Plant Reproduction and Structures

Angiosperms and Gymnosperms Overview

  • Definition of Gymnosperms: The term "gymnosperm" translates to "naked seed", meaning that their seeds are not enclosed in fruits. This is a critical distinction from angiosperms, which have seeds surrounded by fruit.

  • Fertilization Process:

    • Pollen grains are produced by male cones and carried by wind.
    • Upon landing on a female cone scale, pollen grains release sperm to fertilize eggs within ovules.
    • The fertilized ovule develops into a seed, which remains on the scale surface, hence termed "naked seed".

  • Seed Structure:

    • Seeds are diploid, containing two sets of chromosomes from the fusion of an egg and sperm.
    • They consist of:
    • An embryo (baby plant)
    • Nutrients (food) to support growth until germination
    • A protective coat.

  • Seed Dispersal and Survival:

    • Seeds often have adaptations (like wings, or air bladders) for wind dispersal.
    • Seeds can remain dormant for extended periods (up to 50-60 years) before germinating in favorable conditions.

  • Sporophyte and Gametophyte Generations:

    • The visible plant structure is the sporophyte generation (diploid, with two sets of chromosomes).
    • The gametophyte generation (haploid, with one set of chromosomes) develops within cones.

Male and Female Spores

  • Microspores:

    • Develop into pollen grains containing sperm.
    • Any pollen grain includes:
    • Tube cell that creates a pollen tube.
    • Generative cell that divides into two sperm cells.

  • Megaspores:

    • Develop into egg cells within ovules in female cones.
    • Each scale on a female cone typically contains two ovules.

Conifers: Key Features

  • Definition: Conifers are known as "cone bearers".

  • Leaves: Needle-like leaves adapted to survive in dry, cold climates:

    • Shininess and waxy coating to retain water.
    • Sunken stomata to minimize water loss.

  • Reproductive Structures:

    • Male cones (smaller) produce pollen.
    • Female cones (larger) produce seeds.
    • Conifers are typically monoecious: both male and female reproductive structures on the same tree, which reduces self-fertilization risk.

Fertilization Process for Pines

  • Pollen Tube Formation:

    • Upon landing on the scale of a seed cone, pollen grains trigger a sticky droplet from the scale that traps them.
    • The tube cell then elongates, forming a pollen tube through the micropyle leading to the ovule.

  • Fertilization:

    • One sperm fertilizes the egg within the ovule, forming a zygote (diploid).
    • The fertilized ovule later becomes a seed.

Time for Seed Maturation

  • Seeds typically require 1-3 years to mature before germination.

Recap of Key Distinct Concepts

  • Pollination vs. Fertilization:
    • Pollination refers to the transfer of pollen to the ovule, which can involve wind or other means.
    • Fertilization is the process where the sperm fertilizes the egg post pollination.

Cycads, Ginkgo, and Gnetophytes Overview

  • Cycads: Often have large seed cones, dioecious, and were prominent during the age of dinosaurs.
  • Ginkgo (Maidenhair Tree): The only surviving species of its kind, dioecious, known for its ornamental use and resilience but stinky seeds on female trees.
  • Gnetophytes: Include three genera, are considered to have links to angiosperms due to their advanced reproductive characteristics.

Concluding Notes:

  • Emphasize the diversity in reproduction among gymnosperms and the adaptations that enhance their survival and dispersal in various habitats.