Exercises
What is the basis of classification of algae?
Answer
Algae are classified primarily on the basis of:
(i) Nature of pigments: Types and combinations of photosynthetic pigments present. Chlorophyceae have chlorophyll a and b; Phaeophyceae have chlorophyll a, c and fucoxanthin; Rhodophyceae have chlorophyll a, d and phycoerythrin.
(ii) Stored food: Green algae store starch; brown algae store laminarin and mannitol; red algae store floridean starch.
(iii) Cell wall composition: Green algae have cellulose; brown algae have cellulose with algin; red algae have cellulose with pectin.
(iv) Flagella: Number, position and type of flagella on motile cells differ across groups.
(v) Habitat and gross morphological features also contribute to classification.
When and where does reduction division take place in the life cycle of a liverwort, a moss, a fern, a gymnosperm and an angiosperm?
Answer
(i) Liverwort: Reduction division (meiosis) occurs in the sporophyte during spore formation inside the capsule (sporangium). The sporophyte produces haploid spores.
(ii) Moss: Meiosis occurs in the sporophyte's capsule to produce haploid spores.
(iii) Fern: Meiosis occurs in the sporangia (on the underside of sporophyll leaves) to produce haploid spores.
(iv) Gymnosperm: Meiosis occurs in the pollen mother cells within pollen sacs (microsporangia) to form pollen grains, and in megasporangia to form megaspores.
(v) Angiosperm: Meiosis occurs in the anthers (to form pollen grains) and in the ovule (to form the embryo sac). The reduction division is in the pollen mother cells and megaspore mother cells.
Name three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them.
Answer
Three groups that bear archegonia: Bryophytes, Pteridophytes, and Gymnosperms.
Life cycle of a Fern (Pteridophyte)
The dominant phase in the fern life cycle is the diploid sporophyte. It bears sporangia on the underside of leaves (fronds). Meiosis in sporangia produces haploid spores. These spores germinate into a small, heart-shaped, free-living, photosynthetic gametophyte called the prothallus. The prothallus bears antheridia (male organs, producing biflagellate antherozoids) and archegonia (female organs, producing eggs). Fertilisation of the egg by antherozoid (requires water) produces a diploid zygote, which develops into a new sporophyte. The sporophyte remains temporarily attached to the prothallus but eventually becomes independent. This is a heteromorphic alternation of generations with the sporophyte dominant.
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 fern; gemma cell in Marchantia; meristem cell of monocot; ovum of a liverwort; and zygote of a fern.
Answer
• Protonemal cell of a moss: Haploid (n) – the protonema is part of the gametophyte.
• Primary endosperm nucleus in dicot: Triploid (3n) – formed by fusion of two polar nuclei (2n) + one male gamete (n).
• Leaf cell of a moss: Haploid (n) – the leafy moss plant is the gametophyte.
• Prothallus cell of a fern: Haploid (n) – the prothallus is the gametophyte.
• Gemma cell in Marchantia: Haploid (n) – gemmae are produced by the gametophyte.
• Meristem cell of monocot: Diploid (2n) – the plant body is the sporophyte.
• Ovum of a liverwort: Haploid (n) – the ovum is a gamete produced by the gametophyte.
• Zygote of a fern: Diploid (2n) – formed by fusion of two haploid gametes.
Write a note on economic importance of algae and gymnosperms.
Answer
Economic importance of Algae
(i) Algae contribute about 50% of total CO₂ fixation on earth (main source of atmospheric oxygen).
(ii) Chlorella and Spirulina are protein-rich food supplements.
(iii) Laminaria (kelp) and Sargassum are used as food in Japan and coastal regions.
(iv) Agar extracted from red algae (Gelidium, Gracilaria) is used in microbiological culture media, food industry, and pharmaceuticals.
(v) Algin (from brown algae) and carrageenan (from red algae) are used as emulsifiers in ice cream, paints, and cosmetics.
(vi) Diatomaceous earth (from diatoms) is used as a filtering and polishing agent.
Economic importance of Gymnosperms
(i) Wood from conifers (pine, cedar, spruce) is used in timber, paper, and furniture industries.
(ii) Pinus yields resin and turpentine oil used in paints, varnishes, and medicines.
(iii) Seeds of Pinus gerardiana (chilgoza) are edible.
(iv) Cycas provides sago (starch) and the leaves are used as ornamentals.
(v) Taxus yields taxol, used as an anti-cancer drug.
Both gymnosperms and angiosperms bear seeds, then why are they classified separately?
Answer
Although both bear seeds, they differ significantly:
(i) In gymnosperms, seeds are naked (not enclosed in a fruit) – the ovules are borne on megasporophylls that are not fused to form a closed ovary. In angiosperms, seeds are enclosed within a fruit (the ovary wall forms the fruit).
(ii) Angiosperms have flowers with well-differentiated sepals, petals, stamens, and carpels; gymnosperms have only strobili (cones).
(iii) Angiosperms undergo double fertilisation and form a triploid endosperm; gymnosperms lack double fertilisation and their endosperm is haploid (formed before fertilisation).
(iv) The vascular tissue of angiosperms has vessels (more efficient); gymnosperms typically have only tracheids.
What is heterospory? Briefly comment on its significance. Give two examples.
Answer
Heterospory is the production of two types of spores – smaller microspores (which germinate into male gametophytes) and larger megaspores (which germinate into female gametophytes) – by the same plant. This is in contrast to homospory, where only one type of spore is produced.
Significance: Heterospory is considered a significant evolutionary step towards the seed habit. The megaspore is retained on the parent sporophyte, and the female gametophyte develops within the megaspore wall, receiving nourishment from the parent plant. This retention is the precursor to the development of the ovule and ultimately the seed in gymnosperms and angiosperms. It ensures better protection and nourishment of the developing embryo.
Examples: Selaginella (spike moss) and Salvinia (a water fern).
Q8. Explain briefly the following terms with suitable examples
(i) protonema (ii) antheridium (iii) archegonium (iv) diplontic (v) sporophyll (vi) isogamy
Ans.
(i) Protonema: An early, filamentous, green, branched stage that develops on germination of moss spores. It eventually gives rise to the leafy gametophyte. Example: in Funaria (a common moss), the protonema is the first structure to emerge from the spore.
(ii) Antheridium: The male sex organ in bryophytes, pteridophytes, and gymnosperms, which produces motile male gametes (antherozoids). Example: In liverworts like Marchantia, antheridia are borne on antheriophores.
(iii) Archegonium: The flask-shaped female sex organ in bryophytes, pteridophytes, and gymnosperms, producing the egg cell. Example: In ferns, archegonia are present on the prothallus.
(iv) Diplontic: A life cycle in which the dominant photosynthetic phase is the diploid sporophyte (2n). Meiosis produces haploid gametes, which fuse to restore diploidy. The gametophyte is very much reduced or absent. Example: All angiosperms and gymnosperms are diplontic.
(v) Sporophyll: A leaf or leaf-like structure bearing sporangia. In ferns, sporophylls are ordinary leaves bearing sori. In gymnosperms and some pteridophytes like Selaginella, sporophylls are specialised and aggregated into strobili (cones). Example: Leaves of ferns bearing sori; cone scales of Pinus.
(vi) Isogamy: Sexual reproduction involving the fusion of morphologically and physiologically similar (identical in size and form) gametes, both of which are motile. Example: In green algae like Ulothrix and Spirogyra, similar-looking gametes fuse.
Q9. Differentiate between the following
(i) red algae and brown algae (ii) liverworts and moss (iii) homosporous and heterosporous pteridophyte
Ans.
(i) Red algae vs Brown algae:
Red algae (Rhodophyceae): Mostly marine, possess chlorophyll a, d and phycoerythrin (red pigment), store floridean starch, cell wall has cellulose and pectin, mostly non-motile. Example: Gracilaria, Polysiphonia.
Brown algae (Phaeophyceae): Mostly marine, possess chlorophyll a, c and fucoxanthin (brown pigment), store laminarin and mannitol, cell wall has cellulose and algin, motile cells have two laterally attached flagella. Example: Laminaria, Fucus, Sargassum.
(ii) Liverworts vs Moss:
Liverworts: Gametophyte is dorsiventral, thalloid (flat, ribbon-like) or leafy; rhizoids are unicellular. Example: Marchantia.
Mosses: Gametophyte is erect, leafy with stem-like and leaf-like structures; they have an additional protonema stage; rhizoids are multicellular. Example: Funaria, Polytrichum.
(iii) Homosporous vs Heterosporous pteridophyte:
Homosporous: Produces only one type of spore; the gametophyte is bisexual (bears both antheridia and archegonia). Example: Nephrolepis (fern), Equisetum.
Heterosporous: Produces two types of spores – microspores and megaspores; gives rise to separate male and female gametophytes. Example: Selaginella, Salvinia.
Q10. 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
Ans.
(a) Chlamydomonas – (iii) Algae
(b) Cycas – (iv) Gymnosperm
(c) Selaginella – (ii) Pteridophyte
(d) Sphagnum – (i) Moss
Describe the important characteristics of gymnosperms.
Answer
Important characteristics of gymnosperms:
(i) Gymnosperms are plants with naked seeds – seeds are not enclosed in fruits (ovules are not enclosed in an ovary).
(ii) They are mostly tall, evergreen, woody trees or shrubs; the largest living trees are gymnosperms (Sequoia).
(iii) Roots may have mycorrhizae (Pinus) or coralloid roots with nitrogen-fixing cyanobacteria (Cycas).
(iv) Leaves are needle-like (adapted to cold/dry conditions) or broad and pinnate (Cycas).
(v) They bear male and female cones (strobili) – unisexual reproductive structures; some are dioecious.
(vi) Pollen grains are produced in microsporangia and transported by wind (anemophilous).
(vii) Double fertilisation does not occur; a single fertilisation produces a diploid zygote.
(viii) The endosperm is haploid (developed before fertilisation from the female gametophyte).
(ix) Vascular tissue (xylem) mostly consists of tracheids (no vessels except in Gnetum).
Important Extra Questions
What is the significance of the alternation of generations in the plant kingdom?
Answer
Alternation of generations is the alternation between a haploid gametophyte and a diploid sporophyte in the life cycle of plants. It ensures genetic variation (through meiosis and fertilisation), allows organisms to adapt to diverse environments, and represents an evolutionary progression from gametophyte dominance (bryophytes) to sporophyte dominance (angiosperms), which correlates with greater complexity and terrestrial adaptation.
Why are bryophytes called 'amphibians of the plant kingdom'?
Answer
Bryophytes are called amphibians because they can live on land but require water for fertilisation. Their motile antherozoids (sperm) need water as a medium to swim from the antheridia to the archegonia on the same or different plants. They are terrestrial in vegetative phase but depend on water for their sexual reproduction, just like amphibian animals.
What is oogamy? How does it differ from anisogamy?
Answer
Oogamy is sexual reproduction involving the fusion of a large, non-motile female gamete (egg/ovum) with a small, motile male gamete (antherozoid/sperm). Anisogamy involves the fusion of two morphologically different but both motile gametes. In oogamy, only the male gamete is motile; in anisogamy, both gametes are motile but differ in size. Oogamy represents the highest degree of differentiation between gametes and is seen in higher plants and animals.
What are sori? Where are they found?
Answer
Sori (singular: sorus) are clusters of sporangia (spore-bearing structures) found on the underside (abaxial surface) of the fronds (leaves) of ferns. Each sorus is often covered by a flap of tissue called the indusium. Spores produced in the sporangia by meiosis are released and germinate to form the gametophyte (prothallus).
Compare the dominant phase in bryophytes, pteridophytes, gymnosperms and angiosperms.
Answer
In bryophytes, the dominant, conspicuous photosynthetic phase is the haploid gametophyte; the sporophyte is dependent on it. In pteridophytes, both phases are free-living, but the dominant phase is the diploid sporophyte; the gametophyte (prothallus) is small but independent. In gymnosperms and angiosperms, the dominant, elaborate phase is the diploid sporophyte; the gametophyte is highly reduced and completely dependent on the sporophyte.