Sexual reproduction in flowering plants

• All flowering plants show sexual reproduction.
• Several hormonal and structural changes are initiated which lead to the differentiation and further development of the floral primordium.
• Inflorescences are formed which bear the floral buds and then the flowers.
• Male reproductive structure - androecium

  Female reproductive structure - gynoecium
• The male reproductive structures consist of stamens, while the female reproductive structures consist of carpels.
• Two parts of a typical stamen -

  • The long and slender stalk called the filament,
  • The terminal generally bilobed structure called anther.
• The proximal end of the filament is attached to the thalamus or the petal of the flower.
• A typical angiosperm anther is bilobed with each lobe having two theca, i.e; they are dithecous.
• Often a longitudinal groove runs lengthwise separating the theca.
• The anther is a four-sided ( tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.
• The microsporangia develop further and become pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains.
• Structure of microsporangium:

  • In a transverse section, a typical microsporangia appears near circular in outline.
  • Generally surrounded by four wall layers - the 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.
  • The innermost wall layer is the tapetum. It nourishes the developing pollen grains.
  • Cells of the tapetum posses dense cytoplasm and generally have more than one nucleus.
• When the anther is young, a group of compactly arranged homogeneous cells calles sporogenous tissue occupies the centre of each microsporangium.
• As the anther develops , the cells of sporogenous tissue undergoes meiotic divisions to form haploid microspore tetrads.
• The process of formation of microspores from a pollen mother cells (PMO) through meiosis is called microsporogenesis.
• The microspores are arranged in a cluster of four cells - the microspore tetrad.
• As the anther 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 25-50 micrometres in diameter.
• Pollen grains has a prominent two-layered wall.

  • The hard outer layer called the exine 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. No enzyme tha degrades sporopollenin is so far known.
  • Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent.
  • Pollen grain are well-preserved as fossils because of the presence of sporopollenin.
• The inner layer of the pollen wall is called intine. It is a thin and continuous layer made up of cellulose and pectin. Intine is soft and flexible.
• The cytoplasm of pollen grains is surrounded by a plasma membrane.
• When pollen grains mature it contains two cells, the vegetative cell and generative cell.
• • The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus.
  • The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus.
  • In over 60% of angiosperms , pollen grains are shed at this 2-celled stages.
  • In the remaining species, the generative cell divides mitotically to give rise 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 often leading to chronic respiratory disorders - asthma, bronchitis, etc.
• Pollen grains are rich in nutrients. It is used as pollen tablets as food supplements.
• Pollen consumption has been claimed to increase the performance of athletes and race horses.
• In rice and wheat pollen grains lose viability within 30 minutes.

  In some members of Rosaceae, leguminoseae and solanaceae, they maintain viability for months.
• Single pistil - monocarpellary

  More than one pistil - multicarpellary
• When more than one pistils are fused - syncarpous

  When free - apocarpous
• Each pistil has three parts - the stigma, style and ovary.
• Stigma serves as the landing platform for pollen grains.
• The style is the elongated slender part beneath the stigma.
• The basal bulged part of the pistil is the ovary.
• Inside the ovary is the ovarian cavity (locule).

  The placenta is located inside the ovarian cavity.
• Arising from the placenta are the megasporangia, commonly called ovules.

  The number of ovule in an ovary may be one (wheat, paddy, mango) to many (papaya, watermelon,orchids).
• The ovule is a small structure attached to the placenta by means of a stalk calles funicle.
• The body of the ovule fusses with funicle in the region called hilum.
• Hilum represents the junction between ovule and funicle.
• Each ovule has one or two protective envelopes called integuments.