Sexual Reproduction in Flowering Plants

2.1 Introduction:

• Sexual reproduction takes place in angiosperms with the help of flowers. Due to this angiosperms are also called flowering plants.
• Horticulture means the art or practice of garden cultivation and management.
• Floriculture: It is one of the branches of Horticulture to cultivate flowers.
• The end product of sexual reproduction in flowering plants is fruits and seeds.
• In this chapter, let us understand the morphology, structure and the processes of sexual reproduction in flowering plants (angiosperms).

2.1.1 Aids of sexual reproduction in flowering plants:

• The myriads of flower (different shapes and structures)
• The scents and the perfumes
• Attracting petals with different colours and
• Nectar of the flowers.

2.2 Flower- A fascinating (extremely Interesting) Organ of Angiosperms:

• Flower (Lt: ground grain) is a modified stem which functions as a reproductive organ and produces ova and/or pollen.
• Flowers are fascinating organs because they attract pollinators and fruits encourage animals to disperse the seeds.
• Flowers are objects of aesthetic (beauty), ornamental, social, religious and cultural value.
• Flowers are symbols for conveying important human feelings such as love, affection, happiness, grief (response of loss), mourning (remembrance of a dead person), etc.

Note: Flowers are morphological and embryological marvels (wonderful or surprising) according to biologists and it is the site of sexual reproduction because it is attractive and helps in fusion of gametes and this leads to the development of embryo and fruit.

2.2.1 Structure and parts of the flower:

• The flower is commonly borne on short or long stalks called the pedicel. It has an upper swollen region known as receptacle (thalamus or torus).
• A typical angiosperm flower consists of four whorls of floral appendages attached on the receptacle: calyx, corolla, androecium and gynoecium.
• Essential parts: Androecium and Gynoecium (participate in reproduction)
• Non-essential parts: Calyx and corolla (not Participate in reproduction).
• Calyx: It is composed of leaf-like green sepals. The sepals are essentially for protecting flowers at the stage of bud. General calyx in green and if few conditions are coloured is called petaloid.
• Corolla: It consists of a number of petals. Petals are generally brightly coloured and sometimes fragrant which make the flower attract insects for pollination.

L.S. OF A TYPICAL FLOWER

• Androecium: It is the male reproductive organ consisting of stamens. Each stamen is made of filament and anther. The filament supports anther at its tip.
• Gynoecium: This is the female reproductive organ of the flower. It occupies the central position on the receptacle (thalamus) and is composed of ovary, style and stigma together called carpels.

2.3 Events of Sexual reproduction:

1. Pre-fertilization

2. Fertilization

3. Post-fertilization

2.3.1 Pre-fertilization: Structures and events:

1. Androecium structure and events.
2. Gynoecium structure and events.
3. Pollination.

2.3.1.1 General Events of pre-fertilization in flowering plants:

• Hormonal and structural changes are initiated leading to differentiation and development of floral primordium (an organ, structure or tissue in the earliest stage of development).
• Inflorescences are formed which bear floral buds and then flowers.
• Androecium and Gynoecium differentiate and develop.

Note: Flower hormones are called florigen (Auxin, Gibberellins and Cytokinin).
Gene is responsible for flowering is the FT gene (Flower locus Time).

2.4 Androecium structure and events:

An Androecium is a male reproductive part of the flower. It consists as follows;

1. Stamen
2. Microsporangium and
3. Pollen Grain

2.4.1 Stamen:

• Stamen is a male reproductive organ consisting of two parts, a long and slender stalk called filament and a terminal bilobed structure called anther.
• A typical angiosperm anther is bilobed.
• Each lobe has two theca (box or container) i.e. dithecous.
• Often a longitudinal groove runs lengthwise separating the theca called line of dehiscence.
• Anther is a tetragonal structure or tetrasporangiate which consists of four microsporangia (Plural = Microsporangium) located two in each lobe. (or) A typical anther is bilobed, dithecous and tetrasporangiate.

Note: The proximal end of the filament in stamen is attached to the thalamus or the petal of the flower.

2.4.2 Microsporangium: (Gr: Micro = small; spor = spore; and angium = vessel)

Microsporangium is the sporangial structure that produces microspores (later stage is Pollen grains) in pollen sacs that give rise to male gametes in an angiosperm.

2.4.2.1 STRUCTURE OF MICROSPORANGIUM:

A typical microsporangium appears circular in outline in a transverse section. It is surrounded by 4 walls.

1. Epidermis
2. Endothecium
3. Middle layers
4. Tapetum

• Epidermis, Endothecium and middle layers protect and help in dehiscence of anther to release the pollen.
• Tapetum (innermost layer) nourishes the developing pollen grain and possesses more than one nucleus (multinucleated) with dense cytoplasm.

Note:
Why do tapetum (tapetal cells) become bi-nucleated or multinucleated?
The tapetum undergoes either endomitosis or endoreplication, resulting in bi-nucleation. This process is beneficial for nourishing the microspores of pollen grains by enlarging the cell to store resources and increasing metabolic activity.

2.4.2.2 SPOROGENOUS TISSUE:

• It is compactly arranged homogenous cells which are present at the center of each microsporangium when the anther is young.
• After maturity it is converted into pollen sacs by replacing sporogenous cells.

2.4.2.3 MICROSPOROGENESIS:

• The process of formation of microspores from pollen mother cells (sporogenous cells) through meiosis is called microsporogenesis.
• The sporogenous tissue of microsporangium differentiated into microspore mother cell or pollen mother cell (2n).
• Each microspore mother cell undergoes meiosis and gives rise to four haploid microspore tetrads (n).
• On dehydration microspore tetrad dissociates to form microspores.
• Each microspore develops into a pollen grain. Later it is released through dehiscence of anther.

Difference between Microspore tetrad and Microspore:

Microspore Tetrad	Microspore
It is formed from Pollen Mother Cell.	It is formed from Microspore Tetrad.
It is not due to dehydration.	It is due to dehydration.
*Callose wall is present	Callose Wall is absent

*Callose Wall is a temporary wall made of a special sugar molecule called a β-glucan (beta-glucan) found in some plant cells.

• Formation of callose wall with an enzyme called callose synthase and
• Breakdown of callose wall with an enzyme called callase.

What is yellowish powder on anther of Hibiscus?

Ans: Pollen Grains

2.4.3 POLLEN GRAINS:

A pollen grain is a microscopic body that contains the male reproductive cell of a plant and acts as a male gametophyte.

2.4.3.1 STRUCTURE OF POLLEN GRAIN:

• Pollen grains are generally spherical, measuring about 25-50µm in diameter.
• Pollen grains are made of 2 layered walls Exine and Intine.
• Exine: - Made of sporopollenin, the most resistant organic matter known.
• Sporopollenin:- It is derived from Carotenoid. It can withstand high temperature, strong acid and alkali. No enzymes have been discovered that can degrade sporopollenin.
• Intine:- Thin and continuous layer made of cellulose and pectin.
• Germ pores: - Apertures or holes on the exine where sporopollenin is absent and from here pollen tube produces.
• A plasma membrane surrounds cytoplasm of pollen grain.

2.4.3.2 Development of Pollen Grain: 

Development of pollen grain or male gametophyte is consists two stages;

1. Pre-Pollination development of pollen grain and
2. Post-Pollination development of pollen grain.

2.4.3.2.1 Pre-Pollination development of pollen grain:

Pre-pollination development is further divided into two types based on the number of cells in pollen grain as follows;

• Two Cell Stage: This pollen grain consists of one vegetative cell and one generative cell through mitotically.
• Three Cell Stage: This pollen grain consists of one vegetative cell and two generative cells. This is due to the generative nucleus further divides mitotically and forms two male gametes.

2.4.3.2.2 Post-Pollination development of pollen grain:

• The two cell stage pollen grain at the time of shed, its generative cell divides and forms the two male gametes during the growth of pollen tube in the stigma.
• While pollen is shed in the three-celled stage, pollen tubes carry the two male gametes from the beginning itself.

2.4.3.3 Effect of Pollen on Human:

• Pollen grains cause allergy and bronchial afflictions.

• Leading to chronic respiratory disorders like asthma, bronchitis Ex:- Parthenium (carrot grass).

2.4.3.4 POLLEN PRODUCTS:

• Rich in nutrients.

• Pollen tablets and syrup:  Food supplements Ex: Bee Pollen tablet

• Function: Claims to increase performance of athletes and race horses.

2.4.3.5 Period of Viability:

• Viability means whether or not pollen grain is alive after dispersing.
• Once shed the pollen grains have to land on the stigma before they lose viability if they have to bring about fertilization.
• Periods of viability depend on temperature and humidity. Example:- a cereal (like wheat and rice) takes within 30 minutes and members of Rosacea, Leguminosae, Solanaceae take months.
• Pollen grains stored in Cryopreservation (means preserved in liquid nitrogen around -196⁰C) in pollen banks. Pollen grains and Seeds (Pollen and Seed Bank) used in crop breeding programs by using these banks.

Difference between Vegetative cell and Generative Cell of Pollen Grain:

Vegetative Cell	Generative Cell
In Pollen grain, it occupies more space.	In Pollen grain, it occupies a small space.
It consists of an abundant food reservoir.	It does not consist of any food reservoir.
It has a large irregular nucleus.	It has a small regular nucleus.
It is responsible for the development of pollen tube.	It is responsible for syngamy or fertilization.

2.5 Gynoecium structure and events:

The Gynoecium represents the female reproductive part of the flower. It consists as follows;

1. Pistil/Carpel
2. Megasporangium
3. Embryo Sac

2.5.1 Pistil/Carpel:

• Each pistil has three parts, the stigma, style and ovary.
• Inside the ovary is the ovarian cavity called the locule.
• The placenta is located inside the ovarian cavity.
• The Gynoecium may contain a single pistil called monocarpellary. Ex: Wheat and paddy
• May have more than one pistil called multicarpellary. Ex: Papaya, Watermelon
• Fused pistils are called syncarpous and free pistils are called apocarpous.

2.5.2 Megasporangium (Ovule):

In seed plants, the ovule ("small egg") is the structure that contains and gives raises to the female reproductive cells.

2.5.2.1 STRUCTURE OF OVULE:

• Ovule is considered to be an integument megasporangium.
• The ovule consists of the stalk and the body.
• The stalk is called a funicle. One end of the funicle is attached to the placenta and the other end to the body of the ovule.
• The point of attachment of funicles with the body is called hilum.
• Sometimes funicles get fused with the body of the ovule one side and forms a ridge known as raphe.

• The main body of the ovule is covered with one or two envelopes called integuments.
• Integuments leave an opening at the top of the ovule called micropyle and called micropylar end.
• Opposite the micropylar end, is the chalaza (chalaza end), representing the basal part of the ovule.
• The integuments enclose a large parenchymatous tissue or mass of cells known as nucellus. It has abundant reserve food material (later turned into perisperm).
• The central tissue is the nucellus in which one cell of the archesporium converts into the megaspore mother cell. This megaspore mother cell divides meiotically and forms the embryo sac (the female gametophyte).
• An ovule generally has a single embryo sac formed from a megaspore.

2.5.2.2 MEGASPOROGENESIS:

• Process of formation of megaspores from megaspore mother cells is called Megasporogenesis.
• Some cells of nucellus develop into diploid archesporial cells.
• These archesporial cells give rise to Megaspore mother cells.
• Megaspore mother cells divide meiotically to form 4 megaspore cells (haploid).
• Out of 4, only 1 megaspore is functional and forms female gametophyte and the rest degenerate.

Note: Megaspore mother cell is located in the ovule at the Micropylar region/end.

2.5.3 Female Gametophyte (Embryo Sac):

• The embryo sac develops from the functional megaspore (n).

• Formation of the embryo sac from a single megaspore is called monosporic development.

2.5.3.1 FORMATION OF EMBRYO SAC:

• Nucleus of a functional megaspore divides mitotically to form 2 nuclei which move to opposite poles forming 2-nucleate embryo sacs.
• Two more mitotic nuclear divisions result in 4-nucleate and later 8- nucleate stages of embryo sac.
• Subsequently, the cell wall is deposited between nuclei, facilitating the organization of the female embryo sac.

2.5.3.2 STRUCTURE OF EMBRYO SAC:

• Egg apparatus - present at the micropylar end and consist of 2 synergids and 1 egg cell.

Synergids have cellular thickening at micropylar tips called Filiform Apparatus – guides the pollen tube into the synergid.

• Antipodal - 3 cells present at chalaza end.

• A single large central cell has two Polar Nuclei .

• Typical angiosperm embryo sac at maturity, has 8 nucleus and 7 cells.

Note: 

What will be the ploidy of the cells of the nucellus, MMC, the functional megaspore and female gametophyte?
Nucellus = 2n (diploid), MMC = 2n, Functional Megaspore = n (Haploid) and

Female Gametophyte = n.

Difference between Microsporogenesis and Megasporogenesis:

Microsporogenesis	Megasporogenesis
The formation of microspore from pollen mother cells by meiosis.	The formation of megaspore from the megaspore mother cell by meiosis.
It takes place in an anther.	It takes place in an ovary.
Four functional microspores are produced.	One functional megaspore is produced.
Result in the formation of pollen grain.	Result in the formation of the embryo sac.

2.6 Pollination:

The process of transfer of pollen grains from an anther to the stigma of the same flower or of different flowers by the agents is called pollination.
Agents like wind, water, insects, and few birds.

2.6.1 Kinds of Pollination:

Based on the source of pollen, pollination is of 3 types:-

1. Autogamy

2. Geitonogamy

3. Xenogamy

2.6.1.1 AUTOGAMY:

Transfer of pollen grains from anther to stigma of the SAME flower called autogamy.

Features or conditions for Autogamy:-

• The phenomenon where pollen dispersal and stigma receptivity occur simultaneously.

• The anther and the stigma should lie close to each other.

2.6.1.1.1 Types of Autogamy:

• Chasmogamous flowers (Gr: Chasmo = open marriage) - flowers with exposed anthers and stigma. Ex: Pea and Beans.
• Cleistogamous flowers (Gr: Cleisto = closed marriage) - flowers which do not open at all.
• Viola, Oxalis, and Commelina are examples for both Chasmogamous and Cleistogamous.

Note: Cleistogamyis disadvantageous because there is no chance of variation and advantage is pollination brought in absence of pollinators and leads to assured seed set.

2.6.1.2 GEITONOGAMY:- (Gr: Geito = neighbor and gamy = fuse/Marry)

• Transfer of pollen grains from anther to stigma of another flower of the same plant.

• Genetically similar

• Ex:- Cucurbits

Self-pollination:

• This process involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant.
• Autogamy and Geitonogamy comes under self-pollination.

2.6.1.3 XENOGAMY: - (Gr: Xeno = Stranger)

• Transfer of pollen grains from anther to the stigma of another flower of different plants. It is also called Cross Pollination.
• Genetically different pollen grains are brought to the stigma. Ex: Spinach, Grass.

2.6.2 Agents of Pollination:

Anything that can pick up the pollen and deposit it on stigma is known as an agent of pollination / pollinators.

2.6.2.1 Types of Major Agents of Pollination:

1) Abiotic agents

2) Biotic agents

2.6.2.1.1 Types of Pollinators in Abiotic Agents:

The transfer pollen takes with the help of non-living things from one flower to another is known as Abiotic pollinators.
Ex: Wind, and Water, (Gravity)

Ex 1: Wind Pollination: Wind pollination is a form of pollination whereby pollen is distributed by wind. It is also called Anemophily. Wind pollination is common in grasses.

Features of Wind Pollination Flowers or Plants:

• 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

Ex 2: Water Pollination:

Pollen distributed by the flow of water is called water pollination. It is also known as Hydrophily. This type of pollination is observed only in 30 genera and mostly Monocotyledons.

Features of Water Pollination Flowers or Plants:

• Pollen grains are protected from wetting by mucilaginous covering and

• Pollen grains in the form of long, ribbon like structures

Ex: Freshwater pollinators = Vallisneria & Hydrilla.

Sea Water Pollinators or Marine = Sea-grasses such as Zostera.

Hydrophilous species fall into two categories:

• Distribute their pollen to the surface of water. E.g. Vallisneria's male flowers or pollen grains are released on the surface of water, which are passively carried away by water currents; some of them eventually reach the female flower.
• Distribute their pollen beneath the surface or inside the water. E.g. sea-grasses

Common features of wind and water pollinated plants:

• Produce pollen grains in large numbers.
• Flowers are not colourful
• Do not produce nectar.

Note:

• All aquatic plants do not show water pollination.
• Water Hyacinth and Water lily show wind and insect pollination respectively.
• It is achieved by emerging flowers above the level of water.
• Why is algae, bryophyte and pteridophyte (lower plants) distribution limited
• Ans: Due to the necessity of water for the transportation of male gametes during fertilization.

2.6.2.1.2 Types of Pollinators in Biotic Agents:
    The transfer pollen takes with the help of living things from one flower to another is known as biotic pollinators.
Ex: Insects, Birds, Bats, Reptiles, and Mammals

Zoophily is a form of pollination whereby pollen is transferred by animals

1. Ornithophily – pollination by bird. Ex: Hummingbird, Sunbird and Honeyeater.
2. Chiropterophily – pollination by bat
3. But also by monkeys, marsupials, lemurs, bears, rabbits, deer, rodents, and lizards (gecko lizard and garden lizard), take part in pollination
4. Entomophily – pollination by insect

Features of flowers in Insect Pollination:

• Flowers: large, colourful, fragrant, rich in nectar.
• If flowers are small: a number of flowers are clustered into an inflorescence to make them conspicuous (clearly visible).
• The flowers pollinated by flies and beetles secrete foul odours to attract these animals.
• Pollen grains: sticky
• Stigma: sticky

Certain rewards to pollinators:

• Floral Rewards: Nectar and (edible) pollen grains as foods.
• Harvesting Rewards: Provide a safe place for laying eggs. Ex: Amorphophallus (tallest flower 3m’s or 9 ft 10 inc) and Yucca.
• Many insects may consume pollen or nectar without bringing about pollination. Such floral visitors are referred to as pollen/nectar robbers.

Note: Plants produce enormous amounts of pollen when compared to the number of ovules available for pollination to compensate for the uncertainties of pollination.

2.6.3 Outbreeding devices:

• Flowering plants have developed many devices to avoid self-pollination and to encourage cross-pollination. Such devices are called Outbreeding devices.
• Majority of the flowering plants produce hermaphrodite (bisexual) flowers and undergo autogamy.
• Continuous autogamy or self-pollination results in inbreeding depression.

2.6.3.1 Ways to Avoid Self-pollination:

• Pollen release & stigma receptivity – not synchronized [Protandry (pollen mature earlier) and Protogyny (stigma mature earlier)].
• Stigma and anther – placed at different positions (Heterostyly).
• Self-incompatibility or Self sterility is a genetic mechanism in angiosperms that prevents inbreeding (pollen grains do not germinate on stigma of the same flower or another flower of the same plant) and promotes outcrossing. This prevents Geitonogamy.
• Suppression of one sex: In bisexual flowers, stamen or carpel is completely suppressed and becomes sterile.
• Protandry, Protogyny, Heterostyly and Suppression of one sex prevents autogamy.
• Production of unisexual flowers in the same plant Ex: Castor, Maize (prevents autogamy).
• In Papaya, male and female flowers are present on different plants (prevents autogamy & geitonogamy).

Note:

• Staminate: It is a unisexual male flower.
• Pistillate: It is a unisexual female flower.
• Staminode: It is a sterile stamen.
• Pistillode or Carpellode: It is a sterile pistil or carpel.

2.6.4 Pollen Pistil Interaction:

• All the events from pollen deposition on the stigma until pollen tubes enter into the embryo sac or Pollen Inhibition called pollen-pistil interaction.
• Pollination does not guarantee the transfer of the right type of pollen grain to the right type of stigma.
• The pistil or stigma has the ability to recognize or determine the pollen whether it is compatible or incompatible.
• If it is the right type the stigma allows the pollen to germinate.
• If it is the wrong type the stigma rejects the pollen, preventing germination.
• The ability of the pistil to recognize the pollen by continuous dialogue mediated by chemicals like Boron, Inositol and Sucrose level.
• Following compatible pollination, the pollen grain produces a pollen tube through one of the germ pores.

2.6.4.1 Entry of pollen tube into Ovule:

After reaching the ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes:

• Porogamy: When the pollen tube enters the ovule through micropyle is called porogamy. It is the most common type. Ex: Lily.
• Chalazogamy: The entry of a pollen tube into the ovule from the chalazal region is known as chalazogamy. Chalazogamy is less common. Ex: Casuarina, Juglans, Betula, etc. It was first observed by Treub (1981) in Casuarina.
• Mesogamy: The pollen tube enters the ovule through its middle part i.e. through integument (e.g. Cucurbita, Populus) or through funicle (e.g. Pistacia).

2.6.5 Artificial hybridization:

It refers to crosses occurring under controlled conditions by humans to get desired individuals from desired parents.

2.6.5.1 Steps to achieve Artificial hybridization:

• It is one of the major approaches of the crop improvement program.
• Only desired pollen grain used for pollination.
• Stigma is protected from contamination (from unwanted pollen grain).
• Removal of anthers from the flower bud before the anther dehiscence is called emasculation.
• Emasculated flowers covered by bags generally made up of butter paper to prevent contamination of its stigma with unwanted pollen. This step is called bagging.
• If the female flower is unisexual there is no need of emasculation
• Bagged flower- attains receptivity - mature pollen grains- dusted on the stigma – re-bagged - fruits allowed to develop.

2.6.5.2 Significance of Artificial hybridization:

• This technique helps to perform crossing different species and often genera experiments.

• To produce commercially superior varieties of plants.

2.7 Double Fertilization:

This fertilization process occurs in flowering plants, in which one sperm cell of a pollen grain fuses with an egg cell while a second fuses with two polar nuclei.

2.7.1 Fertilized Embryo Sac:

• After double fertilization, the egg is developed into diploid zygote and later turned into an embryo.
• Diploid secondary polar nuclei fertilize with a haploid male gamete to form a triploid (3n) Primary Endosperm Nucleus (PEN), this is also known as triple fusion.
• Antipodals and synergids gradually degenerate.

2.8 Post-fertilization: Structure and Events:

Structures:

1. Development of endosperm

2. Development of embryo

3. Maturation of ovule into seed

4. Maturation of ovary into fruit

Events Occurs in flowering Plants:

• The zygote is formed inside the ovule in flowering plants

• After fertilization the sepals, petals and stamens of the flower fall off except pistil or carpel.

• Zygote develops into an embryo.

• Ovule develops into seed

• Integument of the ovule develops into seed coat.

• Ovary develops into fruit.

• Ovary wall developed into a pericarp.

• Locules develop into endocarp and / or mesocarp.

• Nucellus develop into Perisperm.

• Polar Nuclei develop into Primary Endosperm Nucleus.

• After dispersal, seeds germinate under favorable conditions to produce new plants.

2.8.1 Development of Endosperm:

• Development of endosperm takes place before the embryo development from the primary endosperm cell (PEC).
• Cells are filled with reserve food material and are used for the nutrition of the developing embryo.
• Non - Albuminous / Non-Endospermic seeds:- endosperm completely utilized - before maturation of seeds. Ex: Pea, Beans, and Groundnut.
• Albuminous / Endospermic seeds:- a portion of endosperm remains in mature seeds. Ex: Castor, Coconut, Wheat, Rice and Maize.

2.8.1.1 Types of Endosperm development:

Three main types of endosperm development in flowering plants are:

1. Free Nuclear type

2. Cellular type and

3. Helobial type

(i) Free Nuclear Type: In nuclear type of endosperm the first division of primary endosperm nucleus and few subsequent nuclear divisions are not accompanied by wall formation. Ex: Coconut water
(ii) Cellular type: In this case, there is cytokinesis after each nuclear division of the endosperm nucleus and forms a cell wall. Ex: surrounding white kernel in coconut.
(iii) Helobial type: It is an intermediate type between the nuclear and cellular types. The first division is accompanied by cytokinesis but the subsequent ones are free nuclear. Ex: Asphodelus tenuifolius (onion weed) and most of the Monocotyledons.

2.8.2 Development of Embryo:

An embryo is a basic element that possesses enough potential for further growth and development.

2.8.2.1 Features of embryo development in plants:

• Embryo develops at the micropylar end of the embryo sac.

• Zygote starts its development only after some amount of endosperm is formed.

• An early stage of embryo development called Embryogeny.

• Embryogeny are similar in both monocotyledons and dicotyledons.

• Embryo development takes place in following stages:
• Proembryo
• Globular stage
• Heart shaped and
• Matured embryo
• In higher plants a mature embryo contains precursor tissue for the root (radicle), stem (plumule), leaves, and one or two cotyledons.
• After the embryo germinates, it grows out of the seed and is known as a plantlet or seedling.

2.8.2.2 Dicot Embryo:

• A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
• Embryonal axis: It is the axis or the stalk of the embryo of a seed which lies between radicle and plumule.
• Embryonal axis above the cotyledon is the epicotyls.
• Terminal part of the epicotyls is the plumule (gives rise to the shoot).
• Embryonal axis below the cotyledon is the hypocotyl.
• The terminal part of the hypocotyl is called the radicle (root tip).
• The root tip is covered by the root cap.

2.8.2.3 Monocot Embryo:

• Possesses only one cotyledon
• In the grass family the cotyledon is called scutellum.
• Scutellum is situated towards one side of the embryonal axis.
• The portion of the embryonic axis below the level of attachment of scutellum is called hypocotyls.
• Hypocotyl has in its terminal radicle and the root cap are enclosed by a sheath called coleorhiza.
• The portion of the embryonic axis above the level of attachment of scutellum is called epicotyls.
• Epicotyl has the shoot apex and a few leaf primordia enclosed by a hollow foliar structure called coleoptile.

2.8.3 Seed:

Fertilized and mature ovules develop into seed.

2.8.3.1 Seed consists:

• Cotyledon(s),
• Embryonal axis
• Seed coat - double layered - formed by integuments {Testa (outer coat) and Tegmen (inner coat)}.
• Micropyle: - small opening on seed coat, it facilitates entry of water and oxygen into the seeds for germination.
• Hilum:- scar on seed coat
• Perisperm: Remnants of a nucellus that is persistent or remain. Ex: Black pepper and beet.

2.8.3.2. Germinating Conditions of Seed:

• As the seed matures, its water content is reduced and seeds become relatively dry (10-15 per cent moisture by mass).
• The general metabolic activity of the embryo slows down.
• The embryo or seed may enter a state of inactivity called dormancy.
• In favourable conditions like adequate moisture, oxygen and suitable temperature, seeds start to germinate.
• Seeds store enough nutrients to sustain young seedlings until they can photosynthesize for themselves.

2.8.3.3 Advantages of Seeds:

To plants:

• Seeds - reserve food materials- nourish seedlings.
• Seed coat- protection to young embryos.
• Seeds are large number of species – live for several years
• Seeds - better adaptive strategies - dispersal to new habitats - better survival

To mankind

• used as food - throughout the year
• seed - basis of agriculture

2.8.3.4 Viability of Seeds:

• Viability means whether or not pollen grain / seed is alive after dispersing.
• In a few species the seeds lose viability within a few months.
• Seeds of a large number of species live for several years.
• Some seeds can remain alive for hundreds of years.
• There are several records of very old yet viable seeds. The oldest is that of a lupine, Lupinus arcticus excavated from Arctic Tundra. The seed germinated and flowered 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 archeological excavation at King Herod’s palace near the Dead Sea.

2.8.4 Fruit:

It is the seed-bearing structure in flowering plants formed from the ovary after flowering.

2.8.4.1 Types of fruits:

• True fruit:- Fruit formed from the ovary. Ex: Mango, Cucumber etc

False fruit: any part other than ovary- forms the fruit. Ex: Apple, Strawberry, and Cashew from thalamus or receptacle.

• Parthenogenesis: If the ovary transforms to fruit without fertilization. Parthenocarpy fruit can be induced with growth hormones (gibberellins & auxins) without fertilization. It produces seedless fruits. Ex: Banana, Grapes etc.

Note:

• Orchid fruit has thousands of seeds in it.
• Orobanche and Striga (Parasite Plants) fruits also have thousands of seeds in them.
• Examples for fleshy fruits are guava, mango, orange etc.
• Examples for dry fruits are mustard, dates, groundnuts etc.
• Is there any relationship between the number of ovules in an ovary and the number of seeds present in a fruit?
• Ans: Yes, the number of seeds present in the fruit will be equal to the number of ovules present in the ovary.

2.9 Apomixis and Polyembryony: 

Other modes of reproduction in the formation of seeds as follows;

2.9.1 Apomixis:

Form of asexual reproduction- mimics sexual reproduction- seed formed without fertilization. Ex: Asteraceae (Sunflower family) and grasses.

Advantage: Apomicts have several advantages in horticulture and agriculture in creating hybrid seeds (still in research).

2.9.1.1 Formation of apomictic seeds:

Diploid cells (formed without meiosis) develop into embryos without fertilization.

2.9.2 Polyembryony:

Occurrence of more than one embryo in a seed is called polyembryony. Ex: Citrus and Mango.

2.9.2.1 Formation of Polyembryony:

Cells of nuclleus (2n) surrounding embryo sac protrude into embryo sac and develop into many embryos.

2.10 Hybrid varieties:

    The offspring of two animals or plants of different breeds, varieties, species, or genera, especially as produced through human manipulation for specific genetic characteristics is known as hybrid varieties.
Ex: Sonalika (wheat), Jaya (rice), Pusa shubra (cauliflower)

Importance: Hybrid varieties of several of our food and vegetable crops are being extensively cultivated. Cultivation of hybrids has tremendously increased productivity.

Problems:

• Hybrid seeds have to be produced every year. If the seeds collected from hybrids are sown, the plants in the progeny will segregate and do not maintain hybrid characters.
• Production of hybrid seeds is costly and hence the cost of hybrid seeds becomes too expensive for the farmers.

Overcome or Solution: If these hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. Then the farmers can keep on using the hybrid seeds to raise new crops year after year and they do not have to buy hybrid seeds every year.

Conclusion: Because of the importance of apomixis in the hybrid seed industry, active research is going on in many laboratories around the world to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.