Sexual reproduction in flowering plants

ISC 12>CONTENT>STRUCTURE AND FUNCTION OF PLANTS>5. REPRODUCTION IN ANGIOSPERMS

SYLLABUS

Vegetative reproduction: advantages and disadvantages of vegetative reproduction, micropropagation – plant tissue culture
Sexual reproduction: development of male and female gametophytes, types of ovules, placentation,
Pollination,: Advantages of self and cross-pollination
Fertilisation (Amphimixis) and
Formation of endosperm, embryo.

REPRODUCTION

Reproduction is one of the most important features of living organisms.
Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself.
The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death. Reproduction enables the continuity of the species, generation after generation
There are two basic forms of reproduction.

Asexual reproduction
Sexual reproduction

MODES OF REPRODUCTION IN ANGIOSPERMS

In angiosperms or flowering plants, there are several modes of reproduction.
Generally they are arranged in two groups.

Asexual or vegetative :
Sexual reproduction

A. VEGETATIVE PROPAGATION

Vegetative reproduction is the process of multiplication in which a portion of the plant body becomes detached and develops into a new plant body.
It is the simplest way of reproduction under favorable conditions.
Year after after same variety can be produced.
Since all the plants are genetically alike, they are susceptible to same diseases.
Types : It is of two types.

Natural vegetative propagation
Artificial vegetative propagation

1. Natural vegetative propagation: It can takes place by the following methods.

A) Vegetative propagation by roots.

The swollen tap roots of carrot, turnip and radish have buds at the base of old stems just above the tap root which serve as organ of vegetative propagation.
Adventitious roots of Asparagus, Dahlia, sweet potato, yam etc. serve to propagate plant vegetatively

B) Vegetative propagation by stem

(a) Subaerial stems: Subaerial stems may develop as lateral branches from the mother plant. This may break up from- the parent plant and then, grow into new plants. Example- Runners (Oxalis), sucker - (banana, Chrysanthemum), stolon (Jasmine), offset (Eichhornia)

(b) Underground Stems: In certain plants the underground stems become modified for storage of food during the active phase of the growth. Examples- Rhizome (Ginger), tuber (Potato), bulb (Onion) and corm (colocasia)

a) Subaerial stem

Runner :

These are horizontal stem growing from the parent plant above the soil.
The terminal buds touching the ground develop adventitious roots.
Ex. grass, mint, strawberry etc.

Sucker

A lateral branch arising close to the ground level, traveling underground for some distance.
It then emerges out of the soil obliquely and give rise to a new plant.
Eg. Chrysanthemum, pineapple, raspberry etc.

Stolon

The growing end of a shoot arches over and touches the ground.
The terminal bud touches the ground and develops adventitious roots producing a new plant.
Eg. Blackberry and Jasmine

Offset

An offset is a short thick runner like branch which produces a new plant at its tip.
The offsets grow in all directions from the main stem of the parent plant.
If any accidental injury results in the separation of these units, each is capable of leading an independent existence.
E.g., Pistia, Eichhornia.

b) Underground stem

Stem tuber: Potato

is a swollen underground stem
can produce shoots from buds, also known as ‘eyes’
can be divided by being cut into pieces or grown from a small tuber known as a ‘seed’ tuber.
Once divided, each piece of stem tuber has a bud or eye that will grow to produce roots and shoots. The piece of stem tuber will provide the energy for growth until the new shoot is above ground level.

Rhizomes: Iris, Ginger

are stems that grow under the soil surface
can be divided by cutting the parent plant into sections when the plant is dormant.
Each section must have a bud at a node.
New shoots and roots will form from the buds.

Bulb: Onion

A bulb contains an underground stem.
Leaves are attached to the stem. These leaves contain much stored food.
At the centre of the bulb is an apical bud. Also attached are lateral buds.
The apical bud will produce leaves and a flower while the lateral buds will produce new shoots.
As the plant grows and develops it will form a new bulb underground.

Corm: Colocasia

Corms look like bulbs and are often confused with them. There are no modified leaves on a corm as in the onion. Corms are squashed, compressed stems. They can’t be pulled apart into individual leaf scales.
Corms are flattened underground stems swollen with food and produce cormlets at their bases.
Cormlets can be separated carefully from the parent corm and grown to produce new plants.

C) Vegetative propagation by leaf

The fleshy succulent leaves of Bryophyllum bear adventitious buds in their notches located in the margins.
When the leaves fall on moist soil, these buds develop into small plants completing the process of vegetative propagation.

2. ARTIFICIAL VEGETATIVE PROPAGATION

Vegetative propagation by artificial means may be brought about by the following methods.

A. LAYERING

In layering, the development of adventitious roots is induced on the stem before it is separated from the parent plant. In lemon, rose, jasmine, strawberry, raspberry, grape-vine, etc., the lower branch is bent down and covered under a light layer of moist soil by pushing the tip into the soft ground. After some time, adventitious roots develop and on cutting the branches from the parent plant, these develop into new plants. This is known as layering.

TYPES OF LAYERING:

SIMPLE LAYERING

Simple layering means bending a branch to the ground and getting it to root where it touches This method is used mainly for shrubs with flexible branches,
such as Forsythia, Spirea, and Rambler Rose.

MOUND LAYERING

Mound layering is useful with heavy-stemmed, closely
branched shrubs. It is also useful for fruit root stock production. The original plant may be cut back to encourage many new shoots to grow from the base. Then, the following spring after the new shoots have grown approximately 8- 10 inches, mound soil containing sphagnum peat moss about 7-9 inches deep around the shrub.Roots will grow into the surrounding soil from the new growth. The following autumn or spring, gently dig into the mound, separate and transplant the new plants.

AIR LAYERING OR GOOTEE

It is a modified fom of layering. In this case the cut or injured branch is not buried in the ground but is bound with mud and rags, etc., which is kept moist .The roots develop at this portion within a period of about a month or two. Now the branch is cut and separated from the parent plant from below the tied portion and is planted in the soil. This method is applied for the vegetative propagation of pomegranate, orange, lemon, guava, lokat, litchi, etc.

B. CUTTING

In this process a vegetative part of a plant is taken and rooted to form a new plant.
Several plants like sugarcane, rose, Duranta, Coleus and China-rose, etc., are grown from stem-cutting.
A portion of stem with leaves is cut from the parent and placed in a suitable rooting medium for that particular species of plant. Some suitable media include moist sand, a mixture of peat moss and soil, or water used. After roots have developed from the cut end of the stem the cutting is transplanted to soil.

C. GRAFTING

In this case, a small branch of a plant is inserted into the stem of a rooted plant of the same or allied species. As a result of insertion, organic union or fusion of tissues takes place and both of them grow as one. The inserted plant is called the scion, the rooted plant the stock and the phenomenon is called grafting.
The scion is a shoot, 4 -12 inches in length. Its all the buds are kept intact while all the buds of the stock are removed. The graft is placed on the stock and the joining portion is covered with a layer of wax or clay in order to prevent the evaporation of water and the entry of injurious bacteria. After some time the tissues of the scion and the stock become united. Grafting is of two types:

(a) Splice grafting, and
(b) Whip grafting.

(a) In splice grafting, both graft and stock are cut across obliquely at about the same angle and then firmly tied together.
(b) in whip grafting, both graft and stock are cut diagonally. Now a vertical notch is made in the stock and the graft is cut at one end to make a chisel-shaped structure or tongue. This tongue of the graft is inserted into the notch of the stock and the two are bound. In apple, citrus, mangoes, rose and lemon, new and superior plants are grown in this way.

B. MICROPROPAGATION
(the production of a large number of individual plants from a small piece of plant tissue cultured in a nutrient medium)
Tissue culture:The aseptic culture of plant protoplasts, cells, tissues or organs under conditions which lead to cell multiplication or regeneration of organs or whole plants.

Steps of micropropagation

Selection and maintenance of stock plants for culture initiation
Explant isolation – Virtually any part of the plant can be used as explant like vegetative parts (Shoot tip, meristem, leaves, stems, roots) or reproductive parts (Anthers, pollen, ovules, embryo, seed, spores). Shoot tip and auxiliary buds are most often used.
Surface sterilization – Explants are surface sterilized by treating it with disinfectant solution of suitable concentration for a specific period. Ethyl alcohol, bromine water, mercuric chloride, silver nitrate, sodium hypochlorite, calcium hypochlorite etc. can be used as disinfectant.
Washing – Washed with water.
Establishment of explant on appropriate medium – There is no one universal culture medium; however modifications of Murashige and Skoog basal medium (Murashige and Skoog, 1962) are most frequently used.
Multiplication of shoots or somatic embryo formation (rapid) using a defined culture medium
Rooting of regenerated shoots or germination of somatic embryos in vitro.
Hardening : Transfer of plantlets to sterilized soil for hardening under greenhouse environment

ADVANTAGES OF PROPAGATION BY TISSUE CULTURE TECHNIQUE

This technique enabled us to produce large number of plants in relatively short time.
Disease free plants can be obtained by culturing the shoot apices (meristem) of infected plants.
Very small size explants can be used for micropropagation. This is impossible with conventional technique. Important when limited explant is available.
Material multiplied by micropropagation can be maintained in small place, packing and transport is also easy due to small size.
Micropropagation is the only viable method of multiplying genetically modified cells or cells after protoplast fusion.
In case of dioecious species, where one of the sex is more desirable then under such circumstances plants of desired sex can be selectively multiplied by this technique.
The output is clean, healthy and pathogen free, as during micropropagation, fungi and bacteria are usually eliminated.
Independent of the season; can be carried out through out the year.

LIMITATIONS :

Requirement of sophisticated facilities
High production cost
Requirement of skill in handling and maintenance.
Somaclonal variations may arise during in vitro culture when a callus phase is involved.
For many valuable species suitable micropropagation techniques are not available (e.g. mango).

ADVANTAGES OF VEGETATIVE PROPAGATION

The plants that cannot produce viable seeds such as banana, sugarcane, seedless grapes can be easily grown by vegetative propagation.
It is easier,less expensive and a rapid method of propagation.
Superior quality fruits or flowers can be produced by the method of grafting.
Preservation of desirable characteristics due to no genetic recombination
By tissue culture, a large number of of disease free identical plants can be grown in very short time.

DISADVANTAGES OF VEGETATIVE PROPAGATION

Unwanted characters can not be eliminatedfrom plants.
When plants grown repeatedly they may lose vigor
May become susceptible to diseases.
Vegetative parts such as root, stem leaves, bulbil etc can not be preserved for longer periods as they easily attacked by pathogens.

C. SEXUAL REPRODUCTION

FLOWER AND FLORAL PARTS (Details in practical )

DEVELOPMENT OF THE MALE GAMETOPHYTE (POLLEN)

a) Structure of mature anther

The fertile portion of stamens is called anther.
Each anther consists of two anther lobes connected by a connective.
A young anther comprises of a mass of undifferentiated thin-walled cells bounded by epidermis. Beneath the epidermis is a layer of cells, called endothecium.
Each anther contains two pollen chambers. Thus a typical anther consists of four microsporangia or pollen chambers. This type of anther is known as dithecous.
In Malvaceae, anthers bear only one lobe with two microsporangia. Such a condition is known as monothecous.
Each pollen chamber represents a microsporangium containing innumerable microspores or pollens.

b) Development of microsporangium

Young anther appears to be a homogeneous mass of meristematic cells which is surrounded by epidermis. (Fig-a)
It then becomes four-lobed and four rows of archesporial cells are differentiated. (Fig-b)
The archesporial cells are marked off from the surrounding cells by their more deeply stained cytoplasm and conspicuous nuclei.
Each of the archesporial cells now cuts off a primary parietal cell on the inner side. (Fig-c)
The parietal cell now divides by periclinal and anticlinal walls giving rise to several layers of cells forming the wall of the anther.
Wall of anthers:

a) Epidermis b) Endothecium c) Middle layers d) Tapetum :

Role of tapetum: It provides nourishment to the developing pollen grains. During microsporogenesis, the cells of tapetum produce various enzymes, hormones, amino acids, and other nutritious material required for the development of pollen grains. It also produces the exine layer of the pollen grains, which is composed of the sporopollenin.

The sporogenous cell divides to give rise to a number of microspores or pollen mother cells.

Development of anther(Microsporangium)

c) Microsporogenesis :

During microsporogenesis, the nucleus of each microspore mother cell undergoes meiosis and ultimately gives rise to four haploid nuclei.
These four nuclei are arranged tetrahedrally and are soon invested with cell walls.
These are now called the microspores.
These soon dry up and become powdery while the tapetum becomes absorbed.
The anther now becomes a dry structure, the partition walls between the sporangia are destroyed and the microspores are soon liberated by the dehiscence of the anther.

Microsporogenesis under microscope

Tetrads

Released microspore

Bicellular pollen

d) Structure of pollen grains

The pollen grains represent the male gametophytes.
There is variety of architecture – sizes, shapes, colours, designs – seen on the pollen grains from different species.
Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has a prominent two-layered wall.
The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known.
Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent.
Pollen grains are wellpreserved as fossils because of the presence of sporopollenin.
The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin. The cytoplasm of pollen grain is surrounded by a plasma membrane

e) Development of male gametophyte

Pollen grains begin to germinate before pollination.
Nucleus divide mitotically. (Fig-A)
Two cells: generative cell and vegetative cell. (Figure A)
In over 60 per cent of angiosperms, pollen grains are shed at this 2-celled stage.Generative cells divides after pollination. (Fig:B)
In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
Male gametes consists of large nucleus with thin sheath of cytoplasm.

Figure: A

Figure:B

BQ: Explain the development of an anther and the formation of microspores in angiosperms. 4M (2006)

DEVELOPMENT OF FEMALE GAMETOPHYTE

Female reproductive organ is gynoecium or pistil.
Carpel is a unit of the female part of the flower (gynoecium), consisting of an ovary bearing one or more ovules, a receptivebstigma, and often a stalk-like style between them.

Gynoecium

Ovules:

Structure of the ovule (Megasporangium)

Arises as primordium on placenta.
Parts of ovule:

Funicle: The ovule is a small structure attached to the placenta by means of a stalk called funicle.
Nucellus : The primordium grows into a mass of cells forming nucellus, the body of ovule. Cells of nucellus are rich in reserve food.
Intgegument: The two protective covering of nucellus are integuments.
Micropyle : Integuments encircle the ovule except at the tip where a small opening called the micropyle
Chalaza: Basal part of ovule is called chalaza that lie opposite to micropyle.
Hilum: The body of the ovule fuses with funicle in the region called hilum
Embryo sac: A single embryo sac or female gametophyte located in the nucellus, which is developed from megaspore. It contains egg apparatus, antipodal cells and secondary nucleus.

Development of megasporangium

The ovule develops as a small protuberance of the placental tissue.
In every young ovule a single hypodermal cell is differentiated a the archesporial cell.
The archesporial cell may divide to form a primary parietal cell and a primary sporogenous cell , or it may function directly as the megaspore mother cell.
The primary parietal cell may remain undivided or it may undergo periclinal and anticlinal divisions to form a variable number of wall layers.
The primary sporogenous cell usually functions as the megaspore mother cell without undergoing any further divisions.
Megaspore mother cell undergoes meiosis and a linear row of haploid megaspore cells is formed.

Megasporogenesis

One of the nucellar cell in the micropylar region is differentiated into megaspore mother cell.
The cell is larger, contains dense cytoplasm and a prominent nucleus.
It undergoes meiosis forming 4 haploid cells called megaspore tetrad.
3 megaspores degenerate and only one megaspore become functional.
Functional megaspore is the first cell of female gametophyte.

Megasporogenesis and development of female gametophyte

Development of female gametophyte

The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac.
Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac.
Nuclear divisions are not followed immediately by cell wall formation.
After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.
Six of the eight nuclei are surrounded by cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell.
Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell.
Three cells are at the chalazal end and are called the antipodals.
The large central cell has two polar nuclei. Thus, a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled.

BQ: Describe the development of female gametophyte in angiosperms. (4M)

The ovule or megasporangium develops as a protuberance of placental tissue.
A hypodermal cell (Nucellus) differentiate as archesporial cell.
This cuts off some parietal cells and itself becomes megaspore mother cell.
Megaspore mother cell undergoes meiosis forming a linear row of four megaspore of which three disintegrate and lowermost remains megaspore.
Nucleus divides three times forming eight cells of which four move to each pole of embryo sac.
One cell from each pole converges to centre to form secondary nucleus.
Three cells towards micropyle form egg apparatus (1 egg and 2 synergids) and three cells on the opposite end become antipodal cells.
Outermost cells from the base of ovule develop into integuments

Types of ovules

PLACENTATION

The arrangement of ovules within the ovary is known as placentation.
The placentation are of different types namely, marginal, axile, parietal, basal, central and free central.

Marginal : In this type, the gynoecium is monocarpellary and unilocular and placenta are borne on the fused margins of same carpel. This condition is most common in most of the members of the family leguminosae.
Parietal : In this type the gynoecium is multicarpellary, syncarpous and the ovary is unilocular.The placenta are seen on the inner surface of the ovary, at the junction of the carpels.Sometimes the unilocular ovary is found to be divided by the development of a false septum. Eg. Mustard, Cucurbits
Axile: In this type, the gynoecium is multicarpellary, syncarpous and multilocular, The wall of the carpels in the centre of the ovary are united to form an axis, which bears the placentae. Eg. China rose (Pentalocular), Petunia (Bilocular)
Free central: The ovary is unilocular and the ovules are borne on central axis in the centre of the ovary.Septa are absent. Eg. Dianthus and Primrose
Basal: The ovary is unilocular and a single ovuleis borne at the base of ovary. Eg. Sunflower
Superficial: The gynoecium is multicarpellary, syncarpous and large number of ovules are borne on the walls of loculi without specific order. Eg. Nymphaea

Figure: Ovary Placentation. Ovaries in cross section above ovaries in longitudinal section. A: marginal; B: parietal; C: axile, ovary with 2 loculi; D, axile, ovary with 3 loculi; E: free-central; F:free-central; G: apical; H:basal.

POLLINATION

Definition: Pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or of another flower of same species.
Types: The basic modes of pollination are
A) Self pollination
B) Cross pollination

A) SELF POLLINATION :

Transfer of pollen grains from the anther to stigma of the same flower or another flower on the same plant is said to be self pollination.
Self pollination may autogamous or geitonogamous.

AUTOGAMY (TYPE-I)
When the pollen grains from the anthers are transferred to the stigma of the same flower, it is known as autogamy.
Devices for Autogamy:

Homogamy: Simultaneous maturation of anther and pistil.
Bisexuality: Anther and stigma lie close to each other.
Cleistogamy: Cleistogamous flower: (flower which never open ) Eg.oxalis, commelina, viola

GEITONOGAMY (TYPE-II)
When pollen of a flower pollinate any other flower present on the same plant, it is said to be geitonogamy.

Commelina benghalensis : Both types of flowers

B) CROSS POLLINATION (xenogamy/allogamy)

Transfer of pollen grains from the anther of a flower from one plant to the stigma of the flower on another plant is called cross pollination.
It may need an external agency like wind or insects.
Is geitonogamy a cross pollination?
Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.

Contrivances of cross pollination

1. Unisexuality

Flowers when they are all uni-sexual only allogamy is possible.
Two types of flowers: Male and female flowers
Plant may be monoecious or dioecious.
Monoecious: Male and female organs are found on same plant but in different flower. (Maize)
Dioecious: Male and female organs appear on separate individual.(Mulberry)

2. Dichogamy

Male and female sex organs mature at different time or Pollen release and stigma receptivity are not synchronized.
Two different conditions.
a) Protoandry: Anthers mature earlier than gynoecium.

Eg. China rose, Cotton, Sunflower etc.

b) Protogyny :Gynoecium mature earlier than anthers.

Eg. Brassica, Ficus, Mirabilis etc

3. Heterostyly

In some plants flowers are of two or three forms with anther and stigmas at different levels.
Long styled flowers
Short styled flowers
Based on difference in length, the phenomenon is called heterostyly.
Eg. Primula vulgaris

4. Self incompatibility

The pollen of a flower has no fertilizing effect on the stigma of the same flower.
Eg. Tea (Thea sinensis)

5. Herkogamy:

This is a common strategy employed by hermaphroditic angiosperms to reduce sexual interference between male (anthers) and female (stigma) function by some sort of barrier.
Eg. Calotropis (Madar)

AGENTS OF POLLINATION

1. Pollination by wind (Anemophily)

Flowers: Unisexual
Petals: Absent or small and inconspicuous. Scentless
Nectar: Absent or small and green
Stamen: Long filaments, allowing the anthers to hang freely outside the flower so the pollen is exposed to the wind.
Pollen: Larger amount of smooth and light pollen grains.
Stigma: Long and feathery, hanging outside the flower to catch pollen carried by the wind
Examples: Maize, Grasses

2. Pollination by water (Hydrophily)

Most common in the lower plant groups such as algae, bryophytes and pteridophytes.
Pollination by water is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons.
Not all aquatic plants use water for pollination.
In a majority of aquatic plants such as water hyacinth and water lily, the flowers emerge above the level of water and are pollinated by insects or wind.
It is seen in submerged flowers like Vallisneria, Hydrilla and Zostera.
In Vallisneria male flowers released on water surface and female flowers reaches the surface for pollination.
In sea grasses, pollen grains are long ribbon like and carried passively to submerged female flowers. Mucilage coated pollen grains.

3. Pollination by insects

Flowers: Mostly bisexual and often found in plants which are more or less solitary.
Petals: Present, often large, coloured and scented.
Nectar: Produced by nectaries to attract insects
Stamen: Present inside the flower
Pollen: Smaller amount , often round, sticky or have projection to attach to furry bodies of insects.
Stigma: Small surface area, inside the flower
Examples: Rose, sunflower etc

Special mechanism of insect pollination :
a) Salvia -

It shows lever mechanism.
Anthers are distractile ; lower lobe is sterile and upper lobe is fertile.
The flower is bilabiate and protandrous.
The insect sits on the lower lobe of the corolla; the upper fertile lobe of anther touches the body of insect.
It ruptures and pollen grains are shed on the back of the insect.
When this insect enters the other flower, pollination is affected

4. Pollination by birds(ornithophily)-

Ornithophilous flowers are usually large in size.
They have tubular or funnel shaped corollas.
The flowers are brightly colored(such as red, orange, blue, yellow, etc.) which attract birds from long distances.
Humming bird pollinates while hovering over the flowers and sucking nectar.
The pollination by birds is common in coral tree, bottle brush, Butea monosperma and silk cotton tree.

Figure : Pollination by Hummingbird

5. Pollination by bats (Chiropterophily)

The bats are nocturnal flying mammals, which move swiftly and transport pollen grains to long distances.
The flowers they visit are large, dull coloured and have strong scent.
Chiropterophilous flowers produce abundant pollen grains and secrete more nectar than the ornithophilous flowers.
Bats carry out the pollination in Adansonia and Kigelia.

Figure : Pollination by bat

Difference between insect and wind pollinated flower

Difference between self and cross pollination

BQ: Write four advantages of cross pollination over self pollination. 2M (ISC 2009 )

Cross pollination induces variations
May eliminate harmful characters
Result in hybrid formation
More viable seed formed.

POLLEN PISTIL INTERACTION

Recognition of compatible pollen- It is the interaction between chemical components of pollen and those of stigma.

Pollination does not guarantee the transfer of the right type of pollen stigma).
Often, pollen of the wrong type, either from other species or from the same plant (if it is self-incompatible), also land on the stigma.
The pistil has the ability to recognise the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible).
If it is of the right type, the pistil accepts the pollen and promotes post-pollination changes.
If the pollen is of the wrong type, the pistil rejects the pollen by preventing pollen germination on the stigma.

2. Germination of pollen and development of male gametophyte

The pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores.
The contents of the pollen grain move into the pollen tube
In some plants, pollen grains are shed at two-celled condition (a vegetative cell and a generate cell). In such plants, the generative cell divides by mitosis and forms the two male gametes during the growth of pollen tube in the stigma.
When the pollen is shed from anther it has usually two cells, a generative cells and a tube cell (Vegetative cell).
In plants which shed pollen in the three-celled condition, pollen tubes carry the two male gametes from the beginning.
Pollen tube, after reaching the ovary, enters the ovule.

3. Entry of pollen tube in ovule

The pollen tube is directed towards one of the ovules.It may enter the ovules through one of the following three routes.

Porogamy: When pollen tube enters the ovule through the micropyle,the condition is known as porogamy. This is the most common mode of pollen tube entry into the ovule.
Chalaogmay :When pollen tube enters the ovule through the chalazal end, the condition is said to be chalazogamy. Eg. Betula, Casurina
Mesogamy: When pollen tube enters the ovule through integument, the condition is known as mesogamy. Eg. Cucurbita and Populus

4. Entry of pollen tube in embryo sac

Irrespective of the route of pollen tube into the ovule, it always enters the embryo sac from micropylar end.
Recent studies have shown that filiform apparatus present at the micropylar part of the synergids guides the entry of pollen tube.
The entry of pollen tube into the embryo sac may be

Between the egg cell and one of the synergids
Between the wall of the embryo sac and synergid.
Between the synergids
Directly penetrates one of the synergids

DOUBLE FERTILIZATION

In flowering plants, the process of fertilization was discovered by Strasburger in 1841.
After entering into the embryo sac, the tip of the pollen tube bursts and two male gamete are discharged.
One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy. This results in the formation of a diploid cell, the zygote.
The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN). As this involves the fusion of three haploid nuclei it is termed triple fusion.
Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilization, an event unique to flowering plants.
The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

DEVELOPMENT OF ENDOSPERM

The primary endosperm nucleus (3n) undergoes a series of divisions and ultimately forms endosperm, a highly nutritive tissue which provides nourishment to the developing embryo.
Types of endosperm: On the basis of development, three types of endosperm have been recognised.

1. Nuclear endosperm:

Commonly occurs in polypetalous dicotyledons.
The first few cell divisions are not accompanied by cell wall formation.
Nuclei produced remain free in the cytoplasm of the embryo sac .
Eg. In Cocos nucifera (Coconut), the watery liquid endosperm which fills the large embryo sac contains numerous free nuclei.

2. Cellular endosperm

Commonly occurs in gamopetalous dicotyledons.
Wall formation occurs with first division of primary endosperm nucleus.
After first transverse division, the subsequent division are irregular.
Endosperm tissue cell do not shoe regular arrangement.

3. Helobial endosperm

This type is intermediate between the nuclear and cellular type.
First division of primary emdosperm nucleus accompanied by formation of transverse wall.
This divides the embryo sac the embryo sac unequally into a small chalazal chamber and a large micropylar chamber.
This is followed by free nuclear division in each chamber and than cell wall formation make the endosperm cellular.

BQ: Give reason why endosperm in angiosperms becomes triploid. (ISC 2000: 3M)
BQ: Explain the development of the different types of endosperms in angiosperms. (ISC 2007,4M)

DEVELOPMENT OF EMBRYO

After fertilization, a series of changes takes place in the ovule and as a result seed is is formed.
The process of development of mature embryo from diploid zygote is called embryogenesis.
Most zygotes divide only after certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo.
Though the seeds differ greatly, the early stages of embryo development (embryogeny) are similar in both monocotyledons and dicotyledons.

Development of dicotyledonous embryo :

The normal type of dicot embryo development has been studied in Shepherd's purse (Capsella bursa-pastoris) which belongs to family Cruciferae. This is called as Crucifer or Onagrad type of embryo development.
Zygote (oospore) divides into two unequal cells, larger suspensor cell towards micropyle and a smaller embryonal cell (terminal cell) towards antipodal region.
The suspensor cell undergoes transverse divisions forming 6 - 10 celled long suspensor. The first cell of the suspensor (towards micropyle) is large and called haustorium or vesicular cell. The last cell of suspensor (towards embryo cell) is known as hypophysis. It forms radicle tip.
Embryonal cell divides twice vertically and once transversely to produce a two-tiered eight-celled embryo. The epibasal tier forms two cotyledons and a plumule while the hypobasal tier produces only hypocotyl and most of the radicle.
For this the octant embryo undergoes periclinal divisions producing protoderm, procambium and ground meristem.
It is initially globular but with the growth of cotyledons it becomes heart-shaped and then assumes the typical shape, e.g., Capsella bursa-pastoris.

A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
The portion of embryonal axis 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 terminates at its lower end in the radical or root tip.
The root tip is covered with a root cap. (Figure: A typical dicot embryo)

BQ: Describe formation of embryo from a fertilized egg in angiosperm. 5M .(ISC 2003)

Fertilized egg surrounds itself with a wall and becomes oospore.
Oospore divides into suspensor cell and embryonal cell.
Suspensor cell divides to form a row of cells which form the suspensor.
Terminal cell of the suspensor near the micropyle enlarges and is known as haustorium.
Basal cell of the suspensor is known as hypophysis cell.
Embryonal cell divides and form eight celled octant.
Four cells lying towards the suspensor is known as posterior octant which gives rise to radicle and hypocotyl.
Anterior octant gives rise to plumule and cotyledons
Hypophysis form the apex of radicle.

Development of Embryo in Monocot

The normal type of monocot embryo development has been studied in Luzula forsteri and is called Sagittaria type.

The early development of dicot and monocot embryos are similar upto octant stage. Later on differentiation starts. Suspensor is single celled.
The zygote divides transversely producing a vesicular suspensor cell towards micropylar end and embryo cell towards the chalazal end.
The embryo cell divides transversely again into a terminal and a middle cell. The terminal cell divides vertically and transversely into globular embryo. It forms a massive cotyledon and a plumule.
Growth of cotyledon pushes the plumule to one side.
Remains of second cotyledon occur in some grasses. It is called epiblast.
The single cotyledon of monocots is called scutellum. It is shield shaped and lateral in position but appears terminal.
The middle cell gives rise to hypocotyl and radicle.
It may add a few cells to the suspensor.
Both radicle and plumule develop covering sheaths called coleorrhiza and coleoptile respectively