Sexual Reproduction In Flowering Plants
Morphology of Flower
- Flowers are the primary reproductive organs of the plants. They are attractive, colourful, often fragrant and nectar producing structures of flowering plants.
- The flower is commonly borne on short or long stalk called the pedicel. Above the pedicel lies the thalamus (receptacle), swollen part of the stem. It holds the whorls of a flower.
Parts of a Flower A typical angiospermic flower consists of four whorls attached on the receptacle.
- Calyx: It is the first whorl of the flower; consists of sepals. It is a leaf-shaped structure often green in colour. Its function is to provide support and protection to the flower bud.
- Corolla: The second whorl of the flower and consists of petals. The petals are the colourful and sometimes scented parts of the flower to which pollinators are attracted to.
- Androecium: Also referred to as the male reproductive part of the flower, it consists of stamens. The stamen consists of an anther and a filament. Anther is the pollen-producing part of the plant.
- Gynoecium: It is the female reproductive organ. It occupies the central position on the receptacle. It is made up of stigmas, style and ovary; collectively called carpel.
- Stigma – It is the sticky end of the style which receives the pollen grains.
- Style – It is a thin tube-like structure that holds the stigma and is attached to the ovary at its base.
- Ovary – It is the swollen basal portion of the flower which contains the ovules and ovule contains the female gamete.
- In some flowers, like those of family Liliaceae, the calyx and corolla are fused. Such a structure is called the perianth.
- Clusters or bunches of flowers arranged on a stem form the inflorescence.
Microsporangium and Pollen Grain Stamens
- Stamen is the male reproductive unit of an angiosperm plant. It consists of a long and slender stalk called filament and at the tip of filament, a bilobed structure called anther is found.
- Anther is bilobed, with each lobe having two coverings called theca. Hence, called dithecous. Two lobes are joined together by a tissue called connective.
- The anther is a tetragonal structure. Four microsporangia are located at the four corners of an anther, two in each lobe.
The microsporangia develop further into pollen sacs.
- Development of the anther starts from the homogenous meristematic cells surrounded by an epidermis.
- Four lobes and four layers of archesporial cells are formed.
- Archesporial cells are cells that divide to give rise to cells from which spores are developed.
- Each archesporial cell divides to form primary parietal cell and primary sporogenous cell.
- The division of parietal cells forms the anther walls while that of sporogenous cells forms the pollen mother cells (PMC).
Structure of Microsporangium A typical microsporangium is surrounded by four layers. These are as follows:
- Epidermis: Single layer of cells; perform the function of protection.
- Endothecium: Single layer of cells; helps in dehiscence of anther.
- Middle layer: Multiple layers; degenerates at the time of maturity of anther.
- Tapetum: Innermost layer of cell wall, lies close to PMCs. It surrounds the sporogenous tissue and nourishes the developing pollen grains. Ubisch bodies present in the cells of tapetum, help in the development of pollen grains.
- ‘The pollen mother cell (PMC) through meiosis gives rise to microspores. The process is called microsporogenesis‘.
- Each cell of sporogenous tissue undergoes meiotic divisions (I and II) to give rise to four cells.
- Each cell is a potential pollen or a microspore.
- Microspores are arranged in a group of four cells called the microspore tetrad.
- As anthers dehydrate on maturity, the microspores dissociate from each other and develop into pollen grains.
- Pollen grains are released with the dehiscence of anthers.
Structure of a Pollen Grain
- Pollen grains vary in shapes, sizes, colour and designs.
- A pollen grain is haploid.
- It has a covering of two layers; outer exine and inner intine.
- Exine is thick and made up of sporopollenin, which is a material resistant to biological and chemical decomposition.
- The exine is not continuous; there are prominent apertures where the exine is absent. The apertures are called germ pores.
- The germ pore helps in the formation of the pollen tube and the release of the male gametes during fertilisation.
- Intine, the inner layer, is a thin and continuous layer made up of cellulose and pectin.
- A mature pollen grain generally has two cells, namely, a vegetative cell and a generative cell.
- Vegetative cell is comparatively bigger and has a food reserve.
- Generative cell is small in size and it floats in the cytoplasm of vegetative cell.
- Around 60% of the pollen grains are shed in 2-celled stage while the remaining are shed at 3-celled stage.
- In the 3-celled stage, the generative cell divides mitotically and form two male gametes.
- Palynology is the branch of study of pollens.
- Pollen grains are rich in nutrients. Hence, used as food supplements.
- The period for which pollen remains active and functional is called pollen viability.
- Pollen viability varies.
- Pollen viability depends on humidity and temperature.
- Pollen grains can be preserved in liquid nitrogen (−1960C).
Megasporangium and Embryo SacPistil/s
- Gynoecium consists of pistil/s. It may have a single pistil (monocarpellary) or more than one carpel (multicarpellary).
- Pistils can be fused (syncarpous) or free (apocarpous).
- A pistil comprises three parts, namely, stigma, style and ovary.
- Placenta is present within the ovarian cavity.
- Megasporangia or ovules are present within the placenta.
Structure of a Typical Angiosperm Ovule
- Stalk that connects the ovule to the placenta is called funicle.
- Hilum is the point of attachment of funicle and ovule.
- Each ovule has two protective coverings called integuments.
- Integuments encircle the nucellus except at an opening in the tip, this opening is called micropyle and this end of the ovule is called micropylar end.
- Chalaza represents the basal part of the ovule, opposite the micropylar end.
- The nucellus is mass of cells that forms the part of the inner structure of the ovule, it contains embryo sac.
- Each ovule generally has an embryo sac formed from a megaspore.
- ‘The process of formation of megaspores from the megaspore mother cell is called megasporogenesis‘.
- Ovules form a single megaspore mother cell (MMC) in the micropylar region of the nucellus.
- The MMC divides meiotically to produce four megaspores.
- Only one out of the four megaspores is functional; the other three degenerate.
- Functional megaspore develops into embryo sac.
- Monosporic development is the development of embryo sac from single megaspore.
Formation of Embryo Sac
- The nucleus of functional megaspore undergoes mitosis to form two nuclei. These nuclei move to the opposite side thus forming 2-nucleate embryo sac.
- These nuclei further divide mitotically to produce 4-nucleate embryo sac and again all four divide mitotically to produce 8-nucleate embryo sac.
- Nuclear divisions are not followed by immediate cell division.
- On formation of 8 nuclei, cell walls are laid down forming the typical female gametophyte or embryo sac.
Distribution of Cells inside the Embryo Sac
- Six nuclei surrounded by cell walls form six cells; remaining two nuclei which are called polar nuclei, are situated in the large central cell beneath the egg apparatus.
- Three cells are grouped together in the micropylar end forming the egg apparatus.
- The egg apparatus has two synergid cells and one egg cell.
- The synergids have a filiform apparatus which guides the pollen tube to the synergids.
- Three cells grouped together at the chalazal end, form the antipodals.
- A typical angiospermic embryo sac at maturity is 8-nucleate and 7-celled.
Pollination It is the transfer of pollen grains from the anther of one flower to the stigma of the same or another flower on the same plant or another plant.
Types of Pollination Pollination is of two types:self-pollination and cross pollinationI. Self-pollination It is the transfer of pollen grains from the anther of a flower to the same flower or to another flower but some plants. Self-pollination is further divided into types: 1. Autogamy – In this type of self-pollination, the pollen is transferred from the anthers of one flower to the stigma of the same flower. It is favoured by two adaptations:
- Chasmogamy – Flowers expose their anther and stigma to pollinating agent, eg. Oxalis ,Viola
- Cleistogamy – Flowers do not open, remain close leading to self-pollination. Eg, Oxalis ,Viola
2. Geitonogamy – In this type of self-pollination, the anthers are transferred from the anthers of one flower to the stigma of another flower but on the same plant.
Advantages of Self-pollination
- Purity of the race is maintained.
- Less amount of pollen grains are produced.
- Success rate of pollination is high.
- Self-pollination ensures that recessive characters are eliminated.
Disadvantages of Self-pollination
- No new characters or features are introduced into the lineage of the offspring.
- Reduce the vigour and vitality of the race as there are no new features introduced.
II. Cross pollination (Xenogamy) It is the transfer of pollen grains from the anthers of one flower to the stigma of another flower on a different plant.
Advantages of Cross-pollination
- Introduces new genes into the lineage.
- Improves the resistance of the offspring to diseases and changes in the environment.
- Seeds are good in vigour and vitality.
- Only method by which unisexual flowers can reproduce.
Disadvantages of Cross-pollination
- Wastage of pollen grains.
- There are high chances that the good qualities may get eliminated and unwanted characteristics may get added due to recombination of the genes.
Agents of Pollination For transfer of pollen grains, different agents are utilised. These could be – Abiotic (like water and wind) or Biotic (animals). Different Types of Pollination based on the Pollinating AgentsAnemophily – Flowers are pollinated by the agency of wind.
- Flowers are small and inconspicuous.
- The pollen grains are very light, non-sticky and sometimes winged.
- Stamens are well exposed and large, often with feathery stigma.
- Example: Grasses
Note: Pollen in air causes allergic reaction in some human beings leading to hay fever.
Hydrophily – Flowers are pollinated by means of water.
- Flowers do not have any fragrance or too much colour on their petals.
- Pollen grains are protected from wetting by a mucilaginous covering.
- The pollen is adapted to be able to float in water.
- Example: Hydrilla
Entomophily – Flowers are pollinated by insects.
- Flowers have brightly coloured petals, scented and most of them produce nectar.
- Pollen grains and stigma are sticky.
- Example: Hibiscus
- Ornithophily – Flowers are pollinated by birds.
- Chiropterophily – Flowers are pollinated by bats.
- Malacophily – Flowers are pollinated by snails.
What is better: Self-pollination or Cross-pollination?
- Self-pollination is also called as inbreeding whereas cross-pollination is known as outbreeding.
- Self-pollination does not lead to genetic variation which is required for healthy, productive and better offspring.
- Cross-pollination produces the essential genetic variations.
Thus, cross-pollination is favoured over self-pollination. Outbreeding Devices: To ensure cross-pollination, hermaphrodite flowers show certain adaptations.
- Unisexuality/Dicliny: Plants produce either male or female flowers; no hermaphrodite flowers.
- Dichogamy: The stigma and anthers mature at different times.
- Protandry: In this type, the androecium matures earlier than the gynoecium.
- Protogyny: In this type, the gynoecium matures earlier than the androecium.
- Herkogamy: A natural physical barrier that prevents the pollen of the same flower from entering the ovary.
- Self-sterility: In this condition, there is a gene that recognizes the similar gene and does not allow the pollen grain to germinate.
- Heterostyly: The stigma and the anthers are at different heights in a flower.
- Pollen prepotency: The pollen of a different flower germinates faster than of the same flower thus preventing autogamy.
Artificial Hybridization It is brought by plant breeders by controlling the pollination to produce desired variety.
- Emasculation and bagging are the two steps of artificial hybridization.
- Emasculation is removal of anthers or stamens from the flower bud before the anther dehisces.
- The emasculated flower is covered with a bag to prevent pollination with unwanted pollen; this process is called bagging.
- When the stigma of the bagged flower attains receptivity, then mature pollen grains of desired variety are dusted on the stigma and the flower is rebagged.
Pistil- Pollen Interaction and Double FertilizationPistil-pollen Interaction
- Pollen lands on the stigma, pistil recognizes the compatible pollen and proceeds with the post-pollination events.
- Proteins present on stigma and pollen grain, determine the compatibility and start the process of germination of pollen grain.
- Pollen tube is produced through one of the germ pores.
- The contents of the pollen grain pass into the pollen tube.
- Pollen tube reaches the ovary through style and stigma.
- While the pollen tube grows, the generative cell divides mitotically to produce two sperm nuclei. This is in the case when pollen is shed at the 2-celled stage.
- If pollen was shed at the 3-staged cell, the pollen grain already has two male gametes when it reaches the stigma.
- Pollen tube on reaching the ovary enters the ovule through the micropyle.It then enters the cytoplasm of one of the synergids.
- The Filiform apparatus facilitates the entry of the pollen tube into synergid cells.
- The pollen tube releases male gametes into one of the synergids.
- One of the gametes moves towards the egg cell and fuses with its nucleus to form zygote (2n); the process is called syngamy.
- Another male gamete fuses with the two polar nuclei present in the central cell and forms the triploid endosperm nucleus (PEN). This process is called triple fusion (n + 2n = 3n)
This fertilization in plants is termed as double fertilization because two fusions are taking place, namely, syngamy and triple fusion.
Endosperm and Embryo Development
- Endosperm and embryo development, maturation of ovule(s) into seed(s) and ovary into fruit are the post-fertilization events in an angiosperm.
- Primary endosperm cells divide to form triploid endosperm tissue.
Development of Endosperm
- Endosperm provides nutrition to the developing embryo.
- Three types of endosperm can be formed: nuclear endosperm, cellular endosperm and helobial endosperm.
- In nuclear endosperm formation, free nuclear divisions take place without cell wall formation.
- In cellular endosperm formation, cell wall formation follows each cell division.
- Process of helobial endosperm formation is intermediate between the nuclear and cellular endosperm formation.
- Nuclear endosperm is the most common.
- Primary endosperm nucleus forms a large number of free nuclei by repeated mitosis.
- A central vacuole is formed and it pushes the multinucleate cytoplasm to the periphery.
- Later, cell walls are formed and central vacuole disappears.
- Such type of endosperm is found in cotton, maize, rice, etc.
- In tender coconut, liquid water is free-nuclear endosperm and solid part is cellular endosperm.
Development of Embryo
- Development of the embryo from zygote is called embryogeny.
- Development of embryo occurs at the micropylar end.
- Embryo development occurs after the formation of some amount of endosperm so that it can provide nutrition to the growing zygote.
- There is a difference in the development of embryo in dicot and monocot plants.
1. Development of Dicot Embryo
- Zygote divides into a large suspensor cell and a small embryo cell.
- The suspensor cell undergoes division forming a 6-10 celled suspensor.
- The last cell of the suspensor is called hypophysis which develops into a radicle.
- The embryo cell divides twice to form a two-tired, eight-celled embryo.
- Zygote gives rise to a globular embryo first and then heart-shaped and then finally a mature embryo is formed. A typical dicot embryo consists of one embryonal axis and two cotyledons.
- The part of the embryonal axis above the level of cotyledons is the epicotyl, which ends with the plumule or stem tip. Plumule gives rise to future shoot.
- Beneath the level of cotyledons is hypocotyl that ends in the radicle or root tip. Radicle give rise to futureroot.
- There is a root cap present over the root tip part.
2. Development of Monocot Embryo
- Monocot seeds have one cotyledon, called scutellum, as in the grass family.
- At the lower end, the embryonal axis has radicle and root cap enclosed in a sheath called coleorhiza.
- Epicotyl is the part of the embryonal axis above the level of attachment of scutellum.
- Coleoptile encircles the shoot apex and first leaf.
- In angiosperm, fertilized ovules change into seeds which remain enclosed within the fruit.
- A seed consists of seed coat(s), cotyledon(s) and an embryo axis.
- A mature seed can be of two types:
- Non-Albuminous or Non-endospermic: Food stored in endosperm is completely used by the developing embryo. Example – gram, pea, groundnut.
- Albuminous or Endospermic: Food stored in endosperm is not completely used up by the developing embryo. Example – wheat, maize, barley.
- Some seeds have persistent nucellus called perisperm.
- The ovary develops into fruit. Such fruits are called true fruits.
- Wall of the ovary develops into the wall of fruit which is called pericarp.
- The thalamus part of fruits like strawberry, apple contribute to fruit formation, such fruits are called false fruits.
- Few species develop fruits without fertilization, such fruits are called parthenocarpic fruits. Example – banana.
Apomixis and Polyembryony
- Mechanism of producing seeds without fertilization is called apomixis and seeds are called apomictic.
- It is observed in some species of Asteraceae and grasses.
- Hybrid seeds need to be produced every year to maintain the progeny, so this increases the cost of production.
- In such cases, hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. So, hybrid seeds can be used to grow crops year after year.
Polyembryony refers to the presence of more than one embryo in a seed. It is observed in Citrus fruits.