred to as terrestrial lichens.


Classification Based on Their Internal Structure:

  1. Heteromerous lichens

  2. Homoiomerous lichens.


Classification Based on their Fungal Partner:

  1. Ascolichens

  2. Basidiolichens

  3. Hymenolichens


Lichen Algae

The lichen fungus is classified as a fungus, and the fungal partners are classified as Ascomycota and Basidiomycota. Lichens can also be classified according to their morphology. There are three major types of lichens, though there are others. Crustose lichens are lichens that are tightly attached to the substrate, giving them a crusty appearance. Foliose lichens have leaf-like lobes, can only be attached at one point in the growth form, and have a second cortex below the medulla. Finally, fruticose lichens are known to have rounded structures and a branched appearance in general. Figure 2 depicts an example of each type of lichen.


General Characteristics of Lichens

Let’s discuss lichen characteristics:

  • In general, the major portion of the thallus is occupied by the fungal component and this fungal component produces its own reproductive structures.

  • The algal partner is basically responsible for making the food by the process of photosynthesis. The food diffuses out as well as it is absorbed by the fungal partner.

  • Owing to their symbiotic relationship, lichens can live in a variety of habitats as well as climatic conditions including extreme environments.

  • Lichens can be classified into the following types based on their growing medium.

  • Follicolous (grows on tree barks), Corticolous (grows on tree barks) (grows on leaves surfaces),

  • Saxicolous is a type of saxicolous plant (grows on rock surfaces)

  • Terribly impressive (grows on soil)

  • Musically inclined (grows on mosses)

  • Lichen growth forms are commonly seen on surfaces, forming greyish, greenish, or orange areas. They are divided into three major types based on their morphology and size, namely,

  • Crustose is a sugar (crust like)

  • Foliose is an abbreviation for Foliose (leaf like)

  • Fructose (shrubby)

  • Crustose lichens are known as microlichens whereas foliose and fruticose lichens are known as macrolichens.

  • The thallus is the main plant body of the lichen. The thallus is the vegetative portion, which is similar to that of mosses and liverworts.

  • Mycobionts (Ascomycete or Basidiomycete) form a close symbiotic relationship with phycobionts (green algae or blue-green algae). After association, both phycobionts and mycobionts lose their distinct identities and are referred to as lichens. Lichens now function as a single organism, both morphologically and physiologically.

  • Lichens reproduce in a way that is completely different from that of fungi and algae. Lichens reproduce vegetatively by the formation of special propagules known as diaspores. Soredia and isidia are the most common lichen diaspores.

  • The lichens' fungal partner reproduces sexually. In lichens, sexual reproduction begins with the formation of fruiting bodies, which is followed by the formation of spores known as ascospores. Ascospores are dormant spores that can survive in harsh environments.

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