Seedless Vascular Plants Updated

Seedless Vascular Plants

  • Includes ferns, horsetails, club mosses, and whisk ferns.

  • Characterized by:
      - Sporophyte dominant phase.
      - Gametophyte is small but still tied to water for reproduction.
      - Dominant plants during the late Devonian to early Permian periods.
      - Associated with equatorial coal forests in the past.
      - Today, seedless vascular plants are considered somewhat marginal yet successful.
      - Some exhibit heterospory, the production of two distinct types of spores.

Carboniferous Coal Forest Model

  • Represents the ecosystem from approximately -300 my.

Origin and Evolution of Land Plants

  • Ancestral lineage traced from:
      - Green Algae → Charophyceans → Bryophytes (nonvascular plants) → Mosses
      - Origin of land plants is estimated at about 475 mya.
      - Lycophytes (club mosses, spike mosses, quill worts) emerged.
      - Pterophytes (ferns, horsetails, whisk ferns) evolved from vascular plants around 420 mya.
      - Seed plants appeared around 360 mya, leading to gymnosperms and angiosperms.

Trends in Plant Evolution

  • Continuation of trends includes:
      - Reduction of water-dependent gametophyte phases.
      - Development of vascular tissue enhancing water and food transport capabilities.
      - Increased efficiency of root, stem, and leaf structures.
      - Stomates are present, essential for gas exchange; all advancements are seen predominantly in the sporophyte generation.

Classification of Plant Types

  • Bryophytes:
      - No seeds, roots, or vascular tissue; includes mosses, liverworts, and hornworts.

  • Seedless Vascular Plants:
      - Do not produce seeds; includes ferns, horsetails, club mosses, and whisk ferns.

  • Gymnosperms:
      - Produce seeds and cones; includes conifers, cycads, ginkgo, and gnetophytes.

  • Flowering Plants:
      - Possess seeds, flowers, and fruits; divided into monocots and dicots.

Cross-Sections of Plant Stems (C.S.)

  • Moss Stem:
      - Composed of a central cylinder, phloem, and xylem

  • Fern Stem:
      - Comprises epidermis, cortex (parenchyma), and other structures.

  • Pine Stem:
      - Features phloem, cortex, pith, and xylem.

  • Woody Dicot Stem:
      - Cross-section includes various tissue types for support and conducting function.

Diversity of Seedless Vascular Plants

  • Estimated numbers:
      - Ferns: 11,000 species
      - Club Mosses: 1,000 species
      - Horsetails: 15 species
      - Whisk Ferns: 12 species

  • Seedless vascular plants are defined as plants with specialized tissues for conducting water and nutrients and reproduce via spores rather than seeds.

Phyla of Seedless Vascular Plants

  1. Phylum Psilophyta: Whisk ferns.

  2. Phylum Lycopodophyta: Club mosses.

  3. Phylum Sphenophyta: Horsetails.

  4. Phylum Pterophyta: Ferns.

  • These phyla consist of plants with distinct tissues for conduction and rely on spores for reproduction.

Evolution of Vascular Tissue

  • Vascular plants feature specialized cells for water conduction:
      - Tracheids: Hollow cells with thick walls that can withstand pressure; they connect end-to-end and are key components in xylem.
      - Phloem: Type of vascular tissue responsible for transporting nutrients and carbohydrates produced during photosynthesis.

  • The presence of lignin makes cell walls rigid, allowing plants to grow upright and achieve greater heights.

Characteristics of Seedless Vascular Plants

  • Include club mosses, horsetails, whisk ferns, and ferns, which possess:
      - Roots: Absorb water and nutrients.
      - Leaves: Serve photosynthesis; contain vascular tissue in xylem and phloem veins.
      - Stems: Connective structures for the transportation of resources between roots and leaves.

Whisk Ferns (Psilotum)

Characteristics:

  • Considered to be descendants of early vascular plants, particularly Rhyniophytes.

  • Not classified as true ferns due to the absence of true leaves and roots; possess creeping rhizomes.

  • Primarily homosporous, producing one type of spore.

  • Can grow as epiphytes or on various substrates.

  • Once dominated ecosystems prior to 300 my ago.

Life Cycle of Whisk Fern (Psilotum)

  • Stages:
      - Zygote (2n) develops into a sporophyte (stem) where the embryonic synangium is located.
      - The zygote's fertilization leads to spore production through meiosis.
      - Spores eventually develop into gametophytes (n), bearing antheridia (sperm) and archegonia (eggs).

Club Mosses

Characteristics:

  • Once ruled the landscape approximately 300 my ago.

  • Strobili present for spore production.

  • Exhibit both homosporous and heterosporous reproduction:
      - Homosporous: Produces one type of spore.
      - Heterosporous: Produces both male microspores (n) and female megaspores (n) which yield respective male and female gametophytes.

Life Cycle of Club Mosses

  • After fertilization occurs, the sporophyte (2n) arises and the cycle continues with strobili forming and spore production.

Horsetails (Equisetum)

Characteristics:

  • Once prominent species around 300 my ago with distinct roots, stems, and reduced leaves.

  • Stems are hollow, jointed, and have silica deposits; they are green and participate in photosynthesis.

  • The reproductive branches culminate in terminal strobili.

  • Life cycle parallels that of ferns and necessitates water for fertilization.

Life Cycle of Horsetails

  • Similar to ferns, involving alternation of generations between sporophyte (2n) and gametophyte (n) stages.

  • Fertile shoots emerge from the main vegetative strobilus shoot.

Ferns

Characteristics:

  • Conductive tissues, including xylem and phloem, enable larger structures and support true leaf evolution.

  • Two basic leaf types identified:
      1. Microphylls: Single vascular strand, typically small.
      2. Megaphylls: Multiple vascular strands, generally larger.

  • Sporophyte is the dominant generation, consisting of rhizomes, fronds (which are young leaves known as fiddleheads), and mature sporophytes that produce spores in clusters called sori.

Life Cycle of Ferns

  • The sporophyte stage is the dominant generation.

  • Sporophytes produce haploid spores in sporangia, which upon release germinate into haploid gametophytes.

  • Gametophytes are independent of sporophytes and feature antheridia and archegonia for gamete production.

  • Fertilization occurs when sperm swims to the egg in water, leading to the formation of a zygote which subsequently develops into a new sporophyte.

Relationship Between Sporophyte and Gametophyte

  • The sporophyte continues to grow while the gametophyte phase dies off.

Important Considerations and Applications

  • Understanding seedless vascular plants contributes to knowledge about plant evolution and ecological dynamics.

  • These plants have implications for coal formation and ancient ecosystem structures, along with current biodiversity studies that evaluate their roles within various environments.