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

Sexual Reproduction in Flowering Plants is initiated when the formation of a new organism takes place from two parents using their gametes or sex cells. 

• Flowering plants, also called angiosperms, undergo a sexual mode of reproduction.
• Flowers carry both the male and female reproductive parts, which are known as androecium and gynoecium, respectively. 
• The formation of zygote takes place by fusion of male and female gametes.
• Calyx and corolla are two other different whorls of flowers.
• The male gamete, also known as pollen grain, reaches a female gamete called an egg and undergoes the process of fertilization.

According to the CBSE Syllabus 2023-24, the chapter on Sexual Reproduction in Flowering Plants comes under Unit 1 of Reproduction. NCERT Class 12 Biology Unit Reproduction holds a weightage of around 16 marks.

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Class 12 Biology Chapter 2 Notes – Sexual Reproduction in Flowering Plants

Structure of Flower

• Pedicel: A pedicel is a form of stem that provides structural support to flowers.
• Calyx: Calyx forms the outermost part of the flower that is used to impart a green colour to flowers and petals.
• Petals: Petals are located at the centre of a flower and are used to attract insects for the process of pollination. 
• Sepals: Sepals are special cases of leaves that are used to protect fruits and flowers from predators while they are budding.
• Stamen: Stamen, also known as androecium, is the male reproductive part of the flower that helps in the production of pollen grains.
• Pistil: Pistil, also known as gynoecium, is the female reproductive part of the flower that consists of stigma, style and ovary.
• Receptacle: The receptacle, which is found at the top of a stalk, acts as a bridge between the stalks of a flower.

Structure of Flower

Development of Male and Female Gametophytes

• Gametophytes refers to phases in the life cycles of plants and algae that involve diploid generation called sporophyte and haploid generation called Gametophyte.

Development of Female Gametophyte

• The development of female gametophytes is carried out in two phases, namely megasporogenesis and megagametogenesis.
• Megasporogenesis is a single diploid mother cell which undergoes the process of meiosis to form a haploid megaspore.
• It results in the development of the embryo sac.
• Megagametogenesis undergoes the process of mitosis for the formation of 7-celled, 8-nucleate gametophyte.
• The fusion of diploid cells will take place at the centre.

Development of Female Gametophytes

Development of Male Gametophyte

• In this method, the first pollen grains will undergo the process of germination.
• The process is carried out in a pollen sac, which is known as precocious germination.
• The nucleus will undergo mitosis to form two daughter nuclei.
• The pollen grains falling on the stigma will result in the absorption of nutrients.
• This will cause enlargement of the vegetative cell. 

Pollination

• Pollination is defined as the process of transfer of pollen grains from the anther of one plant to the stigma of the other plant.
• Pollinators are biotic and abiotic agents that will initiate the process of pollination.
• Hydration, activation and pollen tube emergence are three stages of pollen germination.
• It is divided into categories, namely self-pollination and cross-pollination.

Types of Pollination

Self Pollination

• Self-pollination takes place when pollen grains directly fall from one another into the stigma of the flower. 
• Autogamy and geitonogamy are two types of self pollination.
• It will help in the removal of recessive characters.

Cross Pollination

Cross-pollination is a type of sexual reproduction that involves transferring pollen from one flower to the stigma of the same flower.

• Hydrophilous, Anemophilous, Zoophilous, Ornithophilous and Entomophilic flowers are different types of cross-pollination.
• It will help in improving the resistance of future-generation plants.

Pollination

Out Breeding Devices

• Outbreeding devices are defined as the mechanisms that plants use to avoid the process of self-pollination.
• Inbreeding depression, unisexuality and genetic variations are some of the reasons why plants use these devices.
• Plants use these devices to encourage the process of cross-pollination.
• Unisexual flowers, non-synchronization and Self-incompatibility are different factors that will encourage cross-pollination.
• Dichogamy is a type of outbreeding device that is used at the time of maturity of stigma and another.
• Self-sterility is another device that is used to prevent pollen grains from germinating.
• Unisexuality helps plants carry either male or female flowers.

Pollen-Pistil Interaction

• Pollen pistil interaction is a sequential event that is carried out during pollination until the pollen tubes enter the ovules.
• During the process of interaction, the pistil will screen the pollen grains received in the tubes.
• During the course of events, pollens are deposited at stigma.
• The process of fertilization will be initiated when pollen is recognised and accepted by the pistil.

Process of Pollen Pistil Interaction

• First, pollen is inserted inside the compatible pistil.
• In the next step, germination of pollen and formation of pollen tubes take place.
• The pollen tube will grow towards the ovary.
• Lastly, the insertion of male gametes into the ovule takes place.

Pollen Pistil Interaction

Double Fertilization

• Double fertilization refers to the fusion of one female gametophyte with two male gametes.
• It is an important characteristic of angiosperms where one male gamete will fertilize the egg which leads to the formation of zygote or syngamy.
• The other male gamete will fuse with two polar nuclei to form an endosperm.
• The process will convert the ovary into a fruit and the ovules into seeds. 

Double Fertilization in Angiosperms

• Angiosperms is a flowering plant that consists of separate male and female reproductive organs.
• Since fertilization will form two structures in these flowering plants, that is why they are called double fertilisation.
• The two structures formed in angiosperms include zygote and endosperm.
• One of the structures will fuse with the egg cell, and the other will combine with polar nuclei.
• This results in the formation of diploid (2n) zygotes and a triploid (3n) primary endosperm nucleus.

Double Fertilization

Post - Fertilization Events

• Post-fertilization is the third stage of sexual reproduction in flowering plants that leads to zygote formation.
• Pre-fertilisation and fertilization are the first and second stages of reproduction in flowers, respectively.
• It involves embryo development, maturation of ovule into seed, and ovary into fruit.
• Post fertilization also involves development of endosperm.

Endosperm Development

• Endosperm is a form of tissue that is formed during seed development.
• The tissue will store food material for later usage by the plants.
• It is responsible for providing nutrition to embryos in the form of starch.
• Nuclear endosperm formation, Cellular endosperm formation and Helobial endosperm formation are three types of endosperm development.

Embryogeny

• Embryogeny refers to the development and growth of an embryo. 
• The zygote is found at the micropylar end of the embryo sac.
• Epicotyl and Hypocotyl are two parts of the embryonal axis.

Seed and Fruit Development

• Once the process of fertilisation is complete, the flowering plant will produce seeds and fruits.
• Ex-albuminous and non-albuminous are two types of mature seeds.
• Micropyle is found at the pore of the hard coat of seed.
• Fruits are an important source of food for humans.

Post Fertilization Events

Apomixis

• Apomixis is a form of asexual reproduction where the formation of plants will take place without fertilization.
• It will form clones of female parent plants.
• Diplospory, apospory and adventitious embryony are three types of apomixis.
• Gramineae, Rosaceae, Compositae and Rutaceae are some examples of these plants.

Apomixis

Parthenocarpy

• Parthenocarpy is another technique that leads to the formation of fruits without seeds.
• It is a form of artificial development where parthenocarpy is introduced in the hormones of plants during cultivation.
• The maturation of ovaries will form big, fleshy and pulpy fruits.
• Pineapple, banana, cucumber, grape and watermelon are some common examples of this category.
• Vegetative Parthenocarpy and Stimulative Parthenocarpy are two types of parthenocarpy.

Parthenocarpy

Polyembryony

• Polyembryony is a process where two or more embryos will be formed from a single fertilized egg.
• The process is common in plants, animals and humans.
• In plants, this process is carried out due to the fertilisation of more than one embryonic sac.
• Another reason why the process of polyembryony is carried out involves the creation of embryos outside the embryonic sac.

Polyembryony

There are Some important List Of Top Biology Questions On Sexual Reproduction In Flowering Plants Asked In CBSE CLASS XII