Short & Long Answer Questions Sexual Reproduction in Flowering Plants -

Short Answer Questions

Q1: What is the function of the anther in a flower?
Ans: The anther produces and releases pollen grains, each containing male gametes (sperm cells). It therefore provides the male gametophytes necessary for fertilisation and is essential for sexual reproduction in flowering plants.

Q2: Define microsporogenesis and name the cell where it occurs.
Ans: Microsporogenesis is the process by which microspore mother cells (microsporocytes) undergo meiosis to form haploid microspores that develop into pollen grains. It occurs in the microsporangium (pollen sac) of the anther.

Q3: What is the role of synergids in the embryo sac?
Ans: Synergids are two specialised cells at the micropylar end of the embryo sac; they have a filiform apparatus that directs and guides the pollen tube toward the egg cell and help facilitate fertilisation of the egg.

Q4: Differentiate between autogamy and geitonogamy.
Ans: Autogamy is pollination and fertilisation within the same flower (self-pollination in one flower), whereas geitonogamy is transfer of pollen between different flowers of the same plant. Both involve genetically similar gametes, but geitonogamy involves distinct floral units.

Q5: Why is sporopollenin significant in pollen grains?
Ans: Sporopollenin forms the tough outer wall (exine) of pollen grains; it is chemically inert and highly resistant to physical and biological degradation, protecting the pollen during dispersal and allowing pollen to fossilise.

Q6: Comment on viability of pollen grains.
Ans: Pollen viability varies widely with species and environmental conditions such as temperature and humidity. Some crop pollens (e.g., rice, wheat) remain viable for only a short time (about 30 minutes), while others (for example certain species in Rosaceae, Leguminosae and Solanaceae) can remain viable for days or even months under favourable conditions.

Q7: Explain the role of 2 male gametes during double fertilization.
Ans: In double fertilisation, the pollen tube delivers two male gametes into the embryo sac. One male gamete fuses with the egg cell to form the diploid zygote (syngamy), which develops into the embryo. The second male gamete fuses with the two polar nuclei (or the diploid polar nucleus) in the central cell to form the triploid primary endosperm nucleus (PEN), which develops into endosperm to nourish the embryo.

Q8: What is the fate of antipodal cells in the embryo sac?
Ans: Antipodal cells, located at the chalazal end of the embryo sac, generally have a short-lived existence and usually degenerate after fertilisation. They have no essential, universal role in embryo formation, though in some species they may be nutritive or persistent.

Q9: Define apomixis and give one example.
Ans: Apomixis is seed formation without fertilisation, where embryos develop from diploid somatic cells of the ovule (e.g., nucellus or integuments) or from an unfertilised egg cell, producing progeny genetically identical to the mother. Example: Citrus (some species) show apomixis.

Q10: What is the significance of the seed coat in a seed?
Ans: The seed coat, derived from the ovule integuments, protects the embryo from mechanical injury, desiccation and pathogens, and controls water uptake and gas exchange during dormancy and germination.

Long Answer Questions

Q1: Describe the structure of a mature anther and the process of microsporogenesis.
Ans: 
Structure of a mature anther
A mature anther is typically bilobed, each lobe containing two microsporangia (pollen sacs), so an anther usually has four microsporangia. The anther wall is organised in four layers from outside to inside: epidermis (protective outer layer), endothecium (provides mechanical support and helps in dehiscence), one or more middle layers (temporary, usually degenerate), and tapetum (nutritive tissue that supports developing pollen and contributes materials for pollen wall formation). The pollen mother cells are located within the microsporangia surrounded by tapetal cells.

Process of Microsporogenesis  
Microsporogenesis begins when diploid microspore mother cells (microsporocytes) in the microsporangium undergo meiosis. Each microsporocyte undergoes one meiotic division producing a tetrad of four haploid microspores. These microspores separate (or remain temporarily in a tetrad) and each develops into a pollen grain. During pollen development, the microspore differentiates into a vegetative (tube) cell and a generative cell (which may divide to form two sperm cells either before pollen release or within the pollen tube). Pollen grains acquire a two-layered wall: an outer, sculptured exine rich in sporopollenin and an inner intine of cellulose and pectins. Finally, the anther dehisces to release mature pollen grains for dispersal.

Q2: Give a detailed account of the structure of the embryo sac and the process of megasporogenesis.
Ans:  
Structure of Embryo Sac
The typical embryo sac (female gametophyte) in most angiosperms is the Polygonum type and is 7-celled and 8-nucleate. Its components are:

• The egg apparatus at the micropylar end: one egg cell flanked by two synergids; the synergids possess a filiform apparatus to guide the pollen tube.
• Three antipodal cells at the chalazal end, usually short-lived.
• A large central cell containing two polar nuclei (which may fuse to form a diploid secondary nucleus) — this central cell gives rise to the endosperm after fertilisation.

Process of Megasporogenesis

Megasporogenesis starts in the nucellus of the ovule when a single megaspore mother cell (MMC) undergoes meiosis to produce a linear tetrad of four haploid megaspores. Typically, the three megaspores closest to the micropylar end degenerate, and the chalazal-most megaspore remains functional. This functional megaspore enlarges and undergoes three successive mitotic divisions without cytokinesis to form an eight-nucleate, seven-celled embryo sac. The nuclei are then arranged and cellularised to form the mature embryo sac with the egg apparatus, central cell with two polar nuclei and three antipodals.

Q3: Discuss the process of double fertilization and its significance in angiosperms.
Ans: Double fertilisation is a distinctive feature of angiosperms in which two fertilisation events occur in the embryo sac. After pollination, a compatible pollen grain germinates on the stigma and forms a pollen tube that grows through the style to reach the ovule. The pollen’s generative cell divides to form two sperm cells. The pollen tube penetrates the embryo sac (usually via a synergid), releasing the two sperm cells. One sperm fuses with the egg cell nucleus to form the diploid zygote (syngamy), which develops into the embryo. The other sperm fuses with the two polar nuclei in the central cell to form the triploid primary endosperm nucleus (PEN), which gives rise to the endosperm. Significance: double fertilisation ensures that endosperm (nutritive tissue) only develops where the embryo is formed, efficiently allocating resources; it also establishes the nutritional support needed by the developing embryo and is linked to greater seed viability and evolutionary success of flowering plants.

Q4: Explain the development of the embryo and endosperm in a dicot seed.
Ans: After double fertilisation in a dicot, the zygote first divides asymmetrically to form a small terminal cell and a larger basal cell, forming the proembryo. The terminal cell gives rise to the embryo proper and passes through globular, heart-shaped and torpedo stages, during which cotyledons, plumule (future shoot) and radicle (future root) are formed. The basal cell typically forms the suspensor, which anchors and pushes the embryo into nutritive tissue. The primary endosperm nucleus (PEN) undergoes successive free nuclear divisions to form a multinucleate endosperm; later cell walls form to produce a cellular endosperm that nourishes the embryo. In many dicots (for example pea), the endosperm is consumed during embryo development and nutrients are stored in the cotyledons; in others the endosperm persists in the mature seed.

Q5: Describe the structure of a mature seed and the role of its components in germination.
Ans: A mature dicot seed typically consists of:

• Seed coat (testa), derived from ovule integuments — provides protection, regulates water entry and prevents mechanical damage and infection.
• Embryo, consisting of cotyledons (one or two in dicots), plumule (embryonic shoot), radicle (embryonic root) and hypocotyl — the radicle emerges first during germination to form the root, and the plumule develops into the shoot.
• Endosperm (if present) — nutritive tissue that supplies carbohydrates, proteins and lipids to the growing embryo; in many dicots the endosperm is absorbed into the cotyledons during seed maturation.

During germination, the seed coat softens and ruptures, water uptake awakens metabolic processes, stored food is mobilised by the embryo (or by endosperm mobilisation), the radicle protrudes to anchor the seedling and the plumule emerges to begin photosynthesis.

Q6: Discuss apomixis and polyembryony, highlighting their significance in plant breeding.
Ans: Apomixis is the formation of seeds without fertilisation; embryos arise from diploid somatic cells of the ovule (apospory) or from an unfertilised egg (parthenogenesis), bypassing meiosis and syngamy. Progeny are genetically identical to the mother plant. Polyembryony is the occurrence of two or more embryos in a single seed, arising from multiple fertilised eggs or from nucellar or integumentary cells (nucellar embryony). Significance in plant breeding: apomixis allows fixation and clonal propagation of superior hybrids without segregation of traits, maintaining hybrid vigour across generations; polyembryony can produce multiple uniform seedlings from one seed, useful for clonal propagation and mass production of disease-free planting material. Both phenomena can be exploited to maintain desirable genotypes in crop improvement programmes.

Q7: Explain the role of pollinators and the adaptations in flowers that attract them.
Ans: Pollinators (insects, birds, bats, wind and water) transfer pollen from anther to stigma, enabling cross-pollination and genetic recombination. Flowers have evolved numerous adaptations to attract specific pollinators and improve pollination success, including:

• Colour and scent: Brightly coloured petals and specific fragrances attract visual- or scent-oriented pollinators (e.g., bees, butterflies, moths).

• Nectar and pollen rewards: Nectar produced in nectaries and protein-rich pollen reward animals and encourage repeated visits.

• Pollen characteristics: Sticky or spiny pollen adheres to pollinators; dry, powdery pollen suits wind dispersal.

• Flower shape and size: Tubular flowers suit long-billed birds or moths, flat landing platforms suit bees; corolla shape matches the pollinator’s feeding structures.

• Specialised mechanisms: Some plants show co-evolution with pollinators (for example yucca and yucca moth), while wind-pollinated plants (grasses, many trees) have small, inconspicuous flowers with feathery stigmas and abundant light pollen to facilitate wind dispersal.

These adaptations increase pollination efficiency and reproductive success by matching flower traits to the behaviour and morphology of their pollinators.