Sexual Reproduction in Flowering Plants
The overall process of sexual reproduction in Angiosperms plants or flowering plants divided into three major events namely-
- Pre-fertilization events
- Fertilization events
- Post-fertilization events.
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also read in Gametogenesis in human
Pre-fertilization events
The pre fertilization events includes the following points-
- Structure of flower and its parts
- Structure of microsporangium and megasporangium
- Process of Microsporogenesis and Megasporogenesis
- Structure of male and female gametophyte
- Process of pollination and its type
- In breeding and out breeding devices
- some other important terms.
Fertilization events
The fertilization events involve the overall process of double fertilization which includes Syngamy and Triple fusion.
Post fertilization events
It includes the following points
- Zygote or embryo development
- Endosperm development
- Seed formation and structure
- Seed viability and Seed dispersal
- Advantage of seed formation
- Fruits and its types
- Apomixis
- Advantage of Apomixis
- Polyembrony
Pre Fertilization Events in detail
It includes all the processes and the study of reproductive structures before the fertilization, found in all the sexually reproducing organisms.
Structure of Flower
Flower in the plants is a site of sexual reproduction. A flower is the modified condensed shoot developed under the influence of genetic, hormonal and environmental stimulus.
A flower is consists of following parts known as floral appendages, arranged in 4 whorls.
The upper or outer two whorls are Calyx and Corolla which are known non-essential or accessory whorls.
The lower or inner two whorls are called essential or reproductive whorls.
Calyx is the outer most whorls consist of sepals which are green in colour and protect the flower in bud stage.
Corolla is the second outer most whorls consist of brightly colored petals which attract the pollinators and has fragrance.
In case of wind pollinated and air pollinated plants the petals are colorless, nectar less and non fragrance.
The lower or inner two whorls are called essential or reproductive whorls. Because they consist of male reproductive organs and female productive organ called Androecium and Gynoecium respectively.
Androecium consists of one or more stamen.
Gynoecium consists of one or carpel/pistil.
Male Reproductive Organ-Stamen
stamen consists of two parts namely and anther and filament. The proximal end of filament attached to thalamus or sometime with petals.
The anther is the bilobed structure. Each lobe has two theca, it means each lobe is dithecous.
In the transverse section bilobed anther has four microsporangium or pollen sac at the corner.
Microsporangium
It is circular in outline and its wall has four layers namely- outer epidermis, endothecium, middle layers and inner most teptum.
The outer three layers are protective in function and also help in the dehiscence of anther on maturation.
The cells of teptum generally binucleated due to their high metabolic activities. As we know that Tapetum layer are secretary in function and provide nourishment to the developing pollen grain.
Internally microsporangium has sporogenous tissue in the central position.
Sporogenous tissue is the mass of compactly arranged diploid homogeneous cells or pollen mother cell.
Microsporogenesis
Formation of haploid microspore from the diploid microspore mother cell or pollen mother cell (PMC) is known as microsporogenesis.
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The microspore mother cells are found in the sporogenous tissue and contain diploid sets of chromosome.
After meiosis cell division they produce four microspores called microspores tetrad.
The anther when mature and dehydrates the microspores dissociates and release as pollen grain.
Microgametogenesis
Development of male gametophyte from microspore is known as micro gametogenesis.
Structure of Pollen (Male gametophyte)
Pollen grain is spherical in structure and has two layers. The inner one is called intine which consists of cellulose and pectin while the outer one is called exine.
Exine is consisting of most resistance organic substance called is Sporopollenin.
Sporopollenin is highly resistance to strong acid, base, enzymes and high temperature.
Sporopollenin is secreted by Tapetum.
It is important to note that due to the presence of sporopollenin, pollen can be preserve as fossils 25-50 micrometers in diameter.
Internally pollen grain consist of two type of cells the largest cell is called vegetative cell or tube cell
Vegetative cell produce pollen tube while smaller cell is called generative cell which produce two male gametes.
At some places that exine is absent or very thin. This area is known as Germpore. It is the site from where Pollen tube germinated or emerge out.
Pollen Allergy
Pollen grain of some plant cause chronic respiratory disorders or allergy such as Bronchitis, Asthma (hey fever) and bronchial allergy.
For example- Pollen of Parthenium or carrot grass.
Pollen as Nutrient Supplement
Due to high amount of nutritive material present in pollen, it is used as a syrup and tablet form.
The pollen nutrients supplement used by athletes and race horses to improve health and enhance performance.
The chemical composition of nutrients in pollen are-
- Protein is 7 to 26%.
- Carbohydrate is 24 to 48%.
- Fat is 0.9 to 14.5%.
Pollen Viability
The period for which pollen grain retain the ability to germinate is known as pollen viability.
It may vary with the plant to plant species and ranges between few minutes to months.
In case of Rice and Wheat plant the pollen viability after release from anther is about 30 minutes.
In members of family Rosaceae (rose), Leguminoceae (pulses like pea, gram etc.) and Solanaceae (Potato and Brinjals etc.) viability of pollen grain is for few months.
It is also possible to preserve or store the pollen grain for many years by cryopreservation techniques in the pollen banks.
Some pollen related terms
- Palynology- The study of pollen grain is known as well Palynology
- Poroderm- The wall of pollen is called Poroderm.
- Two cell stage pollen- In 60% flowering plants pollen grain shed at two cell stage (vegetative cell + generative cell).
- Three cell stage pollen- In remaining flowering plants pollen grain shed at two cell stage because generative cell divides to form two male gametes (vegetative cell + two male gametes).
- Polyspory- It is the occurrence of more than four microspores tetrad.
- Compound pollen grain- If four poling grains remain attached, and then it is known as compound pollen grain. For example- Jatropha
- Pollen kit- The exine of pollen grain covers with a yellowish, viscous and sticky and substances known as pollen kit. It is protective envelope and also sticky in nature hence easily attached to the body of insect. This facilitates pollination. The yellow and viscous part made up of lipids and carotenoids.
- Pollinium- If all the pollen grains of an anther lobe remain united in the single sac. For example- Calotropis and Orchids.
Female Reproductive organ-Pistil or Carpel
Pistil or Carpel is found in the female reproductive organ that is gynoecium.
Gynoecium is consists of one or more pistil or carpel.
If the carpel or pistil is more than one (multicarpillary) and fuse then it is known as Syncarpous condition, and if they are free then it is known as Apocarpous condition.
Ovule or Megasporangium
Most of the angiospermic ovules are typically anatropous that is inverted in shape.
It is the most common type and found in 82% of angiosperm with two integuments (Bitegmic condition).
Ovule is attached to ovary by the placenta by a fine thread like structure, which communicate between Placenta and the body of ovule is known as funicle.
The point of attachment where the funicle is attached to ovule body is called hilum.
Ovules generally double membranous or Bitegmic (two integuments). Integuments after fertilization converted into seed coat.
Inside the ovule a mass of sporogenous tissue is present called nucellus which contains diploid megaspore mother cell.
Megasporogenesis
The formation of haploid megaspore from megaspore mother cell is known as megasporogenesis.
Development of megaspore in angiosperm plant is monosporic type or polygonum type.
It is the most common type megaspore development, found in 70% flowering plant.
In the process of megasporogenesis the diploid megaspore mother cell undergoes meiotic division to produce four mega spore tetrad which are linearly arrange.
Embryo sac (Female Gametophyte)
The whole process of megasporogenesis takes place at the micropylar end.
After formation of four megaspores, three megaspores degenerated and one survives.
After the rest and growth it is enlarge in size and its nucleus under goes three round mitosis division to produce eight nuclei.
The mitosis involves karyokinesis or nuclear division without the formation of cell wall or cytokinesis.
After formation of eight nuclei stage the cell wall laid down and all the nuclei move towards the respected position.
That is three nuclei at the chalazal end and organized to form three antipodal cells.
At the micropylar end three nuclei organized to form egg apparatus (one egg cell and two synergid cell).
The remaining two nuclei located in the largest cell called as Central cell.
Development of such embryo sac from single megaspore is known as monosporic development.
Megagametogenesis
Formation of mature female gametophyte from functional megaspore is known as Megagametogenesis.
- Monosporic development- Development of embryo sac from single megaspore.
- Bisporic development- When the embryo sac develops from two megaspores.
- Tetrasporic development- Development of embryo sac from four megaspores.
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