Reproduction in flowering plants: Revision notes from studypill,helps you to prepare neet,board,afmc examination,as quick reference as described below
All flowering plants show sexual reproduction.
Alook 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.
2.1 FLOWER — A FASCINATING ORGAN OF
ANGIOSPERMS
To a biologist, flowers are morphological and embryological marvels
and the sites of sexual reproduction.
2.2 PRE-FERTILISATION: STRUCTURES AND EVENTS
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. the androecium consists of a whorl of stamens representing the male reproductive organ and the gynoecium represents the female reproductive organ.
2.2.1 Stamen, Microsporangium and Pollen Grain
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.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. Often a longitudinal groove runs lengthwise separating the theca. 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 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.
‘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.
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. As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.Inside each microsporangium several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther.
Pollen grain: The pollen grains represent the male gametophytes. sizes, shapes. colours, designs – seen on the pollen grains from different species.
Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has a prominent two-layered wall. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids. and alkali. No enzyme that degrades sporopollenin-is so far
known. Pollen grain exine has prominent apertures called germ
pores where sporopollenin is absent. Pollen grains are well-preserved as fossils because of the presence of sporepollenin.The exine exhibits a fascinating array of patternsand designs.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.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. Itis 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 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
When once they are shed. pollen grains have to land on the stigma
before they lose viability if they have to bring about fertilisation.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.artificial insemination,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) or may be free (apocarpous) . Each pistil has three parts 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. 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) :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.Meiosis results in the production of four megaspores.
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.
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. 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 importantrole 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 flowering plants are produced in the pollen grain and embryo sac,respectively. As both types of gametes are non-motile,
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.
Kinds of Pollination : Depending on the source of pollen, pollinationcan 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. 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 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 {lowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers which
do not open at all. 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.
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.
(i) Xenogamy – Transfer of pollen grains from anther to the stigma of a different plant. 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 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
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. 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 current; 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.
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. 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.
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 (o 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.
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 Yiwea 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.
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. 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.
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. 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.
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
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). 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.
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. 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.
Endosperm may ecither 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.
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 émbryo development (embryogeny) are similar in both monocotyledons and
dicotyledons.The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.
A typical dicotyledonous embryo. consists of an embryonal axis and two cotyledons. The portion of embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl that terminates at its lower end in the radicle or root tip. The root tip is covered with a root cap.
Embryos of monocotyledons 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 vartous
parts of the embryo and the seed.
2,4.3 Seed
In angiosperms, the seed is the final product of sexual reproduction. Itis
often described as a fertilised ovule. Seeds are formed inside fruits, A
seed typically consists of seed coal(s). cotyledon(s) and an embryo axis.
The cotyledons of the embryo are simple structures,generally thick and swollen due to storage of food reserves (as in legumes).Mature sceds 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 asblack 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. 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
simultancously. 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.
In most plants, by the time the fruit develops from the ovary, other
foral parts degenerate and fall ofl. However. in a few species such as apple,
strawberry, cashew. etc.. the thalamus also contributes to fruit formation.
Such fruits are called false fruits. 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
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.
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 10000 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.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.
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.
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. 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.
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.
