NCERT Class 12 Biology Chapter 2 Notes Sexual Reproduction in Flowering Plants - Free CBSE Notes

This chapter is renumbered as Chapter 1 in the NCERT Class 12 Biology textbook as per the latest CBSE syllabus 2023–24.

Sexual Reproduction in Flowering Plants Class 12 Notes

NCERT Notes for Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants: In the Sexual Reproduction in Flowering Plants Class 12 Notes, you will understand concepts related to the male and female reproductive systems of flowering plants, as androecium consists of stamens, which represent the male reproductive organs, and gynoecium consists of pistils, which represent the female reproductive organs. Before proceeding towards the NCERT solutions for Class 12 biology chapter 2 sexual reproduction in flowering plants, you have to understand the concepts given in Sexual Reproduction in Flowering Plants Class 12 notes. So, let’s go through the CBSE Notes for Class 12 Sexual Reproduction in Flowering Plants.

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CBSE Notes for Sexual Reproduction in Flowering Plants

CBSE Class 12 Biology Chapter 2 sexual reproduction in flowering plants Notes are given below.

Reproduction is the process that ensures the continuity of species generation after generation as the older individuals undergo aging and die. In the case of flowering plants, they show the sexual mode of reproduction and bear complex male and female reproductive units along with accessory structures.

The flower is a modified stem that functions as a reproductive organ and produces ovum/pollen. A typical angiosperm flower consists of four whorls of floral appendages attached to the receptacle that is:

1. calyx

2. corolla

3. androecium (male reproductive organ consisting of stamens)

4. gynoecium (composed of the ovary, style, and stigma).

Pre-fertilization (Structures and Events)

Before fertilization there are several hormonal and structural changes are initiated which lead to the differentiation and further leads to the development of the floral primordium and then inflorescences are formed which bear the floral buds and then the flowers.

• Stamen, Microsporangium and Pollen Grain-

Stamen:

It consists of a long and slender stalk called filament and generally bilobed anthers. Each lobe contains two thecae (dioecious).

Anther:

It is a four-sided structure consisting of four microsporangia, two in each lobe.

Microsporangia:

It develops further and becomes pollen sacs that contain pollen grains.

Microsporangium: It is generally surrounded by four-layered walls that is:

1. Epidermis

2. Endothecium

3. Middle layer

4. Tapetum.

Whereas the innermost layer of the tapetum provides nourishment to the developing pollen grains.

Microsporogenesis: The process of the formation and differentiation of microspores (pollen grains) from microspore mother cells (MMC) by reductional division is called microsporogenesis. Sporogenous tissues are those which are compactly arranged in the form of homogenous cells and it is present at the center of each microsporangium when the anther is young.

Pollen grains:

It represents the male gametophytes nd made up of two-layered walls that is,

1. Exine- It is made up of sporopollenin and it is the most resistant organic matter is known. It can withstand high temperatures, strong acids, and alkali, as well as no enzyme, can degrade sporopollenin.

2. Intine:-Thin and continuous layer. Made of cellulose and pectin

Germ pore: It apertures on exine where sporopollenin is absent and forms the pollen tube. A plasma membrane surrounds the cytoplasm of the pollen grain and a mature pollen consists of 2 cells with a nucleus (Vegetative and Generative)

1. Vegetative cell:

• Bigger

• Abundant food reserve

• Large irregular nucleus

• Responsible for the development of pollen grain

2. Generative Cells:

• Small

• Involves in syngamy (fuse with an egg)

• Dense cytoplasm and nucleus

Allergies from pollen grains:

For example, Parthenium, cause severe types of allergies and bronchial/lungs diseases in some people and leads to chronic respiratory disorders– asthma, bronchitis, etc.

Pollen grains are very rich in nutrients and they are also used as pollen tablets as food supplements.

The viability of pollen grain varies from species to species and should land on stigma before this period to germinate. Pollen grains of a large number of species are stored in liquid nitrogen at temperature – 196 degree, called pollen bank.

The Pistil, Megasporangium, and Embryo sac

• Gynoecium may consist of a single pistil (monocarpellary) or more than one pistil (polycarpellary) which may be fused (syncarpous) or free (apocarpous). For example:

-Multicarpellary and syncarpous pistil- Papaver

-Multicarpellary and apocarpous pistil- Michelia

• Each pistil has three parts:

  • Stigma

  • style

  • ovary

• Inside the ovary is the ovarian cavity; locule, and the placenta is located inside the ovarian cavity. Whereas megasporangia (ovules) arise from placenta.

Megasporangium

• The ovule is a small structure attached to the placenta.

• Hilum- the junction between ovule and funicle

• Integuments- protective envelops

• Micropyle- small opening at the tip of ovule into where pollen tube enters

• Chalaza- basal part of the ovule

• Nucellus (2n)-mass of cells enclosed in integuments. Has abundant food reserve.

Megasporogenesis: The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in the ovule.

Ovule differentiates a single megaspore mother cell (MMC) in the micropylar region of the nucellus. MMC undergoes meiotic division that results in the production of four megaspores.

• Remember that in most of the flowering plants, total of three megaspores degenerate. one megaspore develops into a female gametophyte that is embryo sac.

• The nucleus of functional megaspore divides mitotically to form two nuclei which move to opposite poles to form a 2-nucleate embryo sac. Two more sequential mitotic division results into the 8-nucleate embryo sac.

• Six of the eight nuclei are surrounded by a cell wall and the remaining two nuclei (polar nuclei) are situated below the egg apparatus.

• Three cells are grouped at the micropylar end to constitute egg apparatus and three cells at the chalazal end form antipodal cells. At maturity, embryosac is 8-nucleate and 7 celled.

Pollination – transfer of pollen grains from anther to stigma.

a) Autogamy: transfer of pollen grain from anther to the stigma of the same flower.

I. Cleistogamous– flowers which does 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.

ll. Chasmogamous– exposed anther and stigma.

b) Geitonogamy: transfer of pollen grains from the anther to the stigma of a different flower of the 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

c) Xenogamy: transfer of pollen grain from anther to the stigma of different plant’s flower of same species.

Agents of pollination include abiotic (water, wind) and biotic (insects, butterfly, honey bee, etc. large number of pollen grains are produced by plants using the abiotic mode of pollination as most the pollen grains are wasted during transfer.

Adaptations in flowers for Pollination

1. Wind Pollination

• pollen grains: light, non-sticky, winged

• anther: well exposed

• stigma: large and feathery

• flower: one ovule, arranged as inflorescence

Ex: corn cob, cotton, date palm

2. Water Pollination

Bryophytes, Pteridophytes, Algae

• pollen grains: protected by mucilaginous covering

Ex: Freshwater plants- Vallisneria, Hydrilla

Seagrass- Zostera

Main features of wind and water pollinated plants

– produce pollen grains in large no.

– do not produce nectar

3. Insect Pollination

– Flowers: large, colorful, fragrant, rich in nectar

– Pollen grains: sticky

– Stigma: sticky

Benifits to pollinators:

• nectar and (edible) pollen grains as foods

• provide a safe place for laying eggs

Ex : 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 a different position.

• Inhibiting pollen germination in the pistil.

• Production of unisexual flowers.

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

Artificial Hybridization

• Crossing different varieties of species to form hybrid organism with desirable characters from both of the parent.

• Desired pollen grains for pollination- stigma protected from contamination

Emasculation: It is the process of removal of the anther

• Bagging: It is a process which is used in the plant breeding to prevent self pollination in bisexual flowers.

Double Fertilisation-

This event takes place when two types of fertilisation takes place; after entering one of the synergids, each pollen grain releases two male gametes and then one male gametes fuse with egg (Syngamy) and other male gametes fuse with two polar nuclei (triple fusion) to produce a triploid primary endosperm nucleus (PEN).

So, when two types of fusion take place in an embryo sac the phenomenon is called double fertilisation. The PEN develops into the endosperm and the zygote develops into the embryo.

Post fertilisation events:

It includes endosperm and embryo development, maturation of ovules into seeds, and ovary into fruits.

Endosperm:

A primary endosperm cell divides many times to forms triploid endosperm tissue having reserve food materials.

Two types of endosperm development :

• Free nuclear type

• Cellular type

Non-albuminous- When the endosperm completely utilize even before the maturation of seeds. For e.g pea, groundnut.

Albuminous- When a portion of endosperm remains in mature seeds. For e.g wheat, maize, castor.

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

Embryogeny – It is the early stages of embryo development. The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped, and mature embryo.

The embryo consists of:

• embryonal axis

• cotyledons

• plumule

• radicle

Monocotyledonous Seed

• Scutellum = Cotyledon

• Coleorrhiza: undifferentiated sheath covering radical & root cap

• Coleoptile: sheath covering plumule

Seed- After fertilisation a mature ovule develops into a seed.

It consists of:

• cotyledon(s)

• embryonal axis

The double layered seed coat is formed by:

• Testa (outer coat)

• Tegmen (inner coat)

Micropyle- small opening on the seed coat, it facilitates entry of H2O & O2 into seeds (for germination)

Hilum- It is the scar on the seed coat.

Perisperm: It is the remnants of nucellus that are persistent. For eg: Black pepper

Dormancy: When the seed reaches a state of inactivity.

Pericarp: The wall of the ovary develops into the wall of fruit which is called the pericarp. In true fruits, the only ovary contributes in fruit formation, and in false fruit, the thalamus contributes in fruit formation.

Apomixis: It is a form of asexual reproduction that mimics sexual reproduction and produce seed without fertilization.

Formation of apomictic seeds: A diploid cell that is formed without meiosis develops into an embryo without fertilization. Ex. Citrus and Mango.

Polyembryony: The occurrence of more than one embryo in one seed is called polyembryony. For eg: Citrus, groundnut.

Chapter-wise Class 12 Biology Notes

Following are the links to the Biology Class 12 notes PDF download for all Chapters:

• Chapter 1 - Sexual Reproduction in Flowering Plants
• Chapter 2 - Human Reproduction
• Chapter 3 - Reproductive Health
• Chapter 4 - Principles of Inheritance and Variation
• Chapter 5 - Molecular Basis of Inheritance
• Chapter 6 - Evolution
• Chapter 7 - Human Health and Disease
• Chapter 8 - Microbes in Human Welfare
• Chapter 9 - Biotechnology: Principles and Processes
• Chapter 10 - Biotechnology and its Applications
• Chapter 11 - Organisms and Populations
• Chapter 12 - Ecosystem
• Chapter 13 - Biodiversity and Conservation

The following three chapters have been Deleted from the NCERT Class 12 Biology textbook 2023-24.

Benefits of Sexual Reproduction in Flowering Plants Class 12 Notes CBSE Biology Chapter 2

Sexual Reproduction in Flowering Plants class 12 Notes Biology chapter 2 are given below:

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