Plant Biology: Double Fertilisation and Seed Formation

Double Fertilisation and Seed Formation

• Ovule swells and wall toughens to form the seed coat (testa) post-fertilisation.
• Zygote develops into the embryo of the seed.
• Triploid endosperm nucleus rapidly divides, creating food storage tissue.
• Integuments thicken, forming the protective testa.
• Seeds become dormant once formed and dried.

Seed Structure

• Embryo: Develops into the new plant.
• Testa: Tough seed coat protects the seed during dormancy.
• Radicle: First part to emerge, becoming the root.
• Plumule: Emerges after the radicle, developing into the shoot.
• Cotyledons: Seed leaves containing food reserves; mono (one) vs. di (two) cotyledons.
• In monocots (e.g., maize, wheat), food is stored in the endosperm.
• In dicots (e.g., broad bean), food is stored in the cotyledons.

Fruit Formation

• Fruits protect seeds and facilitate seed dispersal.
• Seedless fruits, such as watermelons, are produced through genetic selection or asexual propagation.

Plant Reproduction Overview

• Sexual reproduction involves gamete fertilisation; asexual reproduction does not involve gametes.
• Flowers are essential for sexual reproduction in plants.

Functions of Flower Parts

• Sepals: Protect and enclose the flower before blooming, sometimes closing at night.
• Petals: Brightly colored to attract pollinators, producing scent for insects.
• Stamens: Male parts consisting of anther (produces pollen) and filament (supports anther).
• Carpel: Female part consisting of stigma (catches pollen), style (connects stigma to ovary), and ovary (contains ovules).
• Receptacle: Thickened pedicel part where flower parts grow.
• Nectary: Produces nectar at the base of petals.

Gamete Development

• Female gametes arise in the ovule; a diploid megaspore mother cell undergoes meiosis.
• Surviving megaspore divides to form an embryo sac with 8 nuclei: 2 polar nuclei and an egg cell.
• Male gametes develop in the anther from diploid microspore mother cells via meiosis, resulting in four haploid microspores.
• Microspores divide to form pollen grains, consisting of tube nucleus (guides pollen tube) and generative nucleus (forms male gametes).

Pollination Types

• Pollination: Transfer of pollen from an anther to a stigma.
• Self-pollination: Occurs within the same plant, reducing genetic variation.
• Cross-pollination: Involves different plants, promoting genetic diversity.

Pollination Methods

• Insect Pollination: Bees gather nectar and pollen, transferring pollen between flowers.
• Wind Pollination: Plants like wheat produce light pollen that disperses easily in the wind.

Fertilisation Process

• Fusion of male and female gametes results in the diploid zygote.
• Pollen grain germinates on stigma, forming a pollen tube that grows toward the ovule.
• As the pollen tube grows, the generative nucleus divides to produce two sperm nuclei.
• One sperm nucleus fuses with the egg cell to form the zygote; the other fuses with polar nuclei to form the triploid endosperm.