Lichens, Definition, Characteristics, Classification, and Importance

Lichens  are unique organisms formed by a symbiotic relationship between a fungus and an alga or a cyanobacterium. These two distinct organisms coexist in a mutually beneficial association, with the fungus providing protection and structure, while the algae or cyanobacteria contribute by producing food through photosynthesis. This topic is important for the  NEET Exam and is included in the NEET biology syllabus , as it highlights key concepts in symbiosis, ecology, and organism classification.

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What are Lichens?

Classification of Lichen

Classification Based on Habitat

• Saxicolous Lichens : These lichens grow on stones or rocks, such as Dermatocarpon, Xanthoria, and Verrucaria.
• Corticolous Lichens : These lichens thrive on the bark of trees, examples include Graphis, Usnea, and Parmelia.
• Terricolous Lichens : Found growing on soil (terrestrial), such as Cladonia florekeana, Lecidea, and Collema.
• Lignicolous Lichens : These lichens grow directly on wood, like Calicicum.
• Marine Lichens: These grow on rocky shores of the sea, such as Caloplaca and Verrucaria.
• Freshwater Lichens : Found on siliceous rocks in freshwater, such as Ephebe and Hymenelia.

Classification Based on Fungal Partner

• Ascolichens : The fungal partner belongs to Ascomycetes.
• Basidiolichens: The fungal partner belongs to Basidiomycetes. This group includes three genera: Cora, Corella, and Dictyonema.
• Hymenolichens : These lichens have a fungal partner from Deuteromycetes and are known as imperfect lichens.

Classification Based on Growth Forms and Attachment

• Crustose Lichens (Encrusting Lichens): These lichens form a thin or thick crust over rocks, soil, or tree bark and are difficult to separate from the substrate. Only the fruiting bodies may be visible, e.g., Lecanora, Graphis, and Rhizocarpon.
• Foliose Lichens (Leafy Lichens): These lichens have lobed, leaf-like structures attached to the substrate by rhizine-like outgrowths, e.g., Xanthoria, Parmelia, Physcia.
• Fruticose Lichens (Shrubby Lichens) : These lichens have upright or hanging forms, attached at the base by a flat disc. They resemble small shrubs, e.g., Cladonia, Usnea, and Alectoria.

Classification Based on Internal Structure

• Heteromerous Lichens: These lichens have a distinct layering of fungal and algal cells.
• Homoiomerous Lichens : These lichens have a uniform distribution of fungal and algal cells throughout the thallus.

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Characteristics of Lichens

• Lichens consist of a fungus and a green alga or cyanobacterium living together.
• They can grow on various surfaces, including rocks and as epiphytes on plants.
• Symbiotic Relationship: The fungus benefits from photosynthates, while the alga can survive in drier conditions.
• Lichens grow slowly and can live for centuries.
• They are used in food, for dye extraction, and as environmental indicators.
• Lichens have a thallus, an outer cortex, an inner medulla, and rhizines for attachment.

Importance of Lichens

• Certain lichen species help convert rocks into soil, enhancing soil quality by enriching essential nutrients for plant growth.
• Lichens contribute to the nitrogen cycle by fixing atmospheric nitrogen, which benefits surrounding ecosystems.
• Lichens serve as a food source for humans, with species like Iceland moss being important for Northern Europe and the Americas.
• Lichens are valuable sources of natural dyes and coloring agents, such as pH indicators extracted from them.
• They aid in the biodegradation of various pollutants, including polyester, heavy metals like lead and copper, and radionuclides.
• Lichens are utilized in the pharmaceutical industry and in the production of cosmetics.
• Certain lichen species help in degrading pathogens that can cause infectious diseases in humans and plants.
• Lichens act as a keystone species in many ecosystems, providing food and habitat for various animals, including deer, birds, and rodents.
• Lichens protect trees and rocks from harsh environmental elements, including rain, wind, and snow.

MCQs of Lichens

Q1. Lichen is an association between;

1. fungi and bryophyte
2. fungi and algae
3. algae and pteridophyte
4. algae and bacteria

Q2. Lichens are very good pollution indicators because they are sensitive to;

1. Sulfur trioxide
2. Sulfur dioxide
3. Carbon dioxide
4. Global warming

Q3.. Lichens are a well-known combination of an alga and a fungus where the fungus has;

1. a saprophytic relationship with the alga.
2. an epiphytic relationship with the alga.
3. a parasitic relationship with the alga.
4. a symbiotic relationship with the alga.

Answers of MCQs of Lichens