Seedless Vascular Plants

Seedless Vascular Plants

  • Seedless vascular plants dominated the landscape approximately 350 million years ago.

  • Significance:

    • Horticultural significance.

    • Ecological significance:

    • Brake fern (Pteris vittata) is capable of removing arsenic from soil and water.

    • These plants often engage in mutualistic associations.

    • Other Uses:

    • Fiddleheads, which are the young coiled leaves of some ferns, are one of the few edible parts of ferns.

    • Ostrich fern is notable for its edible fiddleheads.

Geological Time Scale of Vascular Plants

  • Major Geological Periods:

    • Age of Dinosaurs:

    • Starts around 230 million years ago.

    • Significant Plants:

    • First roots, leaves, and trees emerged.

    • Evolution of seed plants and flowering plants occurred.

  • Timeline of Major Events:

    • 500 MYA: First land plants (Green Algae).

    • 420 MYA: Appearance of early vascular tissues.

    • 395 MYA: First trees.

    • 345 MYA: First seeds.

    • 280 MYA: Appearance of gymnosperms.

    • 225 MYA: Lower vascular plants.

    • 195 MYA: Angiosperms (flowering plants) evolve.

    • 135 MYA: First flowers and fruits.

  • Diversity in Vascular Plants:

    • Dicots and Monocots arise.

Examples and Adaptations in Ferns

  • Lygodium:

    • Represents a global problem due to its invasive nature.

  • Azolla:

    • Forms a mutualistic association with the cyanobacteria Anabaena azollae, which enhances nitrogen fixation in rice fields.

Fern Adaptations and Research

  • Ferns as Ancient Species:

    • Believed to have coexisted with dinosaurs over 200 million years ago.

    • Recent studies indicate a group of Andean ferns evolved new forms and structures in the last 2 million years to adapt to extreme high-altitude environments.

  • Ecological Importance:

    • The páramo ecosystem was significantly influenced by continental uplift in the Andes, allowing ferns to exploit new ecological niches.

Structure of Vascular Plants

  • Vascular plants are characterized by:

    • Two Apical Meristems:

    • Root meristem

    • Shoot meristem

    • Three Tissue Systems:

    • Dermal Tissue System: Protects the plant.

    • Ground Tissue System: Involved in storage, photosynthesis, and support.

    • Vascular Tissue System: Transports water, nutrients, and food.

Water Transport in Vascular Plants

  • Transpiration Process:

    • Water vapor exits through stomata.

    • Cohesion and adhesion of water molecules assist in lifting water from roots through xylem.

  • Xylem Structure:

    • Contains Tracheary Elements:

    • Tracheids: Lack perforation plates in end walls.

    • Vessel Elements: Have perforation plates in end walls.

Leaf Evolution

  • Microphylls:

    • Simple leaves with a single vein.

  • Megaphylls:

    • Complex leaves with branched veins.

Lifecycle of Vascular Plants

  • Generalized Lifecycle:

    • Alternation of heteromorphic generations (sporophyte vs. gametophyte).

    • Sporophyte generation is larger and more complex than gametophyte.

  • Production Types:

    • Both sporophyte and gametophyte stages in seedless vascular plants exhibit independence at maturity, unlike bryophytes and seed plants where dependency varies.

Phyla of Seedless Vascular Plants

  • Dominated during the Devonian period (350 million years ago):

    • Most important phyla:

    • Rhyniophyta

    • Zosterophyllophyta

    • Trimerophytophyta

  • Homospory vs. Heterospory:

    • Homospory: Production of one type of haploid spore, leading to either separate male and female gametophytes or bisexual gametophytes.

    • Heterospory: Production of two types of spores (microspores and megaspores) in distinct sporangia; microspores lead to male gametophytes, megaspores lead to female gametophytes.

    • Heterospory is a precursor to seed formation.

Modern Classification of Seedless Vascular Plants

  • Phyla:

    • Lycopodiophyta (lycophytes) includes:

    • Lycopodiaceae (club mosses)

    • Selaginellaceae (spike mosses)

    • Isoetaceae (quillworts)

    • Monilophyta includes:

    • Psilotopsida (whisk ferns)

    • Marattiopsida (small tropical ferns)

    • Polypodiopsida (true ferns)

    • Equisetopsida (horsetails)

Extinct Lycophyte Trees

  • Ancient lycophytes dominated moist tropical environments approximately 325-280 million years ago during the Carboniferous period.

  • All lycophytes possess microphylls characterized by a single vein.

Life Cycle of Lycopodium

  • Example of homospory, lifecycle includes:

    • Meiosis occurs in sporophyte, producing spores

    • Gametophyte has a small underground structure (~1 cm)

Life Cycle of Selaginella

  • An example of heterospory:

    • Endosporic development takes place largely inside spore walls.

    • Produces microspores (male) and megaspores (female).

Ecology and Importance of Ferns

  • Ferns play an integral role in plant evolution, leading to diversity in seed plants.

  • Resurrection Plant:

    • Selaginella lepidophylla can recover after rehydration.

Phylum Monilophyta

  • Classes:

    • Featuring four classes

    • Psilotopsida

    • Marattiopsida

    • Polypodiopsida

    • Equisetopsida

Equisetopsida (Horsetails)

  • Represented by Equisetum, which may be the oldest surviving genus of plants.

  • Lifecycle:

    • Homosporous with strobili on fertile shoots.

    • Clusters of sporangia form in umbrella-like structures called sporangiophores.

Reproductive Structures in Ferns

  • Sori:

    • Clusters of sporangia on sporophyte leaves.

  • Indusium:

    • Protective covering over developing sporangia.

    • Sori are primarily located on the underside of leaves.

Alternation of Generations in Ferns

  • Basic Lifecycle:

    • Involves gametes, zygote, embryo, and the multicellular sporophyte and gametophyte stages.

Fertilization and Spores

  • Fertilization generally takes place in the archegonium of the gametophyte.

  • Sporangia produce spores via meiosis, and spore dispersal occurs through structures called annuli.

Questions for Review

  1. Groups of sporangia on fertile fern leaves are called: e) sori.

  2. Seedless vascular plants include: d) ferns.

  3. Ferns in the genus Lygodium: b) have become pests in some areas where they are not native.

  4. Which statement is false about early land life? c) A multicellular embryo is protected within the female parent.

  5. The following statement is correct: e) None of the above.

  6. In bryophytes, fertilization occurs in e) Archegonium.

  7. Tracheids differ from vessel elements in that tracheids b) are less-specialized cells.

  8. The main tissue systems of vascular plants are the d) dermal, vascular, and ground systems.

Acknowledgments

  • Lecture materials and images sourced from: Introduction to Botany by Murray W. Nabors, with contributions from Professor Rowan Sage, University of Toronto.