NCERT Solutions for Class 11 Biology Chapter 3: Plant Kingdom

Concept. Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms, varying greatly in form and size — from colonial Volvox and filamentous Ulothrix and Spirogyra to massive marine kelps. To bring order to this diversity, botanists classify them into three main classes: Chlorophyceae (green algae), Phaeophyceae (brown algae) and Rhodophyceae (red algae).

Main basis. According to the NCERT summary, algae are classified “depending on the type of pigment possessed and the type of stored food”. The two key criteria, therefore, are:

1. Type of photosynthetic pigments. Green algae have chlorophyll a and b (grass-green); brown algae have chlorophyll a, c, carotenoids and xanthophylls (fucoxanthin), giving colours from olive green to brown; red algae have chlorophyll a, d and r-phycoerythrin, which makes them red.
2. Type of reserve (stored) food. Green algae store starch (and sometimes oil droplets, with pyrenoids in chloroplasts); brown algae store mannitol and laminarin; red algae store floridean starch, similar to amylopectin and glycogen.

Supporting characters that also separate the three classes (Table 3.1): cell wall composition (green: cellulose with outer pectose; brown: cellulose plus gelatinous algin; red: cellulose, pectin and polysulphate esters); number and position of flagella (green: 2–8, equal, apical; brown: 2, unequal, lateral; red: absent); habitat (green – fresh, brackish and salt water; brown – mainly marine; red – mostly marine, warmer areas, even great depths); and mode of reproduction (flagellated zoospores in green and brown; only non-motile spores and gametes in red).

Extra depth. Pigments decide the colour and, indirectly, the habitat: red algae with phycoerythrin can absorb the blue-green light that penetrates to great depths, so they grow where little light reaches. Reserve food and cell wall chemistry are stable biochemical characters, which is why they are preferred over mere size or shape, which vary enormously within a class.

Conclusion: algae are classified into Chlorophyceae, Phaeophyceae and Rhodophyceae mainly on the basis of the type of pigments they possess and the type of food they store, supported by cell-wall chemistry, flagella and habitat.

Concept. Reduction division (meiosis) halves the chromosome number (2n → n). In all these land plants the life cycle shows alternation of generations between a haploid gametophyte (n) and a diploid sporophyte (2n). Gametes are produced by the haploid gametophyte by mitosis; hence meiosis occurs in the sporophyte, at the time of spore formation (sporic meiosis), in spore mother cells located in sporangia.

Explanation of each. In bryophytes (liverwort, moss) the zygote does not undergo reduction division immediately; it produces a multicellular sporophyte, and some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores. In pteridophytes the sporangia produce spores by meiosis in spore mother cells. In gymnosperms the megaspore mother cell differentiates from a cell of the nucellus and divides meiotically to form four megaspores, one of which develops into the female gametophyte; the microspores (formed in microsporangia) develop into the highly reduced male gametophyte, the pollen grain. Angiosperms follow the same pattern within the anther and the ovule.

Extra depth. In none of these plants does meiosis occur directly in the zygote or in the gametes (as in some algae); in all, it happens in the sporophyte. This is the reason the gametophyte is always haploid and the sporophyte always diploid.

Conclusion: reduction division occurs in the diploid sporophyte at the time of spore formation — in the capsule of liverworts and mosses, in the sporangia of ferns, in the microsporangia and megasporangium of gymnosperms, and in the anther and ovule of angiosperms.

Concept. The archegonium is the flask-shaped multicellular female sex organ that produces a single egg. Plants that bear archegonia are called archegoniates.

Three groups that bear archegonia: (1) Bryophytes (liverworts and mosses), (2) Pteridophytes (ferns, Selaginella, Equisetum) and (3) Gymnosperms (the female gametophyte bears two or more archegonia). Angiosperms have no archegonia (the embryo sac replaces them).

Life cycle of a moss (Funaria) — a bryophyte (alternation of generations):

1. Spore → protonema. The haploid spore germinates into the first stage of the gametophyte, the protonema — a creeping, green, branched, often filamentous stage.
2. Leafy gametophyte. The leafy stage develops from the secondary protonema as a lateral bud: upright slender axes with spirally arranged leaves, attached by multicellular branched rhizoids. This stage bears the sex organs and is the dominant, haploid phase. It reproduces vegetatively by fragmentation and budding.
3. Sex organs. At the apex of the leafy shoot the male antheridia (producing biflagellate antherozoids) and female archegonia (flask-shaped, single egg) are produced.
4. Fertilisation. Antherozoids are released into water, reach the archegonium and one fuses with the egg → diploid zygote. Water is therefore essential for fertilisation.
5. Sporophyte. The zygote does not undergo reduction division immediately; it develops into a multicellular sporophyte of foot, seta and capsule, which is not free-living — it remains attached to the gametophyte and draws nourishment from it.
6. Meiosis and spores. Some cells of the capsule (spore mother cells) divide by meiosis to form haploid spores; the moss has an elaborate mechanism of spore dispersal. Spores germinate → protonema, and the cycle repeats.

Extra depth. In bryophytes the dominant phase is the gametophyte; in pteridophytes and beyond, the sporophyte dominates. Because the antherozoids swim in water to the archegonium, bryophytes are called the “amphibians of the plant kingdom”.

Conclusion: bryophytes, pteridophytes and gymnosperms bear archegonia; in a moss like Funaria the haploid protonema and leafy gametophyte bear antheridia and archegonia, water-mediated fertilisation forms the zygote, which grows into a dependent sporophyte whose capsule produces haploid spores by meiosis.

Concept. Ploidy is the number of complete chromosome sets in a cell: haploid (n) in the gametophyte generation and its gametes, and diploid (2n) in the sporophyte generation and the zygote. A cell’s ploidy follows from which generation it belongs to and how it was formed.

Extra depth. Rule of thumb: everything of the gametophyte (protonema, leaves of moss, prothallus, gemma, egg, sperm) is n; everything of the sporophyte (zygote, fern frond, monocot plant body) is 2n; the endosperm of angiosperms is the exception, being 3n.

Conclusion: protonemal cell, moss leaf cell, fern prothallus cell, Marchantia gemma cell and liverwort ovum are haploid (n); the dicot primary endosperm nucleus is triploid (3n); the monocot meristem cell and the fern zygote are diploid (2n).

Concept. Both groups are of immense economic value — algae through their role as producers and through commercial products, gymnosperms through timber, resins and medicines.

A. Economic importance of algae (from NCERT):

1. Carbon dioxide fixation and oxygen supply. At least half of the total CO₂ fixation on earth is carried out by algae through photosynthesis; being photosynthetic they raise the dissolved oxygen of their immediate environment.
2. Primary producers. Algae are of paramount importance as primary producers of energy-rich compounds forming the basis of the food cycles of all aquatic animals (fish, etc.).
3. Food. Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. Chlorella, a unicellular alga rich in protein, is used as a food supplement even by space travellers.
4. Hydrocolloids (water-holding substances). Marine brown and red algae produce large amounts of hydrocolloids — algin (brown algae) and carrageen (red algae) — used commercially (e.g., as thickeners and stabilisers).
5. Agar. Obtained from Gelidium and Gracilaria (red algae); used to grow microbes in laboratories and in preparation of ice-creams and jellies.

B. Economic importance of gymnosperms. Gymnosperms (naked-seeded plants such as Pinus, Cedrus, Cycas, Sequoia, Ginkgo) are economically important in several ways (these uses are standard, well-established facts):

1. Timber and wood. Conifers such as Pinus (pine), Cedrus (deodar) and Picea (spruce) supply softwood for furniture, building, packing cases and plywood.
2. Paper pulp. The wood of conifers (pine, spruce) is the main raw material for paper and newsprint.
3. Resins and turpentine. Resin tapped from Pinus yields turpentine oil and rosin, used in paints, varnishes, soaps and polish.
4. Food. Seeds of Pinus gerardiana (chilgoza) are edible; the stem pith of Cycas yields sago-type starch in some regions.
5. Medicines. Ephedra yields the alkaloid ephedrine (used in asthma and cold remedies); Taxus (yew) yields the anticancer drug taxol.
6. Ornamental plants and ecology. Cycas, Thuja, Araucaria, Ginkgo are grown as ornamentals; coniferous forests protect watersheds and soil, and Sequoia (giant redwood) is one of the tallest tree species.

Extra depth. Algae supply the raw material for some of the commonest food and laboratory products (agar, algin), while gymnosperm forests supply most of the world’s softwood and paper pulp.

Conclusion: algae are valuable as major CO₂ fixers, producers and sources of food, algin, carrageen and agar, whereas gymnosperms provide timber, paper pulp, resins and medicines.

Concept. Both gymnosperms and angiosperms are seed-bearing plants (spermatophytes), but they are placed in separate groups because the way the seed is formed and protected is fundamentally different. The key idea is in their names: gymnos = naked, sperma = seed → naked seeds; angeion = vessel/covering → seeds enclosed in a fruit.

Main reason (NCERT). In gymnosperms the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation, so the seeds that develop are naked. In angiosperms (flowering plants) the pollen grains and ovules are developed in specialised structures called flowers, the ovules are enclosed in an ovary, and after fertilisation the seeds are enclosed in fruits.

Extra depth. The differences in the way ovules are protected, in the structure of the female gametophyte and in the fertilisation mechanism (details beyond the NCERT chapter are standard facts you will meet in the reproduction chapters) show that the two groups represent different evolutionary lines of seed plants, justifying separate classification: gymnosperms are the older group, and angiosperms the most advanced and the most successful on land.

Conclusion: gymnosperms and angiosperms are classified separately because gymnosperms have naked ovules and seeds borne on cones, whereas angiosperms have flowers whose ovules are enclosed in an ovary and develop into seeds enclosed in fruits.

Concept. Most pteridophytes produce only one kind of spore. A few — and all seed plants — produce two kinds. This difference is called homospory versus heterospory.

Definition. Heterospory is the production of two kinds of spores — small microspores and large megaspores — by the same sporophyte, instead of spores of one type. Microspores are produced in microsporangia (on microsporophylls) and germinate into male gametophytes; megaspores are produced in megasporangia (on megasporophylls) and germinate into female gametophytes.

Examples: Selaginella and Salvinia (pteridophytes; NCERT); all gymnosperms and angiosperms are also heterosporous.

Significance of heterospory (NCERT):

1. Female gametophyte retained on the parent. In heterosporous plants the megaspores germinate into female gametophytes that are retained on the parent sporophyte for variable periods instead of living independently on the soil.
2. Embryo development within the female gametophyte. The zygote develops into a young embryo within the female gametophyte, which is protected and nourished.
3. Precursor to the seed habit. This event is a precursor to the seed habit, considered an important step in evolution; the seed plants (gymnosperms and angiosperms) evolved from such heterosporous ancestors.
4. Reduced dependence on water and independent gametophytes. The gametophytes are much reduced and do not need a damp, shady habitat of their own, giving the plant a better chance of survival on land.

Conclusion: heterospory is the formation of two kinds of spores (micro- and megaspores) as in Selaginella and Salvinia; it is significant because the female gametophyte and young embryo are retained on the parent, a precursor of the seed habit — a major evolutionary step.

Concept. These six terms are the basic vocabulary of plant life cycles (alternation of generations). Each is explained below with the NCERT example.

(i) Protonema. The first stage of the gametophyte of a moss, developing directly from a haploid spore. It is a creeping, green, branched and frequently filamentous stage. The second (leafy) stage develops from the secondary protonema as a lateral bud and bears the sex organs. Example: Funaria, Polytrichum, Sphagnum.

(ii) Antheridium. The male sex organ of bryophytes and pteridophytes — a multicellular structure that produces biflagellate antherozoids (male gametes) which swim in water to the archegonium. Example: antheridia on the antheridiophore of Marchantia or at the apex of leafy shoots of moss; on the prothallus of ferns.

(iii) Archegonium. The female sex organ — a multicellular, flask-shaped structure producing a single egg. Present in bryophytes, pteridophytes and gymnosperms. Example: archegonia of Funaria and fern prothallus; in Pinus and Cycas the female gametophyte bears two or more archegonia.

(iv) Diplontic. A life-cycle pattern in which the diploid sporophyte (2n) is the dominant, photosynthetic, independent phase and the haploid gametophyte is reduced to a few cells (or only the gametes). Examples: all gymnosperms and angiosperms; among algae, Fucus. (Compare: haplontic – dominant haploid phase, as in Volvox, Spirogyra; haplo-diplontic – both phases independent, as in bryophytes and pteridophytes.)

(v) Sporophyll. A leaf-like appendage that bears sporangia (subtending the sporangia). In some cases sporophylls form compact structures called strobili (cones). Examples: sporophylls of Selaginella and Equisetum (forming strobili); microsporophylls and megasporophylls of gymnosperms (Pinus, Cycas) arranged spirally along an axis to form male and female cones.

(vi) Isogamy. Sexual reproduction by fusion of two gametes similar in size (morphologically alike). The gametes may be flagellated (motile) as in Ulothrix or non-flagellated (non-motile) as in Spirogyra. (Contrast: anisogamy – gametes dissimilar in size, e.g., Eudorina; oogamy – large non-motile female gamete + small motile male gamete, e.g., Volvox, Fucus.)

Conclusion: protonema is the filamentous first stage of the moss gametophyte, antheridium and archegonium are the male and female sex organs, diplontic denotes a dominant diploid life cycle, sporophyll is a spore-bearing leaf-like appendage and isogamy is fusion of similar gametes.

Concept. Differentiation questions are answered best as a table of paired points with examples. Each pair below is based on NCERT Table 3.1 and sections 3.1–3.3.

(i) Red algae (Rhodophyceae) and brown algae (Phaeophyceae)

(ii) Liverworts and mosses (the two classes of bryophytes)

(iii) Homosporous and heterosporous pteridophytes

Extra depth. The pigments decide colour and depth distribution in the algae, the nature of the gametophyte separates the bryophyte classes, and the number of spore types marks a key evolutionary step in pteridophytes.

Conclusion: red algae differ from brown algae in pigments (phycoerythrin vs fucoxanthin), food (floridean starch vs mannitol/laminarin) and flagella; liverworts have a thalloid dorsiventral body while mosses have protonema and a leafy stage; homosporous pteridophytes make one kind of spore while heterosporous ones make micro- and megaspores.

Concept. To match organisms with their groups, recall the group to which each belongs and a defining feature of that group.

Final answer: (a) – (iii); (b) – (iv); (c) – (ii); (d) – (i).

Extra depth. Remember the NCERT examples: algae – Volvox, Ulothrix, Spirogyra, Chara; mosses – Funaria, Polytrichum, Sphagnum; pteridophytes – Selaginella, Equisetum, Dryopteris; gymnosperms – Cycas, Pinus, Cedrus, Ginkgo.

Conclusion: the correct matches are Chlamydomonas – algae, Cycas – gymnosperm, Selaginella – pteridophyte and Sphagnum – moss.

Concept. The term gymnosperm comes from gymnos = naked and sperma = seeds: they are plants in which the ovules are not enclosed by any ovary wall and remain exposed both before and after fertilisation, so the seeds that develop are naked. Examples: Cycas, Pinus, Cedrus, Ginkgo, Sequoia.

Important characteristics (from NCERT):

1. Naked ovules and seeds. Ovules lack an ovary wall; seeds are uncovered (no fruit).
2. Plant body. Medium-sized or tall trees and shrubs; the giant redwood Sequoia is one of the tallest tree species. The main plant body is the sporophyte.
3. Roots. Generally tap roots. Some have a fungal association in the form of mycorrhiza (Pinus), while others (Cycas) have small specialised coralloid roots associated with nitrogen-fixing cyanobacteria.
4. Stems. Unbranched (Cycas) or branched (Pinus, Cedrus).
5. Leaves. Simple or compound; in Cycas the pinnate leaves persist for a few years. Leaves are well adapted to withstand extremes of temperature, humidity and wind; in conifers the needle-like leaves reduce surface area and the thick cuticle and sunken stomata help reduce water loss.
6. Heterosporous. They produce haploid microspores and megaspores in sporangia borne on sporophylls, which are arranged spirally along an axis to form lax or compact strobili (cones).
7. Male cones. The strobili bearing microsporophylls and microsporangia are the microsporangiate (male) strobili; microspores develop into a highly reduced male gametophyte — the pollen grain, formed inside the microsporangia.
8. Female cones. Cones bearing megasporophylls with ovules (megasporangia) are the macrosporangiate (female) strobili. The megaspore mother cell differentiates from a cell of the nucellus (nucellus + envelopes = ovule) and divides meiotically to form four megaspores; one develops into a multicellular female gametophyte bearing two or more archegonia, retained within the megasporangium. Male and female cones are on the same tree (Pinus) or on different trees (Cycas).
9. Gametophytes not free-living. Unlike bryophytes and pteridophytes, the male and female gametophytes remain within the sporangia on the sporophyte.
10. Pollination and fertilisation. Pollen grains are carried by air currents to the ovule opening; the pollen tube carries the male gametes towards the archegonia and discharges them near the archegonial mouth; after fertilisation the zygote develops into an embryo and the ovules into naked seeds.

Extra depth. The retention of the female gametophyte inside the ovule, and the transfer of male gametes through a pollen tube, free gymnosperms from dependence on external water for fertilisation — a major advance over bryophytes and pteridophytes.

Conclusion: gymnosperms are naked-seeded, mostly tall perennial trees with tap roots (mycorrhizal or coralloid), needle-like or pinnate leaves adapted to dry conditions, heterosporous with male and female cones, reduced gametophytes retained on the sporophyte, wind pollination and pollen-tube-mediated fertilisation.