Chapter 25: Seedless Plants

Chapter 25: Seedless Plants

Learning Objectives

  • Explain the evidence linking the ancestors of plants to multicellular algae.

  • Describe plant adaptations key to the transition to land life.

  • Describe the characteristics and diversity of extant Bryophytes.

  • Describe the evolution of key adaptations of early vascular plants.

  • Describe the characteristics and diversity of extant seedless vascular plants.

Topics Overview

  1. Evolution of land plants: from water to land

    • Algal ancestry

    • Shared characteristics in green algae and plants

    • Derived characteristics of plants

  2. Major divisions of the seedless plants

    • Seedless nonvascular plants

    • Seedless vascular plants

Evolution of Land Plants

Algal Ancestry

  • Plants share a common ancestor with charophytes (green algae) within the archaeplastida supergroup of eukaryotes.

Shared Characteristics Between Green Algae and Plants

  • The following characteristics are present in Chlorophytes, Charophytes, and Plants:

    • Multicellularity

    • Cell walls composed of cellulose

    • Chloroplasts containing the same pigments (chlorophyll a and b)

    • Starch as the storage molecule

  • Visual Examples:

    • Chlorophyte example: Ulva

    • Charophyte example: Chara

    • Plant example: Moss

  • Physiological similarities suggest close evolutionary relationships.

Adaptations for Transition From Water to Land

  • By 470 million years ago, multicellular green algae began to migrate from shallow seas into rivers and lakes.

  • Major evolutionary adaptations included:

    • Water Uptake and Support: Evolved vascular tissue (e.g., xylem) to facilitate water absorption and support.

    • Reproductive Adaptations: Development of strategies for gamete dispersal in air rather than water.

    • Desiccation Resistance: Mechanisms to prevent water loss (e.g., waxy cuticle).

Advantages of Life on Land

  • Selection for land life provided several advantages:

    • Higher atmospheric CO2 concentrations

    • Increased light intensity

    • More available minerals

    • Lesser herbivore presence and competition

Challenges of Terrestrial Life

  • Major challenges faced by early land plants included:

    • Desiccation: Risk of drying out when exposed to air.

      • Both gametes and zygotes face the risk of desiccation.

      • Structural support is necessary in an environment lacking the buoyancy of water.

      • Male gametes must evolve new strategies for reaching female gametes, as swimming is not feasible.

    • Support: Plants must develop structural means to maintain integrity outside aquatic environments.

Derived Characteristics of Plants

  1. Alternation of Generations: Life cycle features haploid and diploid stages.

  2. Walled Haploid Spores: Protection for spores during dispersal.

  3. Multicellular Gametangia: Organs where gametes are produced and protected (antheridia and archegonia).

  4. Protected Sporophyte Embryos: Sporophyte embryos grow within the female gametophyte.

  5. Apical Meristems: Regions of continuously dividing cells at root and shoot tips.

  6. Waxy Cuticle: Develops to inhibit water loss and enhance desiccation resistance.

  7. Secondary Compounds: Evolution of chemicals that deter herbivores and competitors (e.g., caffeine, latex).

  8. Mycorrhizae: Mutualistic relationships with fungi for enhanced nutrient absorption.

Plant Anatomy Overview

  • Plant anatomy includes features such as leaves with stomata for gas exchange, waxy coatings to resist desiccation, and specialized functions for water and nutrient transportation.

Life Cycles of Plants
Haplontic vs. Diplontic Life Cycles

  • Haplontic: Dominant haploid stage (e.g., many plants).

  • Diplontic: Dominant diploid stage (e.g., humans).

  • Most plants exhibit alternation of generations, with the gametophyte stage dominant in lower plants and sporophytes becoming more prevalent through evolution.

Comparison to Charophytes

  • Charophytes exhibit a life cycle without alternation of generations; meiosis occurs solely in the zygote to produce four diverse offspring, all of which are haploid.

Seedless Nonvascular Plants (Bryophytes)

General Characteristics

  • Bryophytes are non-woody, small ground-covering plants requiring water for reproduction.

  • Have rhizoids for attachment but lack true roots.

  • Dominant haploid gametophyte form, which produces eggs and motile sperm.

  • The sporophyte depends on the gametophyte for nutrition and is found within the female gametophyte.

  • Includes three major phyla:

    • Marchantiophyta (Liverworts)

    • Anthocerotophyta (Hornworts)

    • Bryophyta (Mosses)

Liverworts (Marchantiophyta)

  • Most possess elevated gametophytes resembling miniature trees (e.g., Marchantia).

  • Sporophytes are reduced, some thalloid, others leafy.

  • Example: Marchantia polymorpha (thalloid liverwort)

Hornworts (Anthocerotophyta)

  • Recognized for their horn-like sporophytes.

  • Synergistic relationship with nitrogen-fixing cyanobacteria.

Mosses (Bryophyta)

  • Most numerous non-vascular plants.

  • Adapted to extreme environments such as mountain tops and tundras.

  • Moss sporophyte grows from the female gametophyte to maximize spore dispersal efficiency.

  • Ecological Importance:

    • Pioneer species in nutrient-poor soils.

    • Major primary producers in cold, high-altitude regions.

    • Sphagnum mosses (peat bogs) act as critical wetlands, harvested for fuel and preservation of organic materials.

Moss Life Cycle

  • Germination process begins as a spore develops into a protonema, differentiating into a ‘bud’ for sexual reproduction.

  • Gametophytes are produced, requiring moisture for growth.

Major Divisions of Seedless Vascular Plants

Overview of Seedless Vascular Plants (SVP)

  • Evolution of vascular tissue leads to taller plants.

  • Oldest SVP fossils date back approximately 425 million years (e.g., club mosses and ferns).

Characteristics of SVPs

  1. Branched Sporophytes: Independent from gametophyte for nutrition.

  2. Diploid Sporophyte Dominance: Diploid stage dominates life cycle.

  3. Transport System: Xylem (water and mineral transport) and phloem (transport of sugars and organic products).

  4. True Roots and Leaves: Adaptations for increased surface area and photosynthetic efficiency.

  5. Microphylls and Megaphylls:

    • Microphylls are small leaves with a single strand of vascular tissue.

    • Megaphylls are larger leaves with a branched vascular system.

Sporophylls in Seedless Vascular Plants

  • Sporophylls are leaves modified to produce sporangia.

    • Ferns have sporophylls with sori on their undersides.

    • Lycophytes have sporophylls modified into strobili (cone-like structures) where meiosis occurs.

Types of Spores in SVPs

  • Homosporous: Most SVPs produce one type of spore that typically develops into a bisexual gametophyte.

  • Heterosporous:

    • Megasporangium on a megasporophyll produces megaspores (female gametophytes).

    • Microsporangium on a microsporophyll produces microspores (male gametophytes).

1. Lycophytes (Club Mosses)

  • Current species are small, primarily tropical and temperate.

  • Well-adapted to desiccation; can survive drying out.

2. Monilophytes (Ferns and Relatives)

  • Diverse group including horsetails and whisk ferns.

  • Ferns are the most widespread and diverse, featuring large megaphylls and typically found in understory habitats.

Importance of Seedless Plants

  • Seedless plants, especially Sphagnum mosses, serve as biological indicators of environmental pollution and contribute to soil formation and nutrient cycling.

  • Historical significance of SVPs in coal formation highlights their role in past environments and climate stability.