NCERT grounding
NCERT places lichens in Section 2.6 — "Viruses, Viroids, Prions and Lichens" — at the close of the Biological Classification chapter, deliberately outside the five kingdoms. The text defines them in a single, dense sentence: "Lichens are symbiotic associations i.e. mutually useful associations, between algae and fungi." The algal partner is the phycobiont (autotrophic) and the fungal partner the mycobiont (heterotrophic). NIOS echoes this, describing a lichen as "a combination of a certain fungus and a green or blue green alga which live in a symbiotic association."
Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner. So close is their association that if one saw a lichen in nature one would never imagine that they had two different organisms within them. Lichens are very good pollution indicators — they do not grow in polluted areas.— NCERT Class 11 Biology, Section 2.6
What a lichen is
A lichen is not an organism in the ordinary sense; it is a partnership. Two genetically distinct life forms — an alga and a fungus — grow so intimately together that they produce a single, stable body called a thallus with a shape, colour and texture found in neither partner alone. The association is so seamless that NCERT remarks one would never guess two organisms were inside. This composite nature is exactly why lichens defy the five-kingdom scheme and are introduced as a special case.
The two partners and their roles
The division of labour is precise and is the single most examined idea in this subtopic. The phycobiont is the algal component — typically a green alga, sometimes a cyanobacterium (blue-green alga). Being autotrophic, it carries chlorophyll and performs photosynthesis, manufacturing carbohydrates that feed the whole association. The mycobiont is the fungal component, usually an ascomycete. Being heterotrophic, it cannot make its own food; instead it weaves a protective web of hyphae that provides shelter, absorbs water and mineral nutrients from the substratum, and anchors the lichen to rock or bark.
Figure 1. A vertical section through a lichen thallus. The fungus forms the protective outer cortex and a water-holding medulla, while a thin band of algal cells just below the surface sits in light to photosynthesise. Food flows from alga to fungus; water, minerals and shelter flow the other way.
Mutualism — both partners benefit
The relationship is mutualism: a form of symbiosis in which both organisms gain. The alga, on its own exposed to dry air and intense sunlight, would dry out and die; inside the fungal mesh it stays moist, shaded and supplied with minerals. The fungus, unable to photosynthesise, would starve; the alga keeps it fed with sugars. Neither partner ordinarily lives as successfully alone in the lichen's harsh habitats. This is why lichens flourish on bare rock, tree bark, walls and tundra where almost nothing else survives.
Phycobiont (alga)
Autotroph
green alga or cyanobacterium
- Contains chlorophyll
- Performs photosynthesis
- Prepares food for the fungus
- Receives shelter, water and minerals
Mycobiont (fungus)
Heterotroph
usually an ascomycete
- Lacks chlorophyll
- Cannot make its own food
- Provides shelter and anchorage
- Absorbs water and mineral nutrients
Growth forms — crustose, foliose, fruticose
Lichens are classified by the shape and attachment of the thallus into three growth forms. The terms describe external morphology only; the internal alga-plus-fungus organisation is the same in all three.
Mnemonic: Crustose lies flat like a crust, foliose is leaf-like (Latin folium = leaf), and fruticose is shrubby (Latin frutex = shrub).
Crustose
A thin crust firmly pressed flat onto rock or bark; cannot be peeled off without taking the substratum with it.
Foliose
Leaf-like and lobed, loosely attached so the flat lobes can be lifted at the edges.
Fruticose
Shrubby or branched, standing erect or hanging in pendulous tufts; attached only at the base.
Figure 2. The three growth forms differ only in how the thallus is shaped and attached: crustose hugs the surface, foliose forms loose leafy lobes, and fruticose stands up as a branched shrub or hangs in tufts.
Reproduction
Because a lichen contains two partners, it must disperse both at once. The commonest route is vegetative: the thallus simply fragments, and any broken piece carrying both alga and fungus can re-establish a new lichen. Many lichens also produce specialised dispersal units — small powdery granules, each a few algal cells wrapped in fungal hyphae — that break away and start fresh colonies. The fungus can additionally reproduce sexually by its own spores, but a spore germinating alone must capture a compatible alga from the environment to re-form the lichen, so the joint, vegetative routes dominate.
Ecological roles
Lichens punch far above their size ecologically, and NEET draws its facts straight from these roles. As pioneer species they are the first colonisers of bare rock in primary succession: their secreted acids slowly weather the rock, and their dead bodies add organic matter, building the first thin film of soil on which mosses and then higher plants can grow. This same activity makes them important agents of soil formation on otherwise lifeless surfaces.
Their second headline role is as pollution indicators. Lichens absorb substances directly from the air across their whole surface and have no protective cuticle or means to excrete toxins, which makes them extremely sensitive to sulphur dioxide (SO₂). They simply do not grow in polluted areas, so a city centre choked with industrial smoke is typically lichen-free, while clean rural air supports rich lichen cover. Ecologists therefore read the presence or absence of lichens as a living gauge of air quality.
The pollutant lichens fear most
Lichens are very sensitive to sulphur dioxide and do not grow in polluted cities — their absence flags poor air quality, their presence flags clean air.
Why lichens close the classification chapter
Whittaker's five kingdoms classify organisms one species at a time, by cell structure, body organisation, nutrition, reproduction and phylogeny. A lichen breaks this logic because it is two organisms from two kingdoms — Fungi and Protista/Monera — fused into one functional body. It is autotrophic and heterotrophic at once and cannot be assigned to any single kingdom. NCERT therefore sets lichens apart at the end of the chapter, beside the acellular viruses, viroids and prions, as biological entities that the tidy kingdom framework cannot accommodate.
Worked examples
In a lichen, the autotrophic partner that prepares food by photosynthesis is the ____, and the heterotrophic partner that absorbs water and minerals is the ____.
The autotroph is the phycobiont (the alga); the heterotroph is the mycobiont (the fungus). Recall the link "phyco → algae, myco → fungus." The alga photosynthesises and feeds the fungus; the fungus shelters the alga and supplies water and minerals.
Why are lichens used as biological indicators of air pollution?
Lichens absorb substances over their whole surface and are very sensitive to sulphur dioxide, so they do not grow in polluted areas. Their absence marks polluted air and their presence marks clean air, making them reliable pollution indicators.
Lichens are often the first organisms to appear on bare rock. What ecological term describes this, and what do they make possible?
They are pioneer species of primary succession. By weathering rock with secreted acids and adding organic matter on death, they begin soil formation, preparing the surface for mosses and higher plants that follow.