Seed Plant Innovations

• Week 6 covers seed plant innovations.

Dominant Vegetation Across Geological Periods

• During the Carboniferous period (~359-299 Mya), lycopods and ferns were the dominant vegetation.
• In the Jurassic period (~252-145 Mya), gymnosperms were the dominant vegetation.

Early Gymnosperms

• Pteridosperms, or "seed ferns," were the first seed-bearing plants, evolving in the Late Devonian period.
• Pteridosperms are a polyphyletic group with multiple lineages that vegetatively resemble ferns.
• They had primitive seeds and secondary growth (true wood).
• Pteridosperms were abundant during the Carboniferous and Permian periods, replacing lycopod scale trees in the late Permian and declining through the Mesozoic era.
• Progymnosperms, known from the Devonian period, are a paraphyletic group of fern-like plants from which seed plants likely evolved.
• Progymnosperms had true wood but reproduced by spores, with Archaeopteris being the most known genus.
• Progymnosperms existed throughout the Carboniferous and Permian but became extinct by the end of the Permian period.
• The existence of Pteridosperms and Progymnosperms suggests wood and seeds evolved multiple times in plant history.

Permian-Triassic Mass Extinction

• The Permian-Triassic Mass Extinction occurred 252 Mya.
• Flood basalt volcanism from the Siberian Traps caused CO₂ levels to rise from ~400ppm to ~2500ppm, leading to an 8°C increase in global mean temperature.
• Release of sulfur dioxide caused oceans to become euxinic, resulting in acid rain and degradation of the ozone layer.
• This mass extinction caused the extinction of over 50% of all biological families, 80% of marine species, and 70% of terrestrial vertebrates.
• The impact on land plants appears less severe at the genus level according to recent analyses.
• The fossil record indicates significant reorganization of ecosystems due to changes in temperature, CO₂ levels, and ultraviolet radiation.
• Dense gymnosperm woodlands in many regions were replaced by herbaceous lycopods and ferns and the recovery of woody trees took 8 million years.

Evolution and Diversification of Seed Plants

• Seed plants first arose near the end of the Devonian period.
• Gymnosperms emerged as a recognizable group by the Carboniferous period.
• Gymnosperms diversified in the Permian and dominated the Mesozoic era (Triassic, Jurassic, Cretaceous) periods.
• Angiosperms did not diversify until the Cretaceous period (~140 Mya).

Traits of Seed Plants

• Seed plants have sporophyte-dominant life cycles with free-living sporophytes.
• Seed plants have a loss of free-living gametophytes, where the haploid generation is wholly dependent on the sporophyte.
• Modern extant seed plants include gymnosperms (four divisions) and angiosperms (Magnoliophyta).
• Both gymnosperm and angiosperm lineages have distinct reproductive and vegetative traits that differ from seedless vascular plants.

Seeds

• A seed is a specialized structure that encases the embryo within a protective seed coat and contains a food reserve (endosperm).
• The sporophyte produces a megasporangium inside an ovule, protected by the integument.
• The diploid megasporangium produces a haploid megaspore.
• The megaspore grows into a multicellular, haploid female gametophyte that produces an egg.
• When the egg is fertilized by pollen entering the micropyle, the zygote grows into a diploid embryo (new sporophyte).
• The seed coat is derived from the diploid integument.
• In gymnosperms, the female gametophyte tissue continues to grow, forming the haploid endosperm that serves as seed food reserves.

Seed Evolution

• Progymnosperms evolved a heterosporous life cycle, producing megaspores and microspores.
• Progymnosperms retained megaspores in the megasporangium, similar to some lycophytes like Selaginella.
• Progymnosperms evolved to have three of the four megaspores produced by meiosis disintegrate leaving only one functional megaspore.
• The integument evolved in progymnosperms as fused lobes of leaf tissue to protect the single functional megaspore as it grows into the megagametophyte within the megasporangium with the integument it is considered an ovule.
• A small opening in the integument, which encases the egg inside the megagametophyte, called the micropyle, requires pollen for fertilization.

Pollen

• Pollen is the immature male gametophyte.
• Seed plants evolved in a way that the microsporangium produces microspores which then develop into microgametophytes (pollen grains) that are retained inside the microsporangium, undergoing only a few mitotic divisions.
• Much of the dispersed pollen (usually by wind) does not find an ovule and perishes.
• Pollen grain enters a micropyle and continues to develop through mitosis, producing two sperm cells and a pollen tube.
• The pollen tube transmits the sperm to the egg.
• Gymnosperms use a pollination drop to capture wind-blown pollen, a sticky liquid that shrinks as it dries, pulling the pollen grains into the micropyle.

Gymnosperm Advantages

• The lack of free-living gametophytes and free-swimming sperm enables reproduction without standing water.
• This was a major advantage as the climate became cooler and drier during the Carboniferous and Permian periods.
• Retention of the gametophyte generation provides long-range dispersal in both generations through seeds (sporophytes) and pollen (gametophytes).
• Spores previously dispersed over long distances, but gametophyte fertilization required short distances.

Gymnosperm Vegetative Traits

• Gymnosperms have megaphylls, originally fern-like in progymnosperms, but now modified into simple leaves, needles, or scales in the four extant divisions.

Gymnosperm Wood Characteristics

• All extant gymnosperms are perennial, long-lived plants with a stem vascular system made of rings of vascular bundles from produced by the vascular cambium.
• The secondary meristem region produces secondary xylem and phloem, rays (made of parenchyma), and bark.
• Secondary xylem is composed of tracheids that vary in size across the growing season, forming annual rings.
• Gymnosperm tracheids feature the torus-margo pit, an important innovation.
• Ferns lack the flexible permeable margo membrane with an impermeable torus found in gymnosperms, which forms a rapid seal to protect against cavitation.
• The torus-margo pit improves drought resistance by slowing air embolisms.
• Cavitation occurs due to:
  • Negative hydrostatic pressure (drought or high transpiration).
  • Freezing (air bubbles coming out of solution upon thaw).
  • Damage allowing air into the xylem.
• Small-diameter tracheids and torus-margo pits reduce cavitation from frost and drought, respectively.