Evolution of Seed Plants

Chapter 26: Seed Plants

The Evolution of Seed Plants

  • Seed plants have dramatically transformed the course of plant evolution, enabling them to become the dominant producers in most terrestrial ecosystems.

  • Originated approximately 360 million years ago.

  • A seed consists of:
      - An embryo
      - Nutrients
      - A protective coat

  • Seeds can disperse over long distances through:
      - Wind
      - Other means (e.g., animals)

Classification of Living Seed Plants

  • Living seed plants can be divided into two clades:
      - Gymnosperms:
        - Appeared early in the fossil record about 305 million years ago.
        - Dominated terrestrial ecosystems during the Mesozoic era (251-65.5 million years ago).
      - Angiosperms:
        - Began to replace gymnosperms near the end of the Mesozoic era.
        - Currently dominate most terrestrial ecosystems.

Timeline of Major Events in Plant Evolution

  • Prehistoric Timeline:
      - Precambrian: Life begins in the sea (approximately 4 billion years ago).
      - Paleozoic Era:
        - 542 MYA: Major diversification of animal life (Cambrian)
        - 359 MYA: First trees and seed ferns (Carboniferous)
        - 299 MYA: Major extinction event (Permian)
        - 251 MYA: First birds and diversification of dinosaurs (Triassic)
        - 145 MYA: First primates and flowering plants (Cretaceous)
      - Mesozoic Era:
        - 251-65.5 MYA: Dominance of gymnosperms
      - Cenozoic Era:
        - 65 MYA - present: Mammals diversify; human evolution follows.

Key Adaptations in Seed Plants

  • The following adaptations are essential for the survival of all seed plants:
      - Reduced Gametophytes: Typically microscopic and dependent on surrounding sporophyte tissue for nutrition.
      - Heterospory: Seed plants are heterosporous; they produce two types of spores.
        - Megasporangia: Produce megaspores which give rise to female gametophytes.
        - Microsporangia: Produce microspores which give rise to male gametophytes.
      - Ovules: Each ovule consists of a megasporangium, a megaspore, and one or more integuments.
        - Gymnosperm megasporangia typically possess one integument.
        - Angiosperm megasporangia usually have two integuments.
      - Pollen: Developed from microspores and serves as the male gametophytes.

Ovules and the Production of Eggs

  • An ovule comprises:
      - Megasporangium
      - Megaspore
      - Protective integument layer of sporophyte tissue.

  • Structure of an unfertilized ovule includes:
      - Integument (2n)
      - Megaspore (n)
      - Megasporangium (2n)
      - Micropyle

  • Upon fertilization, the ovule develops into a seed containing:
      - Seed coat
      - Food supply (n)
      - Embryo (2n)

Pollen and Sperm Production

  • Microspores evolve into pollen grains containing male gametophytes.

  • Pollination involves transferring pollen to the part of the plant containing ovules, thus eliminating the requirement for water films for fertilization.

  • If a pollen grain germinates, it gives rise to a pollen tube that releases sperm into the female gametophyte inside the ovule.

Evolutionary Advantages of Seeds

  • A seed is a sporophyte embryo contained in a protective coat with a food supply.

  • Seeds provide several evolutionary advantages over spores:
      - Dormancy for periods varying from days to years until favorable conditions arise for germination.
      - Stored food (endosperm) for early developmental stages.
      - Potential for long-distance dispersal facilitated by wind or animals.

Seed Dispersal Mechanisms

  • Seeds redistribute through various mechanisms:
      - Wind: E.g., dandelions, milkweed, and maple seeds.
      - Animals: E.g., birds and mammals disperse seeds by consuming fruits.
      - Water: Certain seeds can float.
      - Bursting: Some plants disperse seeds explosively.
      - Human involvement: Interaction with agriculture and gardening practices aids in seed dispersion.

Gymnosperms vs. Angiosperms

  • Gymnosperms:
      - Defined as “naked seeds.”
      - Seeds are exposed on sporophylls that form cones.
      - Most are cone-bearing plants known as conifers.

  • Angiosperms:
      - Seed are encased in fruits, which are mature ovaries.

Life Cycle of Gymnosperms

  • Key features of the gymnosperm life cycle:
      - Miniaturization of gametophytes
      - Development of seeds from fertilized ovules
      - Transfer of sperm via pollen

  • Example life cycle of a pine includes:
      - Microsporangia produce microspores, which develop into pollen grains.
      - Megasporangia contain ovules, producing megaspores that develop into female gametophytes.

  • Time from cone production to maturation of seed is approximately three years.

Gymnosperm Diversity

  • Conifers dominate specific regions, particularly in northern latitudes.

  • The gymnosperms comprise four phyla:
      1. Coniferophyta: Includes species like pine, fir, and redwood.
      2. Cycadophyta: Comprising cycads, often mistaken for palms due to leaf shape.
      3. Ginkgophyta: Contains one living species, Ginkgo biloba.
      4. Gnetophyta: Includes three genera: Gnetum, Ephedra, and Welwitschia.

Angiosperms: Reproductive Adaptations

  • Angiosperms are the most widespread and diverse group of plants, identified by reproductive structures known as flowers and fruits.

  • Flowers:
      - Specialized for sexual reproduction, comprising four floral organs:
        1. Sepals: Enclose and protect the flower.
        2. Petals: Brightly colored to attract pollinators.
        3. Stamens: Produce pollen, consisting of the filament and anther.
        4. Carpels: Produce ovules, with an ovary and style leading to the stigma.

  • Flowers can be classified as:
      - Complete Flowers: Contain all four organs.
      - Incomplete Flowers: Lack one or more organs.

  • Stamens and Carpels can be located within the same flower (perfect/bisexual) or on separate flowers (imperfect/unisexual).

Overview of Angiosperm Life Cycle

  • The angiosperm life cycle involves the following processes:
      - The flower contains male (pollen grains from anthers) and female (ovules in the ovary) structures.
      - Pollen released contains male gametophytes.
      - Female gametophytes (embryo sacs) develop within ovules in the ovary.
      - Mechanisms often exist to promote cross-pollination between flowers of the same species.

  • Significant stages in angiosperm reproduction include meiosis and fertilization leading to the development of a seed.

Angiosperm Diversity

  • House more than 300,000 living species.

  • Historically divided into two main groups:
      - Monocots:
        - Have one cotyledon, parallel leaf venation, scattered vascular tissue, and floral organs in multiples of three.
      - Eudicots:
        - Feature two cotyledons, net-like venation, vascular tissue arranged in a ring, and floral organs typically in multiples of four or five.

  • Monocots: Comprise significant families including orchids, grasses, and palms.

  • Eudicots: Encompass extensive families such as legumes and economically significant rose family.