REPRODUCTION IN FLOWERING PLANTS
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).
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 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?
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.
Stamen, Microsporangium and Pollen Grain
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?
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).
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
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.
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.
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
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 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.
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.
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 shouldlie 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?
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.
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. 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
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?
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 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
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 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.
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.
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.
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.
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 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 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?
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.
Name the parts of an angiosperm flower in which development of male and female
gametophyte take place.
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.
Arrange the following terms in the correct developmental sequence:
Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male
gametes.
With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
What is meant by monosporic development of female gametophyte?
With a neat diagram explain the 7-celled, 8-nucleate nature of the female
gametophyte.
What are chasmogamous flowers? Can cross-pollination occur in cleistogamous
flowers? Give reasons for your answer.
Mention two strategies evolved to prevent self-pollination in flowers.
What is self-incompatibility? Why does self-pollination not lead to seed formation in
self-incompatible species?
What is bagging technique? How is it useful in a plant breeding programme?
What is triple fusion? Where and how does it take place? Name the nuclei involved
in triple fusion.
Why do you think the zygote is dormant for sometime in a fertilised ovule?
Differentiate between:
hypocotyl and epicotyl;
coleoptile and coleorrhiza;
integument and testa;
perisperm and pericarp.
Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
What is meant by emasculation? When and why does a plant breeder employ this
technique?
If one can induce parthenocarpy through the application of growth substances,
which fruits would you select to induce parthenocarpy and why?
Explain the role of tapetum in the formation of pollen-grain wall.
What is apomixis and what is its importance?
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