0CLASS XII BIOLOGY CHAPTER I : REPRODUCTION IN PLANTS
Flower : are known as the reproductive part of a plant.
Flowers can either be
Complete : consists of sepals, petals, stamens and pistils. Or Incomplete.: lacks one or more of these structures.
The calyx, corolla, androecium, and gynoecium are four whorls of modified leaves that constitute the flower. The sepals, petals, stamens, and pistils, respectively, constitute one of the flower parts in each of these whorls.
A complete flower consists of two different parts:
- Vegetative Part B. Reproductive Part
Vegetative Parts
- Petals: bright-coloured part that attracts bees, insects, and birds.
- Sepals: green-coloured part beneath the petals to protect rising buds.
Reproductive Part
- Stamen:
Male reproductive organ
consists of two parts namely:
- a) anther – sac like , produce and store pollens
- b) filaments.- thread like , provide support to anther
- Pistil:
female reproductive organ
Consists of three parts -stigma, style and ovary.
Stigma: the topmost part or receptive tip of carpels
Style: long tube-like slender stalk and connects the stigma and the ovary.
Ovary: contains ovules. Site on many reproductive events.
MICROSPOROGENESIS:
Anther: bilobed with each lobe having two theca, i.e., they are dithecous
(Theca – a sheath or covering )
It 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.
Microsporangium is surrounded by four walls-
1.Epidermis- protects and help in dehiscence of anther.
- Endothecium- protects and help in dehiscence of anther.
- Middle layers - (2 in number) protects and help in dehiscence of anther.
- Tapetum- nourishes the developing pollen grain. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.
Young anther contains compactly arranged homogenous cells called sporogenous tissue.
Microsporogenesis
Site: Microsporangia of anther
Process: The process of formation of microspores from a pollen mother cell through meiosisis called microsporogenesis.
As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads.
On maturation and dehydration of anther, the spores dissociate and develop into pollen grains.
Pollen grains release with the dehiscence of anther.
Pollen Grain
Spherical in shape.
Have two layered wall- outer hard exine layer and inner thin intine. Exine- made up of sporopolenin. Resistant to organic matter, withstand high temperature, acids, alkalis and enzymes. It has prominent apertures called germ pores, where sporopolenin is absent.
Intine- It is thin, continuous layer, made of cellulose and pectin.
In the cytoplasm are present water, protein, fats, carbohydrates,
The pollen grains represent the male gametophytes.
Microgametogenesis :
Maturation of the haploid microspore into a functional, multi-cellular pollen grain.
Vacuole Formation: microspore increases in size, and a large vacuole forms, pushing the nucleus to one side.
Mitosis I: microspore nucleus undergoes an unequal mitotic division, producing two cells: a larger Vegetative Cell (Tube Cell) and a smaller, free-floating Generative Cell.
Pollen Shedding: In 60% of angiosperms, pollen is shed at this 2-celled stage.
Mitosis II (Optional): In the remaining 40% of angiosperms, the Generative Cell undergoes a second mitotic division (often during pollen tube growth) to form two non-motile Male Gametes (Sperm Cells), resulting in a 3-celled pollen grain at maturity.
POLLEN GRAINS – NUTRIENT/COMMERCIAL VALUE
Nutritional Composition
Carbohydrates: 24% to 48%
Proteins: 7% to 26%
Fats: 0.9% to 14.5%
Other Components: High amounts of free amino acids, vitamins (B-complex, folic acid), and essential minerals.
Commercial & Practical Uses
Food Supplements: In many western countries, pollen is processed into tablets and syrups and marketed as a dietary supplement.
Performance Enhancement: Consumption of these supplements is claimed to increase the performance of athletes and race horses.
Pollen grains of many species cause severe allergies.
Pollen viability: depends on both temperature and humidity,
- Pollen of a large number of species can be stored for years in liquid nitrogen (-196°C) in pollen banks for crop breeding programmes.
OVULES – LOCATION AND ORIGIN
Inside ovary ovarian cavity (locule) is present.
The placenta is located inside the ovarian cavity.
Ovules (Megasporangium) arises from placenta.
OVULES – STRUCTURE
Stalk of ovule- Funicle
Region where ovule fuses with funicle- Hilum
Protective envelope of Ovule-Integuments
Nucellus a mass of cells is covered by integuments except at an opening called Micropyle.
Opposite end of micropyle is called Chalazal end.
Nucellus have abundant reserve food materials.
MEGASPOROGENESIS
Process of formation of megaspores from megaspore mother cells is called megasporogenesis.
Ovule differentiate a single megaspore mother cell (MMC) in the micropylar region of nucellus .
MMC undergoes meiotic dividion that results into production of four megaspores
In most of the flowering plants three megaspores dengerates .
Single megaspore develops into female gametophyte (Embryo sac)
Nucleus of functional megaspore divides mitotically to form 2 nuclei which move to opposite poles forming 2- nucleate embryo sacs.
Two more mitotic nuclear division results in 4-nucleate and later 8- nucleate stages of embryo sac.
A mature embryo sac is seven celled eight nucleate, It consists of an egg apparatus having two synergids and one egg cell.
Synergids have special cellular thickenings called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid.
Three cells present at the chalazal end are called the antipodals.
The large central cell contains two polar nuclei.
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