Lecture 22: Seedless Vascular Plants

Seedless Vascular Plants

Lycophytes, Monilophytes (ferns)

Evolution of Vascular Plants

• Vascular tissue is an important adaptation to life on land.

• It enables the transport of water and nutrients.

• Seeds evolved to protect and nourish the sporophyte embryo.

Dominant Sporophyte

• The sporophyte is the dominant and independent stage in vascular plants.

Xylem and Phloem

• Xylem transports water through tracheids.

• Vascular plants are also known as "tracheophytes" because of this.

• Cell walls are fortified with the polymer lignin.

• Phloem transports sugars.

• Rhyniophytes had a "protostele" organization, with phloem surrounding xylem.

Rhyniophytes

• Rhyniophytes were the first vascular plants.

• They possessed sporangia.

Plant Complexity

• Larger, more complex plants evolved branching.

• The first branching was dichotomous, not apical.

• Extant tracheophytes possess leaves and roots unlike Rhyniophytes.

• Rhyniophytes had an underground stem.

Evolution Timeline

• Origin of plants: approximately 470 million years ago (mya).

• Origin of vascular plants: approximately 425 mya.

• Origin of seed plants: approximately 360 mya.

• Lycophytes and Monilophytes diverged before the origin of seed plants.

• Nonvascular plants (bryophytes): Liverworts, Mosses, Hornworts.

• Seedless vascular plants: Lycophytes (club mosses, spikemosses, quillworts), Monilophytes (ferns, horsetails, whisk ferns).

• Seed plants: Gymnosperms, Angiosperms.

Lycophytes

• Have roots and small leaves.

Microphylls vs. Megaphylls

• Leaves of lycophytes are not homologous to leaves of ferns and seed plants.

• Lycophytes have microphylls.

• Monilophytes, gymnosperms, and angiosperms have megaphylls.

Heterospory

• Lycophytes evolved a heterosporous life cycle in some lineages.

• Homosporous spore production (most seedless vascular plants):

  • Sporangium on sporophyll produces a single type of spore.

  • The spore develops into a typically bisexual gametophyte.

  • The gametophyte produces both eggs and sperm.

• Heterosporous spore production (all seed plants):

  • Megasporangium on megasporophyll produces megaspores, which develop into female gametophytes that produce eggs.

  • Microsporangium on microsporophyll produces microspores, which develop into male gametophytes that produce sperm.

• Selaginella kraussiana has a strobilus with megasporangia containing megaspores and microsporangia containing microspores.

Secondary Growth

• Some lycophyte lineages evolved secondary growth analogous to that seen in seed plants.

• Wood Growth and Structure:

  • Key components include outer bark, latewood, earlywood, pith, sapwood, phloem, cambium, and heartwood.

Lycophyte History

• Lycophyte "trees" once dominated the world.

Monilophytes

• As with lycophytes, the dominant stage is the sporophyte.

• The gametophyte is free-living and low to the ground, requiring water for fertilization.

Fern Life Cycle

• Haploid (n) and Diploid (2n) stages alternate.

• Spores are produced by meiosis in the sporangium.

• Spores disperse and develop into young gametophytes.

• Mature gametophytes possess rhizoids and archegonia on the underside.

• Antheridia produce sperm.

• Fertilization results in a zygote (2n), which develops into a new sporophyte.

• The young sporophyte grows from the gametophyte.

• Sporangia are found in sori on the underside of sporophyte leaves (fronds).

• Fiddleheads are young leaves.

• Homospory is common, but heterospory also evolved in some monilophytes.

Monilophyte Leaves

• Compound megaphylls arise from an underground stem.

• Sporophyll is a spore-bearing leaf.

• A strobilus (cone) is a branch with densely packed sporophylls.

Strobili

• Strobili can be macroscopic or microscopic.

• Homosporous strobili are found in some monilophytes.

• Heterosporous lycophytes, such as Selaginella, also possess strobili.

“Euphyllophytes”

• “Euphyllophytes” include ferns and seed plants (plants with megaphylls).

• The difference between microphylls and megaphylls is in origin, not necessarily in size.

Psilotum

• Monilophytes include Psilotum, which exhibits dichotomous branching, lacks leaves and roots and has terminal sporangia.

• Originally thought to be very old but actually an example of convergent evolution.

Equisetum

• Monilophytes include Equisetum (horsetail fern).

• It's considered a "living fossil" and dominated the prehistoric world with tree-like ancestors.

Carboniferous Period

• Large lycophytes, ferns, and equisetum dominated the planet's flora during the Carboniferous Period.

Tree Ferns

• A few tree fern species remain today.

• They do not produce seeds.

Summary: Lycophytes and Monilophytes

• Lycophytes and monilophytes (ferns) are grouped because they lack the homology of vascular tissue.

• They represent an important turning point in the story of land plants.

• Vascular tissue allows the sporophyte life stage to grow BIG.

• The gametophyte life stage remains small and close to the ground due to the need for water fertilization.

• There is a trend of decreasing prominence of the gametophyte life stage.

Diploid Advantage

• Haploid populations have “allele filtration” while mutant alleles persist in a diploid population.