Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants

Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants: Access detailed notes and solutions for Class 12 Biology Chapter 2 on Sexual Reproduction in Flowering Plants. Gain a comprehensive understanding of the topic with clear explanations, diagrams, and important concepts to excel in your studies.

Chapter 2 Sexual Reproduction in Flowering Plants

• Sexual reproduction is the process by which new organisms are formed from the fusion of male and female gametes from two parents.
• The flower is the primary reproductive structure. Within the flowers, the reproductive organs, or sporophylls, are produced.
• Sporophylls are classified into two types: microsporophylls (stamen) and megasporophylls (carpel). 
• A carpel is an ovary that contains an ovule, a style, and a stigma.
• There are three types of stamen: filament, anther, and connective.
• Stamen is distinguished as filament, anther and connective.

Sexual reproduction in flowering plants can be broken down into three steps:

• Pre-fertilization 
• Double fertilization 
• Post-fertilization

Pre-fertilization:

Several structural and hormonal changes lead to formation and development of the floral primordium. Inflorescence is formed that bears floral buds and then flower. In flowers, male (androecium) and female (gynoecium) differentiate and develops in which male and female gametes are produced.

Stamen, Microsporangium and Pollen Grain:

• Stamen consists of long and slender stalk called filament and generally bilobed anthers. Each lobe contains two theca (dithecious).
• The anther is four-sided structure consisting of four microsporangia, two in each lobes.
• Microsporangia develop further and become pollen sacs which contain pollen grains.
• A typical angiosperm anther is bilobed with each lobe having two theca, i.e., they are dithecous.
• The anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.
• The microsporangia develop further and become pollen sacs.

Structure of microsporangium:

A typical microsporangium appears near circular in outline It is generally surrounded by four wall layers epidermis, endothecium, middle layers and the tapetum. The outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen. The innermost wall layer is the tapetum. It nourishes the developing pollen grains. When the anther is young, a group of compactly arranged homogenous cells called the sporogenous tissue occupies the center of each microsporangium.

Microsporogenesis: The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.

Pollen grain: The pollen grains represent the male gametophytes Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has a prominent two-layered wall. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkali. Pollen grain exine has prominent apertures called germ pores the inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin.

When the pollen grain is mature it contains two cells, the vegetative cell and generative cell The generative cell is small and floats in the cytoplasm of the vegetative cell.

The Pistil, Megasporangium (ovule) and Embryo sac:

The gynoecium represents the female reproductive part of the flower.

Monocarpellary or Multicarpellary: The gynoecium may consist of a single pistil Monocarpellary or may have more than one pistil multicarpellary.

Syncarpous or Apocarpous: When there are more than one, the pistils may be fused together syncarpous or may be free apocarpous.

Each pistil has three parts: The stigma, style, and ovary.

The stigma serves as a landing platform for pollen grains. The style is the elongated slender part beneath the stigma. The basal bulged part of the pistil is the ovary.

Inside the ovary is the ovarian cavity The placenta is located inside the ovarian cavity Arising from the placenta are the megasporangia, commonly called ovules.

The Megasporangium:

The ovule is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule fuses with funicle in the region called hilum Each ovule has one or two protective envelopes called integuments. Integuments encircle the ovule except at the tip where a small opening called the micropyle is organized. Opposite the micropylar end, is the chalaza, Enclosed within the integuments is a mass of cells called the nucellus. Located in the nucellus is the embryo sac or female gametophyte.

Megasporogenesis:

The process of formation of megaspores from the megaspore mother cell is called megasporogenesis. Ovules generally differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus. It is a large cell containing dense cytoplasm and a prominent nucleus. The MMC undergoes meiotic division. Meiosis results in the production of four megaspores.

Embryo Sac:

This method of embryo sac formation from a single megaspore is termed monosporic development. The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac. and the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. After the 8-nucleate stage, cell walls are laid down leading to the organization of the typical female gametophyte or embryo sac. Six of the eight nuclei are surrounded by cell walls and organized into cells the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell. Three cells are grouped together at the micropylar end and constitute the egg apparatus. consists of two synergids and one egg cell. Three cells are at the chalazal end and are called the antipodals. a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled.

Pollination

Transfer of pollen grains from anther to stigma.

Autogamy: Transfer of pollen grain from anther to stigma of same flower. Produce two types of flower.

Cleistogamous: Flower which do not open. cleistogamous flowers are autogamous as there is no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce assured seed-set even in the absence of pollinators. e.g., Viola (common pansy), Oxalis, and Commelina.

Chasmogamous: Flowers which are similar to flowers of other species with exposed anthers and stigma

Geitonogamy: Transfer of pollen grains from anther to stigma of different flower of same plant. Geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.

Xenogamy: Transfer of pollen grain from anther to stigma of different plant’s flower of same species.

Agents of pollination: It includes abiotic (water, wind) and biotic (insects, butterfly, honeybee etc. large number of pollen grains are produced by plants using abiotic mode of pollination as most of pollen grains are wasted during transfer.

Adaptations in flowers for Pollination

Wind Pollination:

• Pollen grains: Light, non- sticky, winged
• Anther: Well exposed
• Stigma: Large and feathery
• Flower: One ovule, arranged as inflorescence

Example: corn cob, cotton, date palm

Water Pollination:

Bryophytes, Pteridophytes, Algae.

Pollen grains: protected by mucilaginous covering.

Example: Fresh water plants- Vallisneria, Hydrilla Sea grass- Zostera

Main features of wind and water pollinated plants:

• Produce pollen grains in large no.
• Do not produce nectar.

Insect Pollination:

• Flowers: Large, colorful, fragrant, rich in nectar.
• Pollen grains: Sticky.
• Stigma: Sticky.

Certain rewards to pollinators

• Nectar and (edible) pollen grains as foods.
• Provide safe place for laying eggs.
• Example: Amorphophallus, Yucca.

Outbreeding Devices:

The various mechanisms take discourage self-pollination and encourage cross pollination as continued self-pollination leads to inbreeding depression. It includes

• Pollen release and stigma receptivity not synchronized.
• Anther and stigma are placed at different position.
• Inhibiting pollen germination in pistil.
• Production of unisexual flowers.

Pollen pistil interaction:

The pistil has ability to recognize the compatible pollen to initiate post pollination events that leads to fertilization. Pollen grain produce pollen tube through germ pores to facilitate transfer of male gametes to embryo sac.

Artificial Hybridization:

• Crossing diff varieties of species- Hybrid individual- with desirable characters of the parent plants
• Desired pollen grains for pollination- stigma protected from contamination
• Emasculation: Removal of anther.
• Bagging: Flower covered- bag made up of butter-prevent contamination of stigma from unwanted pollen.
• Bagged flower- attains receptivity- mature pollen grains- dusted on the stigma – rebagged- fruits allowed to develop.

Double Fertilization:

After entering the one of the synergids, each pollen grain releases two male gametes. One male gametes fuse with egg (Syngamy) and other male gametes fuse with two polar nuclei (triple fusion) to produce triploid primary endosperm nucleus (PEN). Since two types of fusion takes place in an embryo sac the phenomenon is called double fertilization. The PEN develops into the endosperm and zygote develops into embryo.

Post fertilization events: Include endosperm and embryo development, maturation of ovules into seeds and ovary into fruits.

Endosperm: The primary endosperm cell divides many times to forms triploid endosperm tissue having reserve food materials.

Two types of endosperm development:

• Free nuclear type (common method).
• Cellular type.

Non-albuminous: Endosperm completely utilized- before maturation of seeds. e.g pea, groundnut.

Albuminous: A portion of endosperm remain in mature seeds. e.g wheat, maize, castor.

Embryo:

Embryo develops at the micropylar end of the embryo sac where the zygote is located.

Embryogeny: Early stages of embryo development. The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped, and mature embryo.

Embryo consists of:

• Embryonal axis.
• Cotyledons.
• Plumule
• Radicle

Seed:

Fertilized and mature ovule develops into seed.

Monocotyledonous Seed:

• Scutellem: Cotyledon
• Coleorrhiza: Undifferentiated sheath covering radical & root cap
• Coleoptile: Sheath covering plumule

Seed consists of: Cotyledon(s), embryonal axis, Seed coat- double layered- formed by integuments. Testa (outer coat), Tegmen (inner coat).

Micropyle: Small opening on seed coat, it facilitates entry of H₂O & O₂ into seeds (for germination).

Hilum: Scar on seed coat.

Seed: Albuminous/ Non-Albuminous.

Perisperm: Remnants of nucellus that is persistent. e.g., Black pepper.

Dormancy: State of inactivity.

Pericarp: The wall of ovary develops into wall of fruit called pericarp. In true fruits only ovary contributes in fruit formation by in false fruit thalamus also contributes in fruit formation.

Apomixis: Form of asexual reproduction- mimics sexual reproduction- seed formed without fertilization

Formation of apomictic seeds: Diploid cell (formed without meiosis)- develop into embryo without fertilization. Cells of nucellus (2n) surrounding embryo sac- protrude into embryo sac- develop into embryos. e.g., Citrus and Mango.

Polyembryony: Occurrence of more than one embryo in a seed, often associated with apomixes. Ex: Citrus, groundnut.