NCERT Solutions Class 12 Biology Chapter 2

NCERT Solutions Class 12 Biology Chapter 2 – Sexual Reproduction in Flowering Plants

The NCERT Solutions Class 12 Biology Chapter 2 is about sexual reproduction in flowering plants. The subject of Biology revolves around learning about various organisms, their functions, anatomy, physiology etc.. It assists students in getting intrinsic knowledge about the life systems of these organisms. NCERT Solutions Class 12 Biology Chapter 2 presents a detailed and in-depth description of the sexual reproduction process in flowering plants. The study materials prepared by the Extramarks on Sexual reproduction in flowering plants have been crafted by the experts at Extramarks as per the new and updated syllabus to help students secure good grades in the examination.

Extramarks intends to assist students of Class 12 by offering all the information about the examinations, important questions, and the CBSE syllabus, including NCERT Solutions Class 12 Biology Chapter 2. The study resources created by the Extramarks offer a complete understanding of all the concepts to streamline your learning process.

Key Topics Covered In NCERT Solutions Class 12 Biology Chapter 2

Sexual Reproduction in Flowering Plants:

• Sexual reproduction is the procedure by which new organisms are formed by fusing male and female gametes from both parents.
• The flower is the critical reproductive structure. 
• Sporophylls are categorised into microsporophylls or stamen and megasporophylls or carpel. 
• A carpel is an ovary that consists of an ovule, a style, and a stigma.
• There are three kinds of stamen: filament, Anther, and connective.
• Stamen is eminent as filament, Anther and connective.

Sexual reproduction in flowering plants can be fragmented into three types:

i) Pre-fertilization.

ii) Post-fertilization.

iii) Double fertilisation.

1. Pre-Fertilisation:

Structure and Events:

The below pre-fertilisation events can be studied: 

i) Pollen grain formation.

ii) Pollen pistil interaction E.

iii) Pollination.

iv) embryo sac formation.

       2. Pollen Grain Formation:

 Male reproductive unit (Stamen):

• A stamen is an angiosperm’s male reproducing unit. It is made of an anther and a filament. 
• Several pollen grains are contained in every pollen sac. A dithecous anther’s four pollen sacs are placed in the four corners.
• Dithecous Anther: An anther with two lobes related by a non-sporangial tissue called the connective. 
• The anther wall is made of four layers of cells.
• To free pollen grains, Anther dehiscence through slits.

Anther Development:

• The growth of an anther starts with a mass of homogeneous meristematic cells surrounded by an epidermis.
• Four lobes are created.
• Archesporial cells: A group of primitive cells that split into two types of cells: a primary parietal cell and a primary sporogenous cell.
• The parietal cell divides multiple times to form the anther wall, whereas a sporogenous cell divides less frequently to create microspores.
• The tapetum is the inside layer of the cell wall that comes across contact with the PMCs. In pollen generation, the tapetum plays a crucial role.
• Tapetum: This is a tissue built within the Anther that feeds the growing spores.
• The endothelium is the layer beneath the epidermis.

Wall Layers of Anther:

The epidermis is one layer of cells that serves as a defensive layer.

Endothecium:

Endothecium is a single-layered following wall. Cells are thickened alongside cellulose and a trace of lignin and pectin. It helps in the dehiscence of anthers.

Middle layers:

Ranges from 1-6. When the Anther develops, the middle layer degenerates.

Tapetum:

a) The anther wall’s innermost layer surrounds the sporogenous tissue.

b) The ubisch bodies settle in the exine of the microspore wall. 

c) They have multiple nuclei and are polyploid.

d) Tapetal cells contain nutrients.

e) There are two kinds of tapetum:

(i)Secretary.

(ii)Amoeboid / periplasmodial.

Microsporogenesis:

Microsporogenesis is the creation and differentiation of microspores.

• PMCs go through meiosis. Each forms tetrahedral tetrads.
• Cytokinesis can be either sequential or simultaneous.
• The cell wall is created in successive types after meiosis –I and meiosis –II, succeeding in an isobilateral pollen tetrad. Monocots own a distinctive trait.
• In the similar kind, each nuclear division in a microspore mother cell is followed by forming a cell wall.

Tetrads are Classified into five types: 

• Tetrahedral.
• Isobilateral.
• Decussate.
• T-shaped.
• Linear.

The most common shape is Tetrahedral.

Pollinium:

A mass of pollen grains formed on each anther lobe. The overall abundance of pollen grains is t as a unit when the pollinium is connected to pollinating agents such as insects.

Pollen Grain:

• Pollen grains arrive in all forms and sizes. The pollen grain owns a haploid, unicellular body with a single nucleus. It has a double-layered exterior wall.
• It is mostly round and has a diameter of 25 – 30m.
• The wall, or sporoderm, is made of a couple of layers.
• The outer layer is quite thick. It is called the exine. It is composed of sporopollenin. 
• The inner wall is thin and is known as the intine. It is composed of pectin-cellulose.
• The tectum aids in pollen grain identification and categorisation by family, genus, or species.
• Pollen grains are classified as monocolpate with one germ pore, bilobate or 
• with two germ pores, tricolpate or three germ pores.
• The study of pollen is termed palynology.

Development of Male Gametophyte:

• Inside the pollen grain, the nucleus develops in size. It breaks up mitotically to give rise to two unequal daughter cells: a giant vegetative cell or tube cell and a smaller generative cell.
• Pollination can occur when the pollen grain is two-celled (generative +tube )or three-celled (two male gametes + tube ).
• The generative cell’s cytoplasm contains tiny conserved food material, while the vegetative cell’s cytoplasm includes carbohydrate, fat, and protein granules.

Pollen Products:

1. Pollen supplements:

The pollen grain is high in carbohydrates and unsaturated fat. They are taken in the form of tablets and syrups to improve essential body functions. Pollen consumption boosts production and is utilised by athletes as well as racehorses.

2. Pollen creams:

Pollen grains give UV protection. Therefore, they are utilised in creams and emulsions to give skin protection and smoothness.

Pollen Viability:

• Pollen viability indicates the amount of time pollen grains remains viable or functional. 
• For 30 minutes, pollen grains are viable.
• It is estimated by temperature and humidity.

Pollen allergy:

• Pollen grains cause severe allergies. It causes fever and general respiratory disorders such as bronchitis and asthma.
• Carrot grass (Parthenium hysterophorus) is a crucial allergen source. 

Female Reproductive Unit (Pistil):

• The female reproducing unit of a flower is the pistil or gynoecium.

Stigma: The part of the body that acquires pollen grains.

• A carpel or pistil comprises three parts: the stigma, the style, and the ovary.
• Ovary: A swollen place at the base of the ovary. One too many ovules are found in the ovary.
• The stalk that relates the stigma to the ovary is the style.
• The ovule is a megasporangium revolved by integuments. The ovule is full-grown into a seed after fertilisation. It is whitish and oval.
• Hilum: The point at which the funicle is linked to the ovule.
• Funiculus: The stalk that connects the ovule and the placenta.
• Fusing the funiculus with the physique of the ovule results in creating a raphe.

(i) Unitegmic:

Using only one integument. It can be located in higher dicots such as Compositae and gymnosperms.

(ii) Bitegmic:

Ovules have two integuments. It can be built in monocots and primitive dicots such as the Malvaceae and Cruciferae.

(iii) Strategic:

Three integuments, as in Asphodelus.

(iv) Ategmic:

No integument. Santalum, Ziriosoma, Loranthus, and Relax are examples of this.

Forms of Ovule:

1. Orthotropous (Erect):

The ovule’s body is linear and lies directly over the Hilum, funicle, chalaza, and micropyle. All share a similar phylogenetic line.

2. Anatropous (Inverted): 

The ovule’s body is inverted. The ovule and the funicle are connected. The raphe is created by the fusion of the ovule and the funicle. The funicle is near Hilum and Micropyle. The chalaza is found on the opposite end of the micropyle. It is the most normal ovule type. 

3. Hemianatropous:

The ovule body is at a straight angle to the funicle, as in Malpighiaceae.

4. Campylotropous:

The physique is curved, but the embryo sac is direct. 

5. Amphitropous:

The ovule bodies and the embryo sac are curved, as in crucifers.

6. Circinotropous:

The ovule revolves at a greater than 360 degrees angle, resulting in the funicle coiling around the ovule. 

7. Megasporogenesis:

• Megasporogenesis refers to the procedure of producing megaspores from megaspore mother cells.
• The MMC goes through meiosis to generate four megaspores.
• Ovules generally form a single megaspore mother cell (MMC) in the nucellar micropylar region. It’s a giant cell with many cytoplasms and a prominent nucleus.
• Monosporic development refers to forming an embryo sac from a single megaspore.
• Only the functional megaspore can grow up into a female gametophyte.
• In the majority of flowering plants, only one of the megaspores is energetic. The other three have devolved.

Formation of Embryo SAC:

• Mitosis happens in the nucleus of the functional megaspore, resulting in the formation of two nuclei. 
• There are two additional sequential mitotic nuclear divisions. 
• Cell wall growth does not take place immediately after nuclear division.
• Cell walls are set up after the eighth nucleate stage. 
• They are located in the substantial central cell beneath the egg apparatus. 
• Three cells are located together at the micropylar end. They make up the egg apparatus.
• The egg apparatus comprises two synergids and one egg cell. 

Pollination:

Pollination is the shift of pollen grains from the Anther to the flower’s stigma. Pollination can be categorised into two distinctive types: cross-pollination and self-pollination.

• Self-pollination indicates the transfer of pollen grains from anthers to stigmas of the same or varied flowers on the same plant. Flowers in self-pollination are genetically similar.

Self-pollination is of two kinds:

1. Autogamy
2. Geitonogamy 

1.Autogamy:

The shift of pollen grains from the Anther to the stigma of the matching flower. It is preferred because of the below adaptations:

1. Chasmogamous apparatuses
2. Cleistogamy

2.Geitonogamy:

It is the movement of a pollen grain from one flower’s Anther to the stigma of another flower of the same or genetically similar plant.

Benefits of Self Pollination:

• It retains the race’s purity.
• The plant does not have to produce a vast number of pollen grains.
• Self-pollination eliminates undesirable recessive traits.
• It ensures seed production. 

Self-pollinated plants have many disadvantages, as shown below:

•  Vitality declines, eventually leading to degeneration.
• Variable, Which reduces adaptability to changing environments.

 Cross-Pollination:

• It is defined as the shift of pollen grains from an anther of one plant to the stigma of another plant of the same or different species. It is also referred to as allogamy.
• Pollination takes place in Xenogamy between two flowers of plants that are genetically and ecologically distinct.

 Cross-Pollination Devices:

1. Dicliny: 

Flowers are categorised into males and females. Plants could be either dioecious or monoecious.

2. Dichogamy:

It takes place when the Anther and stigma mature at various times.

(i) Protandry: 

Anthers mature at a fast rate. examples are Salvia, Sunflower, Clerodendron, and Rose.

(ii) Protogyny: 

Stigmas develop at a younger age. For example, Magnolia, Plantago, and Mirabilis.

3. Self-sterility:

Some crucifers and Tobacco, for example, have pollen grains that are inadequate for growing over the stigma of the same flower.

4. Heterostyly:

The stamens and styles are at varying heights within the flowers. 

5. Herkogamy: 

Self-pollination is avoided by the existence of a natural or physical barrier connecting androecium and gynoecium.

Advantages of Cross-Pollination:

• The race’s flawed character is eliminated and replaced by a better character. 
• Cross-pollination boosts the ability of offspring to adapt to environmental changes.
• Cross-pollination causes genetic recombination and, as a consequence, variation in offspring.

Disadvantages of Cross-Pollination:

• Because an external agency is involved, the risk factor is always present.
• The very best character will most likely be spoiled.
• Plants must generate a vast number of pollen grains.

Agents of Pollination: 

Characteristics of Anemophily (wind pollination):

• Pollen grains are very light in weight. They might have wings or an air sac.
• Flowers are small, colourless, and odorless.
• Stigmas are sticky and feathery in texture.
• Anthers have a long filament and are abundant.
• The pollen grains are dehydrated.

Characteristics of Hydrophily (water pollination):

The flowers are colourless, small, odorless, and nectarless, and the stigma is sticky, long, and unpalatable.

Water Pollination is of two types:

(a) Epihydrophily is a kind of hydrophilic.

(b) Hypohydrophily or inside the water, for example, Zostera and Ceratophyllum.

Characteristics of Entomophily (Insect pollination):

• The flowers are coloured. Bees are attracted to bluish–purplish–violet–yellow flowers and wasps, while butterflies are drawn to reddish flowers.
• Either nectar or pollen nourishes insects that come to visit.
• Pollen grains are sticky as an action of the pollen kit, and stigmas are sticky as well.
• Flowers have an aroma or scent.

Ornithophily (Bird pollination):

• Sunbirds and hummingbirds are two kinds of long-beaked small birds that aid in pollination.
• Bird pollination is common in bottlebrushes, coral, and silk-cotton trees.

Chiropterophily (Pollination by bats):

• They pollinate huge, dull-coloured flowers with a strong aroma. 
• In Adansonia and Kigelia, bats pollinate the flowers. 

Malacophily (pollination by snails):

Snails pollinate Arisaema (snake or cobra plants) and a few arum lily species. 

Myrmecophily (pollination by ants):

Myrmecophily indicates ant pollination of flowers. Few members of the Rubiaceae family are examples. 

Significance of Pollination:

• Pollination is required for fertilisation and, subsequently, seed and fruit production. 
• The seeds and fruits are also nutritious. 
• It leads to the formation of hybrid seeds.
• It promotes ovarian growth.

Post Pollination Events:

• The pollen grain’s nucleus breaks up to produce vegetative and generative cells.
• The generative nucleus split to produce two male nuclei. 
• The region of the arrival of the pollen tube into the ovule determines the type of entry.

They are as follows:

i) Porogamy: 

The approach of a pollen tube into an ovule via a micropyle, as in Ottelia.

ii) Chalazogamy: 

The approach of a pollen tube into an ovule via a chalaza, such as Casuarina.

iii) Mesogamy: 

Pollen tube arrival into the ovule via the funicle or integuments, 

Pollen – Pistil Interaction:

• Female parents with bisexual flowers utilise forceps to remove anthers from the flower bud before the Anther dehisces. 
• Germination is linked to the result of proteins found on pollen grains and the stigma that decides compatibility.
• By manipulating pollination, plant breeders can form hybrids between different species.
• Only compatible pollen from similar species can germinate. 

Double Fertilisation:

Fertilisation is the method by which male and female gametes fuse to form the zygote. 

Post Fertilisation: Structure and Events:

Endosperms:

• The endosperm is a nutritive tissue that enlarges from vegetative fertilisation. 
• The consequence of genes from the male gamete may be seen in the endosperm. The condition is called xenia.
• The direct or indirect pollen reaction on embryo sac structure is limited to the endosperm and is not observed in the embryo.

The endosperm is broken down into three types based on how it develops:

1. Nuclear endosperm.
2. Cellular endosperm.
3. Helobial endosperm.

Functions of Endosperms:

(i) Endosperm nutrients assist in early seedling growth in plants with albuminous seeds.

(ii) Endosperm nourishes the developing embryo.

Embryogeny (embryo formation):

• It is the creation of a mature embryo from a zygote or an oospore.
• The embryo goes through the globular stage.
• Early growth results in an axially symmetric pro-embryo.
• It is globular at first. Later, it takes on a heart shape before resuming its original shape.
• A dicotyledonous embryo is built on an embryonal axis.

Formation of Seed and Fruit:

Fruit indicates a ripened or fertilised ovary. The pericarp is a plump or dry fruit wall initiated by the ovary wall.

Seeds are assorted as follows based on the presence or absence of endosperm:

(i) Non-endospermic or exalbuminous.

(ii) Endospermic or albuminous.

Importance of Seeds:

• Seeds contain enough food reserves to feed the germinating embryo.
• Because of dispersion, seeds can colonise and populate new areas and spread and propagate their species.
• Human seed germination and sowing gave a hike to agriculture, which aided in the advancement of science, civilisation, and technology.
• Because seeds are the action of sexual reproduction, they have a wide range of variations, which aids in adaptation to various environments.
• Evolutionary success: A seed is an evolutionary success. It shields the embryo from any harm.

 Seed Viability:

• The length of time the seeds preserve their ability to germinate.
• Both genetic and environmental elements influence seed viability.
• Humidity and temperature are two natural factors that can influence viability.

NCERT Solutions Class 12 Biology Chapter 2: Exercises and Answer Solutions

Extramarks provides the latest NCERT Solutions Class 12 Biology Chapter 2 as per CBSE regulations. The syllabus of all the other topics of the Class 12 Board examination is also covered by the Extramarks. Students can clarify their queries by clicking on the links below to prepare for the upcoming Biology board examinations.

Detailed Solutions for all the exercises in this Chapter are listed below:

NCERT Solutions Class 12 Biology Chapter 2 Ex 2.1

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Here you can access all the content anytime from anywhere. In addition to the notes, students can also refer to the following NCERT Solutions for different Classes by clicking on the suitable links below. 

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NCERT Exemplar Class 12 Biology

All Solutions and problems are provided to assist students in getting ready for their final examinations. These Exemplar questions are a little more complicated, but they cover every concept stated in the Class 12 Biology Chapters.

By practising this NCERT Exemplar for Class 12 Biology, students will thoroughly comprehend the ideas given in each Chapter.

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Key Features of NCERT Solutions Class 12 Biology Chapter 2

The main key features of NCERT Solutions Class 12 Biology Chapter 2 are as under:

• NCERT Solutions Class 12 Biology Chapter 2 will help students to organise their responses effectively in the examination. 
• The theory is explained in detail. Students can write answers to the point by studying the right notes on the Extramarks’ website.
• The theory is written by experienced subject faculty with significant experience in the relevant subject.