Plant Kingdom — NEET Notes

Five divisions of Plantae

Whittaker's 1969 Five Kingdom scheme placed all eukaryotic, autotrophic, multicellular organisms with cellulose cell walls into Kingdom Plantae. NCERT then resolves Plantae into five groups: Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms. The boundaries are sharp and the diagnostic features are NEET-favourites. Algae are simple, thalloid, largely aquatic. Bryophytes are land plants without vascular tissue. Pteridophytes have vascular tissue but no seeds. Gymnosperms have naked seeds but no flowers. Angiosperms have flowers and seeds enclosed in fruits.

Two warnings worth absorbing at the outset. First, cyanobacteria — once called "blue-green algae" — are no longer algae; they belong to Monera. Second, fungi and protistans with cell walls were once placed in Plantae but have since been excluded. The plant kingdom now contains only true plants: chlorophyll-bearing, eukaryotic, embryophytic organisms.

• Algae
• Three classes of algae
• Economic importance
• Bryophytes
• Liverworts & mosses
• Pteridophytes
• Gymnosperms
• Angiosperms
• Monocots vs dicots
• Alternation of generations
• NEET PYQ snapshot
• Expert FAQs

Algae — the aquatic ancestors

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms. They live in fresh and marine water, but also on moist stones, soils, wood, in association with fungi (lichens), and even on the fur of sloth bears. Their form ranges from the unicellular Chlamydomonas, through the colonial Volvox and filamentous Ulothrix and Spirogyra, all the way to the giant marine kelps which can reach 100 metres. Algae do not have true roots, stems or leaves — the entire plant body is a thallus.

Algae reproduce by three routes. Vegetative reproduction proceeds by fragmentation — each fragment grows into a new thallus. Asexual reproduction occurs by the formation of different types of spores, most commonly flagellated zoospores produced in zoosporangia. Sexual reproduction varies in the type and formation of sex cells: when fusing gametes are flagellated and similar in size (as in Ulothrix) or non-flagellated and similar (as in Spirogyra), reproduction is called isogamous. When the gametes differ in size, as in some species of Eudorina, reproduction is anisogamous. When a large, non-motile female gamete fuses with a smaller motile male gamete, as in Volvox and Fucus, reproduction is oogamous.

Three classes — Chlorophyceae, Phaeophyceae, Rhodophyceae

NCERT classifies algae into three classes on the basis of major photosynthetic pigments, stored food, cell-wall composition, and flagellar arrangement. The pigment-and-food trio is the most heavily tested combination in NEET.

Economic importance of algae

Algae are responsible for at least half of all carbon-dioxide fixation on Earth — a single-line fact that NEET has used repeatedly. They are the primary producers of energy-rich compounds that drive aquatic food cycles, and being photosynthetic, they raise dissolved oxygen in their habitat. Around 70 species of marine algae are eaten by humans, including species of Porphyra (red), Laminaria (brown) and Sargassum (brown). Brown and red algae yield commercially valuable hydrocolloids: algin from brown algae and carrageen from red algae. Agar, used to culture microbes and to make ice creams and jellies, is obtained from Gelidium and Gracilaria — both red algae. Chlorella, a unicellular green alga rich in proteins, has been used as a food supplement even by space travellers.

At least half of the total carbon-dioxide fixation on Earth is carried out by algae.

NCERT Class XI Biology — Algae section

Bryophytes — amphibians of the plant kingdom

Bryophytes — the various mosses, liverworts, and hornworts — are the first true land plants. They grow in damp, humid, shaded localities, usually in the hills. They are called amphibians of the plant kingdom because, although they can live in soil, they remain dependent on water for sexual reproduction: the biflagellate antherozoids must swim through a film of water to reach the archegonium. Bryophytes play an important ecological role in plant succession on bare rocks and soils — together with lichens, mosses are pioneer colonisers, breaking down rock and creating substrate for higher plants. Mosses form dense mats that reduce raindrop impact and prevent soil erosion.

The plant body of a bryophyte is more differentiated than that of an alga but still lacks true roots, stem or leaves. It is thallus-like, prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. It may possess root-like, stem-like or leaf-like structures, but these are not anatomically equivalent to the organs of higher plants. Crucially, bryophytes lack vascular tissue — no xylem, no phloem.

The dominant phase in the bryophyte life cycle is the gametophyte — the main green plant body is haploid and produces gametes. Its sex organs are multicellular. The male sex organ, the antheridium, produces biflagellate antherozoids. The female sex organ, the archegonium, is flask-shaped and produces a single egg. After fertilisation in water, the zygote does not undergo meiosis immediately. Instead it grows into a multicellular sporophyte that remains attached to the gametophyte and depends on it for nutrition. Some cells of the sporophyte then undergo meiosis to produce haploid spores, which germinate into new gametophytes.

Liverworts and mosses

NCERT divides bryophytes into liverworts and mosses (NIOS adds a third group, the hornworts). The two NCERT groups differ in body plan, rhizoid structure, and sporophyte elaboration.

Liverworts grow in moist, shady habitats: stream banks, marshy ground, damp soil, tree bark. The thallus of Marchantia is dorsiventral and closely pressed to the substrate. Asexual reproduction proceeds by fragmentation or by the formation of gemmae — green, multicellular, asexual buds that develop in small receptacles called gemma cups on the upper surface. When detached, each gemma germinates into a new individual. (NEET 2021 asked which group has gemmae — answer: some liverworts. Mosses, pteridophytes, gymnosperms do not.) The sporophyte differentiates into foot, seta, and capsule; meiosis in capsule cells produces spores that germinate to form free-living gametophytes.

Mosses have a two-stage gametophyte. A spore germinates first into a creeping, green, branched, filamentous protonema. The protonema then produces lateral buds that develop into upright, slender axes bearing spirally arranged leaves — the leafy stage. This leafy gametophyte is anchored to soil by multicellular branched rhizoids and bears the sex organs at its apex. The moss sporophyte is more elaborate than that of liverworts and has a sophisticated spore-dispersal mechanism. Species of Sphagnum yield peat, long used as fuel and packing material because of its remarkable water-holding capacity.

Pteridophytes — the first vascular plants

Pteridophytes include the horsetails and ferns. Evolutionarily, they are the first terrestrial plants to possess vascular tissue — xylem and phloem — and the first to be truly independent of water for the bulk of their life. They are used medicinally, as soil-binders, and as ornamentals. Pteridophytes typically grow in cool, damp, shady places, though some flourish in sandy soils.

Unlike bryophytes, the dominant phase in pteridophytes is the sporophyte — the main plant body. It is differentiated into true root, stem and leaves, with well-developed vascular tissues. Leaves may be small (microphylls, as in Selaginella) or large (macrophylls, as in ferns). Sporophytes bear sporangia subtended by leaf-like appendages called sporophylls. In some genera the sporophylls cluster into compact structures called strobili or cones — examples include Selaginella and Equisetum. (NEET 2020 asked which of the listed genera bears cones — answer: Equisetum.) Sporangia produce spores by meiosis in spore mother cells.

Spores germinate into small, multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothalli. These prothalli need cool, damp, shaded habitats — and because fertilisation also requires water, the spread of living pteridophytes is geographically restricted. Antheridia release antherozoids that swim through water to the archegonia. Fusion produces a zygote that develops into the multicellular sporophyte — the dominant phase.

Homospory and heterospory

In most pteridophytes, all spores produced are of one kind — these plants are homosporous. But two NCERT-named genera, Selaginella and Salvinia, produce two kinds of spores: small microspores that develop into male gametophytes, and large megaspores that develop into female gametophytes. These plants are heterosporous. The female gametophyte is retained on the parent sporophyte, and the zygote begins developing into an embryo within it — a precursor to the seed habit and an important step in evolution. NEET has tested this pattern in 2018, 2019, 2021 and 2023.

Pteridophytes are further classified into four classes: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (Dryopteris, Pteris, Adiantum).

Gymnosperms — naked seeds

The name says it all: gymnos means naked, sperma means seed. Gymnosperms are plants whose ovules are not enclosed by any ovary wall — they remain exposed both before and after fertilisation, and the seeds that develop are therefore also naked. There is no fruit. Gymnosperms include medium-sized to tall trees and shrubs. The giant redwood Sequoia is one of the tallest tree species on Earth — a NEET 2016 fact.

Roots are generally tap-rooted. Some genera have specialised root associations: Pinus roots form mycorrhizae with fungi (NEET 2019 asked why Pinus seeds cannot germinate without fungal association — answer: obligate mycorrhizal association); Cycas bears small specialised coralloid roots housing nitrogen-fixing cyanobacteria. Stems are unbranched in Cycas, branched in Pinus and Cedrus. Leaves vary from simple to compound, with pinnate leaves persisting on Cycas for years. The leaves are well-adapted to climatic extremes: conifers have needle-like leaves with thick cuticle and sunken stomata that reduce water loss.

Gymnosperms are heterosporous. Sporophylls are arranged spirally along an axis to form lax or compact strobili (cones). The microsporangiate (male) strobili bear microsporophylls with microsporangia; the macrosporangiate (female) strobili bear megasporophylls with ovules. Male and female cones may be on the same tree (Pinus) or on different trees (Cycas). The microspores develop into highly reduced male gametophytes — each is a pollen grain, confined to only a limited number of cells. In Pinus, the pollen grain carries two large air sacs or wings that help it disperse by wind (NEET 2018 fact: winged pollen grains are characteristic of Pinus).

The megaspore mother cell is differentiated from a cell of the nucellus. The nucellus is protected by envelopes — the composite structure is the ovule. Megasporogenesis yields four megaspores; one of them enclosed within the megasporangium develops into a multicellular female gametophyte bearing two or more archegonia. Crucially, unlike bryophytes and pteridophytes, in gymnosperms male and female gametophytes have no independent free-living existence — they remain inside sporangia on the parent sporophyte.

Pollen grains released from the microsporangium are carried by air currents to the opening of the ovules. A pollen tube carrying the male gametes grows towards the archegonia in the ovule and discharges its contents near the mouth of the archegonia. Fertilisation produces a zygote that develops into an embryo; the ovule matures into a seed — naked, exposed.

Angiosperms — flowers and fruits

Unlike gymnosperms with their naked ovules, in angiosperms — the flowering plants — pollen grains and ovules are developed in specialised structures called flowers, and the seeds are enclosed in fruits (a fruit is a mature, fertilised ovary). Angiosperms are by far the most diverse and successful group of plants, ranging in size from the smallest Wolffia (a few millimetres) to the tallest Eucalyptus (over 100 metres). They provide our food, fodder, fuel, medicines, and a vast range of commercial products. Xylem of angiosperms contains both vessels and tracheids; in gymnosperms it is mostly tracheids.

Monocots vs dicots

Angiosperms are divided into two classes — dicotyledons (dicots) and monocotyledons (monocots) — by the number of cotyledons in the embryo. The vegetative and floral differences cluster reliably enough that the seed-leaf count predicts almost every other feature.

NIOS supplements NCERT with four representative families: Fabaceae and Malvaceae (dicots), Liliaceae and Poaceae (monocots). Fabaceae — the pea family — has zygomorphic flowers with five papilionaceous petals (standard, two wings, two keel), diadelphous (9+1) stamens, and a monocarpellary unilocular ovary developing into a pod. Malvaceae — the China-rose family — has pentamerous actinomorphic flowers with monadelphous stamens fused into a staminal tube. Liliaceae — lilies and onions — has trimerous flowers with a petaloid perianth, six stamens, and a syncarpous tricarpellary ovary. Poaceae — the grass family, which feeds the world — has small inconspicuous flowers in spikelets, with three (sometimes six) stamens and a caryopsis fruit in which seed coat and ovary wall are inseparably fused.

Alternation of generations

Every member of Plantae shows alternation of generations — a life cycle in which a haploid gamete-producing phase (gametophyte) alternates with a diploid spore-producing phase (sporophyte). Gametophytes produce gametes by mitosis; sporophytes produce spores by meiosis. What changes across the kingdom is which phase dominates the life cycle, and how independent each phase is.

Gametophyte (n) → gametes → zygote (2n) → sporophyte (2n) → meiosis → spores (n) → gametophyte (n).

The alternation-of-generations cycle

NCERT recognises three patterns. In the haplontic life cycle, the gametophyte is the dominant, free-living, photosynthetic phase. The sporophyte is reduced to the diploid zygote itself, which immediately undergoes meiosis (zygotic meiosis) to form haploid spores. Most algae — Spirogyra, Volvox, Chlamydomonas, Ulothrix — follow this pattern. (NEET 2017 asked which organism shows zygotic meiosis — answer: Chlamydomonas.)

In the diplontic life cycle, the sporophyte is the dominant, photosynthetic, free-living phase. The gametophyte is reduced to a few cells — sometimes just the gametes themselves. All seed plants (gymnosperms and angiosperms) follow this pattern, as does the brown alga Fucus. Meiosis happens at gamete formation (gametic meiosis).

The haplo-diplontic life cycle has both phases as multicellular, with neither fully reduced. Bryophytes are haplo-diplontic with a dominant gametophyte (the green leafy plant) and an attached, dependent sporophyte. Pteridophytes are haplo-diplontic with a dominant sporophyte (the fern plant) and a small, free-living gametophyte (the prothallus). Some algae — including the brown alga Ectocarpus — are also haplo-diplontic. Meiosis happens at spore formation (sporic meiosis).

The evolutionary ladder — Algae to Angiosperms

Five evolutionary "rungs" mark the major transitions of the plant kingdom — and the NEET-favourite framing is exactly the chapter's spine.

Each transition introduced one new feature that opened a new habitat. Bryophytes climbed onto land but stayed in damp shade. Pteridophytes built plumbing and grew tall, but their gametophyte still needed water. Gymnosperms invented the seed and the pollen tube, finally escaping the need for surface water. Angiosperms refined the seed, wrapped it in a fruit, and built flowers — and proceeded to outcompete every other group across the planet.