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UNIT – VI: REPRODUCTIONCHAPTER-1
SEXUAL REPRODUCTION IN FLOWERING PLANTS
Sexual Reproduction
Topic-1 in Flowering Plants
Concepts Covered Structure of a flower, male and female reproductive structures,
development of male and female gametophytes.
Revision Notes
Flower
l Flowers are the site of sexual reproduction in flowering plants.
l Parts of a typical angiospermic flower are: sepals, petals, stamens and pistils.
l The four whorls of the flower are attached on a central axis called thalamus.
l A flower can be bisexual (contains both male and female reproductive parts) or unisexual (only one of the
reproductive parts is present).
Male Reproductive Structures
Androecium (Whorl of Stamens)
l Androecium consists of a whorl of stamens.
l The number and length of the stamens are variable in flowers of different species.
l A stamen has three parts namely, anther, filament and connective.
(a) Anther
l It is the terminal and bilobed part of stamens attached with filament. A bilobed anther is called dithecous.
l Each lobe has two pollen sacs or microsporangia. Therefore, the anther is tetrasporangiate.
l A longitudinal groove runs lengthwise separating the theca.
(b) Filament
l It is the long and slender stalk part of the stamen.
l Its proximal end is attached to the thalamus or petals of the flower.
(c) Connective
l The structure which connects the anther lobes is known as connective.
Transverse section of an anther
l The anther is tetragonal in a structure consisting of four microsporangia or pollen sacs located at the corners,
two in each lobe.
l The microsporangia develop to become pollen sacs.
l They extend longitudinally throughout the length of an anther.
l These are packed with pollen grains.
Structure of microsporangium or pollen sac
l It is circular and is generally surrounded by wall layers namely,
(a) Epidermis (b) Endothecium
(c) Middle layers (d) Tapetum
l The first two layers perform the function of protection and help in dehiscence of anther to release the pollens.
l The middle layers and the innermost layer, (tapetum) nourishes the developing pollen grains.
l The cells of the tapetum possess dense cytoplasm and more than one nuclei.
l When the anther is young, a group of compactly arranged homogenous cells called sporogenous tissues
occupies the centre of each microsporangium.
Key Words
Homogenous: Common origin or environment. Viability: Ability to survive.
Dehiscence: Splitting or bursting
, 2 Oswaal CBSE Revision Notes Chapterwise & Topicwise, BIOLOGY, Class-XII
Microsporogenesis
l When the anther develops, each cell of sporogenous tissue undergoes meiotic division to form microspore
tetrads.
l Each cell of sporogenous tissue is a microspore mother cell (MMC) or pollen mother cell (PMC).
l The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis.
Dehiscence of anther
l The microspores get arranged in a group of four cells and each group is called microspore tetrad.
l As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen
grains.
l From each microsporangium, thousands of pollen grains are formed and released due to the dehiscence of
anther.
Pollen grain (Male gametophyte)
l Pollen grain germinate and give rise to male gametophyte.
l These are spherical, measuring about 25-50 micrometers in diameter.
l Pollen grains are well preserved as fossils due to the presence of sporopollenin, a tough, resistant and stable
material.
l A pollen grain has a two-layered wall namely, exine and intine.
(a) Exine
l Exine is the hard outer layer which is made up of sporopollenin.
l The sporopollenin is one of the most resistant organic materials.
l It can withstand high temperature and strong acids and alkali.
l It cannot be degraded by enzymes.
l The exine has apertures called germ pores where sporopollenin is absent.
(b) Intine
l It is the inner, thin and continuous layer that is made up of cellulose and pectin.
l A mature pollen grain contains two cells namely, vegetative cell and generative cell.
(i) Vegetative cell
l It is the bigger cell having abundant food reserve and a large irregularly shaped nucleus.
(ii) Generative cell
l It is the smaller cell that floats in the cytoplasm of the vegetative cell.
l It is spindle shaped with dense cytoplasm and a nucleus.
l The pollen grains are generally shed at the 2-celled stage in flowering plants.
l In other plants, the generative cell divides mitotically to give rise to the two male gametes before pollen grains
are shed in a 3-celled stage.
l Once they are shed, pollen grains have to land on the stigma before they lose viability.
l The period of pollen grains remaining viable varies and depends on the prevailing temperature and humidity.
l The viability of pollen grains of some cereals such as rice, wheat, etc. is 30 minutes while some members of
Leguminosae, Rosaceae & Solanaceae have viability for months.
l Pollen grains of some plants like Parthenium are allergic for some people leading to chronic respiratory disorders
such as asthma, bronchitis, etc.
l Pollen grains are rich in nutrients.
l Pollen tablets are used as food supplements.
l Pollen consumption in the form of tablets and syrups increases the performance of athletes and race horses.
l It is possible to store pollen grains for years in liquid nitrogen (–196°C).
l The pollens stored in the pollen banks for crop breeding programmes.
Key Words
Bronchitis: Inflammation of the mucous membrane in the bronchial tubes.
Crop breeding: Deals with the production and selection of superior phenotypes for the development of im-
proved and new varieties.
Fig 1.1: Transverse Section of a young anther
, Oswaal CBSE Revision Notes Chapterwise & Topicwise, BIOLOGY, Class-XII 3
Mnemonics
Concept: Male Reproductive Structures Mnemonic: Eating Tomato
Mnemonic: Ask For Connectivity Interpretation: Endothecium, Tapetum
Interpretation: A
nther, Filament, Connective Concept: Female Reproductive Structures
Concept: Structures of microsporangium or pollen Mnemonic: Small Soft Ornament
sac Interpretation: Stigma, Style, Ovary
Fig 1.2: Enlarged view of an microsporangium
Fig 1.3: Structure of two-celled male gametophyte (pollen grain)
Fig 1.4: Microsporogenesis
Fig 1.5: Stages of a microspore maturing into a pollen grain
Female Reproductive Structures
Gynoecium (Pistil)
l It represents the female reproductive part of the flower.
l If it consists of a single pistil or carpel then, it is known as monocarpellary or if it has more than one pistil or
carpel then, it is called multicarpellary.
l When there is more than one carpel, they may be fused then the pistil is known as syncarpous or may be free
then, it is known as apocarpous.
l Each carpel has three parts namely stigma, style and ovary.
(a) Stigma
It is a landing platform for pollen grains.
, 4 Oswaal CBSE Revision Notes Chapterwise & Topicwise, BIOLOGY, Class-XII
Key Words
Placenta: The surface of the carpel to which the ovules are attached.
Integuments: Outer hard protective layer in plants.
Degenerate: To loose structural or physical ability.
(b) Style
It is an elongated slender part beneath the stigma.
(c) Ovary
l It is the basal swollen part of the carpel.
l Inside the ovary is the ovarian cavity called the locule where the placenta is located.
l Placenta contains the ovules or megasporangia.
l The number of ovules in an ovary may be one as seen in wheat, paddy, mango, etc., or many as seen in papaya,
watermelon, orchids, etc.
Megasporangium (Ovule)
l It is a small structure attached to the placenta by a stalk called the funicle.
l The junction where the body of the ovule and funicle fuse is called the hilum.
l Each ovule has one or two and some times three protective coverings called integuments.
l Integuments encircle the ovule except at the tip where a small opening called micropyle is organised.
l Opposite to the micropylar end is the chalaza which is the basal part of the ovule.
l Within the integuments, there is a mass of cells called nucellus which contains reserve food materials.
l Inside the nucellus there is an embryo sac, which is also called as the female gametophyte.
l An ovule has a single embryo sac usually formed from a single haploid megaspore.
Megasporogenesis
l The formation of haploid megaspores from the diploid megaspore mother cell (MMC) as a results of meiosis is
called megasporogenesis.
l A single megaspore mother cell is differentiated in the micropylar region of the nucellus.
l The megaspore mother cell is a large cell containing dense cytoplasm and a prominent nucleus.
l The megaspore mother cell undergoes meiotic division resulting in the production of four haploid megaspores.
Female gametophyte (Embryo sac)
l In most of the flowering plants, only one megaspore is functional while the other three degenerate.
l The functional megaspore develops into the female gametophyte or embryo sac.
l This method of embryo sac formation from a single megaspore is termed as monosporic development.
Development of Female gametophyte
l The nucleus of the functional megaspore divides mitotically to form two nuclei which move towards the
opposite poles, forming a two-nucleated embryo sac.
l Two more sequential mitotic nuclear divisions result in the formation of the four-nucleated and later the eight-
nucleated stages of the embryo sac
l These divisions are strictly free nuclear, i.e., nuclear divisions are not followed immediately by cell wall
formation.
l After eight-nucleate stage, the organisation of the typical female gametophyte or embryo sac takes place.
l Generally six of the eight nuclei are surrounded by cell walls and organised into cells.
l The remaining two nuclei called the polar nuclei are found below the egg apparatus in the large central cell.
Distribution of the cells within the embryo sac
l The three cells consisting of two synergids and one egg cell which are grouped at the micropylar end constitute
the egg apparatus.
l The synergids have special cellular thickenings at the micropylar tip called filiform apparatus.
l The filiform apparatus helps to guide the pollen tubes into the synergid.
l Three cells at the chalazal end organise as the antipodals.
l Thus, a typical mature angiosperm embryo sac at maturity is eight-nucleate and seven-celled.
Fig 1.6: A diagrammatic view of a
typical anatropous ovule
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