Pre fertilization: structure and events:
- Hormonal and structural changes in plants leads to development of flower,
- Androecium consists of a whorl of stamens represents male sex organ.
- Gynoecium represents the female reproductive organ.
Stamen, Microsporangium and Pollen grain:
- Typical stamen consists of two parts, long and slender stalk called filament and terminal bilobed structure called anther.
- Atypical angiosperm anther is bilobed.
- Each lobe have two theca i.e. dithecous.
- Each anther contains four microsporangia located at the corners, two in each lobe.
- Microsporangia become pollen sacs and are packed with pollen grains.
Structure of microsporangium:
- Each microsporangium surrounded by four wall layers
- Epidermis
- Endothecium
- Middle layer.
- Tapetum.
- The innermost layer is tapetum which is multinucleated, with dense cytoplasm; it nourishes the developing pollen grain.
- The centers of each microsporangium contain homogenous cells called sporogenous tissues.
Microsporogenesis:
- The process of formation of microspores from pollen mother cell through meiosis is called microsporogenesis.
- The sporogenous tissue of microsporangium differentiated into microspore mother cell or pollen mother cell.
- Each microspore mother cell undergoes meiosis and gives rise to haploid microspore tetrad.
- On dehydration microspore tetrad dissociated to form four microspores.
- Each microspore developed into a pollen grain.
Pollen grain:
- Pollen grain represents the male gametophytes.
- It is spherical and measuring about 25-50 micrometer in diameter.
- It is covered by two layers.
- The hard outer layer called the exine is made up of sporopollenin, which is one of the most resistant organic materials known. It can withstand high temperature and strong acids and alkali. No enzyme can degrades sporopollenin is so far known.
- The exine has prominent apertures called germ pore where sporopollenin is absent.
- The inner wall of pollen grain is called intine. It is thin and continuous layer made of cellulose and pectin.
- On maturity the pollen grain 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 vegetative cell.
- In 60% of angiosperms, pollen grains are shed at this 2-celled stage.
- In others the generative cell divides mitotically to form two male gametes before pollen grain are shed (3-celled stage).
Economic importance of pollen grain:
- Pollen grain may cause severe allergies and bronchial afflictions.
- It may cause chronic respiratory disorders – asthma, bronchitis, etc.
- Pollen grain of Parthenium or carrot grass causes pollen allergy.
- Pollen grains are rich in nutrients hence used as pollen tablets for food supplements.
- Pollen consumptions increase performance of athletes and race horses.
- After shedding the viability depends on temperature and humidity.
- In wheat and rice the pollen grain lose viability within 30 min. of their release.
- In Rosaceae, Leguminoseae and Solanaceae they remain viable for months.
- Pollen grain can be preserved for years in liquid nitrogen (-196oC).
The Pistil, Megasporangium (ovule) and Embryo Sac:
- The Gynoecium represents the female reproductive part of the flower.
- The Gynoecium may contain single pistil (monocarpellary) or may have more than one pistil (multicarpellary).
- Fused pistils are called syncarpous and free pistils are called apocarpous.
- Each pistil has three parts the stigma, style and ovary.
- Inside the ovary is the ovarian cavity (locule).
- The placenta located inside the ovarian cavity.
- Megasporongia or ovules arise from the placenta.
- The number of ovule inside the ovary may be single or many.
The Megasporangium (Ovule):
- Ovule is a small structure attached to the placenta of locule with a stalk called funicle.
- The body of the ovule fused with the funicle in the region called hilum.
- Hilum is the junction between the funicle and ovule.
- Each ovule has one or two protective envelops called integuments.
- Integument covered the ovule except an opening at the top called micropyle.
- Opposite of the micropylar end, is the chalaza, representing the basal part of the ovule?
Megasporogenesis:
- The process of formation of megaspores from the megaspore mother cell is called Megasporogenesis.
- In the centre of the ovule there is a mass of tissue called nucellus.
- Cells of nucellus have abundant reserve food materials.
- One cell of the nucellus towards micropylar end differentiated into megaspore mother cell (MMC).
- It is a large diploid cell, dense cytoplasm with prominent nucleus.
- The MMC undergo meiotic division resulting four haploid megaspores.
