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BiologyPlant Reproduction
Arrange the layers present around the female gamete of an angiosperm from outermost to innermost:
A. Parietal cell layer
B. Middle layer
C. Integuments
D. Nucellus
Options
1
C,A,D,B
2
C,A,B,D
3
D,B,A,C
4
A,C,B,D
Correct Answer
Option 2: C, A, B, D
Solution
1

Layers from outside to inside in angiosperm ovule:

C — Integuments (outermost protective layer → becomes seed coat)

A — Parietal cell layer

2

B — Middle layer

D — Nucellus (innermost — contains embryo sac with female gamete)

Sequence: C → A → B → D

Outermost to innermost: Integuments → Parietal cell layer → Middle layer → Nucellus
C, A, B, D
Theory: Plant Reproduction
1. Ovule Structure in Angiosperms

The ovule is the structure inside the ovary that develops into a seed after fertilisation. Structure from outside to inside: Funiculus: stalk attaching ovule to placenta of ovary wall. Hilum: point of attachment. Integuments: protective layers. Most angiosperms = bitegmic (two integuments: outer and inner). Some = unitegmic (one integument — Asteraceae, Solanaceae). Micropyle: opening at the tip where pollen tube enters. Chalaza: base of ovule, opposite micropyle. Nucellus: parenchymatous nutritive tissue enclosed by integuments. Homologous to megasporangium. Embryo sac: female gametophyte develops within nucellus. Parietal cell layer and middle layer: nucellar tissue layers around the embryo sac. After fertilisation: integuments → seed coat (outer integument → testa, inner integument → tegmen). Nucellus → perisperm (in some seeds). Embryo sac → embryo + endosperm.

2. Megasporogenesis

Megasporogenesis: formation of haploid megaspores from diploid megaspore mother cell (MMC) in the nucellus. Process: A single cell in the nucellus differentiates as the MMC (megaspore mother cell). MMC undergoes meiosis → 4 megaspores (megaspore tetrad). In most angiosperms (Polygonum type): 3 of 4 megaspores degenerate. Only 1 functional megaspore (chalazal megaspore — nearest the chalaza) survives. This functional megaspore → female gametophyte (embryo sac) by mitosis. Female gametophyte development (megagametogenesis): functional megaspore → 3 mitotic divisions without cell wall formation → 8-nucleate stage → cell organisation → 7-cell, 8-nucleus embryo sac. Stages of mitosis: 1 nucleus → 2 → 4 → 8. Then cytokinesis: 3 antipodal cells at chalazal end, 2 synergids + 1 egg cell at micropylar end, 1 central cell with 2 polar nuclei.

3. Embryo Sac — 7 Cells, 8 Nuclei

Standard (Polygonum-type) embryo sac has 7 cells and 8 nuclei. Egg apparatus (micropylar end): 1 egg cell (n) — female gamete. 2 synergids (n) — guide pollen tube, filiform apparatus at micropylar end for pollen tube reception. Antipodal cells (chalazal end): 3 cells (n) — degenerate after fertilisation, nutritive role. Central cell: 1 large cell (2n — contains 2 polar nuclei n+n). After double fertilisation: egg + sperm 1 → zygote (2n) → embryo. Polar nuclei + sperm 2 → primary endosperm nucleus (3n) → endosperm. Synergids degenerate after pollen tube penetration. Antipodals degenerate. Total nuclei count: 1 (egg) + 2 (synergids) + 3 (antipodals) + 2 (polar nuclei) = 8 nuclei.

4. Double Fertilisation

Double fertilisation is unique to angiosperms (discovered by Nawaschin, 1898). Two sperm cells from one pollen grain: Syngamy: sperm 1 (n) + egg (n) → zygote (2n) → embryo. Triple fusion: sperm 2 (n) + 2 polar nuclei (n+n=2n) → primary endosperm nucleus (3n) → endosperm. Both events occur simultaneously. Significance: ensures endosperm forms only when fertilisation occurs → resources invested only in seeds that will develop. Triploid endosperm provides nutrition for developing embryo. Pollen tube entry: through micropyle (porogamy — most common), or through chalaza (chalazogamy), or through integuments. Pollen tube enters synergid → releases 2 sperm → one goes to egg, one to central cell.

5. Seed Development

After double fertilisation: zygote (2n) undergoes mitotic divisions → embryo development. Primary endosperm nucleus (3n) → endosperm development. Integuments → seed coat. Embryo development stages: Proembryo → globular embryo → heart embryo → torpedo embryo → mature embryo. In dicots (Capsella bursa-pastoris studied): Globular: spherical mass, no differentiation. Heart: two cotyledon primordia visible. Torpedo: elongated, cotyledons and axis differentiated. Mature embryo: radicle, plumule, cotyledons, hypocotyl, epicotyl. Endosperm: Cellular type: cell walls form after each nuclear division (Gossypium/cotton). Nuclear type: free nuclear divisions first, then cell walls form (Coconut milk = liquid nuclear endosperm). Helobial type: intermediate (in monocots). Seeds: albuminous (endosperm persists) or non-albuminous (endosperm absorbed by embryo, stored in cotyledons).

6. Placentation Types

Placentation = arrangement of ovules on the placenta inside the ovary. Marginal: placenta along ventral suture of monocarpellary ovary. Peas and beans (Fabaceae). Axile: placenta at central axis of multilocular ovary. Tomato, lemon, orange (Solanaceae, Rutaceae). Parietal: placenta on ovary wall (wall) in unilocular ovary. Mustard, cucurbits (Brassicaceae, Cucurbitaceae). Free central: ovules on central column with no septa. Primrose, Dianthus (Primulaceae, Caryophyllaceae). Basal: single ovule at base of ovary. Sunflower, marigold (Asteraceae). Superficial: ovules attached to inner surface all around (Water lily, Nymphaea). Each type corresponds to different ovary structure and is used in plant classification.

7. Fruit Development

After fertilisation: ovary → fruit (pericarp = wall of fruit). Ovules → seeds. True fruit: develops from ovary alone. False fruit (pseudocarp): thalamus or calyx also contributes. Apple, pear: thalamus (receptacle) forms fleshy edible part — ovary forms the core. Types of true fruits: Drupe: fleshy with stony endocarp (mango, peach, cherry, coconut). Berry: fleshy throughout, many seeds (tomato, banana, grapes). Capsule: dry, dehiscent, multiple seeds (poppy, cotton). Legume/pod: Fabaceae family (pea, bean). Achene: dry, single seed, thin pericarp (sunflower). Nut: dry, single seed, hard pericarp (walnut, chestnut). Caryopsis: grain (seed coat fused with pericarp — wheat, rice, maize). Parthenocarpic fruits: develop without fertilisation → seedless (banana, some grapes, cucumber varieties).

8. Polyembryony and Apomixis

Polyembryony: more than one embryo in a seed. Causes: fertilisation of both synergids (rare), adventive embryony (sporophytic tissue of nucellus/integuments → extra embryos), cleavage polyembryony (zygote or proembryo splits). Examples: Citrus (orange, lemon): regular nucellar polyembryony — nucellar cells develop into embryos alongside the zygotic embryo. Onion (Allium), groundnut (Arachis) also show polyembryony. In Citrus: nucellar embryos are genetically identical to mother plant (same as vegetative propagation). Apomixis: seed formation without fertilisation. Types: Diplospory: unreduced egg cell (2n) develops into embryo. Apospory: somatic cells of nucellus form 2n embryo sac. Adventive embryony: nucellar/integument cells directly → embryo (Citrus). Importance: apomictic seeds produce plants genetically identical to mother → maintain hybrid vigour indefinitely → commercially valuable in hybrid crops.

Frequently Asked Questions
1. What are the layers of an ovule from outside to inside? ⌄
From outermost to innermost in a typical angiosperm ovule: (1) Integuments: 2 protective layers in bitegmic ovules (outer and inner integument). Become seed coat after fertilisation. (2) Parietal cell layer: nucellar tissue layer. (3) Middle layer: another nucellar tissue layer. (4) Nucellus: the main nutritive tissue, homologous to megasporangium. Contains the embryo sac (female gametophyte) within it. So the complete order C-A-B-D = Integuments → Parietal cell → Middle layer → Nucellus. The egg cell (female gamete) is inside the embryo sac which is inside the nucellus.
2. What is the difference between micropyle and chalaza? ⌄
Micropyle: the small opening or pore in the integuments at the tip of the ovule, near the egg apparatus. Function: (1) Entry of pollen tube for fertilisation (in most angiosperms = porogamy). (2) Water absorption during seed germination in some species. Chalaza: the base of the ovule where the integuments and nucellus are fused, opposite to micropyle. Function: provides nutritive connection to ovule. Chalazal end is where: megaspore tetrad forms (chalazal megaspore survives). Antipodal cells are located. Funiculus: the stalk connecting ovule to placenta. Raphe: ridge formed by funiculus when fused with ovule body in anatropous ovules.
3. What is the significance of double fertilisation? ⌄
Double fertilisation is unique to angiosperms (flowering plants) and is considered a key innovation in plant evolution. Significance: (1) Efficiency: endosperm (nutritive tissue for embryo) forms ONLY when fertilisation occurs. Unlike gymnosperms where endosperm (female gametophyte) develops regardless of fertilisation — wasted if egg not fertilised. (2) Triploid endosperm: 3n endosperm may be more nutritive than haploid gymnosperm endosperm. (3) Supports agricultural importance: endosperm of cereals (wheat flour, rice starch, corn starch) is the main calorie source for humanity. This triploid tissue, unique to angiosperms, has made angiosperms the dominant food source. (4) Coordinates seed development: ensures embryo and endosperm develop together.
4. What are synergids and what do they do? ⌄
Synergids are 2 haploid (n) cells flanking the egg cell in the egg apparatus at the micropylar end of the embryo sac. Structure: have a unique structure called the filiform apparatus at the micropylar end — finger-like projections of cell wall material that increase surface area. Functions: (1) Guide pollen tube: synergids produce chemicals (LURE peptides) that attract the pollen tube through the micropyle. (2) Pollen tube reception: pollen tube grows into and bursts inside one synergid (usually the larger one) → releases 2 sperm cells. (3) The degenerated synergid then facilitates sperm transfer to egg and central cell. Both synergids degenerate after pollen tube arrival. They are essential for successful double fertilisation.
5. What is apomixis and why is it commercially important? ⌄
Apomixis is the formation of seeds without fertilisation (asexual reproduction through seeds). Types: Diplospory: unreduced megaspore mother cell → embryo (without meiosis). Apospory: somatic cells of nucellus form embryo sac with 2n cells. Adventive embryony: nucellar or integument cells directly develop into embryos (common in Citrus). Agamospermy: general term for seed formation without fertilisation. Commercial importance: Apomictic seeds produce plants GENETICALLY IDENTICAL to the mother plant. F1 hybrid varieties require fresh seed each year (costly) because F2 segregates. If a hybrid could be made apomictic: hybrid vigour maintained indefinitely without re-crossing each year. This would dramatically reduce cost of hybrid seed production → benefit small farmers in developing countries. Active research area: transferring apomixis genes into crop plants (rice, wheat, maize).
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