Class 7 Science Notes Chapter 12 (Reproduction in Plants) – Science Book

Okay, here are the detailed notes for NCERT Class 7 Science, Chapter 12: Reproduction in Plants, structured for government exam preparation.

Chapter 12: Reproduction in Plants

Core Concept: Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parents". It is essential for the continuation of a species.

Modes of Reproduction in Plants:
Plants reproduce through two main modes:

1. Asexual Reproduction: New plants are obtained from a single parent, without the involvement of gametes (sex cells) or seeds. Offspring are genetically identical to the parent.
2. Sexual Reproduction: New plants are obtained from the fusion of male and female gametes, leading to the formation of seeds. Offspring show genetic variation.

1. Asexual Reproduction

• Characteristics: Single parent, no gamete fusion, offspring are clones (identical).

• Methods:

  • (a) Vegetative Propagation:

    • Definition: A type of asexual reproduction where new plants grow from vegetative parts like roots, stems, leaves, or buds.
    • From Stems:
      • Stem Cuttings: A piece of stem with buds is cut and planted (e.g., Rose, Champa, Sugarcane).
      • Tubers: Modified underground stems with "eyes" (buds) (e.g., Potato). Each eye can grow into a new plant.
      • Rhizomes: Horizontal underground stems (e.g., Ginger, Turmeric). They have buds that sprout new shoots and roots.
      • Bulbs: Underground stems with fleshy leaves storing food (e.g., Onion, Garlic). Buds within the bulb develop into new plants.
      • Runners/Stolons: Stems that grow horizontally above ground, developing roots and shoots at nodes (e.g., Grass, Strawberry).
    • From Roots: Some plants can develop new shoots from their roots (e.g., Sweet Potato, Dahlia).
    • From Leaves: Some plants have buds on the margins of their leaves which can develop into new plantlets when the leaf falls on moist soil (e.g., Bryophyllum).
    • From Buds: Vegetative buds can give rise to new plants.
    • Advantages: Faster growth than from seeds, plants bear flowers/fruits earlier, offspring are identical to the parent (maintains desired traits).

  • (b) Budding:

    • Definition: A small bulb-like projection (bud) grows out from the parent cell.
    • Process: The bud gradually grows, detaches from the parent cell, and forms a new individual. Sometimes, a chain of buds may form.
    • Example: Yeast (a single-celled fungus).

  • (c) Fragmentation:

    • Definition: The body of the parent organism breaks into distinct pieces (fragments).
    • Process: Each fragment grows into a new individual.
    • Example: Spirogyra (an alga found in water bodies).

  • (d) Spore Formation:

    • Definition: Reproduction through specialized asexual reproductive bodies called spores.
    • Spores: Microscopic, typically single-celled, covered by a hard protective coat to withstand unfavourable conditions (high temperature, low humidity).
    • Process: Under favourable conditions (moisture, suitable temperature), a spore germinates and develops into a new individual.
    • Examples: Fungi (like Bread mould/Rhizopus, Penicillium), Mosses, Ferns. Spores are often produced in structures called sporangia.

2. Sexual Reproduction

• Characteristics: Involves two parents (or male and female parts), fusion of gametes, formation of seeds, offspring show variation.

• Reproductive Part: The Flower is the reproductive part of a plant (angiosperms).

• Parts of a Flower:

  • Pedicel: Stalk of the flower.
  • Receptacle/Thalamus: The part of the flower stalk where the parts of the flower are attached.
  • Sepals (Calyx): Green, leaf-like structures protecting the flower in the bud stage.
  • Petals (Corolla): Usually brightly coloured to attract insects for pollination.
  • Stamen (Androecium): The male reproductive part.
    • Anther: Contains pollen grains. Pollen grains produce male gametes.
    • Filament: Stalk that supports the anther.
  • Pistil or Carpel (Gynoecium): The female reproductive part, usually located in the centre.
    • Stigma: Sticky tip that receives pollen grains.
    • Style: Tube connecting the stigma to the ovary.
    • Ovary: Swollen base containing one or more ovules. Each ovule contains the female gamete (egg cell).

• Types of Flowers:

  • Bisexual Flowers: Contain both stamens and pistil (e.g., Mustard, Rose, Petunia, Hibiscus).
  • Unisexual Flowers: Contain either only stamens (male flower) or only pistil (female flower) (e.g., Corn, Papaya, Cucumber).

• Pollination:

  • Definition: The transfer of pollen grains from the anther to the stigma.
  • Types:
    • Self-Pollination: Pollen lands on the stigma of the same flower or another flower on the same plant.
    • Cross-Pollination: Pollen lands on the stigma of a flower on a different plant of the same species.
  • Agents of Pollination:
    • Wind: Pollen is light, produced in large quantities; flowers often small, lack bright colours/nectar; stigma may be large and feathery (e.g., Grasses, Maize, Wheat, Rice).
    • Water: Occurs in some aquatic plants; pollen released into water (e.g., Vallisneria, Hydrilla).
    • Insects (and other animals like birds, bats): Flowers are often large, brightly coloured, produce nectar and fragrance; pollen grains may be sticky or spiny (e.g., Rose, Sunflower, Salvia).

• Fertilization:

  • Definition: The fusion of the male gamete (from the pollen grain) with the female gamete (egg cell inside the ovule).
  • Process:
    1. Pollen grain lands on a suitable stigma.
    2. Pollen grain germinates, forming a pollen tube that grows down through the style.
    3. Pollen tube reaches the ovule in the ovary.
    4. Male gamete travels down the pollen tube and fuses with the female gamete (egg) inside the ovule.
    5. The fused cell is called the Zygote.
  • Outcome: The zygote is the first cell of the new individual (embryo).

• Post-Fertilization Changes (Fruit and Seed Formation):

  • The Zygote develops into the Embryo.
  • The Ovule develops into the Seed. (The seed contains the embryo and stored food, protected by a seed coat).
  • The Ovary matures and develops into the Fruit. (The fruit protects the seed(s)).
  • Other parts of the flower (sepals, petals, stamens, style, stigma) usually wither and fall off.
  • Fruit: Can be fleshy (e.g., Mango, Apple, Orange) or dry (e.g., Groundnut, Mustard, Peas).

• Seed Dispersal:

  • Definition: The scattering or transport of seeds away from the parent plant.
  • Importance: Prevents overcrowding, reduces competition for sunlight, water, and minerals, allows colonization of new habitats.
  • Agents of Dispersal:
    • Wind: Seeds are light, small, may have wings (e.g., Drumstick, Maple) or hair-like structures (e.g., Sunflower, Madar/Aak, Cotton).
    • Water: Seeds/fruits can float, may have spongy or fibrous outer coats (e.g., Coconut, Lotus).
    • Animals:
      • Seeds with hooks or spines attach to animal fur/human clothing (e.g., Xanthium, Urena).
      • Fleshy fruits are eaten by animals, and seeds are discarded or pass undigested through the digestive system (e.g., Mango, Guava, Berries).
    • Explosion/Bursting: Some fruits burst open suddenly when dry, scattering the seeds (e.g., Castor, Balsam, Pea).

Key Points for Exams:

• Know the difference between Asexual and Sexual reproduction.
• Memorize examples for each type of vegetative propagation (Potato-Tuber, Bryophyllum-Leaf, Rose-Stem cutting, Ginger-Rhizome, Yeast-Budding, Spirogyra-Fragmentation, Fern/Fungus-Spores).
• Understand the parts of a flower and their functions (Stamen=Male, Pistil=Female, Anther=Pollen, Ovary=Ovules).
• Differentiate between Unisexual and Bisexual flowers with examples.
• Understand Self-pollination vs. Cross-pollination.
• Know the agents of pollination and seed dispersal with corresponding seed/fruit adaptations and examples (e.g., Winged seeds - Wind, Spiny seeds - Animal).
• Understand the process of fertilization (Pollen -> Stigma -> Pollen Tube -> Ovule -> Fusion -> Zygote).
• Know what develops from what after fertilization (Ovary -> Fruit, Ovule -> Seed, Zygote -> Embryo).

This covers the essential details from Chapter 12 for effective preparation. Remember to correlate these points with diagrams in the NCERT textbook for better understanding.