Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 29.3 Ferns and Other Seedless Vascular Plants Were the First Plants to Grow Tall
Seedless vascular plants like ferns were the first to grow tall and form the Earth's earliest forests. These plants developed new structural and reproductive adaptations—such as dominant sporophytes and lignified vascular tissues—that enabled them to thrive in diverse terrestrial environments.
Origins and Traits of Vascular Plants
- Fossils of vascular plants date to ~425 million years ago.
- Early vascular plants had branched, independent sporophytes.
- Key traits:
- Dominant sporophyte generation
- Vascular tissues (xylem and phloem)
- Roots and leaves, including sporophylls
- Advantages:
- Grow tall
- Compete for light
- Disperse spores farther
Life Cycle with Dominant Sporophytes
- Sporophyte is large, photosynthetic, and dominant.
- Ferns: visible leafy plant is sporophyte; gametophytes are small and soil-level.
- Spores form in sporangia and disperse via wind.
- Water still required for fertilization (sperm swim to egg).
Vascular Tissues
- Xylem:
- Transports water/minerals from roots
- Contains lignified tracheids
- Phloem:
- Transports sugars and organic molecules
- Made of living cells
- Lignin strengthens plant bodies for vertical growth
Evolution of Roots and Leaves
- Roots:
- Anchor plants and absorb nutrients
- May have evolved multiple times
- Leaves:
- Microphylls: small, single vein (lycophytes only)
- Megaphylls: larger, branched veins (most vascular plants)
- Sporophylls:
- Bearing sporangia—form sori in ferns, strobili in club mosses
- Some evolved heterospory: separate male and female spores
Diversity of Seedless Vascular Plants
Lycophytes (Phylum Lycophyta)
- Club mosses, spikemosses, quillworts
- Ancient species grew over 40 meters
- Modern ones are small, mostly homosporous
- Often grow as epiphytes
Monilophytes (Phylum Monilophyta)
- Includes ferns, horsetails, whisk ferns
- Ferns are most diverse (~12,000 species)
- Equisetum is the only living horsetail genus
- Whisk ferns lack roots and true leaves
Ecological Impact and the Carbon Cycle
- First forests formed during the Carboniferous period
- Reduced atmospheric CO₂:
- Roots promoted chemical weathering
- Organic remains buried—formed coal deposits
- Helped trigger global cooling, now carbon is re-released by humans
In a Nutshell
Ferns and other seedless vascular plants revolutionized land ecosystems by growing tall and forming the first forests. Their dominant sporophyte stage, vascular tissues, roots, and leaves marked major evolutionary advances. These plants not only shaped plant evolution but also impacted Earth's climate and geology on a global scale.