Biology: Class 12 — Chapter 1: Sexual Reproduction in Flowering Plants

A typical flower consists of four main whorls attached to the thalamus:

• Calyx: Composed of sepals, protecting the flower in the bud stage.
• Corolla: Composed of petals, often brightly colored to attract pollinators.
• Androecium: The male reproductive organ, consisting of a whorl of stamens.
• Gynoecium: The female reproductive organ, consisting of one or more pistils.

The stamen consists of two parts: the long, slender stalk called the filament, and the terminal, typically bilobed structure called the anther.

• Anther Structure: A typical angiosperm anther is bilobed and dithecous (each lobe has two theca). It is a tetragonal structure containing four microsporangia located at the corners.
• Microsporangium Wall Layers: Surrounded by four layers:
  1. Epidermis (protective)
  2. Endothecium (helps in dehiscence)
  3. Middle layers (protective)
  4. Tapetum (innermost layer; nourishes developing pollen grains; cells are often bi-nucleate with dense cytoplasm)
• Microsporogenesis: The process of formation of microspores from a pollen mother cell (PMC) through meiotic division. Microspores are arranged in a cluster of four (microspore tetrad).
• Pollen Grain (Male Gametophyte):
  • Exine: Hard outer layer made of sporopollenin, one of the most resistant organic materials known. It has prominent apertures called germ pores where sporopollenin is absent.
  • Intine: Thin, continuous inner wall made of cellulose and pectin.
  • Cellular Composition: A mature pollen grain contains a large vegetative cell (with abundant food reserves) and a small, spindle-shaped generative cell.
  • In over 60% of angiosperms, pollen is shed at the 2-celled stage; in others, the generative cell divides mitotically to produce two male gametes (3-celled stage).

The gynoecium represents the female reproductive unit and can be monocarpellary (one pistil) or multicarpellary (multiple pistils). Multiple pistils can be fused (syncarpous) or free (apocarpous).

• Structure of a Pistil: Consists of the stigma (landing platform for pollen), style (elongated tube), and ovary (basal bulged part containing the ovarian cavity or locule and placenta).
• Structure of an Anatropous Ovule (Megasporangium):
  • Funicle: Stalk attaching the ovule to the placenta.
  • Hilum: Junction where the body of the ovule fuses with the funicle.
  • Integuments: Protective envelopes encircling the nucellus, except at a small opening called the micropyle.
  • Chalaza: Basal part of the ovule, opposite the micropylar end.
  • Nucellus: Central mass of parenchymatous cells containing abundant reserve food.
• Megasporogenesis: The process of formation of megaspores from the megaspore mother cell (MMC) via meiosis, resulting in four megaspores.
• Female Gametophyte (Embryo Sac) Development: Only one of the four megaspores remains functional, while the other three degenerate. This is called monosporic development.
  • The functional megaspore undergoes three sequential mitotic divisions to form an 8-nucleate, 7-celled mature embryo sac.
  • Organization: Three cells group at the micropylar end to form the egg apparatus (two synergids with filiform apparatus, and one egg cell). Three cells at the chalazal end are antipodals. The large central cell contains two polar nuclei.

Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.

• Kinds of Pollination:
  • Autogamy: Transfer of pollen within the same flower. Some plants (e.g., Viola, Oxalis, Commelina) produce chasmogamous (open) and cleistogamous (permanently closed, ensuring self-pollination) flowers.
  • Geitonogamy: Transfer of pollen from the anther of one flower to the stigma of another flower on the same plant (genetically autogamous, functionally cross-pollination).
  • Xenogamy: Transfer of pollen from anther to stigma of a different plant (brings genetic variation).
• Agents of Pollination:
  • Abiotic: Wind (pollen is light and non-sticky; stigmas are feathery) and Water (rare, occurs in aquatic plants like Vallisneria, Hydrilla, and Zostera).
  • Biotic: Animals, primarily insects (bees, butterflies, moths). Insect-pollinated flowers are typically large, colorful, fragrant, and rich in nectar.
• Outbreeding Devices: Mechanisms to discourage self-pollination and prevent inbreeding depression, including non-synchronous pollen release and stigma receptivity, different positioning of anthers and stigma, self-incompatibility, and production of unisexual flowers.
• Pollen-pistil Interaction: A dynamic dialogue involving chemical recognition where the pistil accepts compatible pollen and promotes pollen tube growth, or rejects incompatible pollen.
• Artificial Hybridisation: A crop improvement technique involving:
  • Emasculation: Removal of anthers from bisexual flower buds before dehiscence.
  • Bagging: Covering the emasculated flower with a bag (usually butter paper) to prevent contamination by unwanted pollen.

Endosperm development always precedes embryo development to ensure a steady nutrient supply. It can be free-nuclear (e.g., coconut water) or cellular (e.g., white coconut kernel). It may be completely consumed during development (e.g., pea, groundnut) or persist in the mature seed (e.g., castor, coconut).

The zygote divides to form a proembryo, which transitions through globular and heart-shaped stages before maturing.

• Dicotyledonous Embryo: Consists of an embryonal axis and two cotyledons. The portion above the cotyledons is the epicotyl (terminates in the plumule), and the portion below is the hypocotyl (terminates in the radicle, protected by a root cap).
• Monocotyledonous Embryo: Possesses only one cotyledon, called the scutellum. The embryonal axis has a shoot apex enclosed in a foliar sheath called the coleoptile, and a radicle enclosed in an undifferentiated sheath called the coleorhiza.

• Seed: The final product of sexual reproduction (fertilised ovule). Consists of seed coats, cotyledons, and an embryo axis.
  • Non-albuminous (Ex-albuminous): No residual endosperm (e.g., pea, groundnut).
  • Albuminous: Retains a part of the endosperm (e.g., wheat, maize, castor).
  • Perisperm: Persistent, residual nucellus found in some seeds (e.g., black pepper, beet).
  • Dormancy: A state of metabolic inactivity entered by the embryo when water content is reduced, helping seeds survive unfavorable conditions.
• Fruit: Developed ovary. The ovary wall becomes the pericarp.
  • True Fruits: Develop only from the ovary.
  • False Fruits: Thalamus also contributes to fruit formation (e.g., apple, strawberry, cashew).
  • Parthenocarpic Fruits: Fruits that develop without fertilisation; they are seedless (e.g., banana).

A form of asexual reproduction that mimics sexual reproduction, where seeds are formed without fertilisation (e.g., in some species of Asteraceae and grasses). It is highly valuable in agriculture to preserve hybrid vigor across generations without segregation of traits.

The occurrence of more than one embryo in a single seed (common in Citrus and Mango varieties), often arising from the division of nucellar cells protruding into the embryo sac.