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Sexual Reproduction in Flowering Plants - Flower Structure

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A flower is the reproductive unit in Angiosperms, often described as a modified shoot. It consists of four whorls: Calyx, Corolla, Androecium, and Gynoecium attached to the thalamus.

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The Androecium consists of stamens. A typical stamen has a long filament and a bilobed, dithecous anther. Each lobe contains two microsporangia, making it tetrasporangiate.

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The microsporangium wall consists of four layers: the epidermis, endothecium, middle layers, and the Tapetum. The tapetum nourishes the developing pollen grains.

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Microsporogenesis: The process of formation of microspores from a Pollen Mother Cell (PMCPMC) through meiosis. Each PMCPMC (2n2n) produces a microspore tetrad (nn).

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Pollen Grain: Represents the male gametophyte. It has a two-layered wall: the outer Exine (made of sporopollenin, C90H142O27C_{90}H_{142}O_{27}) and the inner Intine (cellulose and pectin).

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The Gynoecium consists of the pistil (stigma, style, and ovary). The megasporangium, or ovule, is attached to the placenta by a funicle. The main body of the ovule is the nucellus (2n2n).

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Megasporogenesis: The Megaspore Mother Cell (MMCMMC) undergoes meiosis to form four megaspores. In most plants, three degenerate, and one functional megaspore develops into the female gametophyte (monosporic development).

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The mature Embryo Sac is typically 77-celled and 88-nucleate. It contains the egg apparatus (11 egg cell + 22 synergids), 33 antipodals, and 11 central cell with 22 polar nuclei.

📐Formulae

Number of Meiosis for ′n′ Microspores=n4\text{Number of Meiosis for } 'n' \text{ Microspores} = \frac{n}{4}

Number of Meiosis for ′n′ Megaspores/Seeds=n\text{Number of Meiosis for } 'n' \text{ Megaspores/Seeds} = n

Total Meiosis for ′n′ Seeds=n+n4=5n4\text{Total Meiosis for } 'n' \text{ Seeds} = n + \frac{n}{4} = \frac{5n}{4}

Ploidy: Endosperm=3n, Nucellus=2n, Male Gamete=n\text{Ploidy: Endosperm} = 3n, \text{ Nucellus} = 2n, \text{ Male Gamete} = n

💡Examples

Problem 1:

Calculate the number of meiotic divisions required to produce 400400 seeds in a pea plant.

Solution:

500500 meiotic divisions.

Explanation:

To produce 400400 seeds, we need 400400 pollen grains and 400400 ovules. Meiosis for 400400 pollen grains = 4004=100\frac{400}{4} = 100. Meiosis for 400400 ovules = 400400 (since each meiosis produces only one functional megaspore). Total divisions = 100+400=500100 + 400 = 500.

Problem 2:

If the leaf cell of an angiosperm has 2424 chromosomes, find the chromosome count in the Pollen Grain, Tapetum, and Endosperm.

Solution:

Pollen Grain: 1212; Tapetum: 2424 (or multiple); Endosperm: 3636.

Explanation:

Leaf cell is diploid (2n=242n = 24), so n=12n = 12. 1. Pollen grain is haploid (nn), so it has 1212. 2. Tapetum is a somatic layer of the anther, so it is diploid (2n=242n = 24), though it can become polyploid. 3. Endosperm is triploid (3n=3×12=363n = 3 \times 12 = 36).