Sexual Reproduction in Flowering Plants
Welcome to HSLC Guru. This chapter explores the fascinating world of sexual reproduction in angiosperms (flowering plants). You will learn how a flower acts as the reproductive unit, how pollen and embryo sacs are formed, how pollination and fertilisation occur, and how seeds and fruits develop. This complete guide is prepared strictly according to the ASSEB Class 12 Biology syllabus and is designed for English Medium students of Assam.
Chapter Summary
The flower is the reproductive unit of angiosperms, specialised to perform sexual reproduction. A typical bisexual flower has four whorls — calyx, corolla, androecium and gynoecium. The androecium (male reproductive part) consists of stamens, while the gynoecium (female reproductive part) is composed of one or more carpels (pistils). Sexual reproduction in flowering plants takes place in three sequential phases — pre-fertilisation, fertilisation and post-fertilisation events.
During the pre-fertilisation phase, the stamen produces pollen grains in the anther. A typical anther is bilobed and dithecous, having four microsporangia. The wall of the microsporangium is composed of the epidermis, endothecium, middle layers and tapetum. Inside the microsporangium, microspore mother cells undergo meiosis (microsporogenesis) to form microspore tetrads, which mature into pollen grains. A mature pollen grain is two-celled (vegetative + generative cell) or three-celled and has a two-layered wall — the outer exine (made of sporopollenin) and the inner intine. The pistil contains the ovary, style and stigma. Inside the ovary, ovules develop. The ovule (megasporangium) has a stalk (funicle), nucellus, integuments and a micropyle. A megaspore mother cell within the nucellus undergoes meiosis (megasporogenesis) to form four megaspores; usually only one functional megaspore develops further. The functional megaspore divides mitotically (free nuclear divisions) to form the female gametophyte/embryo sac, which is typically 8-nucleate and 7-celled — comprising one egg apparatus (egg + 2 synergids), three antipodal cells, and a large central cell with two polar nuclei (which fuse to form the secondary nucleus, 2n).
Pollination is the transfer of pollen grains from the anther to the stigma. It is of two main kinds — self-pollination (autogamy and geitonogamy) and cross-pollination (xenogamy). Pollination is achieved through abiotic agents like wind (anemophily) and water (hydrophily) and biotic agents like insects (entomophily), birds (ornithophily) and bats (chiropterophily). Plants have evolved several outbreeding devices to prevent self-pollination, such as dichogamy (protandry/protogyny), herkogamy, heterostyly, self-incompatibility and unisexuality. After pollen lands on a compatible stigma, pollen-pistil interaction begins — a pollen tube grows through the style, carrying two male gametes towards the embryo sac. The phenomenon of double fertilisation, discovered by S.G. Nawaschin (1898) in Lilium and Fritillaria, is unique to angiosperms. One male gamete fuses with the egg (syngamy → zygote, 2n) and the other fuses with the two polar nuclei/secondary nucleus to form the primary endosperm nucleus (PEN, 3n) — this is called triple fusion. Strasburger first observed syngamy in 1884.
In the post-fertilisation phase, the PEN develops into the endosperm (3n), which provides nourishment to the developing embryo. Endosperm development may be nuclear, cellular or helobial. The zygote develops into the embryo. In a typical dicot embryo (e.g. Capsella bursa-pastoris), stages include proembryo, globular, heart-shaped and mature embryo (with two cotyledons, plumule, radicle and hypocotyl). A monocot embryo (e.g. grasses) has only one cotyledon (scutellum), with structures like coleoptile (covering plumule) and coleorhiza (covering radicle). The ovule matures into the seed and the ovary develops into the fruit. Seeds may be albuminous (with persistent endosperm — wheat, maize, castor) or non-albuminous (without endosperm — pea, gram, bean). Some plants exhibit apomixis (seed formation without fertilisation, a form of asexual reproduction) and polyembryony (formation of more than one embryo in a seed, e.g. Citrus, mango). These phenomena are of great importance in plant breeding and hybrid seed industry.
Question and Answers
1-Mark Questions
Q1. What is the reproductive unit of angiosperms?
Answer: The flower is the reproductive unit of angiosperms.
Q2. Define microsporogenesis.
Answer: The process of formation of microspores (pollen grains) from the microspore mother cell through meiosis is called microsporogenesis.
Q3. Name the layer of the anther wall that nourishes the developing pollen grains.
Answer: The tapetum nourishes the developing pollen grains.
Q4. What is the ploidy of the endosperm in angiosperms?
Answer: The endosperm is triploid (3n).
Q5. Who discovered double fertilisation in angiosperms?
Answer: S.G. Nawaschin (1898) discovered double fertilisation in Lilium and Fritillaria.
Q6. What is autogamy?
Answer: Autogamy is self-pollination occurring within the same flower — transfer of pollen from the anther to the stigma of the same flower.
Q7. What is the chemical nature of the exine of pollen grain?
Answer: The exine is made up of sporopollenin, one of the most resistant organic materials known.
Q8. Define apomixis.
Answer: Apomixis is a form of asexual reproduction that mimics sexual reproduction, producing seeds without fertilisation.
Q9. What is polyembryony?
Answer: The occurrence of more than one embryo in a single seed is called polyembryony (e.g. Citrus, mango).
Q10. What is pollination by water called?
Answer: Pollination by water is called hydrophily (e.g. Vallisneria, Zostera).
Q11. Name the scientist who first observed syngamy.
Answer: Eduard Strasburger first observed syngamy in 1884.
Q12. What is parthenocarpy? Give an example.
Answer: Parthenocarpy is the development of fruit without fertilisation, e.g. banana, seedless grapes.
2-3 Marks Questions
Q1. Differentiate between autogamy, geitonogamy and xenogamy.
Answer: Autogamy — pollen is transferred from the anther to the stigma of the same flower; it is true self-pollination both genetically and ecologically. Geitonogamy — pollen is transferred from the anther of one flower to the stigma of another flower of the same plant; it is genetically self-pollination but ecologically cross-pollination as it requires a pollinator. Xenogamy — pollen is transferred from the anther of one plant to the stigma of a flower on a genetically different plant; this is true cross-pollination and brings genetic variation.
Q2. Describe the structure of a typical anther.
Answer: A typical angiosperm anther is bilobed and dithecous (each lobe has two theca). It is tetrasporangiate, having four microsporangia located at the corners — two in each lobe. In transverse section, a microsporangium appears nearly circular and is surrounded by four wall layers — epidermis, endothecium, 1-3 middle layers and the innermost tapetum. The first three layers protect the developing pollen and help in dehiscence, while the tapetum nourishes the developing pollen grains. The centre is filled with sporogenous tissue.
Q3. What are the functions of the tapetum?
Answer: Functions of the tapetum: (i) Provides nutrition to the developing pollen grains; (ii) Cells are characteristically multinucleate and polyploid; (iii) Secretes the enzyme callase that dissolves the callose wall of microspore tetrads; (iv) Contributes to the formation of pollenkitt and the exine layer of pollen; (v) Provides sporopollenin precursors required for exine formation.
Q4. Describe the structure of a mature embryo sac.
Answer: A typical mature embryo sac (Polygonum type) is 7-celled and 8-nucleate. At the micropylar end is the egg apparatus consisting of one egg cell and two synergids (with filiform apparatus that guides the pollen tube). At the chalazal end are three antipodal cells. The large central cell occupies most of the embryo sac and contains two polar nuclei that often fuse to form the diploid secondary nucleus.
Q5. Explain double fertilisation.
Answer: Double fertilisation is a unique feature of angiosperms, discovered by S.G. Nawaschin (1898). After the pollen tube enters the embryo sac, it releases two male gametes. Syngamy: One male gamete (n) fuses with the egg (n) to form the diploid zygote (2n). Triple fusion: The second male gamete (n) fuses with the two polar nuclei/secondary nucleus (2n) to form the triploid primary endosperm nucleus (PEN, 3n). Since two fusion events take place inside one embryo sac, the phenomenon is called double fertilisation.
Q6. Mention any three outbreeding devices found in flowering plants.
Answer: (i) Dichogamy — pollen and stigma mature at different times, either protandry (anther matures first) or protogyny (stigma matures first); (ii) Herkogamy — physical separation between anther and stigma prevents self-pollination; (iii) Self-incompatibility — a genetic mechanism that prevents pollen from the same plant or a genetically similar plant from fertilising the egg, ensuring xenogamy.
Q7. What is pollen-pistil interaction? Mention its significance.
Answer: Pollen-pistil interaction refers to all the events from the deposition of pollen on the stigma till the entry of the pollen tube into the ovule. The pistil has the ability to recognise the right type of pollen (compatible) and to reject incompatible pollen. After landing on a compatible stigma, the pollen germinates to form a pollen tube, which grows through the style and reaches the embryo sac through the micropyle. This interaction prevents fertilisation by foreign or incompatible pollen and is mediated by chemical signals. Knowledge of pollen-pistil interaction helps plant breeders to manipulate hybridisation programmes for crop improvement.
Q8. Differentiate between albuminous and non-albuminous seeds.
Answer: Albuminous (endospermic) seeds retain a part of the endosperm at maturity, as it is not completely consumed during embryo development. The endosperm acts as the food storage tissue. Examples: wheat, maize, rice, castor, coconut. Non-albuminous (non-endospermic) seeds do not retain endosperm at maturity because it is completely consumed by the developing embryo. Food is stored in the cotyledons. Examples: pea, gram, bean, groundnut, mustard.
5-7 Marks Questions
Q1. Describe in detail the process of microsporogenesis and the structure of a mature pollen grain.
Answer: Microsporogenesis is the process by which pollen grains (microspores) are formed from microspore mother cells (MMCs) within the microsporangium of the anther. Inside each microsporangium, sporogenous tissue cells differentiate into microspore mother cells. Each MMC undergoes meiotic (reduction) division to produce a tetrad of haploid microspores arranged in a tetrahedral or isobilateral fashion. As the anther matures, the microspores get separated from the tetrad and develop into pollen grains. Each microspore enlarges, develops a two-layered wall and undergoes mitotic division to produce a vegetative cell and a generative cell.
Structure of pollen grain: A pollen grain is generally spherical, measuring about 25-50 micrometers. It has a two-layered wall — the outer exine, made of the highly resistant biopolymer sporopollenin (which can withstand high temperature, strong acids/alkalis and remains preserved as fossils), and the inner intine, a thin continuous layer of cellulose and pectin. The exine has unthickened regions called germ pores through which the pollen tube emerges. Inside, the cytoplasm contains a large vegetative cell with reserve food and a smaller generative cell that floats in the vegetative cell’s cytoplasm. The generative cell divides to form two male gametes. Pollen is shed at the 2-celled stage in about 60% of angiosperms; in the rest, it is shed at the 3-celled stage.
Q2. Describe megasporogenesis and the development of a typical (Polygonum type) embryo sac with a labelled diagram description.
Answer: Megasporogenesis: The process of formation of megaspores from the megaspore mother cell (MMC) is called megasporogenesis. In a young ovule, a single hypodermal cell of the nucellus near the micropylar end becomes specialised as the megaspore mother cell (2n). The MMC undergoes meiosis to produce four haploid megaspores arranged in a linear tetrad. Out of these four, three megaspores (usually the upper three) degenerate and only one functional megaspore (the chalazal one) develops further into the female gametophyte. This is called the monosporic type of embryo sac development, characteristic of most angiosperms.
Embryo sac formation: The functional megaspore enlarges and its nucleus undergoes three successive free-nuclear mitotic divisions producing 8 nuclei. Of these, four migrate to each pole. After cell wall formation, the mature embryo sac becomes 7-celled and 8-nucleate: at the micropylar end — the egg apparatus consisting of one egg cell flanked by two synergids (synergids have a special thickening called the filiform apparatus that guides the pollen tube to the egg); at the chalazal end — three antipodal cells (which usually degenerate before fertilisation); and at the centre — a large central cell containing two polar nuclei. The two polar nuclei may fuse before fertilisation to form a diploid secondary nucleus.
Q3. What is pollination? Describe the various agents of pollination with examples.
Answer: Pollination is the transfer of pollen grains from the anther of a stamen to the stigma of a pistil. It may be of two main types — self-pollination (autogamy, geitonogamy) and cross-pollination (xenogamy).
Agents of pollination:
(i) Anemophily (wind) — Flowers are small, dull-coloured, non-fragrant, with light, dry, abundant pollen and exposed feathery stigmas. Examples: grasses, maize, wheat, sugarcane.
(ii) Hydrophily (water) — Found in a few aquatic plants. In Vallisneria, female flowers reach the water surface by long stalks; male flowers detach and float to reach the female. In Zostera (sea grass), pollination occurs below the water surface; pollen grains are long, ribbon-like, with no exine and equal density to water.
(iii) Entomophily (insects) — Most common biotic pollination. Flowers are large, brightly coloured, fragrant, often with nectar guides. Pollen is sticky and stigma rough. Examples: Salvia, sunflower, mango, mustard. Yucca and Pronuba moth show obligate mutualism.
(iv) Ornithophily (birds) — Flowers are bright red/orange, large, tubular, scentless, secrete copious watery nectar. Examples: Bignonia, Bombax, Erythrina, Callistemon, pollinated mainly by sunbirds and humming birds.
(v) Chiropterophily (bats) — Flowers are dull, large, sturdy, with strong fermenting odour, open at night. Examples: Adansonia (Baobab), Anthocephalus (Kadam).
Q4. Describe the development of a typical dicot embryo (Capsella bursa-pastoris).
Answer: The development of a dicot embryo from the diploid zygote takes place in successive stages, well-studied in Capsella bursa-pastoris (shepherd’s purse).
(i) Zygote stage: The fertilised egg (zygote) lies at the micropylar end of the embryo sac. It rests for some time and then divides transversely into a smaller terminal cell (towards chalaza) and a larger basal cell (towards micropyle).
(ii) Proembryo stage: The terminal cell divides by two vertical and one transverse division to form an octant (8-celled) proembryo. The basal cell divides transversely to form a 6 to 10-celled suspensor that pushes the embryo into the endosperm. The lowest cell of the suspensor (haustorium) absorbs nutrition; the uppermost cell (hypophysis) gives rise to the radicle tip.
(iii) Globular stage: The octant cells divide periclinally and anticlinally to form a globular embryo with an outer protoderm.
(iv) Heart-shaped stage: Two cotyledon primordia arise on either side, giving the embryo a heart-shape. The plumule develops in the notch between the two cotyledons.
(v) Mature embryo (torpedo to walking-stick stage): The cotyledons elongate, the embryonic axis with plumule and radicle becomes well-defined. The mature embryo bends inside the seed and consists of two cotyledons, an epicotyl bearing the plumule, hypocotyl and radicle.
Q5. Explain post-fertilisation events — endosperm development, seed development and fruit formation.
Answer: Post-fertilisation events include the formation of endosperm and embryo, development of seed from ovule, and formation of fruit from ovary.
Endosperm development: The primary endosperm nucleus (PEN, 3n) divides to form the endosperm tissue, which provides nourishment to the growing embryo. There are three types — (a) Nuclear endosperm (most common; free nuclear divisions occur first, followed by wall formation, e.g. coconut, where the milky water is free-nuclear endosperm and white kernel is cellular endosperm); (b) Cellular endosperm (wall formation occurs from the very first division, e.g. Petunia, balsam); (c) Helobial endosperm (intermediate, e.g. members of Helobiales).
Embryo development: The zygote develops into the embryo. Dicot embryos pass through proembryo, globular, heart-shaped and mature stages, while monocot embryos have a single shield-shaped cotyledon (scutellum) with coleoptile and coleorhiza.
Seed development: The ovule develops into the seed. The integuments harden to form the seed coat (outer testa, inner tegmen). The micropyle persists as a small pore for water absorption during germination. Seeds are of two types — albuminous (with persistent endosperm — wheat, maize, castor, coconut) and non-albuminous (where endosperm is consumed during embryo development — pea, gram, bean, groundnut).
Fruit development: The ovary wall develops into the pericarp, forming the fruit. True fruits develop only from the ovary (e.g. mango, tomato), while false fruits involve other floral parts like the thalamus (e.g. apple, strawberry). When fruits develop without fertilisation, they are called parthenocarpic fruits (e.g. banana, seedless grapes).
Q6. Explain the structure of a typical anatropous ovule with a labelled description, and list the different types of ovules.
Answer: An ovule (megasporangium) is a small structure attached to the placenta of the ovary by a stalk called the funicle. The point at which the funicle is attached to the body of the ovule is called the hilum.
Parts of a typical ovule:
(i) Funicle — Stalk that connects the ovule to the placenta.
(ii) Hilum — Junction of the body of the ovule with the funicle.
(iii) Integuments — One or two protective coverings surrounding the body of the ovule, leaving a small opening called the micropyle.
(iv) Micropyle — A narrow pore through which the pollen tube enters; it also helps in water absorption during seed germination.
(v) Chalaza — The basal end of the ovule, opposite to the micropyle.
(vi) Nucellus — The mass of parenchymatous tissue enclosed within the integuments; it nourishes the embryo sac.
(vii) Embryo sac — The female gametophyte embedded inside the nucellus.
Types of ovules based on orientation: (a) Orthotropous — ovule is straight, with funicle, chalaza and micropyle on the same axis (e.g. Polygonum); (b) Anatropous — completely inverted, micropyle lies close to the hilum, most common type (about 80% of angiosperms); (c) Hemianatropous — ovule is at right angles to the funicle; (d) Campylotropous — ovule is curved with embryo sac slightly bent (e.g. legumes); (e) Amphitropous — ovule and embryo sac are both curved; (f) Circinotropous — funicle is very long and forms a complete circle around the ovule (e.g. Opuntia).
Multiple Choice Questions (MCQs)
Q1. The number of nuclei in a mature embryo sac of an angiosperm is —
(a) 6 (b) 7 (c) 8 (d) 10
Q2. The hard outer layer of pollen grain is composed of —
(a) Cellulose (b) Pectin (c) Sporopollenin (d) Lignin
Q3. Double fertilisation was discovered by —
(a) Strasburger (b) Nawaschin (c) Maheshwari (d) Hofmeister
Q4. Endosperm in angiosperms is —
(a) Haploid (b) Diploid (c) Triploid (d) Tetraploid
Q5. Pollination by water (hydrophily) is found in —
(a) Mango (b) Maize (c) Lotus (d) Vallisneria
Q6. Filiform apparatus is found in —
(a) Antipodals (b) Egg cell (c) Synergids (d) Polar nuclei
Q7. Tapetum nourishes —
(a) Megaspores (b) Pollen grains (c) Embryo sac (d) Endosperm
Q8. Formation of seeds without fertilisation is called —
(a) Parthenocarpy (b) Apomixis (c) Polyembryony (d) Autogamy
Q9. The product of triple fusion is —
(a) Zygote (b) Embryo (c) Primary Endosperm Nucleus (d) Suspensor
Q10. Wind-pollinated flowers are called —
(a) Entomophilous (b) Anemophilous (c) Hydrophilous (d) Ornithophilous
Fill in the Blanks
Q1. The reproductive unit of a flowering plant is the __________. (flower)
Q2. The outermost wall of the pollen grain is called __________. (exine)
Q3. The fusion of one male gamete with two polar nuclei is called __________. (triple fusion)
Q4. The single cotyledon of a monocot seed is called __________. (scutellum)
Q5. Pollination by birds is called __________. (ornithophily)
True or False
Q1. The endosperm of angiosperms is haploid. (False — it is triploid)
Q2. Tapetum nourishes the developing pollen grains. (True)
Q3. Geitonogamy is genetically a type of cross-pollination. (False — genetically self-pollination)
Q4. Sporopollenin is the most resistant organic substance found in pollen exine. (True)
Q5. Apple is a true fruit. (False — apple is a false fruit)
Glossary
| Term | Meaning |
|---|---|
| Androecium | The male reproductive whorl of a flower made up of stamens. |
| Gynoecium | The female reproductive whorl of a flower made up of carpels (pistils). |
| Microsporogenesis | Formation of microspores (pollen grains) from microspore mother cells through meiosis. |
| Megasporogenesis | Formation of megaspores from the megaspore mother cell through meiosis. |
| Tapetum | Innermost nutritive layer of the anther wall that supplies food to developing pollen. |
| Sporopollenin | Highly resistant biopolymer that forms the exine of pollen grains. |
| Embryo sac | Female gametophyte of angiosperms; typically 7-celled and 8-nucleate. |
| Filiform apparatus | Special thickening in the synergids that guides the pollen tube to the egg. |
| Pollination | Transfer of pollen grains from anther to stigma. |
| Autogamy | Self-pollination within the same flower. |
| Geitonogamy | Pollination between two flowers of the same plant. |
| Xenogamy | Pollination between two flowers of genetically different plants (true cross-pollination). |
| Anemophily | Pollination by wind. |
| Hydrophily | Pollination by water (e.g. Vallisneria, Zostera). |
| Entomophily | Pollination by insects. |
| Ornithophily | Pollination by birds. |
| Dichogamy | Pollen and stigma maturing at different times to prevent self-pollination. |
| Herkogamy | Physical barrier between anther and stigma to prevent self-pollination. |
| Self-incompatibility | Genetic mechanism that prevents fertilisation by self pollen. |
| Syngamy | Fusion of one male gamete with the egg, forming the diploid zygote. |
| Triple fusion | Fusion of the second male gamete with two polar nuclei to form the PEN (3n). |
| Double fertilisation | Two fusion events (syngamy + triple fusion) inside one embryo sac, unique to angiosperms. |
| Endosperm | Triploid (3n) nutritive tissue formed after triple fusion. |
| Suspensor | Group of cells that pushes the developing embryo into the endosperm and absorbs nutrients. |
| Scutellum | Single, shield-shaped cotyledon of monocot embryos. |
| Coleoptile | Protective sheath covering the plumule in monocots. |
| Coleorhiza | Protective sheath covering the radicle in monocots. |
| Albuminous seed | Seed retaining endosperm at maturity (e.g. wheat, maize, castor). |
| Non-albuminous seed | Seed without endosperm at maturity (e.g. pea, gram, bean). |
| True fruit | Fruit developed only from the ovary (e.g. mango). |
| False fruit | Fruit developed with the involvement of other floral parts (e.g. apple). |
| Parthenocarpy | Formation of fruit without fertilisation (e.g. banana). |
| Apomixis | Formation of seeds without fertilisation; a form of asexual reproduction. |
| Polyembryony | Occurrence of more than one embryo in a single seed (e.g. Citrus, mango). |
Important Points to Remember
- Flower is a modified shoot and acts as the reproductive unit of angiosperms.
- Anther is bilobed, dithecous, tetrasporangiate; pollen develops inside microsporangia.
- Tapetum is the innermost nutritive layer, multinucleate and polyploid.
- Pollen wall — exine (sporopollenin) and intine (cellulose, pectin).
- Embryo sac (Polygonum type) is monosporic, 7-celled and 8-nucleate.
- Pollination types — autogamy, geitonogamy and xenogamy.
- Wind pollination — small, dull, light flowers; water pollination — Vallisneria, Zostera.
- Outbreeding devices — dichogamy, herkogamy, heterostyly, self-incompatibility.
- Double fertilisation — syngamy + triple fusion (unique to angiosperms).
- Endosperm is triploid (3n), embryo is diploid (2n).
- Apomixis is asexual seed formation; polyembryony is multiple embryos in one seed.
This complete chapter on Sexual Reproduction in Flowering Plants covers all the major concepts required for ASSEB Class 12 Biology examinations. Students are advised to revise the diagrams of L.S. of anther, T.S. of ovule, embryo sac, and stages of dicot/monocot embryo development for full preparation. Keep practising with HSLC Guru — your trusted companion for ASSEB Class 12 Biology English Medium.