Sexual Reproduction in Flowering Plants

2.1 Core concepts

• A typical angiosperm anther is bilobed, dithecous (two theca per lobe) and tetrasporangiate, i.e. four microsporangia located at the four corners (NCERT §1.2.1, p. 5).
• Each microsporangium is surrounded by four wall layers — epidermis, endothecium, middle layers and tapetum; the tapetum nourishes the developing pollen grains and its cells have dense cytoplasm and often more than one nucleus (NCERT §1.2.1, p. 5).
• Sporogenous tissue at the centre of a young microsporangium gives rise to pollen mother cells (PMC); meiosis of the PMC (microsporogenesis) yields a microspore tetrad, and microspores mature into pollen grains released on anther dehiscence (NCERT §1.2.1, pp. 5–6).
• Pollen grains are generally spherical, 25–50 µm across, with a two-layered wall: a hard exine made of sporopollenin (highly resistant to acids, alkali and high temperature; no known enzyme degrades it) bearing germ pores, and an inner intine of cellulose and pectin (NCERT §1.2.1, p. 7).
• At shedding, pollen is 2-celled in over 60% of angiosperms (a large vegetative cell + a small spindle-shaped generative cell); in the remaining species the generative cell divides mitotically before shedding to give the 3-celled stage (NCERT §1.2.1, p. 7).
• Pollen viability varies — only ~30 minutes in rice and wheat, but months in some Rosaceae, Leguminosae and Solanaceae; pollen can be stored for years in liquid nitrogen at −196°C as pollen banks (NCERT §1.2.1, p. 8).
• The gynoecium may be monocarpellary, or multicarpellary with pistils fused (syncarpous, e.g. Papaver) or free (apocarpous, e.g. Michelia); each pistil has stigma, style and ovary, with ovules (megasporangia) borne on the placenta (NCERT §1.2.2, pp. 8–9).
• A typical ovule has a funicle, hilum, one or two integuments, a micropyle, a chalaza, a nucellus, and an embryo sac (female gametophyte) (NCERT §1.2.2, p. 9).
• Megasporogenesis: the megaspore mother cell (MMC) in the micropylar nucellus undergoes meiosis to form four megaspores; typically one is functional and the other three degenerate — monosporic development (NCERT §1.2.2, pp. 9–10).
• The functional megaspore nucleus undergoes three free-nuclear mitotic divisions (2 → 4 → 8 nuclei); cell walls then form, organising a typical mature embryo sac that is 8-nucleate but 7-celled — 2 synergids + 1 egg (egg apparatus at micropylar end), 3 antipodals at chalazal end, and 1 large central cell with 2 polar nuclei (NCERT §1.2.2, pp. 10–11).
• Synergids bear filiform apparatus at the micropylar tip that guides the pollen tube into the synergid (NCERT §1.2.2, p. 11; §1.2.3, p. 17).
• Pollination = transfer of pollen from anther to stigma; by source, it is autogamy (same flower), geitonogamy (different flower of same plant — functionally cross-pollination but genetically autogamous), or xenogamy (different plant — the only type that brings genetically different pollen) (NCERT §1.2.3, pp. 11–12).
• Cleistogamous flowers (e.g. some Viola, Oxalis, Commelina) never open and are invariably autogamous, ensuring seed-set even without pollinators; chasmogamous flowers open and expose anthers and stigma (NCERT §1.2.3, p. 12).
• Agents of pollination are abiotic (wind, water) and biotic (animals); majority of plants use biotic agents. Wind pollination needs light, non-sticky pollen, well-exposed stamens and large feathery stigma; common in grasses, e.g. corn cob (NCERT §1.2.3, pp. 12–13).
• Water pollination is rare (~30 genera, mostly monocots), e.g. Vallisneria, Hydrilla and the marine sea-grass Zostera; pollen is often long, ribbon-like and protected by a mucilaginous covering (NCERT §1.2.3, p. 13).
• Insect-pollinated flowers tend to be large, colourful, fragrant and rich in nectar; rewards include nectar, pollen, and even safe egg-laying sites (Amorphophallus; the obligate Yucca–moth mutualism) (NCERT §1.2.3, pp. 14–15).
• Outbreeding devices to prevent inbreeding depression: (i) non-synchronous pollen release and stigma receptivity, (ii) different positions of anther and stigma, (iii) self-incompatibility (genetic — prevents self-pollen from fertilising by inhibiting germination/tube growth), (iv) unisexual flowers — monoecy (prevents autogamy only) and dioecy (prevents both autogamy and geitonogamy) (NCERT §1.2.3, p. 15).
• Pollen-pistil interaction is a chemical dialogue allowing the pistil to accept compatible pollen or reject incompatible pollen; compatible pollen germinates through a germ pore, the tube grows through the style, enters the ovule via the micropyle, and enters one synergid via the filiform apparatus (NCERT §1.2.3, pp. 15–17).
• Artificial hybridisation tools: emasculation (removal of anthers from a bisexual flower bud before dehiscence) and bagging (covering the emasculated flower with butter paper to exclude unwanted pollen); unisexual female flowers need only bagging (NCERT §1.2.3, p. 17).
• Double fertilisation (unique to flowering plants): one male gamete fuses with the egg (syngamy → diploid zygote); the other fuses with the two polar nuclei in the central cell (triple fusion → triploid primary endosperm nucleus, PEN). The central cell becomes the primary endosperm cell (PEC) (NCERT §1.3, p. 18).
• Endosperm development precedes embryo development: in the common pattern, the PEN undergoes successive free-nuclear divisions (free-nuclear endosperm), then cell walls form (cellular endosperm). Coconut water = free-nuclear endosperm; surrounding white kernel = cellular endosperm (NCERT §1.4.1, pp. 18–19).
• Embryogeny: zygote → proembryo → globular → heart-shaped → mature embryo; early stages are similar in dicots and monocots (NCERT §1.4.2, p. 19).
• A typical dicot embryo has an embryonal axis with two cotyledons; the part above cotyledons is the epicotyl (ending in plumule), the part below is the hypocotyl (ending in radicle with root cap) (NCERT §1.4.2, p. 19).
• A monocot embryo (grass family) has one cotyledon called scutellum (lateral); the radicle and root cap are enclosed in a coleorrhiza, and the shoot apex with leaf primordia is enclosed in a coleoptile (NCERT §1.4.2, pp. 19–20).
• Seed = fertilised ovule; consists of seed coat(s), cotyledon(s) and an embryo axis. Mature seeds are non-albuminous (endosperm fully consumed, e.g. pea, groundnut) or albuminous (endosperm persists, e.g. wheat, maize, barley, castor). Persistent nucellus, when present, is the perisperm (e.g. black pepper, beet) (NCERT §1.4.3, p. 20).
• Integuments harden into the seed coat; the micropyle remains as a pore allowing entry of oxygen and water during germination (NCERT §1.4.3, p. 20).
• Ovary wall develops into the fruit wall (pericarp). False fruits (e.g. apple, strawberry, cashew) involve the thalamus in fruit formation; true fruits develop only from the ovary (NCERT §1.4.3, p. 20).
• Parthenocarpic fruits form without fertilisation (e.g. banana) and can be induced by growth hormones — such fruits are seedless (NCERT §1.4.3, p. 21).
• Record seed longevity: Lupinus arcticus seed from Arctic Tundra germinated after ~10,000 years; a 2,000-year-old viable date palm (Phoenix dactylifera) seed was recovered near the Dead Sea (NCERT §1.4.3, p. 22).
• Apomixis: production of seeds without fertilisation (a form of asexual reproduction mimicking sexual), occurring in some Asteraceae and grasses; mechanisms include a diploid (unreduced) egg developing without fertilisation, and nucellar cells dividing into embryos (common in many Citrus and Mango varieties) (NCERT §1.5, pp. 22–23).
• Polyembryony: occurrence of more than one embryo in a seed — common in the nucellar-embryony Citrus/Mango varieties (NCERT §1.5, p. 23).
• Apomixis is being researched intensively because, if hybrid varieties could be made apomictic, farmers would not need to buy hybrid seed every year as character segregation in the progeny would be avoided (NCERT §1.5, p. 23).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 1.1 (p. 4) — L.S. of a flower showing thalamus, sepals, petals, stamens and pistil.
• Figure 1.2 (p. 5) — Typical stamen (filament + bilobed anther) and 3-D cut section of an anther showing four microsporangia.
• Figure 1.3 (p. 6) — T.S. of a young anther; enlarged microsporangium with the four wall layers; mature dehisced anther.
• Figure 1.5 (p. 7) — Pollen tetrad and stages of microspore maturation into a 2-celled pollen grain (vegetative + generative cell).
• Figure 1.7 (p. 9) — Hibiscus pistil; syncarpous Papaver; apocarpous Michelia; anatropous ovule (funicle, hilum, integuments, micropyle, nucellus, embryo sac, chalaza).
• Figure 1.8 (p. 10) — MMC, dyad and megaspore tetrad; 2-, 4-, 8-nucleate embryo sac and the mature 7-celled/8-nucleate sac (egg apparatus, central cell, antipodals).
• Figure 1.9 (p. 12) — Self-pollinated, cross-pollinated and cleistogamous flowers.
• Figure 1.11 (p. 14) — Pollination by water (Vallisneria) and by insects.
• Figure 1.12 (p. 16) — Pollen germination on stigma; pollen tube growth through style; entry of pollen tube into a synergid via filiform apparatus; discharge of two male gametes — one into the egg, the other into the central cell.
• Figure 1.13 (p. 18) — Fertilised embryo sac with zygote and PEN; stages of dicot embryo development.
• Figure 1.14 (p. 19) — Typical dicot embryo (epicotyl, hypocotyl, cotyledons, radicle); L.S. of a grass embryo (scutellum, coleoptile, coleorrhiza).
• Figure 1.15 (pp. 20–21) — Structures of some seeds; false fruits of apple and strawberry.

2.4 Common confusions / NTA trap points

• 7-celled vs 8-nucleate: the mature embryo sac has 8 nuclei but only 7 cells, because the two polar nuclei sit together inside one large central cell.
• Geitonogamy looks like cross-pollination (a pollinator is involved, pollen comes from a different flower) but is genetically equivalent to autogamy — the pollen comes from the same plant. Only xenogamy brings genetically different pollen.
• Dioecy prevents both autogamy and geitonogamy; monoecy (e.g. castor, maize) prevents autogamy but not geitonogamy.
• Triple fusion involves three haploid nuclei (one male gamete + two polar nuclei), but it is a single fusion event yielding a triploid PEN — not three separate fusions.
• "Pollination" is just transfer; "fertilisation" is fusion. CUET often disguises this in stems about Vallisneria, corn cob, etc.
• False fruit (apple, strawberry, cashew) involves the thalamus, not just the ovary; true fruit develops only from the ovary.
• Albuminous seeds (wheat, maize, barley, castor) retain endosperm; non-albuminous seeds (pea, groundnut) consume it entirely.
• Coleoptile encloses the shoot apex (above), coleorrhiza encloses the radicle (below) — students often swap them.