Are we not lucky that plants reproduce sexually? The

myriads of flowers that we enjoy gazing at, the scents and

the perfumes that we swoon over, the rich colours that

attract us, are all there as an aid to sexual reproduction.

Flowers do not exist only for us to be used for our own

selfishness. All flowering plants show sexual reproduction.

A look at the diversity of structures of the inflorescences,

flowers and floral parts, shows an amazing range of

adaptations to ensure formation of the end products of

sexual reproduction, the fruits and seeds. In this chapter,

let us understand the morphology, structure and the

processes of sexual reproduction in flowering plants

(angiosperms).

2.1 FLOWER – A FASCINATING ORGAN OF

ANGIOSPERMS

Human beings have had an intimate relationship with

flowers since time immemorial. Flowers are objects of

aesthetic, ornamental, social, religious and cultural value

– they have always been used as symbols for conveying

important human feelings such as love, affection,

happiness, grief, mourning, etc. List at least five flowers

of ornamental value that are commonly cultivated at

2.1 Flower – A Fascinating

Organ of Angiosperms

2.2 Pre-fertilisation : Structures

and Events

2.3 Double Fertilisation

2.4 Post-fertilisation: Structures

and Events

2.5 Apomixis and

Polyembryony

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SEXUAL REPRODUCTION IN

FLOWERING PLANTS

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homes and in gardens. Find out the names of five more flowers that are

used in social and cultural celebrations in your family. Have you heard

of floriculture – what does it refer to?

To a biologist, flowers are morphological and embryological marvels

and the sites of sexual reproduction. In class XI, you have read the various

parts of a flower. Figure 2.1 will help you recall the parts of a typical

flower. Can you name the two parts in a flower in which the two most

important units of sexual reproduction develop?

2.2 PRE-FERTILISATION: STRUCTURES AND E

VENTS

Much before the actual flower is seen on a plant, the decision that the plant

is going to flower has taken place. Several hormonal and structural changes

are initiated which lead to the differentiation and further development of

the floral primordium. Inflorescences are formed which bear the floral buds

and then the flowers. In the flower the male and female reproductive

structures, the androecium and the gynoecium differentiate and develop.

You would recollect that the androecium consists of a whorl of stamens

representing the male reproductive organ and the gynoecium represents

the female reproductive organ.

Figure 2.1 A diagrammatic representation of L.S. of a flower

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2.2.1 Stamen, Microsporangium and Pollen Grain

Figure 2.2a shows the two parts of a typical stamen – the long and slender

stalk called the filament, and the terminal generally bilobed structure

called the anther. The proximal end of the filament

is attached to the thalamus or the petal of the flower.

The number and length of stamens are variable in

flowers of different species. If you were to collect a

stamen each from ten flowers (each from different

species) and arrange them on a slide, you would

be able to appreciate the large variation in size seen

in nature. Careful observation of each stamen

under a dissecting microscope and making neat

diagrams would elucidate the range in shape and

attachment of anthers in different flowers.

A typical angiosperm anther is bilobed with

each lobe having two theca, i.e., they are dithecous

(Figure 2.2 b). Often a longitudinal groove runs

lengthwise separating the theca. Let us

understand the various types of tissues and their

organisation in the transverse section of an anther

(Figure 2.3 a). The bilobed nature of an anther is

very distinct in the transverse section of the anther.

The anther is a four-sided (tetragonal) structure

consisting of four microsporangia located at the

corners, two in each lobe.

The microsporangia develop further and

become pollen sacs. They extend longitudinally

all through the length of an anther and are packed

with pollen grains.

Structure of microsporangium: In a transverse

section, a typical microsporangium appears near

circular in outline. It is generally surrounded by four wall layers (Figure

2.3 b)– the epidermis, endothecium, middle layers and the tapetum. The

outer three wall layers perform the function of protection and help in

dehiscence of anther to r

elease the pollen. The innermost wall layer is

the tapetum. It nourishes the developing pollen grains. Cells of the

tapetum possess dense cytoplasm and generally have more than one

nucleus. Can you think of how tapetal cells could become bi-nucleate?

When the anther is young, a group of compactly arranged homogenous

cells called the sporogenous tissue occupies the centre of each

microsporangium.

Microsporogenesis : As the anther develops, the cells of the sporogenous

tissue undergo meiotic divisions to form microspore tetrads. What would

be the ploidy of the cells of the tetrad?

Figure 2.2 (a) A typical stamen;

(b) three–dimensional cut section

of an anther

(b)

(a)

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As each cell of the sporogenous tissue is capable of giving rise to a

microspore tetrad. Each one is a potential pollen or microspore mother

cell. The process of formation of microspores from a pollen mother cell (PMC)

through meiosis is called microsporogenesis. The microspores, as they

are formed, are arranged in a cluster of four cells–the microspore tetrad

(Figure 2.3 a). As the anthers mature and dehydrate, the microspores

dissociate from each other and develop into pollen grains (Figure 2.3 b).

Inside each microsporangium several thousands of microspores or pollen

grains are formed that are released with the dehiscence of anther

(Figure 2.3 c).

Pollen grain: The pollen grains represent the male gametophytes. If you

touch the opened anthers of Hibiscus or any other flower you would find

deposition of yellowish powdery pollen grains on your fingers. Sprinkle

these grains on a drop of water taken on a glass slide and observe under

(c)

(a) (b)

Figure 2.3 (a) Transverse section of a young anther; (b) Enlarged view of one microsporangium

showing wall layers; (c) A mature dehisced anther

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a microscope. You will really be amazed at the variety of architecture –

sizes, shapes, colours, designs – seen on the pollen grains

from different species (Figure 2.4).

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 well-

preserved as fossils because of the presence of sporopollenin.

The exine exhibits a fascinating array of patterns and designs.

Why do you think the exine should be hard? What is the

function of germ pore? 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. When the pollen grain is

mature it contains two cells, the vegetative cell and generative

cell (Figure 2.5b). The vegetative cell is bigger, has abundant

food reserve and a large irregularly shaped nucleus. The

generative cell is small and floats in the cytoplasm of the

vegetative cell. It is spindle shaped with dense cytoplasm and

a nucleus. In over 60 per cent of angiosperms, pollen grains

are shed at this 2-celled stage. 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).

Pollen grains of many species cause severe allergies and bronchial

afflictions in some people often leading to chronic respiratory

disorders– asthma, bronchitis, etc. It may be mentioned that Parthenium

or carrot grass that came into India as a contaminant with imported wheat,

has become ubiquitous in occurrence and causes pollen allergy.

Figure 2.5 (a) Enlarged view of

a pollen grain tetrad; (b) stages

of a microspore maturing into a

pollen grain

Figure 2.4 Scanning electron micrographs of a few pollen grains

(a)

(b)

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When once they are shed, pollen grains have to land on the stigma

before they lose viability if they have to bring about fertilisation. How long

do you think the pollen grains retain viability? The period for which pollen

grains remain viable is highly variable and to some extent depends on the

prevailing temperature and humidity. In some cereals such as rice and

wheat, pollen grains lose viability within 30 minutes of their release, and

in some members of Rosaceae, Leguminoseae and Solanaceae, they

maintain viability for months. You may have heard of storing semen/

sperms of many animals including humans for artificial insemination. It

is possible to store pollen grains of a large number of species for years in

liquid nitrogen (-196

C). Such stored pollen can be used as pollen banks,

similar to seed banks, in crop breeding programmes.

2.2.2 The Pistil, Megasporangium (ovule) and Embryo sac

The gynoecium represents the female reproductive part of the flower. The

gynoecium may consist of a single pistil (monocarpellary) or may have

more than one pistil (multicarpellary). When there are more than one,

the pistils may be fused together (syncarpous) (Figure 2.7b) or may be

free (apocarpous) (Figure 2.7c). Each pistil has three parts (Figure 2.7a),

the stigma, style and ovary. The stigma serves as a landing platform

for pollen grains. The style is the elongated slender part beneath the

stigma. The basal bulged part of the pistil is the ovary. Inside the ovary

is the ovarian cavity

(

( (

(locule). The placenta is located inside the ovarian

cavity. Recall the definition and types of placentation that you studied in

Figure 2.6 Pollen products

Pollen grains are rich in nutrients. It has become a fashion in recent

years to use pollen tablets as food supplements. In western countries, a

large number of pollen products in the form of tablets and syrups are

available in the market. Pollen consumption has been claimed to increase

the performance of athletes and race horses (Figure 2.6).

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Class XI. Arising from the placenta are the megasporangia, commonly

called ovules. The number of ovules in an ovary may be one (wheat,

paddy, mango) to many (papaya, water melon, orchids).

The Megasporangium (Ovule) : Let us familiarise ourselves with the

structure of a typical angiosperm ovule (Figure 2.7d). The ovule is a small

structure attached to the placenta by means of a stalk called funicle.

The body of the ovule fuses with funicle in the region called hilum. Thus,

hilum represents the junction between ovule and funicle. Each ovule has

one or two protective envelopes called integuments. Integuments encircle

the nucellus except at the tip where a small opening called the micropyle

is organised. Opposite the micropylar end, is the chalaza, representing

the basal part of the ovule.

Enclosed within the integuments is a mass of cells called the nucellus.

Cells of the nucellus have abundant reserve food materials. Located in the

nucellus is the embryo sac or female gametophyte. An ovule generally has

a single embryo sac formed from a megaspore.

Megasporogenesis :

The process of formation of megaspores from the

megaspore mother cell is called megasporogenesis. Ovules generally

differentiate a single megaspore mother cell (MMC) in the micropylar region

Stigma

Style

Ovary

Thalamus

Figure 2.7 (a) A dissected flower of Hibiscus showing pistil (other floral parts have been removed);

(b) Multicarpellary, syncarpous pistil of Papaver; (c) A multicarpellary, apocarpous

gynoecium of Michelia; (d) A diagrammatic view of a typical anatropous ovule

(a)

(b)

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Figure 2.8 (a) Parts of the ovule showing a large megaspore mother cell, a dyad and a tetrad of

megaspores; (b) 2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac; (c) A

diagrammatic representation of the mature embryo sac.

(a)

(b)

(c)

of the nucellus. It is a large cell containing dense cytoplasm and a

prominent nucleus. The MMC undergoes meiotic division. What is the

importance of the MMC undergoing meiosis? Meiosis results in the

production of four megaspores (Figure 2.8a).

Female gametophyte : In a majority of flowering plants, one of the

megaspores is functional while the other three degenerate. Only the

functional megaspore develops into the female gametophyte (embryo

sac). This method of embryo sac formation from a single megaspore is termed

monosporic development. What will be the ploidy of the cells of the nucellus,

MMC, the functional megaspore and female gametophyte?

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Let us study formation of the embryo sac in a little more detail.

(Figure 2.8b). 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. It is of interest to note that these mitotic divisions are

strictly free nuclear, that is, 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. Observe the distribution of cells inside the embryo sac

(Figure 2.8b, c). 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.

There is a characteristic distribution of the cells within the embryo

sac. 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. The synergids have special cellular thickenings at the

micropylar tip called filiform apparatus, which play an important role in

guiding the pollen tubes into the synergid. Three cells are at the chalazal

end and are called the antipodals. The large central cell, as mentioned

earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at

maturity, though 8-nucleate is 7-celled

2.2.3 Pollination

In the preceding sections you have learnt that the male and female gametes

in flowering plants are produced in the pollen grain and embryo sac,

respectively. As both types of gametes are non-motile, they have to be

brought together for fertilisation to occur. How is this achieved?

Pollination is the mechanism to achieve this objective. Transfer

of pollen grains (shed from the anther) to the stigma of a pistil is

termed pollination. Flowering plants have evolved an amazing array

of adaptations to achieve pollination. They make use of external

agents to achieve pollination. Can you list the possible external

agents?

Kinds of Pollination : Depending on the source of pollen, pollination

can be divided into three types.

(i) Autogamy : In this type, pollination is achieved within the same

flower. Transfer of pollen grains from the anther to the stigma of the

same flower (Figure 2.9a). In a normal flower which opens and

exposes the anthers and the stigma, complete autogamy is rather

rare. Autogamy in such flowers requires synchrony in pollen release

and stigma receptivity and also, the anthers and the stigma should

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lie close to each other so that self-pollination

can occur. Some plants such as Viola

(common pansy), Oxalis, and Commelina

produce two types of flowers –

chasmogamous flowers which are similar to

flowers of other species with exposed anthers

and stigma, and cleistogamous flowers which

do not open at all (Figure 2.9c). In such flowers,

the anthers and stigma lie close to each other.

When anthers dehisce in the flower buds,

pollen grains come in contact with the stigma

to effect pollination. Thus, cleistogamous

flowers are invariably autogamous as there is

no chance of cross-pollen landing on the

stigma. Cleistogamous flowers produce

assured seed-set even in the absence of

pollinators. Do you think that cleistogamy is

advantageous or disadvantageous to the

plant? Why?

(ii) Geitonogamy – Transfer of pollen grains from

the anther to the stigma of another flower of

the same plant. 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.

(iii) Xenogamy – Transfer of pollen grains from

anther to the stigma of a different plant (Figure

2.9b). This is the only type of pollination which

during pollination brings genetically different

types of pollen grains to the stigma.

Agents of Pollination : Plants use two abiotic (wind

and water) and one biotic (animals) agents to achieve

pollination. Majority of plants use biotic agents for

pollination. Only a small proportion of plants use

abiotic agents. Pollen grains coming in contact with

the stigma is a chance factor in both wind and water

pollination. To compensate for this uncertainties and

associated loss of pollen grains, the flowers produce

enormous amount of pollen when compared to the

number of ovules available for pollination.

Figure 2.9 (a) Self-pollinated flowers;

(b)Cross pollinated flowers;

(c)Cleistogamous flowers

(a)

(c)

(b)

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Pollination by wind is more common

amongst abiotic pollinations. Wind pollination

also requires that the pollen grains are light

and non-sticky so that they can be

transported in wind currents. They often

possess well-exposed stamens (so that the

pollens are easily dispersed into wind currents,

Figure 2.10) and large often-feathery stigma

to easily trap air-borne pollen grains. Wind-

pollinated flowers often have a single ovule in

each ovary and numerous flowers packed into

an inflorescence; a familiar example is the corn

cob – the tassels you see are nothing but the

stigma and style which wave in the wind to

trap pollen grains. Wind-pollination is quite

common in grasses.

Pollination by water is quite rare in

flowering plants and is limited to about 30

genera, mostly monocotyledons. As against

this, you would recall that water is a regular

mode of transport for the male gametes among

the lower plant groups such as algae,

bryophytes and pteridophytes. It is believed,

particularly for some bryophytes and

pteridophytes, that their distribution is limited

because of the need for water for the transport

of male gametes and fertilisation. Some

examples of water pollinated plants are Vallisneria and Hydrilla which

grow in fresh water and several marine sea-grasses such as Zostera. 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 as in most of the land

plants. In Vallisneria, the female flower reach the surface of water by the

long stalk and the male flowers or pollen grains are released on to the

surface of water. They are carried passively by water currents (Figure

2.11a); some of them eventually reach the female flowers and the stigma.

In another group of water pollinated plants such as seagrasses, female

flowers remain submerged in water and the pollen grains are released

inside the water. Pollen grains in many such species are long, ribbon like

and they are carried passively inside the water; some of them reach the

stigma and achieve pollination. In most of the water-pollinated species,

pollen grains are protected from wetting by a mucilaginous covering.

Both wind and water pollinated flowers are not very colourful and do

not produce nectar. What would be the reason for this?

Figure 2.10 A wind-pollinated plant showing

compact inflorecence and well-

exposed stamens

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Majority of flowering plants use

a range of animals as pollinating

agents. Bees, butterflies, flies,

beetles, wasps, ants, moths, birds

(sunbirds and humming birds) and

bats are the common pollinating

agents. (Figure 2.11b). Among the

animals, insects, particularly bees

are the dominant biotic pollinating

agents. Even larger animals such

as some primates (lemurs), arboreal

(tree-dwelling) rodents, or even

reptiles (gecko lizard and garden

lizard) have also been reported as

pollinators in some species.

Often flowers of animal-

pollinated plants are specifically

adapted for a particular species of

animal.

Majority of insect-pollinated

flowers are large, colourful, fragrant

and rich in nectar. When the flowers

are small, a number of flowers are

clustered into an inflorescence to

make them conspicuous. Animals

are attracted to flowers by colour

and/or fragrance. The flowers

pollinated by flies and beetles

secrete foul odours to attract these

animals. To sustain animal visits,

the flowers have to provide rewards

to the animals. Nectar and pollen

grains are the usual floral rewards.

For harvesting the reward(s) from

the flower the animal visitor comes

in contact with the anthers and the

stigma. The body of the animal gets

a coating of pollen grains, which are

generally sticky in animal pollinated flowers. When the animal carrying

pollen on its body comes in contact with the stigma, it brings about

pollination.

In some species floral rewards are in providing safe places to lay eggs;

an example is that of the tallest flower of Amorphophallus

(the flower

itself is about 6 feet in height). A similar relationship exists between a

species of moth and the plant Yucca where both species – moth and the

(a)

(b)

Figure 2.11 (a) Pollination by water in Vallisneria;

(b) Insect pollination

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plant – cannot complete their life cycles without each other. The moth

deposits its eggs in the locule of the ovary and the flower, in turn, gets

pollinated by the moth. The larvae of the moth come out of the eggs as

the seeds start developing.

Why don’t you observe some flowers of the following plants (or any

others available to you): Cucumber, Mango, Peepal, Coriander, Papaya,

Onion, Lobia, Cotton, Tobacco, Rose, Lemon, Eucalyptus, Banana? Try to

find out which animals visit them and whether they could be

pollinators.You’ll have to patiently observe the flowers over a few days

and at different times of the day. You could also try to see whether there

is any correlation in the characteristics of a flower to the animal that

visits it. Carefully observe if any of the visitors come in contact with the

anthers and the stigma as only such visitors can bring about pollination.

Many insects may consume pollen or the nectar without bringing about

pollination. Such floral visitors are referred to as pollen/nectar robbers.

You may or may not be able to identify the pollinators, but you will surely

enjoy your efforts!

Outbreeding Devices : Majority of flowering plants produce hermaphrodite

flowers and pollen grains are likely to come in contact with the stigma of

the same flower. Continued self-pollination result in inbreeding depression.

Flowering plants have developed many devices to discourage self-

pollination and to encourage cross-pollination. In some species, pollen

release and stigma receptivity are not synchronised. Either the pollen is

released before the stigma becomes receptive or stigma becomes receptive

much before the release of pollen. In some other species, the anther and

stigma are placed at different positions so that the pollen cannot come in

contact with the stigma of the same flower. Both these devices prevent

autogamy. The third device to prevent inbreeding is self-incompatibility.

This is a genetic mechanism and prevents self-pollen (from the same flower

or other flowers of the same plant) from fertilising the ovules by inhibiting

pollen germination or pollen tube growth in the pistil. Another device to

prevent self-pollination is the production of unisexual flowers. If both male

and

female flowers are present on the same plant such as castor and maize

(monoecious), it prevents autogamy but not geitonogamy. In several species

such as papaya, male and female flowers are present on different plants,

that is each plant is either male or female (dioecy). This condition prevents

both autogamy and geitonogamy.

Pollen-pistil Interaction : Pollination does not guarantee the transfer

of the right type of pollen (compatible pollen of the same species as the

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 events that

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leads to fertilisation. If the pollen is of the wrong type, the pistil rejects the

pollen by preventing pollen germination on the stigma or the pollen tube

growth in the style. The ability of the pistil to recognise the pollen followed

by its acceptance or rejection is the result of a continuous dialogue

between pollen grain and the pistil. This dialogue is mediated by chemical

components of the pollen interacting with those of the pistil. It is only in

recent years that botanists have been able to identify some of the pollen

and pistil components and the interactions leading to the recognition,

followed by acceptance or rejection.

As mentioned earlier, following compatible pollination, the pollen grain

germinates on the stigma to produce a pollen tube through one of the

germ pores (Figure 2.12a). The contents of the pollen grain move into the

Figure 2.12 (a) Pollen grains germinating on the stigma; (b) Pollen tubes growing through the

style; (c) L.S. of pistil showing path of pollen tube growth; (d) enlarged view of an

egg apparatus showing entry of pollen tube into a synergid; (e) Discharge of male

gametes into a synergid and the movements of the sperms, one into the egg and

the other into the central cell

(e)

(d)

(a)

(b)

(c)

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pollen tube. Pollen tube grows through the tissues of the stigma and

style and reaches the ovary (Figure 2.12b, c). You would recall that in

some plants, pollen grains are shed at two-celled condition (a vegetative

cell and a generative cell). In such plants, the generative cell divides and

forms the two male gametes during the growth of pollen tube in the stigma.

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 through the micropyle and then enters one of

the synergids through the filiform apparatus (Figure 2.12d, e). Many recent

studies have shown that filiform apparatus present at the micropylar part

of the synergids guides the entry of pollen tube. All these events–from

pollen deposition on the stigma until pollen tubes enter the ovule–are

together referred to as pollen-pistil interaction. As pointed out earlier,

pollen-pistil interaction is a dynamic process involving pollen recognition

followed by promotion or inhibition of the pollen. The knowledge gained

in this area would help the plant breeder in manipulating pollen-pistil

interaction, even in incompatible pollinations, to get desired hybrids.

You can easily study pollen germination by dusting some pollen from

flowers such as pea, chickpea, Crotalaria, balsam and Vinca on a glass slide

containing a drop of sugar solution (about 10 per cent). After about 15–30

minutes, observe the slide under the low power lens of the microscope. You

are likely to see pollen tubes coming out of the pollen grains.

As you shall learn in the chapter on plant breeding (Chapter 9), a

breeder is interested in crossing different species and often genera to

combine desirable characters to produce commercially ‘superior’ varieties.

Artificial hybridisation is one of the major approaches of crop

improvement programme. In such crossing experiments it is important

to make sure that only the desired pollen grains are used for pollination

and the stigma is protected from contamination (from unwanted pollen).

This is achieved by emasculation and bagging techniques.

If the female parent bears bisexual flowers, removal of anthers from

the flower bud before the anther dehisces using a pair of forceps is

necessary. This step is referred to as emasculation. Emasculated flowers

have to be covered with a bag of suitable size, generally made up of butter

paper, to prevent contamination of its stigma with unwanted pollen. This

process is called bagging. When the stigma of bagged flower attains

receptivity, mature pollen grains collected from anthers of the male parent

are dusted on the stigma, and the flowers are rebagged, and the fruits

allowed to develop.

If the female parent produces unisexual flowers, there is no need for

emasculation. The female flower buds are bagged before the flowers open.

When the stigma becomes receptive, pollination is carried out using the

desired pollen and the flower rebagged.

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2.4 POST-FERTILISATION : STRUCTURES AND EVENTS

Following double fertilisation, events of endosperm and embryo

development, maturation of ovule(s) into seed(s) and ovary into fruit, are

collectively termed post-fertilisation events.

2.4.1 Endosperm

Endosperm development precedes embryo development. Why? The

primary endosperm cell divides repeatedly and forms a triploid

Figure 2.13 (a) Fertilised embryo sac showing zygote and Primary Endosperm Nucleus (PEN);

(b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)]

(a)

(b)

2.3 DOUBLE FERTILISATION

After entering one of the synergids, the pollen tube releases the two male

gametes into the cytoplasm of the synergid. 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) (Figure 2.13a). 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

fertilisation, 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.

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endosperm tissue. The cells of this tissue are filled with

reserve food materials and are used for the nutrition of

the developing embryo. In the most common type of

endosperm development, the PEN undergoes successive

nuclear divisions to give rise to free nuclei. This stage of

endosperm development is called free-nuclear endosperm.

Subsequently cell wall formation occurs and the

endosperm becomes cellular. The number of free nuclei

formed before cellularisation varies greatly. The coconut

water from tender coconut that you are familiar with, is

nothing but free-nuclear endosperm (made up of

thousands of nuclei) and the surrounding white kernel is

the cellular endosperm.

Endosperm may either be completely consumed by the

developing embryo (e.g., pea, groundnut, beans) before seed

maturation or it may persist in the mature seed (e.g. castor

and coconut) and be used up during seed germination. Split

open some seeds of castor, peas, beans, groundnut, fruit of

coconut and look for the endosperm in each case. Find out

whether the endosperm is persistent in cereals – wheat, rice

and maize.

2.4.2 Embryo

Embryo develops at the micropylar end of the embryo sac where

the zygote is situated. 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. Figure 2.13 depicts the stages of embryogeny in

a dicotyledonous embryo. The zygote gives rise to the

proembryo and subsequently to the globular, heart-shaped

and mature embryo

A typical dicotyledonous embryo (Figure 2.14a), 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 radicle or root tip. The root

tip is covered with a root cap.

Embryos of monocotyledons (Figure 2.14 b) possess only

one cotyledon. In the grass family the cotyledon is called

scutellum that is situated towards one side (lateral) of the

embryonal axis. At its lower end, the embryonal axis has the

Figure 2.14 (a) A typical dicot

embryo; (b) L.S. of an

embryo of grass

(a)

(b)

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radical and root cap enclosed in an undifferentiated sheath called

coleorrhiza. The portion of the embryonal axis above the level of

attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a

few leaf primordia enclosed in a hollow foliar structure, the coleoptile.

Soak a few seeds in water (say of wheat, maize, peas, chickpeas,

ground nut) overnight. Then split the seeds and observe the various

parts of the embryo and the seed.

2.4.3 Seed

In angiosperms, the seed is the final product of sexual reproduction. It is

often described as a fertilised ovule. Seeds are formed inside fruits. A

seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.

The cotyledons (Figure 2.15a) of the embryo are simple structures,

generally thick and swollen due to storage of food reserves (as in legumes).

Mature seeds may be non-albuminous or ex-albuminous. Non-

albuminous seeds have no residual endosperm as it is completely

consumed during embryo development (e.g., pea, groundnut).

Albuminous seeds retain a part of endosperm as it is not completely used

up during embryo development (e.g., wheat, maize, barley, castor).

Occasionally, in some seeds such as black pepper and beet, remnants of

nucellus are also persistent. This residual, persistent nucellus is the

perisperm.

Integuments of ovules harden as tough protective seed coats

(Figure 2.15a). The micropyle remains as a small pore in the seed coat.

This facilitates entry of oxygen and water into the seed during germination.

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 may enter a state of

inactivity called dormancy, or if favourable conditions are available

(adequate moisture, oxygen and suitable temperature), they germinate.

As ovules mature into seeds, the ovary develops into a fruit, i.e., the

transformation of ovules into seeds and ovary into fruit proceeds

simultaneously. The wall of the ovary develops into the wall of fruit called

pericarp. The fruits may be fleshy as in guava, orange, mango, etc., or

may be dry, as in groundnut, and mustard, etc. Many fruits have evolved

mechanisms for dispersal of seeds. Recall the classification of fruits and

their dispersal mechanisms that you have studied in an earlier class. Is

there any relationship between number of ovules in an ovary and the

number of seeds present in a fruit?

In most plants, by the time the fruit develops from the ovary, other

floral parts degenerate and fall off. However, in a few species such as apple,

strawberry, cashew, etc., the thalamus also contributes to fruit formation.

Such fruits are called

false fruits (Figure 2.15b). Most fruits however

develop only from the ovary and are called true fruits. Although in most

of the species, fruits are the results of fertilisation, there are a few species

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in which fruits develop without fertilisation. Such fruits are called

parthenocarpic fruits. Banana is one such example. Parthenocarpy can

be induced through the application of growth hormones and such fruits

are seedless.

Seeds offer several advantages to angiosperms. Firstly, since

reproductive processes such as pollination and fertilisation are

independent of water, seed formation is more dependable. Also seeds have

better adaptive strategies for dispersal to new habitats and help the species

Figure 2.15 (a) Structure of some seeds. (b) False fruits of apple and strawberry

(b)

(a)

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to colonise in other areas. As they have sufficient food reserves, young

seedlings are nourished until they are capable of photosynthesis on their

own. The hard seed coat provides protection to the young embryo. Being

products of sexual reproduction, they generate new genetic combinations

leading to variations.

Seed is the basis of our agriculture. Dehydration and dormancy of

mature seeds are crucial for storage of seeds which can be used as food

throughout the year and also to raise crop in the next season. Can you

imagine agriculture in the absence of seeds, or in the presence of seeds

which germinate straight away soon after formation and cannot be stored?

How long do the seeds remain alive after they are dispersed? This

period again varies greatly. 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.

After completing a brief account of sexual reproduction of flowering

plants it would be worth attempting to comprehend the enormous

reproductive capacity of some flowering plants by asking the following

questions: How many eggs are present in an embryo sac? How many

embryo sacs are present in an ovule? How many ovules are present in

an ovary? How many ovaries are present in a typical flower? How many

flowers are present on a tree? And so on...

Can you think of some plants in which fruits contain very large

number of seeds. Orchid fruits are one such category and each fruit

contain thousands of tiny seeds. Similar is the case in fruits of some

parasitic species such as Orobanche and Striga. Have you seen a tiny

seed of Ficus? How large is the tree of Ficus developed from that tiny

seed. How many billions of seeds does each Ficus tree produce? Can

you imagine any other example in which such a tiny structure can

produce such a large biomass over the years?

2.5 APOMIXIS AND POLYEMBRYONY

Although seeds, in general are the products of fertilisation, a few flowering

plants such as some species of Asteraceae and grasses, have evolved a

special mechanism, to produce seeds without fertilisation, called apomixis.

What is fruit production without fertilisation called? Thus, apomixis is a

form of asexual reproduction that mimics sexual reproduction. There are

several ways of development of apomictic seeds. In some species, the

diploid egg cell is formed without reduction division and develops into

the embryo without fertilisation. More often, as in many Citrus and Mango

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SUMMARY

Flowers are the seat of sexual reproduction in angiosperms. In the flower,

androecium consisting of stamens represents the male reproductive

organs and gynoecium consisting of pistils represents the female

reproductive organs.

A typical anther is bilobed, dithecous and tetrasporangiate. Pollen

grains develop inside the microsporangia. Four wall layers, the

epidermis, endothecium, middle layers and the tapetum surround the

microsporangium. Cells of the sporogenous tissue lying in the centre of

the microsporangium, undergo meiosis (microsporogenesis) to form

tetrads of microspores. Individual microspores mature into pollen grains.

Pollen grains represents the male gametophytic generation. The

pollen grains have a two-layered wall, the outer exine and inner intine.

The exine is made up of sporopollenin and has germ pores. Pollen grains

may have two cells (a vegetative cell and generative cell) or three cells (a

vegetative cell and two male gametes) at the time of shedding.

The pistil has three parts – the stigma, style and the ovary. Ovules

are present in the ovary. The ovules have a stalk called funicle, protective

integument(s), and an opening called micropyle. The central tissue is

the nucellus in which the archesporium differentiates. A cell of the

archesporium, the megaspore mother cell divides meiotically and one of

the megaspores forms the embryo sac (the female gametophyte). The

mature embryo sac is 7-celled and 8-nucleate. At the micropylar end is

varieties some of the nucellar cells surrounding the embryo sac start

dividing, protrude into the embryo sac and develop into the embryos. In

such species each ovule contains many embryos. Occurrence of more

than one embryo in a seed is referred to as polyembryony. Take out

some seeds of orange and squeeze them. Observe the many embryos of

different sizes and shapes from each seed. Count the number of embryos

in each seed. What would be the genetic nature of apomictic embryos?

Can they be called clones?

Hybrid varieties of several of our food and vegetable crops are being

extensively cultivated. Cultivation of hybrids has tremendously increased

productivity. One of the problems of hybrids is that 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 become too expensive for the farmers. 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 crop year after year and he does not have to buy hybrid seeds every

year. Because of the importance of apomixis in 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.

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EXERCISES

1. Name the parts of an angiosperm flower in which development of male

and female gametophyte take place.

2. Differentiate between microsporogenesis and megasporogenesis. Which

type of cell division occurs during these events? Name the structures

formed at the end of these two events.

3. Arrange the following terms in the correct developmental sequence:

Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell,

male gametes.

4. With a neat, labelled diagram, describe the parts of a typical angiosperm

ovule.

5. What is meant by monosporic development of female gametophyte?

6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female

gametophyte.

the egg apparatus consisting of two synergids and an egg cell. At the

chalazal end are three antipodals. At the centre is a large central cell

with two polar nuclei.

Pollination is the mechanism to transfer pollen grains from the

anther to the stigma. Pollinating agents are either abiotic (wind and

water) or biotic (animals).

Pollen-pistil interaction involves all events from the landing of pollen

grains on the stigma until the pollen tube enters the embryo sac (when

the pollen is compatible) or pollen inhibition (when the pollen is

incompatible). Following compatible pollination, pollen grain germinates

on the stigma and the resulting pollen tube grow through the style,

enter the ovules and finally discharges two male gametes in one of the

synergids. Angiosperms exhibit double fertilisation because two fusion

events occur in each embryo sac, namely syngamy and triple fusion.

The products of these fusions are the diploid zygote and the triploid

primary endosperm nucleus (in the primary endosperm cell). Zygote

develops into the embryo and the primary endosperm cell forms the

endosperm tissue. Formation of endosperm always precedes

development of the embryo.

The developing embryo passes through different stages such as

the proembryo, globular and heart-shaped stages before maturation.

Mature dicotyledonous embryo has two cotyledons and an embryonal

axis with epicotyl and hypocotyl. Embryos of monocotyledons have a

single cotyledon. After fertilisation, ovary develops into fruit and ovules

develop into seeds.

A phenomenon called apomixis is found in some angiosperms,

particularly in grasses. It results in the formation of seeds without

fertilisation. Apomicts have several advantages in horticulture and

agriculture.

Some angiosperms produce more than one embryo in their seed.

This phenomenon is called polyembryony.

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7. What are chasmogamous flowers? Can cross-pollination occur in

cleistogamous flowers? Give reasons for your answer.

8. Mention two strategies evolved to prevent self-pollination in flowers.

9. What is self-incompatibility? Why does self-pollination not lead to seed

formation in self-incompatible species?

10. What is bagging technique? How is it useful in a plant breeding

programme?

11. What is triple fusion? Where and how does it take place? Name the

nuclei involved in triple fusion.

12. Why do you think the zygote is dormant for sometime in a fertilised

ovule?

13. Differentiate between:

(a) hypocotyl and epicotyl;

(b) coleoptile and coleorrhiza;

(d) perisperm and pericarp.

14. Why is apple called a false fruit? Which part(s) of the flower forms the

fruit?

15. What is meant by emasculation? When and why does a plant breeder

employ this technique?

16. If one can induce parthenocarpy through the application of growth

substances, which fruits would you select to induce parthenocarpy

and why?

17. Explain the role of tapetum in the formation of pollen-grain wall.

18. What is apomixis and what is its importance?

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