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2. Sexual Reproduction in Flowering Plants

Pre-Fertilisation Events 

  1. Several hormonal and structural changes result in the development of a flower.

  2. Inflorescences bear the flower buds, and then the flowers.

  3. Flowers are the reproductive parts of a plant.

  4. In the flowers, the androecium (male reproductive part) and the gynoecium (female reproductive part) develop.

Androecium

  1. The androecium consists of whorls of stamen.

  2. The stamen consists of the filament (long and slender stalk) and anther (bilobed structure).

  3. Filament is attached to the thalamus or to the petal.

Anther:

  1. A typical anther is bilobed and each lobe is dithecous (consists of two theca).

  2. Theca are separated by a longitudinal groove running lengthwise.

  3. The microsporangia are located at the corners, two in each theca. They further develop to form pollen sacs, which contain the pollen grains.

Structure of microsporangium

  1. The microsporangium is surrounded by four wall layers (epidermis, endothecium, middle layers, and tapetum).

  2. The outer three layers are protective and help in dehiscence of anther to release the pollen grains. The tapetum provides nourishment to the developing pollen grains.

  3. In the young anther, the sporogenous tissue forms the centre of each microsporangium.

Microsporogenesis

  1. It is the process of formation of microspore from PMC (Pollen Mother Cells).

  2. As development occurs in the anther, the sporogenous tissue undergoes meiosis to form microspore tetrad.

  3. Each cell of sporogenous tissue has capacity to give rise to a tetrad. Hence, each cell is a potential pollen or PMC.

  4. As the anther matures, the microspores get detached from each other and develop into pollen grains.

Pollen grains
  1. Represent the male gamete and are spherical, having a two-layered wall:

  2. Exine (outer) − Hard layer made of sporopollenin, which is extremely resistant and can withstand high temperatures, acidic and alkaline conditions, and enzymes

  3. Intine (inner) − Thin and continuous layer made up of cellulose and pectin

  4. Mature pollen grain contains two cells:

    1. Vegetative cell − Large with irregular nucleus, contains food reserves

    2. Generative cell − Small and floats in the cytoplasm of the vegetative cell

    3. In 60% of the angiosperms, pollen grains are shed at 2-celled stage while in others generative cell undergoes mitosis to form two male gametes (3-celled stage).

    4. The viability of pollen grains after they are shed depends upon temperature and humidity. It ranges from 30 minutes to few months.

Gynoecium and Formation of Female Gametophyte 

  1. The gynoecium represents the female reproductive part of a flower.

  2. It may be mono-carpellary (one pistil) or multi-carpellary (many pistils). In multi-carpellary, the pistils may be fused in one (syncarpous) or free (apocarpous).

Each pistil consists of:

  1. Stigma − Receives the pollen grains

  2. Style − Elongated, slender part below the stigma

  3. Ovary − Bulged basal part containing the placenta, which is located inside the ovarian locule (cavity)

  4. The placenta contains the megasporangia or ovules.

Megasporangium

  1. The ovule is attached to the placenta by the funicle. The junction of the ovule and the funicle is called hilum.

  2. Each ovule has one or two protective layers, called integuments, which cover the rest of the ovule, except for a small opening called micropyle.

  3. The chalaza lying on the opposite side of the micropyle end represents the basal part of the ovule.

  4. Nucellus is present within the integuments and contains reserved food. The embryo sac or female gametophyte is located within the nucellus.

Megasporogenesis 

  1. The megaspore mother cell (MMC) gets converted into megaspores by the process of megasporogenesis.

  2. The MMC is large and contains a dense cytoplasm and a prominent nucleus. It undergoes meiosis to produce four megaspores.

Female Gametophyte 

  1. In most flowering plants, only one megaspore is functional while the other three degenerate.

  2. The single functional megaspore develops into the female gametophyte. This kind of development is called monosporic development.

  3. The nucleus of the functional megaspore divides mitotically to form 2 nuclei, which move towards the opposite ends, forming a 2-nucleate embryo sac. Two more mitotic divisions ensue, leading to the formation of 4-nucleate and 8-nucleate embryo sacs.

  4. After the 8-nucleate stage, the cell walls are laid down and the typical female gametophyte (embryo sac) gets organised.

  5. Six of the 8-nuclei get surrounded by the cell wall and the remaining two, called polar nuclei, are situated below the egg apparatus in the large central cell.

  6. Three of the six cells are placed at the micropylar end and constitute the egg apparatus (2 synergids + 1 egg cell).

  7. The synergids have special thickenings at the micropylar end. These are together called the filiform apparatus. It helps in leading the pollen tubes into the synergids.

  8. Three cells are at the chalazal end, and are called antipodal cells.

  9. A typical angiosperm female gametophyte is 7-celled and 8-nucleated at maturity.

Pollination 

  1. It is the process of transfer of pollen grains from the anther to the stigma.

Depending on the source of pollen, pollination can be divided as follows:

  1. Autogamy − It is the transfer of pollen grains from the anther to the stigma of the same flower. Autogamy requires the anther and the stigma to lie close. It also requires synchrony in the pollen release and stigma receptivity.

    1. Plants like Viola, Oxalis, etc., produce two kinds of flowers--chasmogamous flowers (with exposed anther and stigma) and cleistogamous flowers (which do not open at all and only autogamy occurs).

  2. Geitonogamy − It is the transfer of pollens from the anther of one flower to the stigma of another flower in the same plant. Genetically, it is similar to autogamy, but it requires pollinating agents.

  3. Xenogamy − It is the transfer of pollen grains from the anther to the stigma of a different plant. Pollination causes genetically different types of pollens to be brought to a plant.

Agents of Pollination

  1. Plants use air, water (abiotic agents) and animals (biotic agents) for pollination.

Pollination by wind
  1. It is the most common form of abiotic pollination.

  2. Plants possess well-exposed stamens and large, feathery stigma.

  3. Pollens should be light and non-sticky to be carried easily by winds.

  4. Wind-pollinated flowers often have single ovule in the ovary and numerous flowers packed in an inflorescence.

  5. It is common in grass.

Pollination by water
  1. It is rare in flowering plants, except for some aquatic plants like Vallisneria and Hydrilla.

  2. In most water-pollinated plants, the pollen grains are long and ribbon-like, and are protected from wetting by mucilaginous covering.

  3. In a majority of water plants like water hyacinth and water lily, flowers emerge above the water level and are pollinated by insects.

Pollination by animals
  1. Majority of flowering plants use butterflies, bees, wasps etc., for pollination.

  2. Most of the insect-pollinated flowers are large, colourful, fragrant, and contain nectar to attract the animal pollinators. These are called floral rewards.

  3. Floral reward can be in the form of providing safe places to lay eggs (example: the tallest flower, Amorphophallus)

  4. A symbiotic relationship exists between the plant, Yucca and its pollinator moth. The moth is dependent on the plant since the moth deposits its eggs in the locule of the ovary of the plant, and in return, the plant is pollinated by the moth.

REVISION NOTES

 

CLASS 11 BIOLOGY

UNIT I – DIVERSITY IN THE LIVING WORLD

Chapter 1 – The Living World 

Chapter 2 – Biological Classification 

Chapter 3 – Plant Kingdom 

Chapter 4 – Animal Kingdom 

UNIT II – STRUCTURAL ORGANISATION IN PLANTS AND ANIMALS

Chapter 5 – Morphology of Flowering Plants 

Chapter 6 – Anatomy of Flowering Plants 

Chapter 7 – Structural Organisation in Animals 

UNIT III – CELL : STRUCTURE AND FUNCTIONS

 

Chapter 8 – Cell: The Unit of Life 

Chapter 9 – Bio-Molecules 

Chapter 10 – Cell Cycle and Cell Division 

UNIT IV – PLANT PHYSIOLOGY 

Chapter 11 – Transport in Plants 

Chapter 12 – Mineral Nutrition 

Chapter 13 – Photosynthesis in higher plants 

Chapter 14 – Respiration in Plants 

Chapter 15 – Plant Growth and Development 

UNIT V – HUMAN PHYSIOLOGY 

Chapter 16 – Digestion And Absorption 

Chapter 17 – Breathing and Exchange of Gases 

Chapter 18 – Body fluids and circulation 

Chapter 19 – Excretory Products and their Elimination 

Chapter 20 – Locomotion and Movement 

Chapter 21 – Neural Control and Coordination 

Chapter 22 – Chemical Coordination and Integration 

CLASS 12 BIOLOGY

Unit-VI Reproduction
  

Chapter 1 : Reproduction in Organisms 

Chapter 2 : Sexual Reproduction in Flowering Plants 

Chapter 3 : Human Reproduction 

Chapter 4 : Reproductive Health 

Unit-VII Genetics and Evolution

Chapter 5 : Principles of Inheritance and Variation 

Chapter 6 : Molecular Basis of Inheritance 

Chapter 7 : Evolution 

Unit-VIII Biology and Human Welfare

Chapter 8 : Human Health and Disease 

Chapter 9 : Strategies for Enhancement in Food Production 

Chapter 10 : Microbes in Human Welfare 

Unit-IX Biotechnology  

Chapter 11 : Biotechnology Principles and Processes 

Chapter 12 : Biotechnology and its Applications 

Unit-X Ecology and Environment 

Chapter 13 : Organisms and Populations 

Chapter 14 : Ecosystem 

Chapter 15 : Biodiversity and Conservation 

Chapter 16 : Environmental Issues 

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