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NCERT Solutions · Class 11 Biology Plant Kingdom

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Exercises 3.1–3.11

  1. Exercise 3.1

    What is the basis of classification of algae?

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    Algae are classified into three classes — Chlorophyceae, Phaeophyceae and Rhodophyceae — chiefly on the type of pigments they possess and the type of stored food.
    The other characters used are the cell wall composition, the number and position of insertion of the flagella, and the habitat (Table $\displaystyle 3.1$).
    Chlorophyceae (green algae): chlorophyll a and b; food stored as starch; cellulose wall; $\displaystyle 2$–$\displaystyle 8$ equal, apical flagella; fresh, brackish and salt water.
    Phaeophyceae (brown algae): chlorophyll a, c, carotenoids and xanthophylls (fucoxanthin); food stored as mannitol and laminarin; wall of cellulose and algin; $\displaystyle 2$ unequal, laterally attached flagella; mostly brackish and salt water, rarely fresh water.
    Rhodophyceae (red algae): chlorophyll a, d and r-phycoerythrin; food stored as floridean starch; wall of cellulose, pectin and polysulphate esters; flagella absent; mostly marine, some fresh water.

  2. Exercise 3.2

    When and where does reduction division take place in the life cycle of a liverwort, a moss, a fern, a gymnosperm and an angiosperm?

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    Reduction division (meiosis) takes place in the spore mother cells of the sporophyte in every case — that is, at the point where the diploid phase gives rise to haploid spores.
    Liverwort (e.g. Marchantia): in the capsule of the sporophyte; "after meiosis, spores are produced within the capsule", and these spores germinate into free-living gametophytes.
    Moss (e.g. Funaria): in the capsule of the sporophyte, which consists of foot, seta and capsule; the spores in the capsule are formed after meiosis.
    Fern: in the spore mother cells inside the sporangia, which are borne on the sporophylls of the sporophyte (the dominant phase).
    Gymnosperm: at two places on the sporophyte — in the microspore mother cells within the microsporangia borne on microsporophylls (giving haploid microspores, i.e. pollen grains), and in the megaspore mother cell within the megasporangium of the ovule, which "divides meiotically to form four megaspores". The megaspore mother cell is differentiated from one of the cells of the nucellus.
    Angiosperm: the chapter states only that in angiosperms the pollen grains and ovules are developed in specialised structures called flowers; it does not describe where reduction division occurs. (Taken up later, in the chapter on sexual reproduction in flowering plants: meiosis occurs in the microspore mother cells of the anther and in the megaspore mother cell of the ovule.)

  3. Exercise 3.3

    Name three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them.

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    Three groups that bear archegonia: bryophytes, pteridophytes and gymnosperms.
    In all three the archegonium is the female sex organ; in bryophytes it is flask-shaped and produces a single egg, and in gymnosperms the female gametophyte bears two or more archegonia.
    Life cycle of a moss (a bryophyte, e.g. Funaria):
    The main plant body is the haploid gametophyte, and it passes through two stages.
    Protonema stage — develops directly from a spore; creeping, green, branched and frequently filamentous.
    Leafy stage — develops from the secondary protonema as a lateral bud; upright, slender axes bearing spirally arranged leaves, attached to the soil by multicellular, branched rhizoids. This stage bears the sex organs.
    Sex organs: antheridia and archegonia are produced at the apex of the leafy shoots. The antheridium produces biflagellate antherozoids; the archegonium produces a single egg.
    Fertilisation: antherozoids are released into water, reach the archegonium, and one fuses with the egg to form the zygote.
    The zygote does not undergo reduction division immediately; it develops into a sporophyte of foot, seta and capsule — more elaborate than that of a liverwort. The sporophyte is not free-living but stays attached to the photosynthetic gametophyte and draws nourishment from it.
    Spores are formed in the capsule after meiosis, and mosses have an elaborate mechanism of spore dispersal.
    Each spore germinates to form the protonema, completing the cycle.
    Vegetative reproduction also occurs, by fragmentation and by budding in the secondary protonema.

  4. Exercise 3.4

    Mention the ploidy of the following: protonemal cell of a moss; primary endosperm nucleus in dicot, leaf cell of a moss; prothallus cell of a ferm; gemma cell in Marchantia; meristem cell of monocot, ovum of a liverwort, and zygote of a fern.

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    Protonemal cell of a moss — haploid (n); the protonema is the first stage of the gametophyte and develops directly from a spore.
    Primary endosperm nucleus in a dicot — triploid (3n).
    Leaf cell of a moss — haploid (n); the leafy stage is part of the gametophyte, which is the main plant body of a bryophyte.
    Prothallus cell of a fern — haploid (n); the prothallus is the gametophyte that grows from a spore formed by meiosis.
    **Gemma cell in *Marchantia* — haploid (n)**; gemmae are asexual buds borne in gemma cups on the gametophytic thallus.
    Meristem cell of a monocot — diploid (2n); the flowering plant body is the sporophyte.
    Ovum of a liverwort — haploid (n); the egg is produced in the archegonium of the haploid gametophyte.
    Zygote of a fern — diploid (2n); formed by fusion of the male gamete with the egg, and it develops into the sporophyte.

  5. Exercise 3.5

    Write a note on economic importance of algae and gymnosperms.

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    (a) Economic importance of algae
    At least half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
    Being photosynthetic, they increase the level of dissolved oxygen in their immediate environment.
    They are of paramount importance as primary producers of energy-rich compounds, which form the basis of the food cycles of all aquatic animals.
    Food: many species of Porphyra, Laminaria and Sargassum are among the $\displaystyle 70$ species of marine algae used as food.
    Hydrocolloids (water-holding substances) are produced in large amounts by certain marine brown and red algae — algin from brown algae and carrageen from red algae — and are used commercially.
    Agar, obtained from Gelidium and Gracilaria, is used to grow microbes and in preparations of ice-creams and jellies.
    *Chlorella*, a unicellular alga rich in proteins, is used as a food supplement, even by space travellers.
    (b) Economic importance of gymnosperms
    The chapter gives no account of the economic uses of gymnosperms — this is the half of the question the chapter does not answer.
    All it offers that bears on use: gymnosperms are medium-sized to tall trees and shrubs, and the giant redwood Sequoia is one of the tallest tree species — i.e. they are the timber-sized members of the plant kingdom.
    Beyond this chapter (the usual board answer): conifers such as Pinus and Cedrus yield softwood timber and paper pulp; Pinus yields resin and turpentine; the seeds of Pinus gerardiana (chilgoza) are edible; Ephedra is the source of the drug ephedrine; Cycas and Ginkgo are grown as ornamentals.

  6. Exercise 3.6

    Both gymnosperms and angiosperms bear seeds, then why are they classified separately?

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    They are classified separately because of where the ovules and seeds lie.
    In gymnosperms (gymnos: naked, sperma: seeds) the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop after fertilisation are therefore naked.
    In angiosperms the pollen grains and ovules are developed inside specialised structures called flowers, and after fertilisation the seeds are enclosed in fruits.
    So gymnosperms are the naked-seeded plants, while angiosperms are the flowering plants whose seeds are covered — a difference in the protection of the ovule and seed, not merely in the presence of a seed.
    Bearing a seed is a character the two share, but a shared character alone does not put plants in one group; the groups are separated by this consistent difference in the enclosure of the ovule.

  7. Exercise 3.7

    What is heterospory? Briefly comment on its significance. Give two examples.

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    Heterospory is the production of two kinds of spores — macro (large) or megaspores, and micro (small) spores — by the same plant, as against the majority of pteridophytes in which all the spores are of similar kind (homosporous).
    Examples: Selaginella and Salvinia. (Gymnosperms are also heterosporous.)
    Significance
    The megaspores and microspores germinate to give rise to female and male gametophytes respectively, so the sexes are separated at the spore stage itself.
    The female gametophytes are retained on the parent sporophyte for variable periods, instead of being shed.
    The development of the zygote into the young embryo takes place within the female gametophyte, on the parent plant.
    This event is a precursor to the seed habit, considered an important step in evolution.

  8. Exercise 3.8

    Explain briefly the following terms with suitable examples:-
    (i)
    protonema
    (ii)
    antheridium
    (iii)
    archegonium
    (iv)
    diplontic
    (v)
    sporophyll
    (vi)
    isogamy

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    (i) Protonema
    The first stage of the moss gametophyte, which develops directly from a spore.
    It is creeping, green, branched and frequently filamentous; the leafy stage arises from the secondary protonema as a lateral bud. Example: Funaria.
    (ii) Antheridium
    The male sex organ of bryophytes and pteridophytes; multicellular, as all bryophyte sex organs are.
    It produces biflagellate antherozoids, which are released into water and swim to the archegonium. Examples: Marchantia, Funaria, fern prothallus.
    (iii) Archegonium
    The female sex organ; flask-shaped and multicellular, and it produces a single egg.
    Borne by bryophytes and by the pteridophyte prothallus; in gymnosperms the female gametophyte bears two or more archegonia. Examples: Funaria, fern prothallus, Pinus.
    (iv) Diplontic
    A life-cycle pattern in which the diploid sporophyte is the dominant, photosynthetic, independent phase and the haploid gametophyte is reduced to a few cells (the gametes, or a few-celled gametophyte). Examples: gymnosperms and angiosperms; among algae, Fucus.
    This term is not defined anywhere in this chapter — see the note.
    (v) Sporophyll
    A leaf-like appendage that bears (subtends) the sporangia on the sporophyte.
    In some pteridophytes the sporophylls form compact structures called strobili or cones (Selaginella, Equisetum); in gymnosperms the microsporophylls and megasporophylls are arranged spirally along an axis to form male and female cones respectively.
    (vi) Isogamy
    Sexual reproduction by the fusion of two gametes similar in size.
    The gametes may be flagellated, as in Ulothrix, or non-flagellated (non-motile) but still similar in size, as in Spirogyra. Contrast anisogamy (Eudorina) and oogamy (Volvox, Fucus).

  9. Exercise 3.9

    Differentiate between the following:-
    (i)
    red algae and brown algae
    (ii)
    liverworts and moss
    (iii)
    homosporous and heterosporous pteridophyte

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    (i) Red algae (Rhodophyceae) vs brown algae (Phaeophyceae)
    Pigments: red algae have chlorophyll a, d and the red pigment r-phycoerythrin; brown algae have chlorophyll a, c, carotenoids and xanthophylls, chiefly fucoxanthin.
    Colour: red, from the predominance of r-phycoerythrin; brown algae vary from olive green to various shades of brown with the amount of fucoxanthin.
    Stored food: floridean starch (similar to amylopectin and glycogen); mannitol or laminarin.
    Cell wall: cellulose, pectin and polysulphate esters; cellulose, usually covered outside by a gelatinous coating of algin.
    Flagella: absent in red algae — spores and gametes are non-motile; brown algae have $\displaystyle 2$ unequal, laterally attached flagella on pear-shaped zoospores and gametes.
    Plant body: red thalli are mostly multicellular, some with complex organisation; the brown algal body is attached by a holdfast and has a stipe and a frond, and ranges from filamentous Ectocarpus to kelps $\displaystyle 100$ m tall.
    Sexual reproduction: red algae are oogamous, with non-motile gametes and complex post-fertilisation development; brown algae may be isogamous, anisogamous or oogamous.
    Habitat: red algae are mostly marine, in greater concentration in warmer areas, and grow both near the surface and at great depths where little light penetrates; brown algae are found primarily in marine habitats.
    Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium; Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.
    (ii) Liverworts vs mosses
    Plant body: the liverwort gametophyte is thalloid and dorsiventral, closely appressed to the substrate (Marchantia), the leafy members having tiny leaf-like appendages in two rows; the moss gametophyte has upright, slender axes bearing spirally arranged leaves.
    Stages of the gametophyte: liverworts have a single thalloid gametophyte; the moss gametophyte has two stages — the protonema and the leafy stage.
    Rhizoids: mosses are attached to the soil through multicellular and branched rhizoids.
    Asexual reproduction: liverworts reproduce by fragmentation of thalli and by gemmae borne in gemma cups; mosses by fragmentation and by budding in the secondary protonema.
    Sex organs: borne on the thallus in liverworts (on antheridiophores and archegoniophores in Marchantia), and at the apex of the leafy shoots in mosses.
    Sporophyte: both are differentiated into foot, seta and capsule, but the sporophyte in mosses is more elaborate than that in liverworts.
    Spore dispersal: mosses have an elaborate mechanism of spore dispersal.
    Examples: Marchantia; Funaria, Polytrichum, Sphagnum.
    (iii) Homosporous vs heterosporous pteridophytes
    Spores: all spores are of similar kind in homosporous plants — the majority of pteridophytes; heterosporous plants produce two kinds, macro (large) and micro (small) spores.
    Gametophyte: one kind of gametophyte (prothallus), bearing both antheridia and archegonia; in heterosporous forms the megaspores give female gametophytes and the microspores give male gametophytes.
    Fate of the gametophyte: the homosporous prothallus is free-living; the female gametophyte is retained on the parent sporophyte for variable periods in heterosporous forms.
    Embryo: develops after the spore is shed and the prothallus is independent; in heterosporous forms the zygote develops into the young embryo within the female gametophyte, on the parent plant.
    Evolutionary significance: heterospory is a precursor to the seed habit, an important step in evolution; homospory is not.
    Examples: most ferns and other pteridophytes; Selaginella and Salvinia.

  10. Exercise 3.10

    Match the following (column I with column II) Column I
    (a)
    Chlamydomonas
    (b)
    Cycas
    (c)
    Selaginella
    (d)
    Sphagnum Column II
    (i)
    Moss
    (ii)
    Pteridophyte
    (iii)
    Algae
    (iv)
    Gymnosperm

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    (a) Chlamydomonas — (iii) Algae; it is listed in the chapter among the commonly found green algae (Chlorophyceae).
    (b) Cycas — (iv) Gymnosperm; the chapter names Cycas for its unbranched stem, persistent pinnate leaves and coralloid roots associated with \(\displaystyle N_{2}\)-fixing cyanobacteria, and for bearing male cones and megasporophylls on different trees.
    (c) Selaginella — (ii) Pteridophyte; it is the chapter's example of microphyllous, heterosporous pteridophytes (class Lycopsida).
    (d) Sphagnum — (i) Moss; the chapter gives Sphagnum as a common moss and as the moss that provides peat.
    Note that the two columns are not printed in matching order — each pair has to be worked out from the chapter, not read off the row it sits beside.

  11. Exercise 3.11

    Describe the important characteristics of gymnosperms.

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    Important characteristics of gymnosperms (gymnos: naked, sperma: seeds)Habit and vegetative structure
    The ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation; the seeds that develop after fertilisation are therefore naked.
    They are medium-sized to tall trees and shrubs; the giant redwood Sequoia is one of the tallest tree species.
    Roots are generally tap roots. Some genera have a fungal association as mycorrhiza (Pinus); in Cycas small specialised coralloid roots are associated with \(\displaystyle N_{2}\)-fixing cyanobacteria.
    Stems are unbranched (Cycas) or branched (Pinus, Cedrus).
    Leaves may be simple or compound; in Cycas the pinnate leaves persist for a few years. They are well adapted to withstand extremes of temperature, humidity and wind — in conifers the needle-like leaves reduce surface area, and a thick cuticle and sunken stomata reduce water loss.
    Reproduction
    Gymnosperms are 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 or cones.
    Strobili bearing microsporophylls and microsporangia are male (microsporangiate) strobili; cones bearing megasporophylls with ovules or megasporangia are female (macrosporangiate) strobili.
    The two kinds of cones may be borne on the same tree (Pinus), or male cones and megasporophylls may be borne on different trees (Cycas).
    The microspore develops into a highly reduced male gametophyte, confined to a limited number of cells — the pollen grain, which develops within the microsporangium.
    The megaspore mother cell is differentiated from one of the cells of the nucellus; the nucellus with its envelopes is the ovule. The megaspore mother cell divides meiotically to form four megaspores; one of them develops into a multicellular female gametophyte bearing two or more archegonia, retained within the megasporangium.
    Unlike bryophytes and pteridophytes, the male and female gametophytes have no independent, free-living existence — they remain within the sporangia retained on the sporophyte.
    Pollen grains released from the microsporangium are carried by air currents to the opening of the ovules; the pollen tube carrying the male gametes grows towards the archegonia and discharges its contents near their mouth.
    After fertilisation the zygote develops into an embryo and the ovules into seeds, and these seeds are not covered.