BIOL 108: Seedless Vascular Plants Study Notes

Bryophyte Dominance:
- Bryophytes dominated terrestrial vegetation for approximately 100 million years during plant evolution.
- The earliest fossils of vascular plants date back to around 425 million years ago (mya).
- Early vascular plants, like Aglaophyton, displayed anatomical features that were intermediate between bryophytes and fully developed vascular plants:
- They had independent, branching sporophytes.
- These sporophytes did not rely continuously on gametophytes for sustenance.
- Nevertheless, comparable to bryophytes, they lacked both true leaves and roots, and did not contain true vascular tissues.
- Subsequent fossil findings demonstrated:
- The gradual evolution of vascular tissue.
- The eventual emergence of leaves and roots.

Vascular Tissues:
- Vascular tissue evolved exclusively within the sporophytes of vascular plants.
  - Evolutionary Advantage: The presence of vascular tissue allowed sporophytes to attain greater heights, thereby gaining an evolutionary advantage over nonvascular plants.
  - Contrast with Gametophytes: Gametophytes do not possess vascular tissues.
- Components of Vascular Tissues:
  - Xylem:
  - Conducts water and minerals through dead, hollow cells, forming continuous conduits throughout the plant.
  - These water-conducting cells, reinforced with lignin, provide structural support.
  - Phloem:
  - Composed of living cells, facilitating the distribution of nutrients and organic products.
Life Cycles with Dominant Sporophytes:
- Sporophytes are predominant in terms of size, complexity, and longevity.
- They are independent and not consistently reliant on gametophytes.
Well-Developed Roots and Leaves:
- Complex multicellular roots serve to anchor vascular plant sporophytes and assist in the absorption and transportation of water and nutrients from the soil.
  - Roots may have evolved from subterranean stems.
- Complex multicellular leaves enhance the surface area of vascular plant sporophytes, increasing solar energy capture for photosynthesis.
- In contrast, bryophyte sporophytes do not develop true roots and leaves.

Types of Leaves:
- Microphylls:
- Small leaves characterized by a singular vein; potentially evolved as outgrowths of stems.
- Megaphylls:
- Larger leaves characterized by highly branched vascular systems; possibly evolved from webbing between flattened branches.
Sporophylls:
- Vascular plants possess sporophylls, which are modified leaves that bear sporangia (spore-producing organs).
- Sori:
  - Clusters of sporangia located on the undersides of sporophylls, observed in entities like ferns.
- Strobili:
  - Cone-like structures formed from groups of sporophylls, notable in lycophytes and most gymnosperms.
Variation in Spore Sizes Ubiquitously Across Taxa:
- Most seedless vascular plants are homosporous, producing a singular type of spore that develops into a bisexual gametophyte.
- Conversely, all seed plants and some seedless vascular plants are heterosporous, producing:
- Megaspores: Developing into female gametophytes.
- Microspores: Developing into male gametophytes.

Vascular tissue facilitates the height growth of seedless vascular plants.
Seedless vascular plants, associated with flagellated sperm, require a thin layer of water for fertilization.
- Similar to bryophytes, these plants predominantly thrive in relatively moist habitats.

Historical Dominance:
- Giant lycophyte trees once dominated the Carboniferous swamps but went extinct as climatic conditions changed (cooled and dried).
Contemporary Survivors:
- Existing lycophyte species are small herbaceous plants.
- All lycophytes possess microphylls and can be either homo- or heterosporous.
- It's crucial to note that club mosses and spike mosses are vascular plants, contradicting their names.

Morphological Diversity:
- Monilophytes exhibit a diverse morphology, including whisk ferns, horsetails, and ferns.
- Whisk Ferns:
  - Resemble ancestral vascular plants; however, they have lost true roots and leaves secondarily.
- Horsetails:
  - Characterized by brushy stems, once thriving during the Carboniferous (grew to 15 m), now represented predominantly by the genus Equisetum (comprises 15 species).
  - Horsetails feature leaves with a single vein, showing secondary microphyllous characteristics; this indicates a simplification compared to ancestral complex leaf venation.

Diversity:
- Ferns are the most diverse group of seedless vascular plants, encompassing around 12,000 species.
- They are predominantly found in tropical regions but also flourish in temperate forests.
- Some ferns might reach tree-sized dimensions (tree ferns).
Leaf Structure:
- Ferns possess megaphylls, which are large leaves with branched vascular systems.
- Clusters of sporangia (sori) are usually found on the undersides of sporophylls.
- Spores are dispersed via wind; many ferns are homosporous.

Dominance during Geological Eras:
- Ancestors of modern lycophytes, horsetails, and ferns dominated the Devonian and Carboniferous periods, contributing to the formation of the first forests.
- The plant material from these Carboniferous forests underwent partial decay, leading to the formation of coal deposits.
- Increased rates of growth and photosynthesis among these plants during this epoch possibly aided in global cooling by reducing atmospheric CO₂ levels:
- Recorded an approximately 5-fold decrease in atmospheric CO₂ during the Carboniferous period.