Lecture Notes on Seeded Plants

Evolution of Seed Plants

  • Historical Context
  • Once dominated by seedless vascular plants, today's landscapes are primarily made up of seed plants.
  • Seeds are defined as embryos encased with nutrients and a protective coat, allowing for dispersal and survival in challenging environments.
  • Key adaptation contributing to plant abundance.

Derived Traits of Seeded Plants

  • Reduced Gametophytes

  • Gametophytes, which are now mostly microscopic in seed plants, develop and remain protected within the sporophyte.

  • Examples: In gymnosperms, these are cones; in angiosperms, these are flowers.

  • Heterospory

  • All seed plants are heterosporous, producing two types of spores: mega (female) and micro (male).

  • Megasporophylls produce one megaspore that develops into female gametophytes.

  • Microsporophylls produce multiple microspores that grow into male gametophytes.

  • Ovules

  • Seed plants retain megasporangia within the parent sporophyte, protected by an integument layer (1 in gymnosperms, 2 in angiosperms).

  • An ovule consists of the megasporangium, megaspore, and integument.

  • Pollen

  • Male gametophytes develop into pollen grains that are encased in a protective pollen wall.

  • Pollination is the transfer of pollen to the ovule.

  • Male gametophyte travels through a pollen tube, facilitating fertilization.

  • Key distinction: unlike bryophytes and seedless vascular plants with flagellated sperm, many seed plants do not require water for sperm transport.

  • Seeds

  • Seeds contain embryos, a food supply, and integument.

  • Advantages of seeds over spores:

    • Multicellular structure
    • Can remain dormant
    • Consistent food supply
    • Capable of longer-distance travel.

Groups of Seed Plants

  • Clades
  • Gymnosperms: Seeds exposed on sporophylls that form cones.
    • Features include miniaturized gametophytes, resistant and dispersive seed stages, and pollen for fertilization.
  • Angiosperms: Seeds develop in mature ovaries (fruits).
    • Represents ~90% of all plant species, belonging to phylum Anthophyta.

Gymnosperms Overview

  • Adaptations:

  • Miniaturization of gametophytes and the capacity for persistence in the dry climate.

  • Fertilization can take up to three years to form seeds.

  • Diversity of Gymnosperms

  • Phylum Cycadophyta: Large cones, flagellated sperm; many at risk.

  • Phylum Ginkgophyta: Deciduous leaves; only species is Ginkgo biloba.

  • Phylum Gnetophyta: Diverse forms with pollen tube fertilization.

  • Phylum Coniferophyta: Woody and fleshy cones; includes pines, firs, and redwoods.

Angiosperms Overview

  • Key Adaptations:

  • Presence of flowers and fruits for reproduction.

  • Most diverse group today among plants.

  • Flower Structure:

  • Comprises sepals, petals, stamens (male), and carpels (female).

  • Flower diversity reflects pollinator interactions (symmetry types: radial and bilateral).

  • Fruits:

  • Develop from the ovary post-fertilization to protect seeds and assist with their dispersal across various mediums.

Life Cycle of an Angiosperm

  • Mechanisms for maximizing cross-pollination.
  • Germination process where pollen tube penetrates ovule via micropyle, allowing fertilization and subsequent nutrient formation.

Importance and Threats to Plant Diversity

  • Importance:

  • Primary source of food, fuel, and medicine; key crops contribute major calories in human diets.

  • Artificial selection plays a crucial role in crop development.

  • Threats:

  • Habitat destruction poses a significant risk, particularly rainforest clearance, impacting biodiversity and climate change resilience.