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.