Chapter 1: Sexual Reproduction in Flowering Plants Long Questions | biology Class 12 CBSE

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Chapter 1: Sexual Reproduction in Flowering Plants Long Questions | biology Class 12 CBSE | ByteNotes.in

Long Answer

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Question 1

Long Form

Describe the structure of a microsporangium and explain the process of microsporogenesis. How does the mature pollen grain differ in structure from the microspore?

A microsporangium appears nearly circular in transverse section and is surrounded by four wall layers: epidermis (protection), endothecium (dehiscence), middle layers (protection), and tapetum (nourishes pollen). The sporogenous tissue occupies the centre.
During microsporogenesis, cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads — clusters of four haploid microspores.
As the anther matures and dehydrates, microspores dissociate from the tetrad and develop into pollen grains. Each pollen grain develops a two-layered wall: outer exine (sporopollenin, with germ pores) and inner intine (cellulose and pectin).
A mature pollen grain is either 2-celled (vegetative cell + generative cell) — in over 60% of angiosperms — or 3-celled (vegetative cell + 2 male gametes) if the generative cell divides before shedding.
Thus, the microspore matures into a male gametophyte (pollen grain) that is structurally complex, carrying the cells needed to achieve fertilisation.

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Long Answer

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Question 2

Long Form

Compare the development of the male gametophyte and the female gametophyte in angiosperms with respect to location, cell divisions, and final structure.

The male gametophyte develops inside the microsporangia of the anther. Pollen mother cells (PMC) undergo meiosis to form microspore tetrads; each microspore matures into a pollen grain (male gametophyte).
The female gametophyte (embryo sac) develops inside the nucellus of the ovule. The megaspore mother cell (MMC) undergoes meiosis; only one functional megaspore is retained.
The functional megaspore divides mitotically three times (free nuclear divisions) to form 8 nuclei; cell wall formation produces the 7-celled, 8-nucleate embryo sac.
The mature male gametophyte is 2-celled (or 3-celled) and microscopic; the mature female gametophyte contains the egg apparatus (egg + 2 synergids), 3 antipodals, and a central cell with 2 polar nuclei.
Both gametophytes are highly reduced and dependent on the sporophyte; the male gametophyte is produced in large numbers, while only one embryo sac typically forms per ovule.

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Long Answer

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Question 3

Long Form

Explain the various outbreeding devices in flowering plants. How do these devices promote genetic diversity in plant populations?

Pollen release and stigma receptivity may be unsynchronised (protandry or protogyny), ensuring pollen from the same flower cannot fertilise its own ovules, favouring cross-pollination.
Herkogamy: anthers and stigma may be placed at different positions within the same flower so that self-pollination is mechanically impossible.
Self-incompatibility is a genetic mechanism that inhibits pollen germination or pollen tube growth in the pistil when self-pollen lands, preventing fertilisation.
Unisexual flowers (monoecy — e.g., maize, castor) prevent autogamy; dioecy (e.g., papaya) prevents both autogamy and geitonogamy, ensuring obligate cross-pollination.
These devices collectively promote xenogamy (cross-pollination), introducing new genetic combinations in the progeny and reducing inbreeding depression, thereby increasing the adaptive potential and genetic diversity of plant populations.

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Question 4

Long Form

Describe the events of double fertilisation in angiosperms. What are the products formed and what role does each play in seed development?

After reaching the embryo sac, the pollen tube enters one synergid through the filiform apparatus and releases two male gametes.
Syngamy: one male gamete fuses with the egg cell to form a diploid (2n) zygote, which develops into the embryo.
Triple fusion: the other male gamete fuses with the two polar nuclei in the central cell to form the triploid (3n) primary endosperm nucleus (PEN).
Since two fusion events occur within a single embryo sac, the process is called double fertilisation — a unique feature of angiosperms.
The zygote develops into the embryo (the future plant) while the PEN forms the endosperm, a triploid nutritive tissue that provides food reserves for the developing embryo. Endosperm development always precedes embryo development.

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Question 5

Long Form

Analyse the structural differences between a dicotyledonous and a monocotyledonous embryo. How are these differences reflected in the seedling structure?

A dicotyledonous embryo consists of an embryonal axis with two cotyledons. The portion above the cotyledons is the epicotyl (bearing the plumule/shoot tip); the portion below is the hypocotyl (terminating in the radicle covered by the root cap).
A monocotyledonous embryo (e.g., grass) has one lateral cotyledon called the scutellum. The embryonal axis has the radicle enclosed in the coleorrhiza and the shoot apex enclosed in the coleoptile.
In dicots, the two cotyledons are thick and store food reserves; in monocots, the scutellum is a shield-shaped absorptive cotyledon that digests endosperm during germination.
Coleoptile (protective sheath around the shoot apex) and coleorrhiza (protective sheath around the radicle) are unique to monocot embryos, absent in dicots.
These structural differences are directly reflected in the seedling: dicot seedlings show two seed leaves (cotyledons), while monocot seedlings emerge with a single leaf; root emergence patterns also differ due to coleorrhiza and radicle organisation.

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Long Answer

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Question 6

Long Form

Describe the process of pollen-pistil interaction. How does the pistil distinguish between compatible and incompatible pollen, and what happens in each case?

Pollen-pistil interaction includes all events from pollen landing on the stigma to pollen tube entry into the ovule. The pistil recognises incoming pollen through chemical signals.
Compatible pollen: the pistil accepts the pollen, the pollen grain germinates on the stigma producing a pollen tube through a germ pore, and the tube grows through the style toward the ovary.
Incompatible pollen: the pistil rejects the pollen by inhibiting pollen germination on the stigma or arresting pollen tube growth within the style.
The recognition is mediated by chemical interactions between molecular components of pollen and the pistil — a continuous molecular dialogue that determines the outcome.
The pollen tube enters the ovule through the micropyle, guided by the filiform apparatus of the synergid, and discharges the two male gametes into the synergid to initiate fertilisation.

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Question 7

Long Form

Explain the role of endosperm in angiosperm seed development. Describe its origin, development, and fate in different types of seeds.

Endosperm originates from the primary endosperm nucleus (PEN, 3n) formed by triple fusion of one male gamete and two polar nuclei. It is triploid.
In the most common type, PEN undergoes repeated free nuclear divisions forming free nuclei (free-nuclear endosperm), followed by cell wall formation to produce cellular endosperm. Coconut water is free-nuclear endosperm; coconut kernel is cellular endosperm.
The endosperm provides nutrition (food reserves) to the developing embryo; its development always precedes embryo development.
In non-albuminous seeds (pea, groundnut), the endosperm is completely consumed by the developing embryo before seed maturation, leaving no residual endosperm at maturity.
In albuminous seeds (wheat, maize, castor, coconut), endosperm persists in the mature seed and serves as a food reserve during seed germination.

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Long Answer

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Question 8

Long Form

Evaluate the significance of apomixis in agriculture. How does it help solve the problem associated with hybrid seed production?

Apomixis is asexual seed production without fertilisation. Apomictic seeds are genetically identical to the parent plant (clones), ensuring that hybrid characters are maintained without segregation in the progeny.
Hybrid varieties of many crops show increased productivity (hybrid vigour), but hybrid seeds must be produced fresh every year because seeds collected from hybrids segregate and do not maintain hybrid characters.
Production of hybrid seeds is costly, making them expensive for farmers. If hybrids were converted into apomicts, farmers could save and reuse seeds year after year while maintaining hybrid vigour.
Active research is ongoing globally to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.
Apomixis would also ensure seed uniformity in crops, reduce dependence on seed companies, lower production costs, and make improved crop varieties more accessible to farmers in developing countries.

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Question 9

Long Form

Compare pollination by wind (anemophily) and pollination by insects (entomophily) with respect to floral adaptations, pollen characteristics, and efficiency.

Wind-pollinated flowers: small, inconspicuous, not colourful, no nectar or fragrance; stamens are well-exposed with feathery stigmas to trap airborne pollen.
Insect-pollinated flowers: large, brightly coloured, fragrant, and nectar-rich to attract pollinators; anthers and stigmas are positioned to contact the insect body.
Wind pollen is light, non-sticky, and produced in enormous quantities to compensate for the random, wasteful transfer; insect pollen is sticky and produced in smaller quantities.
Anemophilous plants often have single ovules per ovary and flowers in dense inflorescences (e.g., corn cob); entomophilous plants have fewer flowers but higher precision in pollen transfer.
Wind pollination is less efficient in terms of pollen utilisation but works well in open habitats; insect pollination is more targeted and efficient, especially in dense vegetation, and the mutualistic relationship benefits both plant and pollinator.

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Long Answer

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Question 10

Long Form

Describe the structure of a typical angiosperm ovule. Explain how the ovule transforms into the seed after fertilisation.

A typical anatropous ovule is attached to the placenta by the funicle; funicle meets the ovule body at the hilum. It is surrounded by one or two integuments enclosing the nucellus; micropyle is the opening at the tip and chalaza is the basal part.
Inside the nucellus, the embryo sac (female gametophyte) is located, containing the egg apparatus, antipodals, and central cell with polar nuclei.
After fertilisation, the zygote develops into the embryo and PEN develops into the endosperm within the nucellus.
The integuments harden to form the protective seed coat (testa and tegmen in bitegmic ovules); the micropyle persists as a small pore in the seed coat facilitating entry of water and oxygen during germination.
Water content of the developing seed decreases; metabolic activity slows; the embryo may enter dormancy. The funicle leaves a scar (hilum) on the mature seed coat.

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Chapter 1: Sexual Reproduction in Flowering Plants | Long Questions

Describe the structure of a microsporangium and explain the process of microsporogenesis. How does the mature pollen grain differ in structure from the microspore?

Compare the development of the male gametophyte and the female gametophyte in angiosperms with respect to location, cell divisions, and final structure.

Explain the various outbreeding devices in flowering plants. How do these devices promote genetic diversity in plant populations?

Describe the events of double fertilisation in angiosperms. What are the products formed and what role does each play in seed development?

Analyse the structural differences between a dicotyledonous and a monocotyledonous embryo. How are these differences reflected in the seedling structure?

Describe the process of pollen-pistil interaction. How does the pistil distinguish between compatible and incompatible pollen, and what happens in each case?

Explain the role of endosperm in angiosperm seed development. Describe its origin, development, and fate in different types of seeds.

Evaluate the significance of apomixis in agriculture. How does it help solve the problem associated with hybrid seed production?

Compare pollination by wind (anemophily) and pollination by insects (entomophily) with respect to floral adaptations, pollen characteristics, and efficiency.

Describe the structure of a typical angiosperm ovule. Explain how the ovule transforms into the seed after fertilisation.

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