Plant Kingdom

This is Chapter 3 of NCERT's Class 11 Biology (pages 23–36 of the 2026-27 reprint) in Unit I, Diversity in the Living World. It has five sections — 3.1 Algae, 3.2 Bryophytes, 3.3 Pteridophytes, 3.4 Gymnosperms and 3.5 Angiosperms — with Figures 3.1–3.5, Table 3.1, a summary and 11 exercises. Earlier editions ended with a section on plant life cycles and alternation of generations; the reprint has dropped it, though exercise 8 still asks about ‘diplontic’ — so section 9 of this page covers it as Beyond NCERT. Whittaker's five kingdoms and why Chlamydomonas sits in Protista are in Chapter 2, Biological Classification (kebo102); the ranks and binomial names are in Chapter 1 (kebo101).

• Neighbouring chapters: Biological Classification (Chapter 2) places Plantae among five kingdoms; Animal Kingdom (Chapter 4) does for animals what this chapter does for plants; Morphology and Anatomy of Flowering Plants (Chapters 5–6) open up the angiosperm body and its xylem and phloem; Sexual Reproduction in Flowering Plants (Class 12, Chapter 1) completes the angiosperm life cycle that this reprint leaves out; Organisms and Populations (Class 12, Chapter 11) treats mycorrhiza and the coralloid-root partnership as mutualism.
• Prelims — science and environment: UPSC has asked whether Spirogyra and Ulothrix are ‘blue-green algae’ (2003) and which organisms colonise bare surfaces without soil — mosses and lichens (2021). Expect plant-group features, algal products (agar, algin, carrageen), seaweeds, peat-forming mosses, gymnosperm adaptations and heterospory.
• GS3 — Environment, Science & Technology, Agriculture: seaweeds as food, hydrocolloids and a coastal livelihood; algae as the planet's main carbon fixers; Sphagnum peatlands as carbon stores; mosses in succession and erosion control; conifer forests and their drought-proof leaves; nitrogen-fixing partnerships in Cycas roots.

• Plantae now excludes fungi and walled Monera and Protista — cyanobacteria are not algae. Systems: artificial (Linnaeus; few characters, androecium) → natural (Bentham and Hooker; internal + external features) → phylogenetic (common ancestry), aided by numerical taxonomy, cytotaxonomy, chemotaxonomy.
• Algae: chlorophyll-bearing, thalloid, mostly aquatic; colonial Volvox, filamentous Ulothrix, Spirogyra, massive kelps. Sex: isogamous (Ulothrix, Spirogyra), anisogamous (Eudorina), oogamous (Volvox, Fucus). Fix at least half of Earth's CO₂; ~70 marine species eaten; algin (brown), carrageen (red), agar (Gelidium, Gracilaria); Chlorella for space travellers.
• Green (Chlorophyceae): chl a, b, starch, pyrenoids, cellulose wall, 2–8 equal apical flagella. Brown (Phaeophyceae): chl a, c, fucoxanthin, laminarin/mannitol, algin, 2 unequal lateral flagella; holdfast, stipe, frond. Red (Rhodophyceae): chl a, d, r-phycoerythrin, floridean starch, no flagella, oogamous, deep water.
• Bryophytes (amphibians of plants): no true roots, stems, leaves; haploid gametophyte dominant; antheridium (biflagellate antherozoids), flask-shaped archegonium (one egg); sporophyte = foot, seta, capsule, fed by the gametophyte. Liverworts: dorsiventral thallus, gemmae in gemma cups (Marchantia). Mosses: protonema → leafy stage (Funaria, Polytrichum, Sphagnum — peat). Pioneers on rock with lichens.
• Pteridophytes: first terrestrial plants with xylem and phloem; sporophyte dominant (true root, stem, leaves); sporophylls, strobili; free-living prothallus; water for fertilisation. Heterosporous Selaginella, Salvinia → embryo inside retained female gametophyte = precursor to the seed habit. Classes: Psilopsida, Lycopsida, Sphenopsida, Pteropsida.
• Gymnosperms: naked ovules and seeds; heterosporous; male and female cones (same tree in Pinus, different in Cycas); pollen grain = reduced male gametophyte; female gametophyte with 2+ archegonia; coralloid roots (Cycas, N₂-fixing cyanobacteria), mycorrhiza (Pinus); Sequoia. Angiosperms: flowers, seeds in fruits, Wolffia to Eucalyptus (>100 m); dicots and monocots.

The earliest systems used only gross superficial morphology — habit, colour, number and shape of leaves — based mainly on vegetative characters or, in the system given by Linnaeus, on the structure of the androecium (the stamens). These were artificial: built on a few characters, they separated closely related species, and they gave vegetative and sexual characters equal weight, which is unsound because vegetative characters are more easily affected by the environment. Natural systems followed, based on natural affinities and on internal as well as external features — ultrastructure, anatomy, embryology and phytochemistry; such a classification of flowering plants was given by George Bentham and Joseph Dalton Hooker. At present phylogenetic systems, based on evolutionary relationships, are accepted: they assume that organisms in the same taxon share a common ancestor.

Taxonomists now draw on many other sources, especially where there is no fossil evidence. Numerical taxonomy, done easily with computers, uses all observable characteristics: each is given a number and code, so every character carries equal importance and hundreds can be considered at once. Cytotaxonomy uses cytological information — chromosome number, structure and behaviour; chemotaxonomy uses the plant's chemical constituents to resolve confusions.

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (fresh water and marine) organisms. They also grow on moist stones, soil and wood, with fungi (in lichens, kebo102) and on animals. Their form and size vary hugely — colonial Volvox, filamentous Ulothrix and Spirogyra, and massive marine kelps.

Algae reproduce vegetatively by fragmentation (each fragment grows into a thallus), asexually by spores — most commonly flagellated, motile zoospores — and sexually by fusion of two gametes. Equal-sized gametes, flagellated (Ulothrix) or non-flagellated (Spirogyra), give isogamy; gametes of different size, as in Eudorina, give anisogamy; a large, non-motile female gamete fused by a small motile male gamete is oogamy (Volvox, Fucus).

Algae matter enormously. At least half of all carbon dioxide fixation on Earth is by algae; they raise dissolved oxygen around them and are the primary producers at the base of all aquatic food cycles. Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. Brown and red algae produce hydrocolloids (water-holding substances) — algin from brown algae, carrageen from red. Agar, from Gelidium and Gracilaria, is used to grow microbes and in ice-creams and jellies. Chlorella, a unicellular, protein-rich alga, is a food supplement even for space travellers. The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.

Chlorophyceae — green algae: unicellular, colonial or filamentous; grass green from chlorophyll a and b held in definite chloroplasts, which may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped. Most have pyrenoids — storage bodies in the chloroplast containing protein besides starch; some store oil droplets. The rigid wall has an inner layer of cellulose and an outer layer of pectose. Vegetative reproduction by fragmentation; asexual by flagellated zoospores in zoosporangia; sex isogamous, anisogamous or oogamous. Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.

Phaeophyceae — brown algae: mainly marine, from simple branched filaments (Ectocarpus) to profusely branched kelps up to 100 metres. Pigments: chlorophyll a, c, carotenoids and xanthophylls; olive green to brown according to the amount of fucoxanthin. Food: complex carbohydrates — laminarin or mannitol. Walls of cellulose coated outside with gelatinous algin; the protoplast has a central vacuole and nucleus besides plastids. The body is attached by a holdfast, with a stalk (stipe) and a leaf-like photosynthetic frond. Asexual reproduction by pear-shaped biflagellate zoospores with two unequal, laterally attached flagella; sex isogamous, anisogamous or oogamous, in water or within the oogonium; gametes pyriform with two lateral flagella. Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.

Rhodophyceae — red algae: red from r-phycoerythrin; mostly marine, concentrated in warmer areas, both near the surface and at great depths where little light penetrates. Most thalli are multicellular, some complex. Food: floridean starch, very like amylopectin and glycogen. Vegetative reproduction by fragmentation; asexual by non-motile spores; sexual by non-motile gametes — oogamous, with complex post-fertilisation development. Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.

Bryophytes — mosses and liverworts — grow in moist, shaded places, commonly in the hills. They are the amphibians of the plant kingdom: they live in soil but depend on water for sexual reproduction. The body is more differentiated than in algae — thallus-like, prostrate or erect, attached by unicellular or multicellular rhizoids — but there are no true roots, stems or leaves, only root-, stem- or leaf-like structures. The main plant body is haploid and produces gametes: the gametophyte. The multicellular sex organs are the antheridium, producing biflagellate antherozoids, and the flask-shaped archegonium with a single egg. Antherozoids released into water reach the archegonium; one fuses with the egg to form the zygote. The zygote does not undergo meiosis at once: it grows into a multicellular sporophyte, which is not free-living but attached to and nourished by the gametophyte. Some sporophyte cells undergo meiosis to form haploid spores, which germinate into gametophytes.

Bryophytes are of little economic importance in general, but some mosses feed herbivorous mammals, birds and other animals. Species of Sphagnum provide peat, long used as fuel and, because it holds water, as packing material for shipping living material. Mosses, with lichens, are the first organisms to colonise rocks: they decompose rock and make the substrate fit for higher plants — hence their ecological importance in plant succession. Dense moss mats reduce the impact of falling rain and prevent soil erosion. Bryophytes are divided into liverworts and mosses.

Liverworts grow in moist, shady places — stream banks, marshy ground, damp soil, tree bark, deep woods. The body is thalloid (Marchantia): dorsiventral and closely pressed to the substrate; leafy members have tiny leaf-like appendages in two rows. Asexual reproduction by fragmentation or by gemmae — green, multicellular asexual buds formed in gemma cups on the thallus, which detach and grow into new plants. Male and female sex organs are on the same or different thalli. The sporophyte has a foot, seta and capsule; spores formed after meiosis in the capsule grow into free-living gametophytes.

Mosses: the predominant stage is the gametophyte, in two stages. The protonema develops directly from a spore — creeping, green, branched, often filamentous. The leafy stage develops from the secondary protonema as a lateral bud: upright slender axes with spirally arranged leaves, attached by multicellular, branched rhizoids, and bearing the sex organs at the apex. Vegetative reproduction by fragmentation and budding in the secondary protonema. The sporophyte — foot, seta and capsule — is more elaborate than in liverworts, and mosses have an elaborate mechanism of spore dispersal. Examples: Funaria, Polytrichum, Sphagnum.

Pteridophytes include horsetails and ferns; they are used medicinally, as soil-binders and as ornamentals. Evolutionarily they are the first terrestrial plants with vascular tissues — xylem and phloem. They grow in cool, damp, shady places, though some flourish in sandy soil. Unlike bryophytes, the main plant body is a sporophyte, differentiated into true root, stem and leaves with well-differentiated vascular tissue. Leaves are small (microphylls, Selaginella) or large (macrophylls, ferns). Sporangia are subtended by leaf-like sporophylls, which in some form compact strobili or cones (Selaginella, Equisetum). Spores form by meiosis in spore mother cells and germinate into small, multicellular, free-living, mostly photosynthetic thalloid gametophytes — the prothallus — which need cool, damp, shady places. The prothallus bears antheridia and archegonia; water carries the antherozoids to the archegonium's mouth; the zygote grows into the dominant sporophyte. Because of the prothallus's needs and the need for water in fertilisation, living pteridophytes are restricted to narrow geographical regions.

In most pteridophytes all spores are alike — homosporous. Selaginella and Salvinia produce two kinds — large macro (mega)spores and small microspores — and are heterosporous. Megaspores and microspores give female and male gametophytes; the female gametophyte is retained on the parent sporophyte for a while, and the zygote develops into a young embryo within it — a precursor to the seed habit, considered an important step in evolution. Pteridophytes are classified into four classes:

Gymnosperms (gymnos, naked; sperma, seeds) have ovules not enclosed by any ovary wall, exposed before and after fertilisation, so their seeds are naked. They are medium or tall trees and shrubs; the giant redwood Sequoia is one of the tallest trees. Roots are generally tap roots; some have mycorrhiza (Pinus), others (Cycas) have small specialised coralloid roots with N₂-fixing cyanobacteria. Stems are unbranched (Cycas) or branched (Pinus, Cedrus); leaves simple or compound — Cycas's pinnate leaves persist for a few years. Leaves withstand extremes of temperature, humidity and wind: conifers' needle-like leaves reduce surface area, and a thick cuticle and sunken stomata cut water loss.

Gymnosperms are heterosporous. Microspores and megaspores form in sporangia on sporophylls, arranged spirally into lax or compact strobili (cones). Male strobili bear microsporophylls and microsporangia; each microspore develops into a highly reduced male gametophyte of a few cells — the pollen grain, formed inside the microsporangium. Female strobili bear megasporophylls with ovules (megasporangia). Male and female cones may be on the same tree (Pinus); in Cycas, male cones and megasporophylls are on different trees. A megaspore mother cell differentiates from a nucellus cell; the nucellus with its protective envelopes is the ovule. The mother cell divides by meiosis into four megaspores; one develops, inside the megasporangium, into a multicellular female gametophyte bearing two or more archegonia. Neither gametophyte is free-living. Pollen, carried by air currents, reaches the opening of the ovule; a pollen tube carries the male gametes to the archegonia; after fertilisation the zygote becomes an embryo and the ovule a seed — not covered.

In angiosperms (flowering plants) pollen grains and ovules develop in specialised structures called flowers, and seeds are enclosed in fruits. They are an exceptionally large group in a wide range of habitats, from the smallest Wolffia to tall Eucalyptus trees over 100 metres. They give food, fodder, fuel, medicines and many commercial products. They are divided into two classes: dicotyledons and monocotyledons. The reprint stops here; the angiosperm life cycle — double fertilisation, endosperm and embryo — is in Class 12, Chapter 1.

This chapter asks no questions in its running text and has no activities; its cross-references point to Chapter 6 (vascular tissues) and Chapter 8 (cell structure). The exercises follow.

Numbers

One-liners

• Cyanobacteria are no longer counted as algae.
• Linnaeus's system was artificial, based partly on the androecium.
• Bentham and Hooker gave a natural classification of flowering plants.
• Phylogenetic systems assume members of a taxon share a common ancestor.
• Numerical taxonomy gives equal weight to all observable characters.
• Algae fix at least half of the carbon dioxide fixed on Earth.
• Agar is obtained from Gelidium and Gracilaria.
• Algin comes from brown algae and carrageen from red algae.
• Chlorella is a protein-rich food supplement used by space travellers.
• Pyrenoids contain protein besides starch.
• Fucoxanthin gives brown algae their colour.
• Red algae store floridean starch.
• Bryophytes are called amphibians of the plant kingdom.
• In bryophytes the main plant body is the haploid gametophyte.
• Liverworts reproduce asexually by gemmae in gemma cups.
• The moss gametophyte begins as a protonema.
• Sphagnum provides peat.
• Mosses and lichens are the first colonisers of bare rock.
• Pteridophytes are the first terrestrial plants with xylem and phloem.
• The fern gametophyte is called a prothallus.
• Selaginella and Salvinia are heterosporous.
• Cycas has coralloid roots with nitrogen-fixing cyanobacteria.
• In gymnosperms the pollen grain is the reduced male gametophyte.
• Wolffia is the smallest flowering plant.

Traps to avoid

• Spirogyra and Ulothrix are green algae, not blue-green algae.
• Isogamous gametes can be non-flagellated, as in Spirogyra.
• Holdfast, stipe and frond are not true root, stem and leaf.
• A moss leaf and a Marchantia gemma are haploid.
• The fern plant is the sporophyte; the prothallus is the gametophyte.
• Heterospory is a precursor to seeds, not seed formation itself.
• Naked seed means no ovary wall, not no seed coat.
• Angiosperms do not bear archegonia; gymnosperms do.
• Pinus bears both cone types on one tree; Cycas on different trees.
• Coralloid roots host cyanobacteria; Pinus roots host fungi.
• Chlamydomonas is a green alga here but a protist in Whittaker's scheme.
• NCERT's 'sloth bear' should be the sloth.
• Table 3.1 says 'Divisions' but means classes.
• Haplontic, diplontic and haplo-diplontic are no longer in the reprint.

Prelims — what UPSC asks

• Asked so far: whether Spirogyra and Ulothrix are ‘blue-green algae’ found in ponds and oceans (2003 — they are green algae); which organisms survive on surfaces without soil — lichens and mosses, not ferns or mushrooms (2021). Related questions on lichens, blue-green algae and Spirogyra as a non-biofertiliser sit on kebo102.
• Likely next: plant-group features in statement form (vascular tissue, dominant phase, archegonia, naked seeds); Table 3.1 pigments, stored food and flagella; agar, algin and carrageen sources; seaweeds in the news (cultivation, invasive species); Sphagnum and peatlands; heterospory; Cycas coralloid roots and Pinus mycorrhiza; gymnosperm adaptations (needles, sunken stomata); smallest and tallest plants; life-cycle patterns.

Mains — where it appears and how to answer

• GS3 Environment / Science & Technology: no Mains question has been set on this chapter. Use it for answers on the seaweed economy and coastal livelihoods, algae in carbon fixation and ocean health, peatland conservation and carbon storage, mosses and lichens in ecological succession and erosion control, and the drought-resistant design of Himalayan conifer forests.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.