Sexual Reproduction In Flowering Plants

• Biology is the study of life on Earth
• Species continue to live through millions of years unless threatened by extinction
• Reproduction is vital for species survival
• Individuals leave progeny through asexual or sexual means
• Sexual reproduction creates new variants, enhancing survival advantage
• This unit explains reproductive processes in flowering plants and humans as representative examples
• Human reproductive health and how to avoid reproductive ill health are also discussed
• Panchanan Maheshwari was a distinguished botanist who made significant contributions to embryology and tissue culture
• He popularized the use of embryological characters in taxonomy
• Maheshwari's work on test tube fertilization and intra-ovarian pollination gained worldwide acclaim
• Flowers play a crucial role in sexual reproduction in plants
• Flowers have aesthetic, ornamental, social, religious, and cultural value
• Flowers are morphological and embryological marvels and sites of sexual reproduction
• The pre-fertilization stage involves hormonal and structural changes leading to the development of floral primordium
• The androecium and gynoecium differentiate and develop in the flower as male and female reproductive structures
• The stamen consists of a filament and an anther, which produces pollen grains
• Microsporangia develop into pollen sacs packed with pollen grains
• Microspores are formed from pollen mother cells through meiosis in microsporogenesis
• Pollen grains represent male gametophytes and have a two-layered wall with a hard outer layer called exine
• The exine is made of sporopollenin, a resistant organic material
• Pollen grains are well-preserved as fossils due to sporopollenin
• Pollen grains of many species cause severe allergies and bronchial afflictions in some people, leading to chronic respiratory disorders like asthma and bronchitis
• Pollen grains must land on the stigma before losing viability to bring about fertilization
• The viability period of pollen grains varies depending on temperature and humidity:

  • Some cereals like rice and wheat lose viability within 30 minutes
  • Some members of Rosaceae, Leguminoseae, and Solanaceae maintain viability for months
• Stored pollen grains can be kept for years in liquid nitrogen (-196°C) for use in pollen banks, similar to seed banks, in crop breeding programs
• The gynoecium in flowering plants represents the female reproductive part of the flower
• The pistil may consist of a single (monocarpellary) or multiple (multicarpellary) pistils
• Each pistil has three parts:

  • Stigma: landing platform for pollen grains
  • Style: elongated slender part beneath the stigma
  • Ovary: basal bulged part of the pistil
• The ovary contains the ovarian cavity (locule) where the placenta is located
• The ovule is attached to the placenta by a stalk called funicle, with the body fusing with the funicle at the hilum
• Each ovule has protective envelopes called integuments, with a small opening called the micropyle at the tip
• The nucellus inside the integuments contains the embryo sac or female gametophyte, formed from a megaspore
• Megasporogenesis is the process of forming megaspores from the megaspore mother cell in the ovule
• One of the megaspores becomes functional, developing into the female gametophyte (embryo sac) through monosporic development
• The functional megaspore nucleus divides mitotically to form the 2-nucleate embryo sac, which further develops into the 4-nucleate and 8-nucleate stages
• After the 8-nucleate stage, cell walls are laid down, leading to the organization of the typical female gametophyte or embryo sac
• The mature embryo sac is 7-celled, with specific cell distributions including the egg apparatus, synergids, antipodals, and polar nuclei
• Pollination is the transfer of pollen grains from the anther to the stigma of a pistil
• There are three types of pollination based on the source of pollen:

  • Autogamy: within the same flower
  • Geitonogamy: to another flower of the same plant
  • Xenogamy: to a different plant
• Plants use abiotic (wind and water) and biotic (animals) agents for pollination, with most plants relying on biotic agents