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UNIT VI
REPRODUCTION
Index
Chapter 1 :
Sexual Reproduction in flowering Plants
Chapter 2 : Human Reproduction
Chapter 3 :
Reproductive Health
Biology in essence is the story of life on
earth. While individual organisms die without fail, species continue to live
through millions of years unless threatened by natural or anthropogenic extinction.
Reproduction becomes a vital process without which species cannot survive for
long. Each individual leaves its progeny by asexual or sexual means. Sexual
mode of reproduction enables creation of new variants, so that survival advantage
is enhanced. This unit explains the details of reproductive processes in
flowering plants and humans as easy to relate representative examples. A
related perspective on human reproductive health and how reproductive ill
health can be avoided is also presented to complete our understanding of
biology of reproduction.
Born in November 1904 in Jaipur (Rajasthan)
Panchanan Maheshwari rose to become one of the most distinguished botanists not
only of India but of the entire world. He moved to Allahabad for higher
education where he obtained his D.Sc. During his college days, he was inspired by
Dr W. Dudgeon, an American missionary teacher, to develop interest in Botany
and especially morphology. His teacher once expressed that if his student
progresses ahead of him, it will give him a great satisfaction. These words
encouraged Panchanan to enquire what he could do for his teacher in return.
He worked on
embryological aspects and popularised the use of embryological characters in
taxonomy. He established the Department of Botany, University of Delhi as an
important centre of research in embryology and tissue culture. He also
emphasised the need for initiation of work on artificial culture of immature
embryos. These days, tissue culture has become a landmark in science. His work
on test tube fertilisation and intra-ovarian pollination won worldwide acclaim.
He was honoured with fellowship of Royal
Society of London (FRS), Indian National Science Academy and several other
institutions of excellence. He encouraged general education and made a
significant contribution to school education by his leadership in bringing out
the very first textbooks of Biology for Higher Secondary Schools published by
NCERT in 1964.
CHAPTER 1
SEXUAL REPRODUCTION IN FLOWERING
PLANTS
1.1 Flower – A Fascinating Organ of
Angiosperms
1.2 Pre-fertilisation : Structures and Events
1.3 Double Fertilisation
1.4 Post-fertilisation: Structures and Events
1.5 Apomixis and Polyembryony
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).
1.1 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 :
Figure 1.1 A diagrammatic representation of
L.S. of a flower
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 earlier classes,
you have read the various parts of a flower. Figure 1.1 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?
1.2 PRE-FERTILISATION: STRUCTURES
AND EVENTS
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.
1.2.1 Stamen, Microsporangium and Pollen
Grain
Figure 1.2 (a) A typical stamen;
(b) three–dimensional cut section of an
anther
Figure 1.2a 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 1.2b). 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 1.3a). 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 1.3b)– 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 release 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 1.3 (a) Transverse section of a young
anther; (b) Enlarged view of one microsporangium showing wall layers; (c) A
mature dehisced anther
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 1.3a). As the anthers
mature and dehydrate, the microspores dissociate from each other and develop
into pollen grains (Figure 1.3 b). Inside each microsporangium several
thousands of microspores or pollen grains are formed that are released with the
dehiscence of anther (Figure 1.3c).
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
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 1.4).
Figure 1.5 (a) Enlarged view of a pollen
grain tetrad; (b) stages of a microspore maturing into a pollen grain.
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 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 1.5b). 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 1.6 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 1.6).
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
(-1960C). Such stored pollen can be used as pollen banks, similar to seed
banks, in crop breeding programmes.
1.2.2 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 1.7b)
or may be free (apocarpous) (Figure 1.7c). Each pistil has three parts (Figure
1.7a), 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 1.7 (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
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
1.7d). 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
Figure 1.8 (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
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 1.8a).
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?
Let us study about the formation of the
embryo sac in detail. (Figure 1.8b). 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 1.8b, 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.
1.2.3 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 1.9a). 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
Figure 1.9
(a) Self-pollinated flowers;
(b)Cross pollinated flowers;
(c) Cleistogamous flowers
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 1.9c). 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 1.9b). 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.
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 1.10) and large often-feathery stigma to easily trap
air-borne pollen grains. Windpollinated 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 1.11a); 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 1.11
(a) Pollination by water in Vallisneria;
(b) Insect pollination
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 1.11b). 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 animalpollinated 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 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 selfpollination 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
Figure 1.12 (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
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 1.12a). The contents of the pollen grain
move into the pollen tube. Pollen tube grows through the tissues of the stigma
and style and reaches the ovary (Figure 1.12b, 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 1.12d, 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.
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.
1.3 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 1.13a). 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.
Figure 1.13 (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)]
1.4 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.
1.4.1 Endosperm
Endosperm
development precedes embryo development. Why? The primary endosperm cell
divides repeatedly and forms a triploid 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.
1.4.2 Embryo
Figure 1.14 (a) A typical dicot embryo; (b)
L.S. of an embryo of grass
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 1.13 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 1.14a), 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 1.14 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 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.
1.4.3 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 1.15a) 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. Nonalbuminous 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 1.15a). 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 1.15b). 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
Figure 1.15 (a) Structure of some seeds. (b)
False fruits of apple and strawberry
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 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?
1.5 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
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.
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 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.
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.
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 fertilized ovule?
13. Differentiate between:
(a) hypocotyl and epicotyl;
(b) coleoptile and coleorrhiza;
(c) integument and testa;
(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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