Cbse Class 12 Biology Revision Notes Chapter 2

Class 12 Biology Chapter 2 notes

The Class 12 Biology Chapter 2 notes describe various topics that fall under sexual reproduction in flowering plants. Referring to these notes, students would be able to learn about the morphology of flowers, male and female reproductive organs(androecium and gynoecium), the male and female gametophyte( pollen grains and embryo sac), Pollination, the process of fertilisation, embryo, endosperm, development of fruits, parthenocarpy and apomixis. 

Class 12 Biology chapter 2 notes cover all the topics and offer a detailed explanation of the topics covered since it is prepared by our panel of qualified teachers by following the CBSE syllabus and referring to NCERT books. Chapter 2 Biology Class 12 notes reduce the burden on the students and provide them with a simple way to study and revise. Extramarks’ academic team has created Class 12 Biology Chapter 2 notes by referring to the NCERT textbook and other books.

Students may register on Extramarks to access Class 12 Biology Chapter 2 notes. Along with the notes curated by our experts, students may also refer to CBSC sample papers, CBSC revision notes, CBSC important questions, CBSC previous year question papers and CBSC additional questions. 

Key Topics Covered In Class 12 Biology Chapter 2 Notes 

Chapter 2 Biology Class 12 Notes cover all important topics and sub-topics from Class 12 Biology Chapter 2. 

Following are the topics included in Class 12 Biology Chapter 2 Notes:

• Flower – A Fascinating organ of Angiosperms
• Pre-fertilisation: Structures and Events
  • Stamen, Microsporangium and Pollen Grain
  • The Pistil, Megasporangium (ovule) and Embryo sac
  • Pollination
  • Kinds of Pollination
  • Agents of Pollination
  • Outbreeding devices
  • Pollen Pistil interaction
  • Artificial Hybridisation
  • Emasculation 
  • Bagging
•  Fertilisation
  • Syngamy
  • Zygote 
  • Triple fusion

• Double fertilisation
• Post-fertilisation: Structures and Events
  • Embryo
  • Endosperm
  • Seed
  • Seed dormancy
  • True and false fruits
  • Parthenocarpy
• Apomixis and  Polyembryony
• Usage of Apomixis and Polyembryony

For every topic and subtopic included in Class 12 Biology Chapter 2 Notes, below is a self-explanatory note prepared by the expert panel of teachers at Extramarks.

Class 12 Biology Chapter 2 notes – Sexual Reproduction In Flowering Plants

The myriads of flowers, the scents and the perfumes that we use, and the rich colours that can attract anyone, are all present in flowers as an aid to sexual reproduction. All flowering plants reproduce by sexual reproduction. The diverse structures of the flowers, floral parts and inflorescences show amazing adaptations to ensure the formation of the fruits and seeds. Here areClass 12 Biology Chapter 2 notes to understand the structure, morphology and processes of sexual reproduction in flowering plants (angiosperms).

FLOWER- A FANTASTIC ORGAN OF ANGIOSPERM

Human beings have had a significant relationship with flowers since olden times. Flowers have been unforgettable objects of aesthetic, ornamental, religious, social, and cultural value – they have always been used by humans as symbols of important human feelings such as love, happiness, grief etc. 

Flowers are the sites of sexual reproduction, and biologists can study morphological and embryological details from them.

Reproduction- Reproducing offspring enables the continuity of the species, generation after generation. Sexual reproduction facilitates evolution by genetic variation through the intermingling of species and inheritance.

• Flowering plants show the sexual mode of reproduction, and flowers are sites of sexual reproduction.
• Flowers function as a reproductive organ of plants, and the end product of sexual reproduction is fruits and seeds.
• All flowering plants show sexual reproduction.
• An angiospermic flower has four whorls of floral appendages: calyx, corolla, androecium (male reproductive organ) and gynoecium (female reproductive organ), all attached to the receptacle.

PRE-FERTILISATION: STRUCTURES AND EVENTS

Under this section of Class 12 Biology chapter 2 notes, students learn about pre-fertilization. Several structural and hormonal changes are initiated, which lead to the differentiation and development of the floral primordium. Inflorescences are grown, which bear the floral buds and then the flowers. In flower, the female and male reproductive structures, the gynoecium and the androecium differentiate and develop. The androecium consists of a whorl of stamens representing the male reproductive organ, and the gynoecium is the female reproductive organ.

• Several changes in hormone and structure lead to the differentiation and development of the floral primordium.
• Inflorescences( arrangement of flowers) are formed, which bear the floral buds and then the flowers.
• The androecium (male reproductive structure) and the gynoecium (female reproductive structure) differentiate and develop in flowers.

Stamen, Microsporangium and Pollen Grain

Students may refer to various study materials in addition to Class 12 Biology Chapter 2 notes to learn about stamen, pollen grain and microsporangium. A brief if it is as below.

• Generally, stamen can be divided into two parts  – the long and slender stalk (filament) and the terminal bilobed structure (anther). 
• The anther is bilobed, with each lobe having two theca (dithecous)
• A longitudinal groove is present lengthwise, separating the theca.
•  The proximal part of the filament joins the thalamus or the petal of the flower.
• The length and number of stamens are different in flowers of different species. 

• The anther is a tetragonal structure that includes four microsporangia located at each

corner (two in each lobe).

• The microsporangia develop and become pollen sacs and are packed with pollen grains at maturity.
• Microsporangium is generally surrounded by four wall layers- the epidermis, endothecium, middle layers and the tapetum.
• The outer three wall layers are for protection and help in the dehiscence of anther to release the pollen. 
• The innermost tapetum gives nourishment to the developing pollen grains.
• In the early stage, the centre of each microsporangium is filled with sporogenous tissue.

Microsporogenesis:  The Class 12 Biology chapter 2 notes provided by Extramarks briefly explains microsporogenesis. The sporogenous tissue further undergoes meiosis (reduction division) to form microspore tetrads. Out of these tetrads, each one is a microspore mother cell or potential pollen.” The process of production of microspores from a pollen mother cell (PMC) is called microsporogenesis”.

As soon as the anthers mature and dehydrate, the microspores dissociate from tetrad and develop into pollen grains.

Pollen Grain: Pollen grains are male gametophytes generally spherical.

It has a prominent two-layered wall: 

• Exine (outer layer)-  is hard and made up of sporopollenin(the most resistant organic material). It is resistant to high temperatures and strong acids and alkali. No enzyme can degrade sporopollenin. In the exine of a pollen grain, a prominent aperture is present, which is called germ pores, where sporopollenin is absent. A pollen tube emerges from this pore.
• Intine ( inner wall)- It is a cellulose and pectin made thin and continuous layer. 

As mentioned in Class 12 Biology Chapter 2 notes, Pollen grain’s cytoplasm is surrounded by the plasma membrane. A mature pollen grain contains two cells: 

Vegetative cell: It is the bigger cell, has abundant food material and has a large irregularly shaped nucleus. It is responsible for the development of pollen grain.

Generative cell: It is a small cell and floats in the cytoplasm of the vegetative cell. Involves in syngamy(fuses with egg).

• Pollen grains of some species(e.g., Parthenium or carrot grass) cause severe allergies and respiratory disorders in many people leading to chronic diseases like bronchitis, asthma etc. 
• Pollen grains have great nutritive value and are used as pollen food supplements and tablets. In western countries, 

many pollen products in the form of 

syrup and tablets are available on the market. It has been seen that Pollen consumption increases

the performance of athletes and racehorses.

More information is available in Class 12 Biology Chapter 2 notes.

Once the pollen grains are shed, they have to land on the stigma before losing viability because they have to bring about fertilisation. The time period for which pollen grains stay viable is highly variable and depends on the prevailing humidity and temperature. In cereals such as wheat and rice, pollen grains lose viability within 30 minutes of their shedding, and in some members of Leguminosae, Rosaceae and Solanaceae, they remain viable for months.

Pollen grains have to land on pistil’s stigma before they lose viability. Pollen viability varies from species to species and depends on temperature and humidity.

Pollen grains of many species can be stored for years in liquid nitrogen (-1960C). Stored pollen grains are used as pollen banks, similar to seed banks, in crop breeding programmes.

Pollinium: It is a coherent mass of pollen grains that are transmitted as a single unit by insects during Pollination.

The Pistil, Megasporangium (ovule) and Embryo sac

A detailed description of Chapter 2 also includes the structure of pistil, female reproductive organ gynoecium, ovule and embryo sac, which are described in Extramarks chapter 2 Biology class 12 notes. 

• Flower’s female reproductive organ is the gynoecium. The gynoecium may have a single pistil (monocarpellary) or more than one pistil (multicarpellary). In multicarpellary conditions, the pistils may be fused (syncarpous) or free (apocarpous). 

1. Multicarpellary, syncarpous pistil – E.g., Papaver
2. Multicarpellary, apocarpous gynoecium – E.g., Michelia

• Every pistil has three parts: stigma, style and ovary.
• Pollen grains, when released, land on stigma. 
• Beneath the stigma, there is the elongated slender part called style.
• The bulged basal part of the pistil is the ovary. Inside the ovary is the locule (ovarian cavity).
• The placenta is present inside the ovarian cavity. The Megasporangia (commonly called ovules) arise from the placenta. The ovule number in an ovary may vary. E.g. one ovule in Wheat and Mango, many ovules in Papaya.

Students may refer to Class 12 Biology chapter 2 notes to understand the various aspects related to ovule and embryo sac.

The Megasporangium (Ovule)

Students may refer to chapter 2, Biology Class 12 notes on Extramarks, to study in detail ovule and their structure for their examination. 

The ovule is a tiny structure attached to the placenta with the help of a stalk called a funicle.

The ovule’s body fuses with the funicle in the region known as the hilum. Thus, the hilum act as the junction between the funicle and ovule. The ovule has one or two protective layers known as integuments. Integuments surround the nucellus leaving at the tip where a small micropyle opening is situated. The chalaza is the basal part of the ovule organised at the opposite end of the micropyle. Inside the integuments is a mass of cells called the nucellus. Nucellus cells have abundant reserve food materials. Inside the nucellus is a female gametophyte or embryo sac. An ovule generally has a single embryo sac formed from a megaspore.

• Funicle:  It is a stalk through which the ovule is attached to the placenta. 
• Hilum: It is the junction point between the ovule and funicle.
• Integuments: Each ovule has one or two protective envelopes called integuments.
• Nucellus: Integuments enclose the nucellus; it is a mass of cells having abundant food materials.
• Micropyle: The integument encircles the nucellus except at the tip, where a small opening called the micropyle is organised. Pollen tube enters through this opening.
• Chalaza: Opposite the micropylar end is the chalaza, representing the basal part of the ovule. 
• Embryo Sac: Inside the nucellus is the female gametophyte or embryo sac. Generally, a single embryo sac formed from a megaspore is present in an ovule.

 More information is available in Class 12 Biology Chapter 2 notes provided by Extramarks.

Megasporogenesis: This is the process of formation of megaspores from megaspore mother cell (MMC) by reduction division or meiosis.

In ovules, there is a differentiation of a single megaspore mother cell (MMC) in the micropylar region of the nucellus. The MMC undergoes meiotic (reduction) division and results in the production of four megaspores.

Female gametophyte: 

Under this section of Class 12 Biology Chapter 2 notes, students learn about female gametophytes. In most flowering plants, one megaspore is functional while the other three degenerate. Functional megaspore forms the female gametophyte (embryo.

Sac). The formation of an embryo sac from a single megaspore is termed monosporic development. The functional megaspore nucleus divides by mitotic division to form two nuclei which go to the opposite poles, making the 2-nucleate embryo sac. The other two sequential mitotic nuclear divisions form the 4-nucleate and later the 8-nucleate stages of the embryo sac. These mitotic divisions are free nuclear; nuclear divisions do not follow cell wall formation immediately. At the 8-nucleate stage, cell walls are formed, leading to the formation of the typical embryosac or female gametophyte. Out of the eight, six nuclei are surrounded by cell walls and organised into cells, and two nuclei, called polar nuclei, are organised below the egg apparatus in the large central cell. 

There is a specific distribution of the cells within the embryo sac as explained in Class 12 Biology Chapter 2 notes. At the micropylar end, three cells are grouped together and constitute the egg apparatus. In the egg apparatus, there are two synergids and one egg cell. The synergids have special cellular thickenings called filiform apparatus at the micropylar tip, which guides the pollen tubes into the synergid. At the chalazal end also, there are a group of three cells known as the antipodals. The central cell has two polar nuclei. Thus, a typical angiosperm embryo sac is  8-nucleate but 7-celled at maturity.

• Out of four, one megaspore is functional, while the other three degenerate mostly. The functional megaspore develops into the embryo sac ( female gametophyte ).
• The formation of a female gametophyte from a single megaspore is called monosporic development. 

• The megaspore nucleus divides mitotically and forms two nuclei that move to the opposite poles, forming the 2-nucleate embryo sac.
• Two more subsequent mitotic nuclear divisions or free cell divisions (nuclear divisions do not follow cell wall formation immediately) result in the formation of the  8-nucleate embryo sac.
• At the 8-nucleate stage, cell walls are laid down.
• Out of eight, six nuclei are surrounded by cell walls, and the remaining two, called polar nuclei, are situated near the egg apparatus in the large central cell. Three cells together at the micropylar end form the egg apparatus, which has two synergids and one egg cell. The synergids have a  filiform apparatus, which guides the pollen tubes into the synergid.
• At the chalazal end, three cells grouped together are called the antipodals. 
• The big central cell has two polar nuclei.
• Thus, an 8-nucleate and 7-celled embryo sac are formed.

Pollination: The Class 12 Biology Chapter 2 notes give detailed information on Pollination, its type, agents of Pollination and its significance as under:

When pollen grains get transferred ( from the anther) to the stigma of the pistil is called Pollination. There are different kinds of Pollination:

Depending on the source of pollen:

• Autogamy- In this type, transfer of pollen grains occurs from the anther to the stigma of the same flower. 
  • Cleistogamous flower: These flowers do not open at all. In this type of flower, autogamy takes place as there is no chance of other pollen landing on the stigma.
  • Chasmogamous flower: Flower which opens and exposes the anthers and the stigma; complete autogamy is rare in these flowers. Oxalis, Voila (common pansy), and Commelina produce both kinds of flowers; hence they produce assured seed sets even without pollinators.

• Geitonogamy – Pollen grain transfers from the anther of one flower to the stigma of another flower but of the same plant. Thus geitonogamy is functionally cross-pollination involving a pollinating agent, but it is similar to autogamy genetically since both the flowers are on the same plant.

• Xenogamy – Pollen grains transfer from the anther of one plant to the stigma of a different plant. It brings a genetically different type of pollen to the stigma. 

To know more, students may access Class 12 Biology Chapter 2 notes at Extramarks.

Agents of Pollination: 

Plants use two abiotics (water and wind) and one biotic (animals like a butterfly, honey Bee etc.) agent for Pollination. Most plants use biotic agents for Pollination. Only a few use abiotic agents. Pollen coming in contact with the stigma by the pollinating agents is a chance factor; hence, the flowers form a large amount of pollen compared to the number of ovules to compensate for the loss of pollen grains during Pollination.

Pollination by wind (Anemophily): 

Characteristics or requirements as mentioned in Class 12 Biology Chapter 2 notes include

• Lightweight pollen grains
• Powdery (non-sticky) so that they can be transported by wind current.
• Well exposed stamens for dispersal of pollen into the wind.
• Larg and feathery stigma for an easy trap.

E.g., Corncob, grasses.

Pollination by water (Hydrophily)

Characteristics or requirements include Algae, bryophytes, and pteridophytes need water to transfer male gamete and fertilisation. The flowers are colourless, small, odourless, and nectarless in higher plants, and the stigma is long, sticky, and unpalatable.

Water Pollination is of Two Types –

• Epihydrophily (on the surface of the water ), e.g. Vallisneria
• Hypohydrophily (inside the water), e.g., Zostera (seagrass) and Ceratophyllum.

The exine is not found in pollen grains and sand are frequently elongated (ribbon-like) and covered with mucilaginous covering. 

As per Class 12 Biology Chapter 2 notes, a Vallisneria (a dioecious plant), male plants produce a large number of male flowers, which grow upwards in a closed position and open on the surface of the water. The female plant forms flowers that float on the water’s surface with the help of long pedicels. After Pollination, the female flower is again submerged in water.

Pollination by animals

Characteristics and requirements include

• A variety of animals such as butterflies, bees, flies, beetles, wasps, ants, moths, birds (sunbirds and hummingbirds) and bats are the common agents for Pollination.
• Larger animals such as some primates (lemurs), rodents, arboreal (tree-dwelling) or even reptiles have also been reported as pollinators in some species.

• Flowers of animal pollinated plants are large, colourful, fragrant and rich in nectar and sometimes foul odour so that animals can be attracted.
• Pollen grains and nectar are the usual floral rewards to the animals by flowers to make them visit again and again.
• The animal visitors come in touch with the anthers and the stigma both. 
• The pollen grains and stigma are sticky.
• When the pollen on an animal’s body comes in contact with the stigma, it brings about Pollination.

Significance of Pollination, as mentioned in Class 12 biology Chapter 2 notes.

• Pollination is necessary for fertilisation and the production of seeds and fruits. 
• It promotes ovarian growth.
• It helps in the raising of hybrid seeds. 

Outbreeding Devices: Most flowering plants produce flowers that are hermaphrodite, and pollen grains usually come in contact with the stigma of the same flower.

Flowering plants have many devices to discourage self-pollination and encourage cross-pollination because continued self-pollination results in inbreeding depression. In some species, stigma receptivity and pollen release are not at the same time. The stigma and anther are placed at different positions in the same flower in some other species. Both these devices prevent autogamy.

Self-incompatibility is a genetic mechanism that prevents ovules from fertilising with self-pollen by inhibiting pollen germination. Production of unisexual flowers (monoecious)- prevents autogamy. In species like Papaya, male and female flowers are present on different plants (Dioecious)- preventing autogamy as well as geitonogamy.

Pollen-pistil Interaction: Students can refer to the NCERT solution for Class 12 Biology chapter 2 notes to learn about pollen pistil interaction.

Transfer of the correct type of pollen is not guaranteed after Pollination (compatible pollen of the same species ). Often, the wrong type of pollen, either from other species or from the same plant, also lands on the stigma. The pistil recognises the pollen and whether it is compatible or not. If it is of the right type, the pistil lets the pollen grow and promotes post-pollination events. If the pollen is of the wrong type, the pistil prevents the pollen germination on the stigma or the growth of the pollen tube in style. The acceptance or rejection of pollen is mediated by chemical components of the pollen interacting with those of the pistil. The compatible pollen grain germinates on the stigma.

• Starting from pollen deposition on the stigma until pollen tubes enter the ovule–they are together called pollen-pistil interaction.
• The pistil is able to recognise the pollen, whether it is compatible or incompatible, followed by its acceptance or rejection. This recognition is mediated by chemical components of the pollen interacting with those of the pistil.

• After compatible Pollination, the pistil accepts the pollen and promotes post pollination events.

Post Pollination Events 

As mentioned in Class 12 Biology Chapter 2 notes, the pollen grain germinates through one of the germ pores to produce a pollen tube on the stigma, and pollen grain contents move into the pollen tube. The pollen tube grows within the tissues of the stigma and style and reaches the ovary. 

In some plants, pollen grains are land on stigma under two-celled conditions (a vegetative

cell and a generative cell). In that case, the generative cell divides and forms the two male gametes in the growing pollen tube in the stigma.

In plants where pollen is shed in the three-celled condition, pollen tubes already carry the two male gametes.

After reaching the ovary, the pollen tube enters the ovule through the micropyle and then enters one of the two synergids. Filiform apparatus at the micropylar part of the synergids guides the entry of the pollen tube.

Pollen-pistil interaction is a process that involves pollen recognition and then promotion or inhibition of the pollen. With the help of knowledge gained in this area, plant breeders are able to manipulate pollen-pistil interaction, even in incompatible Pollination, to get desired hybrids. Students may refer to class 12 Biology Chapter 2 notes to know more. 

• In some plants, the generative cell divides and forms the two male gametes during the growth of the pollen tube in the stigma. Whereas in some other plants, pollen is shed in three celled stages, and the pollen tube carries the two male gametes from the beginning.
• Once it reaches the ovary, the pollen tube enters the ovule through the micropyle and then enters one of the synergids with the help of filiform apparatus, which guides the entry of the pollen tube.

Artificial Hybridisation: Students can refer to the short notes, CBSE sample paper, important questions, and specific solutions in addition to Class 12 Biology Chapter 2 notes to understand the concept of artificial hybridisation properly.

Artificial hybridisation is an approach to improve crop varieties by crossing different species and often genera to combine desirable characters of parent plants to form commercially ‘superior’ varieties called hybrids. Only the wanted pollen grains are used for Pollination to produce these hybrids, and the stigma is protected from unwanted pollen. Artificial hybridisation is done by emasculation and bagging techniques.

• Emasculation- If the flower of the female parent is bisexual, there is the anther removal from the flower before the anther dehisces by a pair of forceps. 
• Bagging- Emasculated flowers are covered with a bag (made up of butter paper) to restrict contamination of stigma by unwanted pollen. If the female parent is unisexual, there is no need for emasculation; flowers are bagged before the buds open.

When the stigma of a bagged flower becomes receptive, mature pollen grains obtained from anthers of the male parent are introduced on the stigma, the flowers are rebagged, and the fruits are allowed to develop.

Double Fertilisation:

For the overview of double fertilisation, students may refer to NCERT solutions Class 12 Biology Chapter 2 notes.

Two male gametes are released after entering one of the synergid’s cytoplasm. One of the two male gametes moves and fuses with the egg cell nucleus, thus completing the syngamy. Fusion causes the formation of a diploid cell, the zygote. The second male gamete moves toward the two polar nuclei located in the central cell and fuses with them to produce a triploid (3n) structure called the primary endosperm nucleus (PEN). It is because this event involves the fusion of three haploid nuclei; it is termed triple fusion. 

Since syngamy and triple fusion both take place in an embryo sac, the phenomenon is termed double fertilisation. Double fertilisation is a unique event for flowering plants.

 The zygote develops into an embryo, and the central cell, after triple fusion, becomes the primary endosperm cell (PEC) and grows into the endosperm.

POST-FERTILISATION: STRUCTURES AND EVENTS

Embryo and endosperm development, maturation of ovules into seeds and ovary into fruit, are collectively called post-fertilisation events.

 Endosperm: Students can access the Extramarks short notes and different solutions apart from Class 12 Biology Chapter 2 notes for endosperm development while studying.

The primary endosperm cell (PEC) divides and forms a triploid endosperm tissue. The cells are filled with reserved food nutrients and are used for the nutrition of the embryo that is developing.

• The development of endosperm precedes embryo development.
• The primary endosperm cell (PEC) repeatedly divides to make a triploid tissue in which the cells are filled with reserve food materials that are used for the nourishment of the developing embryo.

There are two types of Endosperm:

• Free Nuclear Endosperm: It is the most common type of endosperm development; the primary endosperm nucleus undergoes successive nuclear divisions to give free nuclei. This stage of endosperm is called free-nuclear endosperm.
• Cellular Endosperm: After free nuclear division, subsequently cell wall formation occurs, and the endosperm becomes cellular. 

E.g., The coconut water from tender coconut is a free-nuclear endosperm (made up of thousands of nuclei), and the surrounding white kernel is the cellular endosperm.

The endosperm may either be completely consumed by the embryo (e.g., pea, groundnut, beans) before seed maturation, or it may remain in the mature seed (e.g. castor and coconut), which is to be used up during germination of the seed.

Embryo: 

In this section of Class 12 Biology Chapter 2, curated by Extramarks, students will learn about embryos.

The zygote is situated at the micropylar end of the embryo sac, where the embryo develops. Most zygotes divide only after a small amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo.

Though the monocot and dicot seeds are very different, the early stages of embryo development are similar in both. The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.

A  dicotyledonous embryo consists of an embryonal axis and two cotyledons. The portion above the level of cotyledons is the epicotyl, which terminates with the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl that ends at its lower end in the radicle or root tip. The root tip is enveloped with a root cap.

Embryos of monocotyledons have only one cotyledon. In the grasses, the cotyledon is called the scutellum, which is situated laterally (towards one side ) of the embryonal axis. The portion of the embryonal axis above the level of attachment of the scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia surrounded in a hollow foliar structure, the coleoptile.   At the lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath. i.e., coleorhiza. 

• Embryogeny (embryo development) are similar in both monocotyledons and dicotyledons. 
• The zygote forms the proembryo and subsequently the globular, heart-shaped and mature embryo. 

Difference between monocot and dicot embryo:

The difference between monocot and dicot embryo is best explained in Extramarks Class 12 Biology Chapter 2 notes.

Seeds: In angiosperms, Seeds are formed inside the fruits and are the final product of sexual reproduction. 

Structure of seed:

• Seed coat(s)
• Cotyledon(s)
• An embryo axis.

The cotyledons of the embryo are thick and simple swollen structures it contains storage food material. E.g. Legumes

Mature seeds can be:

Exalbuminous or Non-albuminous: In this kind of seeds, the endosperm is completely consumed during embryo development (e.g., pea, groundnut).

Albuminous seeds: In these seeds, a part of endosperm is retained as it is not completely used during embryo development (e.g., wheat, maise, barley, castor).

Perisperm-Occasionally, remnants of nucellus are also persistent in some seeds such as black pepper. The residual, persistent nucellus layer is the perisperm.

Seed Coat – Seed coat is hardened, tough and protective integuments.

Microplyle: It remains as a small pore in the seed coat to enter water and oxygen during germination.

• At maturity the seed’s water content is reduced, and it becomes dry (10 to 15 % by mass). The embryo becomes dormant (inactive), or if there is adequate moisture, oxygen and suitable temperature, the embryo germinates.
• The formation of the ovary into fruit and ovules into seeds proceeds simultaneously. The wall of the ovary develops into a fruit covering called pericarp. 
• The fruits may be fleshy, as in orange, mango, etc., or dry, as in mustard and groundnut etc. 
• Many fruits evolve seed dispersal mechanisms.

• Most fruits develop only from the ovary and are called true fruits.
• In a few species, such as strawberries, apples and cashew, etc., the thalamus also contributes to fruit formation. Such fruits are called false fruits.

• Parthenocarpic Fruits: Fruits are the results of fertilisation, but in a few species, fruits develop without fertilisation. Such fruits are called parthenocarpic fruits. Parthenocarpy can be achieved by growth hormones, and parthenocarpic fruits are seedless. E.g., Banana

Importance of seeds:

• Seed formation is more reliable because Pollination and fertilisation do not depend on water.
• Seeds have their mechanism for dispersal to new places and help the species to grow in other areas. 
• Seedlings are nourished by the food reserves of seeds until they are capable of photosynthesis on their own.
• The young embryo is protected by a hard seed coat.
• Seeds generate new genetic combinations and lead to variations because they are the product of sexual reproduction.
• Seed is the basis of our agriculture. Seeds are dehydrated and dormant structures. Because of this, storage of seeds is easy and can be used as food throughout the year and also to raise a crop in the next season.

Students may refer to various other study materials in addition to Class 12 Biology chapter 2 notes to understand the importance of seeds.

Seed Viability: Students learn about seed viability under this section of Class 12 Biology chapter 2 notes. Seed viability is how long a seed remains alive after they are dispersed. It may be from a few months to several years. There are records of very old yet viable seeds. The oldest seed in record is Lupinus arcticus found in Arctic Tundra. The seed germinated and flowered as well after an estimated record of 10,000 years of dormancy. A recent record of 2000 years old viable seed is of the date palm, Phoenix dactylifera, discovered during the archaeological excavation at King Herod’s palace near the Dead Sea.

Apomixis: The formation of a seed without fertilisation by flowering plants is called apomixis. E.g.spicies of the Asteraceae family and grasses. Thus, it is a form of asexual reproduction that mimics sexual reproduction.

In a few cases, the diploid egg cell is formed without reduction division in the megaspore mother cell(MMC) and develops into the embryo without fertilisation.

More often, some of the nucellar cells (2n) surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos. In such a species, each ovule contains many embryos.

Polyembryony- The occurrence of more than one embryo in a seed is referred to as polyembryony.

Usage of Apomixis- Hybrid seeds increase productivity and are of superior quality, but it has to be produced every year by artificial hybridisation. That is why the production of hybrid seeds is costly and is too expensive for farmers. 

If the seeds produced by hybrid plants are sown again, the plants in progeny will segregate and do not maintain the hybrid characteristics ( because of the involvement of sexual reproduction). If hybrid seed production involves the process of apomixis, there is no segregation of characters in progeny. Then the farmers can keep using these seeds year after year, and the seeds will not lose their hybrid properties. 

Class 12 Biology Chapter 2 Notes: Exercises & Solutions

Extramarks provides a Biology syllabus where Class 12 students can get proper knowledge of every chapter with ease. Referring to the syllabus helps students to be prepared with the necessary study materials. Subjects like Biology will become so easy to study once students access the syllabus, refer to the notes and concentrate on important questions. Students may register on Extramarks to access the Class 12 Biology chapter 2 notes any time they need them on Extramarks.

Class 12 Biology Chapter 2 notes Sexual Reproduction in Flowering Plants is provided here with a simple explanation. The notes are based on the CBSE syllabus and can benefit the students immensely. All questions and solutions are from the NCERT books of Class 12, Chapter 2. In addition to Class 12 Biology Chapter 2 notes, students may access study materials by clicking on the respective link below. 

• CBSE revision notes
• CBSE sample papers
• CBSE previous year’s question papers
• CBSE extra questions

The Class 12 Biology Chapter 2 notes and study materials provided by Extramarks are prepared by experts and are 100% accurate. Students may refer to these study materials along with Class 12 Biology Chapter 2 notes to clear their doubts and for a better understanding of the respective chapters. These study materials must be focused properly to understand chapters included in the CBSE Class 12 textbooks which, if followed, can help the students to get excellent marks in Class 12 exams.

Extramarks platform provides NCERT Class 12 Biology Chapter 2 Exemplar, which has a wide range of CBSE extra questions for students to increase their knowledge. With the help of these notes, students would have all the study material related to Chapter 2, Biology, in one place. 

Solving CBSE sample papers will also help the students to understand how much and which content should be given more time to study. Study material such as Class 12 Biology Chapter 2 notes enable quick and thorough revision for subjects like Biology. Students can score really well in Class 12 CBSE board exams by getting access to academic materials from Extramarks.

Key Features of Class 12 Biology Chapter 2 Notes

Subjects like Biology will become easy to study once you access Class 12 Biology notes on our Extramarks website. Students may register on Extramarks to access Biology Chapter 2 Class 12 notes any time they need.

Class 12 Biology Chapter 2 notes describe various topics regarding sexual reproduction in flowering plants. Referring to these notes, students would be able to learn about the morphology of flowers, male and female reproductive organs(androecium and gynoecium), the male and female gametophyte( pollen grains and embryo sac), Pollination, artificial hybridisation, hybrid seeds and their importance, the process of fertilisation, double fertilisation, embryo development (both in monocot and dicot), endosperm, development of fruits, parthenocarpy and apomixis, usage of parthenocarpy and apomixis.

The key features of Class 12 Biology Chapter 2 Notes provided by Extramarks are as under.

• It follows the CBSE Syllabus.
• It is prepared by the experts at Extramarks.
• It provides a broad understanding of all the concepts.
• It is focused on preparing short and pointwise notes for quick revision.
• It provides authentic and reliable revision notes in an easy language.
• It enhances time management skills.

The students can refer to Class 12 Biology Chapter 2 notes to clarify their doubts and better understand the chapter on sexual reproduction in plants. These solutions must be adequately focused to understand all the topics included in the CBSE Class 12 text books. Here, the most effective Class 12 NCERT Solutions are given, which, if followed, can help the students to get excellent marks in Class 12 exams.