# Sexual Reproduction in Flowering Plants (NEET Biology Class 12): Embryo Sac & Double Fertilisation Source: https://tracknation.in/story/sexual-reproduction-in-flowering-plants-neet-biology-class-12-embryo-sac-double-fertilisation (TrackNation β€” free UPSC notes with PYQs) > NEET Class 12 Sexual Reproduction, mined from NCERT: the anther and pollen grain (microsporangium, sporopollenin), the ovule and the mature embryo sac (7-celled, 8-nucleate β€” egg apparatus, antipodals, central cell), pollination types (autogamy, geitonogamy, xenogamy), double fertilisation (syngamy, triple fusion), endosperm and embryo development, seed, fruit, apomixis and polyembryony. With 12 MCQs, 8 NEET PYQs and an Active Recall prompt. ## The Anther, Microsporogenesis & Pollen Grain 🎯 **NEET priority: High-yield.** *4–6 questions every year. The embryo sac (7-celled, 8-nucleate) and double fertilisation are asked almost every year β€” learn the numbers exactly.* A flower is built for one job: sexual reproduction. This chapter follows pollen and ovule from formation to the finished seed. ### The anther & pollen grain - A typical anther is **bilobed** and **tetragonal (4-sided)**, with **4 microsporangia** (2 per lobe). - A microsporangium wall has 4 layers: **epidermis, endothecium, middle layers** and the innermost **tapetum** β€” the tapetum nourishes developing pollen. - Each microspore mother cell (MMC) undergoes **meiosis** to form a **microspore tetrad**; the pollen wall's hard outer layer, the **exine**, is made of **sporopollenin** β€” one of the most resistant organic materials known, undegraded by any known enzyme. - Pollen is shed either at the **2-celled stage** (vegetative + generative cell) or, after the generative cell divides mitotically, at the **3-celled stage** (vegetative cell + 2 male gametes). ## The Ovule & the Mature Embryo Sac (7-celled, 8-nucleate) ### The ovule (megasporangium) - Attached to the placenta by a **funicle**; their junction is the **hilum**. **Integuments** encircle the **nucellus** except at the **micropyle** (opening); the base opposite the micropyle is the **chalaza**. - Inside the nucellus, a **megaspore mother cell (MMC)** undergoes meiosis to give **4 megaspores** β€” usually only **one survives** and forms the embryo sac (**monosporic development**). ### The mature embryo sac β€” the most-asked number in this chapter Two sequential mitotic divisions of the functional megaspore give a mature embryo sac that is **8-nucleate but only 7-celled**: - **Micropylar end:** the **egg apparatus** β€” 1 egg cell + **2 synergids** (with a **filiform apparatus** that guides the pollen tube in). - **Chalazal end:** **3 antipodal cells**. - **Centre:** one large **central cell** with **2 polar nuclei** (2 nuclei in 1 cell β€” hence 8 nuclei in 7 cells). ## Pollination Types & Pollen-Pistil Interaction ### Types of pollination | **Type** | **Meaning** | **Note** | | Autogamy | Self-pollination within the same flower | True self-pollination | | Geitonogamy | Pollen from one flower to another on the SAME plant | Genetically self, functionally cross | | Xenogamy | Pollen from a flower on a DIFFERENT plant | True cross-pollination | **Cleistogamous flowers** (e.g. some *Viola, Oxalis, Commelina*) never open β€” guaranteeing autogamy. **Out-breeding devices** that prevent self-pollination / encourage cross-pollination: **dichogamy** (anther and stigma mature at different times), **herkogamy** (physical separation), **unisexual flowers** (monoecious, e.g. castor, maize β€” prevents autogamy but not geitonogamy; dioecious β€” prevents both), and **self-incompatibility** (a genetic block against self pollen germinating/growing). Continued self-pollination causes **inbreeding depression**. ### Pollen–pistil interaction & pollen tube growth The pistil recognises compatible pollen; the pollen grain germinates on the stigma to form a **pollen tube** that grows through the style, enters the ovule, and releases its two male gametes near/into a **synergid** (guided by the filiform apparatus). ## Double Fertilisation & Post-Fertilisation Development ### Double fertilisation β€” the defining angiosperm event - **Syngamy:** one male gamete (n) fuses with the egg cell (n) β†’ **diploid zygote (2n)**. - **Triple fusion:** the other male gamete (n) fuses with the **2 polar nuclei** β†’ **triploid primary endosperm nucleus (3n)**. - Because two fusions occur, angiosperm fertilisation is termed **double fertilisation** β€” **unique to flowering plants**. ### Endosperm & embryo development - The primary endosperm cell divides repeatedly. In **nuclear (free-nuclear)** type, divisions occur without wall formation (e.g. coconut water = liquid endosperm). In **cellular** type, wall formation follows each division. - Embryo passes through **proembryo β†’ globular β†’ heart-shaped β†’ mature** stages. A mature **dicot** embryo has **2 cotyledons**, an **epicotyl** (β†’ plumule) and a **hypocotyl** (β†’ radicle). A **monocot** embryo has **1 cotyledon (scutellum)**, with the plumule covered by the **coleoptile** and the radicle by the **coleorrhiza**. ## Seed, Fruit, Apomixis & Polyembryony ### Seed & fruit - Seeds are **albuminous** (retain some endosperm β€” wheat, maize, castor) or **non-albuminous / ex-albuminous** (endosperm fully used up β€” pea, groundnut, beans). - The ovary wall becomes the **pericarp**. A **true fruit** develops only from the ovary; a **false fruit** also involves other floral parts (e.g. the **thalamus** in apple, strawberry, cashew). - **Parthenocarpy** = fruit development **without fertilisation**, usually seedless (e.g. **banana**). ### Apomixis & polyembryony - **Apomixis** = seed production **without fertilisation** β€” a form of asexual reproduction dressed as sexual. - **Polyembryony** = more than one embryo in a single seed (e.g. **Citrus**, where extra embryos arise from nucellar cells). - Both are commercially important β€” apomictic hybrids don't segregate, so a farmer can reuse hybrid seed without buying fresh every year. ### Pollination β€” the complete type table | **Type** | **Definition** | **Key point / example** | | Autogamy | Pollen transferred to the stigma of the SAME flower | Needs synchrony of anther dehiscence and stigma receptivity. CLEISTOGAMOUS flowers (e.g. Viola, Oxalis, Commelina) never open and are INVARIABLY autogamous β€” assured seed set but NO genetic variation | | Geitonogamy | Pollen from one flower to the stigma of ANOTHER flower on the SAME plant | Functionally cross-pollination (needs a pollinator) but GENETICALLY it is autogamy β€” pollen comes from the same plant | | Xenogamy | Pollen from a flower on a DIFFERENT plant | The ONLY type that brings genetically different pollen to the stigma β€” true cross-pollination | | **Agent** | **Name** | **Flower characteristics** | | Wind | Anemophily | Well-exposed stamens, large feathery stigma, light non-sticky pollen, often single-ovule flowers packed into an inflorescence. Common in grasses; maize corn-silk = the stigma+style | | Water | Hydrophily | RARE β€” only in some aquatic plants (Vallisneria, Hydrilla, Zostera). Pollen protected by a MUCILAGINOUS covering. Note: most aquatic plants like water hyacinth and water lily are actually INSECT- or wind-pollinated | | Insects/animals | Entomophily / zoophily | Large, colourful, fragrant, nectar-rich flowers. Pollinators include bees, butterflies, birds, bats. In small flowers, several group into an inflorescence to be visible | ### Why this matters for NEET - The embryo sac composition (**7 cells / 8 nuclei**) and the double fertilisation products (**zygote 2n**, **PEN 3n**) are asked almost every year β€” memorise the exact numbers. - Traps: geitonogamy is **genetically self** but **functionally cross**-pollination; sporopollenin is **indestructible** (no known enzyme degrades it); monoecious β‰  prevents geitonogamy. ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** The hard, resistant outer layer of the pollen grain wall is called the: - (a) Intine - (b) Exine - (c) Nucellus - (d) Tapetum **Answer: (b)** β€” The exine is made of sporopollenin, one of the most resistant known organic materials β€” no enzyme degrades it. **Q2.** A mature angiosperm embryo sac is: - (a) 8-celled and 8-nucleate - (b) 7-celled and 8-nucleate - (c) 4-celled and 8-nucleate - (d) 7-celled and 7-nucleate **Answer: (b)** β€” The central cell has 2 polar nuclei in one cell, making the embryo sac 8-nucleate but only 7-celled. **Q3.** The egg apparatus of the embryo sac consists of: - (a) 1 egg cell only - (b) 1 egg cell and 2 synergids - (c) 3 antipodals - (d) 2 polar nuclei **Answer: (b)** β€” The egg apparatus (at the micropylar end) has one egg cell flanked by two synergids. **Q4.** The filiform apparatus is found in the: - (a) Central cell - (b) Egg cell - (c) Antipodal cells - (d) Synergids **Answer: (d)** β€” Synergids have a filiform apparatus at their micropylar tip that guides the pollen tube's entry. **Q5.** Transfer of pollen grains from the anther to the stigma of another flower on the SAME plant is: - (a) Autogamy - (b) Geitonogamy - (c) Cleistogamy - (d) Xenogamy **Answer: (b)** β€” Geitonogamy is genetically self-pollination but functionally cross-pollination (needs a pollinator). **Q6.** In double fertilisation, triple fusion produces the: - (a) Zygote - (b) Suspensor - (c) Synergid - (d) Primary endosperm nucleus (PEN) **Answer: (d)** β€” A male gamete fuses with the two polar nuclei to form the triploid primary endosperm nucleus (triple fusion). **Q7.** Coconut water is an example of which type of endosperm? - (a) Helobial - (b) Cellular - (c) Nuclear (free-nuclear) - (d) Non-albuminous **Answer: (c)** β€” Coconut water is liquid, free-nuclear endosperm β€” divisions occur without immediate wall formation. **Q8.** A mature dicot embryo has: - (a) Two cotyledons, an epicotyl and a hypocotyl - (b) No cotyledons - (c) One cotyledon and a coleoptile - (d) A scutellum **Answer: (a)** β€” Dicot embryos have two cotyledons and an embryonal axis with epicotyl (β†’plumule) and hypocotyl (β†’radicle). **Q9.** Fruits that develop without fertilisation are called: - (a) Aggregate fruits - (b) False fruits - (c) True fruits - (d) Parthenocarpic fruits **Answer: (d)** β€” Parthenocarpy is fruit development without fertilisation, usually giving seedless fruit β€” e.g. banana. **Q10.** Polyembryony, seen in Citrus, means: - (a) Multiple endosperms - (b) Multiple pollen tubes in one ovule - (c) Fusion of two zygotes - (d) More than one embryo in a single seed **Answer: (d)** β€” Polyembryony is the occurrence of more than one embryo in a single seed, often from nucellar tissue in Citrus. **Q11.** The number of microsporangia in a typical angiosperm anther is: - (a) Six - (b) Two - (c) Four - (d) Eight **Answer: (c)** β€” A bilobed, tetragonal anther has 4 microsporangia, two in each lobe. **Q12.** The tapetum layer of the anther wall functions to: - (a) Nourish the developing pollen grains - (b) Protect the anther - (c) Release pollen - (d) Form the exine **Answer: (a)** β€” The innermost anther wall layer, the tapetum, has dense cytoplasm and nourishes developing pollen grains. **Q13.** In the embryo sac shown, the primary endosperm nucleus (PEN) is formed by the fusion of a male gamete with the structure marked - (a) B - (b) A - (c) C - (d) D **Answer: (c)** β€” C marks the two polar nuclei of the central cell. In double fertilisation one male gamete fuses with the egg [B] to form the zygote and the other fuses with the two polar nuclei (triple fusion) to form the triploid PEN. **Q14.** The cells marked A, which have a filiform apparatus and guide the pollen tube into the embryo sac, are the - (a) egg cells - (b) synergids - (c) antipodals - (d) polar nuclei **Answer: (b)** β€” The two synergids and the egg form the egg apparatus at the micropylar end. The filiform apparatus of the synergids guides the entry of the pollen tube. **Q15.** The three cells at the chalazal end, marked D, are called - (a) megaspores - (b) antipodals - (c) synergids - (d) egg apparatus **Answer: (b)** β€” The three cells at the chalazal end are antipodals. With the 3 cells of the egg apparatus (egg + 2 synergids) and the central cell, they make the 7 cells of the mature embryo sac, which has 8 nuclei. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q16.** The chalazal end of the embryo sac has: (NEET PYQ) - (a) Egg apparatus - (b) Synergids - (c) Two polar nuclei - (d) Three antipodal cells **Answer: (d)** β€” Three antipodal cells sit at the chalazal end; the egg apparatus and synergids are at the micropylar end. **Q17.** Zygote formed after syngamy is: (NEET PYQ) - (a) Haploid - (b) Tetraploid - (c) Diploid - (d) Triploid **Answer: (c)** β€” Syngamy fuses two haploid (n) gametes to form a diploid (2n) zygote. **Q18.** Which of the following shows apomixis? (NEET PYQ) - (a) Pea - (b) Wheat only - (c) Some Citrus and grasses - (d) Maize only **Answer: (c)** β€” Apomixis (seed formation without fertilisation) occurs in some Citrus, grasses (Asteraceae/Poaceae), and is important for hybrid seed technology. **Q19.** Sporopollenin, found in the pollen exine, is resistant to: (NEET PYQ) - (a) UV light only - (b) Only physical damage - (c) Acids, alkalis and enzymatic degradation - (d) High temperature only **Answer: (c)** β€” Sporopollenin withstands high temperature, strong acids and alkalis, and no enzyme is known to degrade it. **Q20.** Self-incompatibility is a mechanism that prevents: (NEET PYQ) - (a) Cross-pollination - (b) Self-pollination from producing seeds - (c) Pollen germination on any stigma - (d) Pollen formation **Answer: (b)** β€” Self-incompatibility is a genetic device that blocks self pollen from fertilising the same flower, even though it may germinate. **Q21.** Non-albuminous seeds are seen in: (NEET PYQ) - (a) Pea and groundnut - (b) Wheat and maize - (c) Barley - (d) Castor **Answer: (a)** β€” Pea, groundnut and beans are non-albuminous (ex-albuminous) β€” the endosperm is fully consumed during embryo development. **Q22.** In which type of pollination is the pollen transferred to a flower on a genetically different plant? (NEET PYQ) - (a) Cleistogamy - (b) Autogamy - (c) Geitonogamy - (d) Xenogamy **Answer: (d)** β€” Xenogamy transfers pollen to a flower on a different plant β€” true cross-pollination with new genetic combinations. **Q23.** The megaspore mother cell undergoes meiosis to form: (NEET PYQ) - (a) Eight megaspores - (b) Two megaspores - (c) One megaspore - (d) Four megaspores **Answer: (d)** β€” The MMC undergoes meiosis to give four haploid megaspores; usually only one survives to form the embryo sac. ### Active Recall Prompt Write everything you can recall, naming each topic first. Cover: the **anther and pollen grain** (microsporangium layers, exine/sporopollenin, 2-celled/3-celled pollen); the **ovule and mature embryo sac** (7-celled, 8-nucleate β€” egg apparatus, antipodals, central cell); **pollination types** (autogamy, geitonogamy, xenogamy) and out-breeding devices; **double fertilisation** (syngamy, triple fusion, zygote, PEN); **endosperm and embryo development**; and **seed, fruit and apomixis/polyembryony**. Begin each fact with its topic and end it with a full stop.