Study Notes on Sexual reproduction in flowering plants

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  • All flowering plants show sexual reproduction.
  • The diversity of structures of the inflorescences, flowers and floral parts shows a range of adaptions.
  • This ensures formation of end products of sexual reproduction, the fruits and seeds.
  • Several hormonal and structural changes are initiated which lead to the differentiation amd further development of floral primordium.
  • Inflorescences bear the floral buds and then the flowers.
  • In the flower the male and the female reproductive structures, androecium and the gynoecium differentiate and develop.
  • Androecium consists of a whorl of stamens representing the male reproductive organ.
  • Gynoecium represents the female reproductive organ.

STAMEN, MICROSPORANGIUM AND POLLEN GRAIN

  • Stamen has two parts : the long slender stalk called the filament and the terminal bilobed structure called the anther.
  • Proximal end of filament is attached to the thalamus or the petal of the flower.
  • Different species of flowers have different length and number of stamens.
  • Angiosperm anther is bilobed with each lobe having theca .
  • They are dithecous.
  • A longitudinal groove runs lengthwise separating the theca.
  • Anther is a four sided structure consisting of four microsporangia .
  • They are located at the corners, two in each lobe.
  • They develop further and become pollen sacs.
  • They are packed with pollen grains.
  • In transverse section a microsporangium appears circular in outline.
  • It is surrounded by four wall layers.
  • They are : epidermis, endothecium, middle layers and the tapetum.
  • Outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen.
  • Inner most layer nourishes the developing pollen grains.
  • Cells of the tapetum possess dense cytoplasm and have more than one nucleus.
  • A group of compactly arranged homogenous cells are present in a young anther.
  • They are called sporogenous tissue.
  • It occupies the centre of each microsporangium.
  • The process of formation of microspores from pollen mother cell through meiosis is called microsporogenesis.
  • When anther develops , the cells of sporogenous tissue undergo meiotic divisions to form microspore tetrads.
  • Each tetrad is a potential pollen mother cell.
  • As the anthers mature and dehydrate the microspores dissociate from each other and develop into pollen grains.
  • The pollen grains represent the male gametophytes.
  • Pollen grains are generally spherical measuring about 25 to 50 micrometres in diameter.
  • It has a prominent two layered wall.
  • The hard outer layer is called exine.
  • It 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.
  • The exine has prominent apertures called jump force where sporopollenin is absent.
  • The inner wall of pollen grain is called intine.
  • It is a thin and continuous layer made up of cellulose and pectin.
  • The cytoplasm of pollen grains 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. It has abundant food reserve and a large irregularly shaped nucleus.
  • The generative cell is small and floats in the cytoplasm of the vegetative cell.
  • This cell is spindle shaped with dense cytoplasm and a nucleus.
  • In 60% of angiosperms pollen grains are shade at this two celled stage.
  • In the remaining species The generated cells divide mitotically to give rise to 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 leading to chronic respiratory disorders like asthma, bronchitis, etc.
  • Pollen grains are rich in nutrients.
  • A large number of pollen products in the form of tablets and syrups are available in the market.
  • Poland consumption has been claimed to increase the performance of athletes and race horses.
  • To bring about fertilization The pollen grains wants they are shed have to land on the stigma before they loss viability.
  • The viability duration of the polling greens depend upon the prevailing temperature and humidity.

THE PISTIL, MEGASPORANGIUM AND THE EMBRYO SAC

  • The gynoecium may consist of a single pistil a may have more than one pistil.
  • With single pistil it is called monocarpellary.
  • With more than one pistils it is called multicarpellary.
  • In multicarpellary the pistils may be fused together or may be free.
  • Fused pistils are called syncarpous.
  • Free pistils are called apocarpus.
  • Each pistils have three parts : the stigma, style, ovary.
  • Stigma serves as the landing platform for pollen grains.
  • Style is the slender elongated part beneath the stigma.
  • Ovary is the basal bulged part of the pistil.
  • Ovarian cavity is present inside the ovary.
  • Placenta is located inside the ovarian cavity.
  • The megasporangia arise from the placenta.
  • It is also called as ovules.
  • Number of ovules in ovary varies.
  • Ovule is a small structure.
  • It is attached to placenta by means of a stalk called funicle.
  • Ovule fuses with funicle in the region called hilum.
  • Hilum represents the junction between ovule and funicle.
  • Ovule has one or two protective envelope called integuments.
  • This and circle the oval except at the tip where a small opening call the micropyle is organised.
  • Opposite the micropylar end, is the chalaza.
  • It represents the basal part of the ovule
  • A mass of cells called nucellus is enclosed within the integuments.
  • Cells of the nucellus have abundant reserve food materials.
  • The embryo sac is located in the nucellus.
  • An ovule generally has a single embryo sac formed from a megaspore through reduction division.
  • The process of formation of megaspores from the megaspore mother cell is called megasporogenesis.
  • Ovules generally differentiate a single megaspore mother cell in the micropylar region.
  • The MMC undergoes meiotic division.
  • Meiosis results in the production of 4 megaspores.
  • One of the megaspores is functional while the other three degenerates.
  • Only the functional megaspore develops into the female gametophyte.
  • The 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.
  • This forms the 2 – nucleate embryo sac.
  • Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8 nucleate stages of the embryo sac.
  • Nuclear division is not followed by cell wall formation.
  • After 8 nucleate stage, cell walls are laid down leading to 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 the polar nuclei are situated below the egg apparatus in the large central cell.
  • Three cells are grouped together at the micropylar end and constitute the egg apparatus .
  • The egg apparatus consists of two synergids and one egg cell.
  • The synergids have special cellular thickenings at the micropylar tip called filiform apparatus.
  • It plays an important role in guiding the pollen tubes into the synergid.
  • The three cells at the chalazal end are called the antipodals.
  • The large central cell has two polar nuclei.
  • A typical angiosperm embryo sac at maturity though 8 nucleate is 7 celled.

POLLINATION

  • Transfer of pollen grains to the stigma of a pistil is called pollination.
  • Depending on the source of pollen pollination can be divided into three types.
  • Autogamy ,Geitonogamy and Xenogamy.
  • In autogamy pollination is achieved within the same flower.
  • The pollen grains are transferred from the anther to the stigma of the same flower.
  • Complete autogamy is rare.
  • Autogamy requires synchrony in pollen release and stigma receptivity and also the anther and stigma should lie close to each other so that the self pollination can occur.
  • Some plants produce two types of flowers.
  • Chasmogamous and cleistogamous.
  • Chasmogamous flowers have exposed anther and stigma.
  • In cleistogamous flowers the anther and stigma lie close to each other.
  • When the anther dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination.
  • cleistogamous flowers are invariably autogamous  there is no chance of cross-pollen landing on the stigma.
  • In Geitonogamy  transfer of pollen grains from the anther to the stigma of another flower of the same plant occurs.
  • geitonogamy is genetically similar to autogamy since the pollen grains come from the same plant.
  • In  Xenogamy transfer of pollen grains from anther to the stigma of a different plant occurs . This is the only type of 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.
  • 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.
  • Wind pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence
  • For example is the corn cob.
  • Wind-pollination is quite common in grasses.
  • Pollination by water is quite rare in flowering plants
  • Limited to about 30 genera, mostly monocotyledons.
  • Some examples of water pollinated plants are Vallisneria and Hydrilla
  • They grow in fresh water.
  • Not all aquatic plants use water for pollination.
  • In Aquatic plants such as water hyacinth and water lily, the flowers are pollinated by insects or wind.
  • Pollen grains in some water pollinated species are long, ribbon like.
  • They are carried passively inside the water.
  • 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
  • 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.
  • 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.
  • Animals are attracted to flowers by colour / fragrance.
  • The flowers pollinated by flies and beetles secrete foul odours to attract these animals.
  • The body of the animal gets a coating of pollen grains.
  • Which are generally sticky in animal pollinated flowers.
  • In some species floral rewards are in providing safe places to lay eggs.
  • Many insects may consume pollen or the nectar without bringing about pollination.
  • Such floral visitors are referred to as pollen/nectar robbers.

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.
  • 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
  • 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 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 recognize 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 fertilization.
  • 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.
  • Following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores.
  • The contents of the pollen grain move into the pollen tube. Pollen tube grows through the tissues of the stigma and style .
  • Then reaches the ovary.
  • In some plants, pollen grains are shed at two-celled condition (a vegetative cell and a generate cell).
  • In such plants, the generative cell divides 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 .
  • 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.
  • Pollen-pistil interaction is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen.
  • Artificial hybridization 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
  • The other male gamete moves towards the two polar nuclei located in the central cell.
  • Fuses with them to produce a triploid primary endosperm nucleus .
  • 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 .
  • The central cell after triple fusion becomes the primary endosperm cell (PEC).
  • Develops into the endosperm while the zygote develops into an embryo.

 

POST FERTILISATION

  • 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.
  • The primary endosperm cell divides repeatedly and forms a triploidendosperm tissue.
  • The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo.
  • In endosperm development, the primary endosperm nucleus undergoes successive nuclear divisions to give rise to free nuclei.
  • This stage of endosperm development is called free-nuclear endosperm.
  • 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 is the free-nuclear endosperm (made up of thousands of nuclei)
  • 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.

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.
  • The early stages of embryo 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
  • The portion of embryonal axis above the level of cotyledons is the
  • It terminates with the plumule or stem tip.
  • The cylindrical portion below the level of cotyledons is
  • That terminates at its lower end in the radical 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 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

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 consists of seed coat, cotyledon and an embryo axis.
  • The cotyledons of the embryo are simple structures.
  • They are thick and swollen due to storage of food reserves.
  • Mature seeds may be non-albuminous or 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, sunflower).
  • In some seeds such as black pepper and beet, remnants of nucellus are also present.
  • This residual, nucellus is the
  • 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 simultaneously.
  • The wall of the ovary develops into the wall of fruit called pericarp.
  • The fruits may be fleshy or may be dry.
  • Many fruits have evolved mechanisms for dispersal of seeds.
  • In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off.
  • 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.
  • 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.
  • 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 colonize 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 through out the year and also to raise crop in the next season.
  • This period for the seed to be alive after dispersion  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.
  • The oldest is that of a lupine, Lupinus arcticus excavated from Arctic Tundra.

 

APOMIXIS AND POLYEMBRYONY

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 varieties some of the nuclear 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 as polyembryony.

Hybrid seeds are 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.