Sexual reproduction of 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.

  
• Flowers of ornamental value that are commonly cultivated at homes and in gardens

  • List at least five
• Flowers used in social and cultural celebrations in your family


  • Find out the names of five more
• Floriculture


  What does it refer to?
• To a biologist, flowers are morphological and embryological marvels and the sites of sexual reproduction.
• Stamen


  The male reproductive organ in a flower
• Anther


  The terminal generally bilobed structure of a stamen
• Anther


  • Bilobed with each lobe having two theca (dithecous)
  • A four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe
• Microsporangium


  Develops into pollen sacs packed with pollen grains
• Microsporangium


  • Surrounded by four wall layers - epidermis, endothecium, middle layers and the tapetum
  • Tapetum nourishes the developing pollen grains
• Microsporogenesis


  1. Cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads

  2. Microspores develop into pollen grains
• Pollen grain


  Represents the male gametophyte
• Pollen grain


  • Has a prominent two-layered wall - the hard outer exine made of sporopollenin and the inner intine made of cellulose and pectin
  • Contains a vegetative cell and a generative cell
• Pollen grains of many species cause severe allergies and bronchial afflictions in some people often leading to chronic respiratory disorders– asthma, bronchitis, etc.
• Pollen grains can be stored in liquid nitrogen (-1960C) for years and used as pollen banks in crop breeding programmes.
• Gynoecium


  The female reproductive part of the flower
• Pistil


  • Has three parts - stigma, style and ovary
  • Inside the ovary is the ovarian cavity with the placenta, from which the ovules arise
• Ovule


  The megasporangium
• Ovule


  • Has one or two protective integuments encircling the nucellus, except at the micropyle
  • Contains the embryo sac or female gametophyte
• Megasporogenesis


  The process of formation of megaspores from the megaspore mother cell
• Megaspore mother cell


  Undergoes meiosis to produce four megaspores
• Female gametophyte


  Develops from one of the megaspores
• Megasporogenesis


  The process where the megaspore mother cell is formed
• Ovules


  • Generally differentiate a single megaspore mother cell (MMC) in the micropylar region
• Parts of a flower


  • Stigma
  • Style
  • Ovary
  • Thalamus

  
• Dissected flower of Hibiscus


  • Showing pistil (other floral parts have been removed)
• Megaspore mother cell (MMC)


  • A large cell containing dense cytoplasm and a prominent nucleus
• Meiosis of the MMC


  Results in the production of four megaspores
• Monosporic development


  The method of embryo sac formation from a single megaspore
• Formation of the embryo sac


  1. Nucleus of the functional megaspore divides mitotically to form 2-nucleate, 4-nucleate and 8-nucleate stages

  2. Cell walls are laid down after the 8-nucleate stage to organise the typical female gametophyte or embryo sac
• Cells in the mature embryo sac


  • Egg apparatus (2 synergids and 1 egg cell)
  • 3 antipodal cells
  • Large central cell with 2 polar nuclei
• Pollination


  Transfer of pollen grains (shed from the anther) to the stigma of a pistil
• Types of pollination


  • Autogamy (within the same flower)
  • Geitonogamy (between flowers of the same plant)
  • Xenogamy (between flowers of different plants)
• Autogamy


  • Requires synchrony in pollen release and stigma receptivity, and close proximity of anthers and stigma
• Cleistogamous flowers


  Flowers that do not open at all, allowing autogamy
• Cleistogamous flowers produce assured seed-set even in the absence of pollinators