Seedless Vascular Plants Overview

Overview of Seedless Vascular Plants

Introduction to Seedless Vascular Plants

• Seedless vascular plants emerged around 425 million years ago.

• Key advancements from primitive land plants (liverworts, hornworts, and mosses) include:

  • Development of vascular tissue (xylem and phloem), enabling taller growth.

  • Cuticle presence, aiding in adaptation to terrestrial environments.

Dominant Life Cycle Forms in Seedless Vascular Plants

• Photosynthetic nature: All seedless vascular plants are photosynthetic.

• Life cycle phase shifts:

  • Dominance of sporophyte over gametophyte (contrary to bryophytes where gametophyte is dominant).

  • Sporophytes are free-living and not dependent on gametophytes.

• Gametophyte reduction: More simplified and short-lived compared to bryophytes.

Reproductive Strategies

• Euagamous reproduction:

  • Sperm are motile; eggs are larger and non-motile.

  • Require water for sperm motility towards egg.

• Spores are dispersed as part of reproduction.

Structure of Sporophytes

• Dichotomous branching: Key feature of early sporophytes, enabling growth into two branches (Y-shaped).

• Lack of true roots and leaves in early forms; development of these structures occurred later through specialization.

• Examples:

  • Cooksonia: Early vascular plant, lacks true stems, leaves, or roots.

  • Lycophytes (e.g. club mosses): Retain primitive features such as simple structures and protostelic arrangement.

Evolution of Roots and Stems

• Root evolution: Transition from simple anchoring structures to complex rosettes, enhancing resource acquisition.

• Dimorphic rhizomes: Two distinct forms performing different functions (anchorage vs. nutrient absorption).

• Development of true roots with increased surface area for better absorption efficiency.

Leaf Types: Microphylls and Megaphylls

• Microphylls:

  • Simple leaves with single vascular strand.

  • No leaf gap initially; evolved from stem outgrowths.

• Megaphylls:

  • Complex leaves with multiple branched vascular strands and a leaf gap associated with stems.

• Evolutionary origin: Megaphylls theorized to have evolved from flattening and fusion of branches.

Vascular Tissue Types

• Xylem: Water/mineral transport, includes:

  • Tracheids: Thin, tapered water-conducting cells, present in non-flowering plants (primitive).

  • Vessel elements: Wider and longer, primarily in angiosperms (derived).

• Phloem: Food conduction, made up of sieve elements.

• Complexity of vascular tissue is correlated with evolutionary advancements in plants.

Types of Steles

• Protostele:

  • Simple cylindrical arrangement; present in primitive vascular plants.

• Siphonostele:

  • More evolved; features pith surrounded by vascular tissue.

• Eustele:

  • Discrete vascular bundles with pith; typical in seed plants.

Spore Production: Homosporous vs. Heterosporous

• Homosporous:

  • Produces a single spore type; results in bisexual gametophytes (e.g., most ferns).

• Heterosporous:

  • Produces both male and female spores; leads to unisexual gametophytes (e.g., some lycophytes).

  • Megaspores for female gametophyte, microspores for male gametophyte.

Major Groups of Seedless Vascular Plants

Lycopodiophyta

• Originated around 408 million years ago; ancient lineage.

• Includes club mosses, with homosporous life cycle and microphyll leaves.

• Strobilus present at tip, where sporophylls are located.

Monilophyta (Ferns)

• Over 12,000 species, emerging about 360 million years ago.

• Common characteristics: Rhizomes, megaphylls, homosporous gametophytes.

• Sori on the underside of leaves (fronds) contain sporangia for reproduction.

Equisetophyta (Horsetails)

• Characterized by jointed stems and solid nodes.

• Homosporous, producing spores at the strobilus tip.

Psilotophyta (Whisk Ferns)

• Simplest aerial structure, lacking true leaves and roots.

• Contains fused sporangia in threes.

• Represents primitive features among vascular plants.

Key Takeaways for Exam

• Recognize shared traits of seedless vascular plants.

• Distinguish between primitive and advanced forms, gametophyte evolution, and vascular tissue types.

• Be able to outline major plant groups within seedless vascular plants including their characteristics and evolutionary traits.

• Understand life cycle variations between homosporous and heterosporous plants.