Pre-Fertilization: Structure and Events
Formation of a flower occurs as a result of a number of sequential changes.
(i) Hormonal and structural changes are initiated.
(ii) This leads to the differentiation and development of floral primordium.
(iii) These floral primordia give rise to the floral buds which take the shape of flowers.
(iv) The male (androecium) and female (gynoecium) reproductive structures differentiates and develops.
Stamen, Microsporangium and Pollen Grain
Stamen: It is the male reproductive unit of angiosperm consisting of anther and filament.
Structure of Anther: Anther is a bi-lobed structure having two theca in each lobe. Therefore it is called dithecous.
The anther is a four sided structure consisting of four microsporangia located at the corners, two in each lobe.
Microsporangia develops into pollen sacs in which pollen grains are produced.
Structure of Microsporangium: A typical microsporangium appears to have a circular outline. It is surrounded by four wall layers. Center of a microsporangium has compactly arranged homogenous cells called sporogenous cells or microsporocytes. A microsporocyte has abundant cytoplasm and a prominent nucleus.
Four layers surrounding the pollen-sac are
1. Epidermis: It is the outermost single layer. The cells of epidermis are generally stretched and flattened and perform the function of protection.
2. Endothecium/ fibrous layer: Inner to epidermis, there is a single layer of radially elongated cells called endothecium. Cells of endothecium develops fibrous thickening(made up of cellulose with a little pectin and lignin). This is mainly responsible for the dehiscence of mature anther.
3. Middle Layer: Three to four layers of thin allied cells situated just below the endothecium are known as middle layers. Cells of this layer are ephemeral and degenerate to provide nourishment to growing microspore mother cells.
4. Tapetum: This is the innermost layer. Cells of tapetum have dense cytoplasm and generally possess more than one nucleus. It nourishes the developing pollen grains.
Microsporogenesis
$\displaystyle \small \bullet$ Each microsporocyte undergoes mitotic division and produces a number of microspore mother cells or pollen mother cells which are diploid.
$\displaystyle \small \bullet$ Later they undergo meiotic division to form haploid microspores.
$\displaystyle \small \bullet$ The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis.
$\displaystyle \small \bullet$ The micropspores when formed are present in a cluster of four cells called the microspore tetrad.
$\displaystyle \small \bullet$ As the anthers mature and dehydrate, each microspore gets separated and forms a thick outer wall called exine and an inner wall called intine.
$\displaystyle \small \bullet$ Each microspore develops into a pollen grain. as the pollen grains mature, the anther splits open along the two lines of dehiscence and the pollen grains are released.
Structure of Pollen grain
$\displaystyle \small \bullet$ The pollen grains represents the male gametophytes.
$\displaystyle \small \bullet$ Pollen grains are generally spherical in structure, about 20-50 mm in diameter.
$\displaystyle \small \bullet$ Each pollen grain is covered by a thick wall having two layers - exine and intine.
Exine
$\displaystyle \small \circ$ It is outer, thick and tough sculptured layer which is often provided with spinous outgrowths.
$\displaystyle \small \circ$ It is made up of sporopollenin which is one of the most resistant organic material.
$\displaystyle \small \circ$ It enables the pollen grains to survive in unfavorable conditions like high temperatures, strong acids and alkalis.
$\displaystyle \small \circ$ The regions where exine is thin or absent are known as germ pores.
$\displaystyle \small \circ$ A pollen tube emerges from these germ pores during pollen germination.
Intine
$\displaystyle \small \circ$ This is a thin and smooth layer.
$\displaystyle \small \circ$ It is a continuous layer made up of cellulose and pectin.
$\displaystyle \small \circ$ Being thin, it gives rise to pollen tube during pollen germination.
$\displaystyle \small \circ$ When the pollen grain is mature, it contains two cells, the vegetative cell and generative cell.
Vegetative cell
$\displaystyle \small \circ$ It is the large cell consisting of vascolated cytoplasm with rich food reserve.
$\displaystyle \small \circ$ It helps in the formation of pollen tube.
Generative cell
$\displaystyle \small \circ$ It is small and floats in the cytoplasm of vegetative cell.
$\displaystyle \small \circ$ It is spindle shaped with dense cytoplasm and prominent nuclei.
$\displaystyle \small \bullet$ In over 60% of angiosperms, the pollen grins are shed at this two-celled stage.
$\displaystyle \small \bullet$ In the remaining species the generative cell divides mitotically to give rise to two male gametes. Hence the pollen grains in these are shed at the three-celled stage.
Picture
Pollen grains and allergies
$\displaystyle \small \bullet$ Pollen grains of a number of species are known to cause severe allergies and brochial afflications in people.
$\displaystyle \small \bullet$ This may lead to chronic respiratory disorders like asthma, bronchitis, etc.
$\displaystyle \small \bullet$ Ex: Parthenium or carrot grass
Pollen grains as rich sources of nutrients
$\displaystyle \small \bullet$ Pollen grains are very rich in nutrients.
$\displaystyle \small \bullet$ They are being used as food supplements, in the form of pollen tablets and syrups.
$\displaystyle \small \bullet$ Pollen consumption has been claimed to increase the performance of athletes and race horses.
Pollen grains and their viability
$\displaystyle \small \bullet$ The period for which the pollen grains retain the ability to germinate is known as pollen viability.
$\displaystyle \small \bullet$ It is highly variable in different plant species and depends on temperature and humidity.
$\displaystyle \small \bullet$ In cereals like rice and wheat, pollen grains loose viability within 30 minutes of their release and in some members of Solanaceae, Rosaceae they maintain viability for several months.
$\displaystyle \small \bullet$ It is possible to store pollen grains of a large number of species for years in liquid nitrogen (-196$\displaystyle \small ^{0}C$).
Formation of a flower occurs as a result of a number of sequential changes.
(i) Hormonal and structural changes are initiated.
(ii) This leads to the differentiation and development of floral primordium.
(iii) These floral primordia give rise to the floral buds which take the shape of flowers.
(iv) The male (androecium) and female (gynoecium) reproductive structures differentiates and develops.
Stamen, Microsporangium and Pollen Grain
Stamen: It is the male reproductive unit of angiosperm consisting of anther and filament.
Structure of Anther: Anther is a bi-lobed structure having two theca in each lobe. Therefore it is called dithecous.
The anther is a four sided structure consisting of four microsporangia located at the corners, two in each lobe.
Microsporangia develops into pollen sacs in which pollen grains are produced.
Structure of Microsporangium: A typical microsporangium appears to have a circular outline. It is surrounded by four wall layers. Center of a microsporangium has compactly arranged homogenous cells called sporogenous cells or microsporocytes. A microsporocyte has abundant cytoplasm and a prominent nucleus.
Four layers surrounding the pollen-sac are
1. Epidermis: It is the outermost single layer. The cells of epidermis are generally stretched and flattened and perform the function of protection.
2. Endothecium/ fibrous layer: Inner to epidermis, there is a single layer of radially elongated cells called endothecium. Cells of endothecium develops fibrous thickening(made up of cellulose with a little pectin and lignin). This is mainly responsible for the dehiscence of mature anther.
3. Middle Layer: Three to four layers of thin allied cells situated just below the endothecium are known as middle layers. Cells of this layer are ephemeral and degenerate to provide nourishment to growing microspore mother cells.
4. Tapetum: This is the innermost layer. Cells of tapetum have dense cytoplasm and generally possess more than one nucleus. It nourishes the developing pollen grains.
Microsporogenesis
$\displaystyle \small \bullet$ Each microsporocyte undergoes mitotic division and produces a number of microspore mother cells or pollen mother cells which are diploid.
$\displaystyle \small \bullet$ Later they undergo meiotic division to form haploid microspores.
$\displaystyle \small \bullet$ The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis.
$\displaystyle \small \bullet$ The micropspores when formed are present in a cluster of four cells called the microspore tetrad.
$\displaystyle \small \bullet$ As the anthers mature and dehydrate, each microspore gets separated and forms a thick outer wall called exine and an inner wall called intine.
$\displaystyle \small \bullet$ Each microspore develops into a pollen grain. as the pollen grains mature, the anther splits open along the two lines of dehiscence and the pollen grains are released.
Structure of Pollen grain
$\displaystyle \small \bullet$ The pollen grains represents the male gametophytes.
$\displaystyle \small \bullet$ Pollen grains are generally spherical in structure, about 20-50 mm in diameter.
$\displaystyle \small \bullet$ Each pollen grain is covered by a thick wall having two layers - exine and intine.
Exine
$\displaystyle \small \circ$ It is outer, thick and tough sculptured layer which is often provided with spinous outgrowths.
$\displaystyle \small \circ$ It is made up of sporopollenin which is one of the most resistant organic material.
$\displaystyle \small \circ$ It enables the pollen grains to survive in unfavorable conditions like high temperatures, strong acids and alkalis.
$\displaystyle \small \circ$ The regions where exine is thin or absent are known as germ pores.
$\displaystyle \small \circ$ A pollen tube emerges from these germ pores during pollen germination.
Intine
$\displaystyle \small \circ$ This is a thin and smooth layer.
$\displaystyle \small \circ$ It is a continuous layer made up of cellulose and pectin.
$\displaystyle \small \circ$ Being thin, it gives rise to pollen tube during pollen germination.
$\displaystyle \small \circ$ When the pollen grain is mature, it contains two cells, the vegetative cell and generative cell.
Vegetative cell
$\displaystyle \small \circ$ It is the large cell consisting of vascolated cytoplasm with rich food reserve.
$\displaystyle \small \circ$ It helps in the formation of pollen tube.
Generative cell
$\displaystyle \small \circ$ It is small and floats in the cytoplasm of vegetative cell.
$\displaystyle \small \circ$ It is spindle shaped with dense cytoplasm and prominent nuclei.
$\displaystyle \small \bullet$ In over 60% of angiosperms, the pollen grins are shed at this two-celled stage.
$\displaystyle \small \bullet$ In the remaining species the generative cell divides mitotically to give rise to two male gametes. Hence the pollen grains in these are shed at the three-celled stage.
Picture
$\displaystyle \small \bullet$ Pollen grains of a number of species are known to cause severe allergies and brochial afflications in people.
$\displaystyle \small \bullet$ This may lead to chronic respiratory disorders like asthma, bronchitis, etc.
$\displaystyle \small \bullet$ Ex: Parthenium or carrot grass
Pollen grains as rich sources of nutrients
$\displaystyle \small \bullet$ Pollen grains are very rich in nutrients.
$\displaystyle \small \bullet$ They are being used as food supplements, in the form of pollen tablets and syrups.
$\displaystyle \small \bullet$ Pollen consumption has been claimed to increase the performance of athletes and race horses.
Pollen grains and their viability
$\displaystyle \small \bullet$ The period for which the pollen grains retain the ability to germinate is known as pollen viability.
$\displaystyle \small \bullet$ It is highly variable in different plant species and depends on temperature and humidity.
$\displaystyle \small \bullet$ In cereals like rice and wheat, pollen grains loose viability within 30 minutes of their release and in some members of Solanaceae, Rosaceae they maintain viability for several months.
$\displaystyle \small \bullet$ It is possible to store pollen grains of a large number of species for years in liquid nitrogen (-196$\displaystyle \small ^{0}C$).
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