Sexual Reproduction In Flowering Plant
Sexual Reproduction in Flowering Plants - Complete NEET Guide with Diagrams & Practice Questions
Table of Contents
- Introduction
- Key Concepts: Pre-Fertilisation Structures & Events
- Key Concepts: Double Fertilisation
- Key Concepts: Post-Fertilisation Structures & Events
- Key Concepts: Apomixis and Polyembryony
- Important Ploidy Levels to Remember
- Memory Techniques (Mnemonics)
- Previous Year Questions (NEET)
- Key Takeaways for Quick Revision
Introduction
The vibrant colors, enchanting scents, and intricate structures of flowers are not just for aesthetic pleasure; they are sophisticated marvels of nature designed for one primary purpose: sexual reproduction. This chapter is one of the most important and high-weightage topics in the NEET Biology syllabus, with an average of 3-4 questions appearing every year. A thorough understanding of the structures of the male and female gametophytes, the processes of microsporogenesis and megasporogenesis, and the unique phenomenon of double fertilisation is non-negotiable for a high score.
This guide will take you on a detailed journey through the entire process of sexual reproduction in angiosperms. We will dissect the flower, trace the development of pollen grains and the embryo sac, understand the strategies plants use for pollination, unravel the elegant mechanism of double fertilisation, and follow the transformation of the ovule into a seed and the ovary into a fruit.
Key Concepts: Pre-Fertilisation Structures & Events
1. The Flower: A Fascinating Organ
A flower is a modified shoot that serves as the reproductive unit in angiosperms. A typical flower has four main whorls: calyx, corolla, androecium, and gynoecium. For reproduction, we focus on the androecium (male reproductive part) and gynoecium (female reproductive part).
2. Stamen, Microsporangium, and Pollen Grain
The androecium is composed of stamens. Each stamen consists of a long, slender filament and a terminal, typically bilobed anther.
- Structure of Anther: A typical anther is bilobed and dithecous (each lobe has two theca). It is a four-sided (tetragonal) structure consisting of four microsporangia, which develop into pollen sacs.
- Structure of Microsporangium: A microsporangium is surrounded by four wall layers:
- Epidermis: Outermost protective layer.
- Endothecium: Helps in anther dehiscence (bursting to release pollen).
- Middle Layers: 2-3 layers that also aid in protection and dehiscence.
- Tapetum: The innermost nutritive layer. It provides nourishment to the developing pollen grains. Tapetal cells are dense in cytoplasm and are generally multinucleate.
- Microsporogenesis: The process of formation of microspores from a Pollen Mother Cell (PMC) through meiosis.
- The sporogenous tissue (2n) in the center of the microsporangium differentiates into PMCs (2n).
- Each PMC undergoes meiosis to produce a microspore tetrad (four haploid microspores, n).
- Pollen Grain (Male Gametophyte):
- It is a spherical structure with a two-layered wall:
- Exine: The hard outer layer made of sporopollenin, the most resistant organic material known. It has apertures called germ pores where sporopollenin is absent.
- Intine: The thin inner wall made of cellulose and pectin.
- A mature pollen grain contains two cells:
- Vegetative Cell: Large, with abundant food reserve and an irregularly shaped nucleus.
- Generative Cell: Small, spindle-shaped, and floats in the cytoplasm of the vegetative cell.
- In over 60% of angiosperms, pollen is shed at the 2-celled stage. In the remaining species, the generative cell divides mitotically to form two male gametes, and pollen is shed at the 3-celled stage.
- It is a spherical structure with a two-layered wall:
Figure: Transverse section of a mature anther showing the four wall layers and the pollen grains within the pollen sacs.
3. The Pistil, Megasporangium (Ovule), and Embryo Sac
The gynoecium represents the female reproductive part and is made up of one or more pistils (or carpels). A pistil has three parts: stigma (receptive platform for pollen), style (elongated tube), and ovary (basal bulged part containing ovules).
- Structure of the Megasporangium (Anatropous Ovule):
- The ovule is attached to the placenta by a stalk called the funicle. The junction between the ovule body and the funicle is the hilum.
- It has one or two protective envelopes called integuments, which encircle the ovule except at the tip, where a small opening called the micropyle is present.
- The chalaza is the basal part of the ovule, opposite the micropylar end.
- The main body of the ovule is composed of a mass of cells called the nucellus, which contains reserve food materials.
- Megasporogenesis: The process of formation of megaspores from the Megaspore Mother Cell (MMC).
- A single MMC (2n) differentiates in the micropylar region of the nucellus.
- The MMC undergoes meiosis to produce four haploid megaspores (n).
- Female Gametophyte (Embryo Sac):
- In most flowering plants, only one of the four megaspores is functional, while the other three degenerate.
- This development from a single megaspore is termed monosporic development.
- The functional megaspore undergoes three successive free-nuclear mitotic divisions, resulting in an 8-nucleate stage.
- Cell walls are then laid down, resulting in a mature embryo sac which is 7-celled and 8-nucleate.
- Egg Apparatus (at micropylar end): Consists of one egg cell and two synergids. The synergids have special cellular thickenings called the filiform apparatus, which guides the pollen tube.
- Antipodal Cells: Three cells at the chalazal end.
- Central Cell: A large central cell with two polar nuclei.
Figure: A diagram of a mature embryo sac (female gametophyte) showing the 7-celled, 8-nucleate structure.
4. Pollination: The Transfer of Gametes
Pollination is the transfer of pollen grains from the anther to the stigma.
- Types of Pollination:
- Autogamy (Self-pollination): Transfer of pollen within the same flower.
- Chasmogamous flowers: Exposed anthers and stigma.
- Cleistogamous flowers: Flowers that do not open, ensuring autogamy (e.g., Oxalis, Viola, Commelina).
- Geitonogamy: Transfer of pollen from the anther of one flower to the stigma of another flower on the same plant. Functionally cross-pollination, but genetically self-pollination.
- Xenogamy (Cross-pollination): Transfer of pollen to the stigma of a flower on a different plant. This is the only type that brings genetic variation.
- Autogamy (Self-pollination): Transfer of pollen within the same flower.
- Outbreeding Devices: Mechanisms to prevent self-pollination and promote cross-pollination.
- Pollen release and stigma receptivity are not synchronized.
- Anther and stigma are placed at different positions.
- Self-incompatibility: A genetic mechanism to prevent self-pollen from fertilizing the ovules.
- Production of unisexual flowers.
- Pollen-Pistil Interaction: The ability of the pistil to recognize, accept, or reject pollen. If compatible, the pollen grain germinates on the stigma, and the pollen tube grows through the style to reach the ovule.
Key Concepts: Double Fertilisation
This is an event unique to flowering plants.
- After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid.
- Syngamy: One male gamete (n) fuses with the egg cell (n) to form the zygote (2n).
- Triple Fusion: The second male gamete (n) fuses with the two polar nuclei (n+n) in the central cell to form the triploid Primary Endosperm Nucleus (PEN) (3n).
Since two types of fusions (syngamy and triple fusion) occur in the embryo sac, the phenomenon is termed double fertilisation.
Key Concepts: Post-Fertilisation Structures & Events
1. Endosperm and Embryo Development
- Endosperm: The PEN develops into the endosperm, a nutritive tissue for the developing embryo. Endosperm development precedes embryo development.
- Albuminous/Endospermic seeds: Retain endosperm at maturity (e.g., castor, wheat, maize).
- Non-albuminous/Ex-albuminous seeds: Completely consume the endosperm during development (e.g., pea, groundnut, bean).
- Embryo (Embryogeny): The zygote develops into the embryo.
- Dicot Embryo: Consists of an embryonal axis and two cotyledons. The part of the axis above the cotyledons is the epicotyl (terminates in the plumule), and the part below is the hypocotyl (terminates in the radicle).
- Monocot Embryo: Possesses one large, shield-shaped cotyledon called the scutellum. The radicle is protected by the coleorrhiza, and the plumule is protected by the coleoptile.
Figure: L.S. of a Dicot embryo (e.g., bean) and a Monocot embryo (e.g., grass/maize).
2. Seed and Fruit Formation
- Seed: The fertilized ovule. The integuments harden to form the protective seed coat (outer testa, inner tegmen). In some seeds like black pepper, remnants of the nucellus persist, called perisperm.
- Fruit: The mature or ripened ovary. The wall of the ovary develops into the fruit wall, called the pericarp.
- True Fruits: Develop only from the ovary.
- False Fruits: Floral parts other than the ovary, such as the thalamus, contribute to fruit formation (e.g., apple, strawberry).
- Parthenocarpic Fruits: Fruits that develop without fertilization (e.g., banana). They are seedless.
Key Concepts: Apomixis and Polyembryony
- Apomixis: A special mechanism to produce seeds without fertilization. It is a form of asexual reproduction that mimics sexual reproduction. Its use in the hybrid seed industry is of great importance as it prevents the segregation of characters in the hybrid progeny.
- Polyembryony: The occurrence of more than one embryo in a seed. Often seen in citrus and mango varieties where some nucellar cells surrounding the embryo sac start dividing and develop into embryos.
Important Ploidy Levels to Remember
| Structure | Ploidy |
|---|---|
| Nucellus, Integuments, MMC, PMC | Diploid (2n) |
| Megaspore, Microspore | Haploid (n) |
| Egg, Synergids, Antipodals | Haploid (n) |
| Male Gametes | Haploid (n) |
| Polar Nuclei | Haploid (n+n) |
| Zygote | Diploid (2n) |
| Primary Endosperm Nucleus (PEN) | Triploid (3n) |
| Embryo | Diploid (2n) |
| Endosperm | Triploid (3n) |
| Perisperm | Diploid (2n) |
Memory Techniques (Mnemonics)
- Anther Wall Layers (from outside in): "Every Ending Meets the Target" - Epidermis, Endothecium, Middle layers, Tapetum.
- Components of a Mature Embryo Sac: "An Excited Syrian Ant Cried" - Egg (1), Synergids (2), Antipodals (3), Central Cell with polar nuclei (1). Total 7 cells, 8 nuclei.
- Double Fertilisation:
- Syngamy: Male Gamete (n) + Egg (n) = Zygote (2n)
- Triple Fusion: Male Gamete (n) + 2 Polar Nuclei (n+n) = PEN (3n)
- Post-Fertilisation Fates:
- Ovary → Fruit
- Ovule → Seed
- Zygote → Embryo
- PEN → Endosperm
- Integuments → Seed Coat
Previous Year Questions (NEET)
Q1. In some members of which of the following pairs of families, pollen grains retain their viability for months after release? (NEET 2021) a) Poaceae; Rosaceae b) Poaceae; Leguminosae c) Rosaceae; Leguminosae d) Poaceae; Solanaceae
Explanation: The NCERT textbook states that in cereals like rice and wheat (Poaceae), pollen grains lose viability within 30 minutes. In contrast, members of Rosaceae, Leguminosae, and Solanaceae maintain viability for months. Therefore, the correct pair is Rosaceae and Leguminosae. Answer: (c) Rosaceae; Leguminosae
Q2. The plant parts which consist of two generations - one within the other, are: (NEET 2020) (1) Pollen grains inside the anther (2) Germinated pollen grain with two male gametes (3) Seed inside the fruit (4) Embryo sac inside the ovule a) (1), (2), and (3) b) (3) and (4) c) (1) and (4) d) (1) only
Explanation: (1) Pollen grain (n, gametophyte) is inside the anther (2n, sporophyte). (4) Embryo sac (n, gametophyte) is inside the ovule (2n, sporophyte). In both cases, a haploid gametophytic generation is developing within a diploid sporophytic generation. Answer: (c) (1) and (4)
Q3. What is the fate of the male gametes discharged in the synergid? (NEET 2019) a) One fuses with the egg, other fuses with synergid. b) One fuses with the egg and other fuses with the central cell nuclei. c) One fuses with the egg and other degenerates in the synergid. d) All fuse with the egg.
Explanation: This question directly tests the definition of double fertilisation. One male gamete fuses with the egg (syngamy) to form the zygote, and the other male gamete fuses with the two polar nuclei in the central cell (triple fusion) to form the PEN. Answer: (b) One fuses with the egg and other fuses with the central cell nuclei.
Q4. Persistent nucellus in the seed is known as: (NEET 2019) a) Perisperm b) Hilum c) Tegmen d) Chalaza
Explanation: In some seeds, such as black pepper and beet, remnants of the nucellus are not fully consumed during embryo development and remain in the mature seed. This residual, persistent nucellus is called the perisperm. Answer: (a) Perisperm
Key Takeaways for Quick Revision
- Microsporogenesis (Meiosis) forms haploid microspores from a diploid PMC.
- Megasporogenesis (Meiosis) forms a haploid functional megaspore from a diploid MMC.
- The pollen grain has a tough outer exine (sporopollenin) and inner intine.
- A mature embryo sac is 7-celled and 8-nucleate.
- Double Fertilisation is the fusion of one male gamete with the egg (Syngamy) and the other with the two polar nuclei (Triple Fusion).
- The Zygote is diploid (2n) and develops into the embryo.
- The Primary Endosperm Nucleus (PEN) is triploid (3n) and develops into the nutritive endosperm.
- Apomixis is the formation of seeds without fertilization, creating clones.
- False fruits (e.g., apple) develop from the thalamus in addition to the ovary.