Topic 14: Seedless Vascular Plants

Vocabulary flashcards covering the origin, anatomical characteristics, life cycles, classification, and ecological significance of seedless vascular plants.

Aglaophyton major

An ancient relative of present-day vascular plants from ~425 mya that exhibited independent, branched sporophytes not continuously reliant on gametophytes for sustenance, but lacked leaves, roots, and true vascular tissues.

Xylem

Vascular tissue in sporophytes that conducts water and minerals through dead, hollow cells reinforced with lignin for structural support.

Phloem

Vascular tissue composed of living cells that distributes nutrients and organic products throughout the plant.

Dominant sporophyte generation

A shared derived trait of vascular plants where the diploid sporophyte is larger, more complex, branched, and longer-lived than the gametophyte, intercepting light better and living independently.

Roots

Complex multicellular organs that anchor vascular plant sporophytes and absorb and transport water and nutrients from the soil, possibly having evolved from underground stems.

Microphylls

Small leaves supported by a single unbranched vein, which possibly evolved as outgrowths of stems; found in lycophytes.

Megaphylls

Large leaves with highly branched vascular systems that potentially evolved from webbing between flattened branches; found in monilophytes and seed plants.

Sporophylls

Modified leaves in vascular plants that bear sporangia, the organs responsible for producing spores.

Sori

Clusters of sporangia located on the undersides of sporophylls, typical of ferns.

Strobili

Cone-like structures formed from grouped clusters of sporophylls, found in lycophytes and most gymnosperms.

Homosporous spore production

The condition where a sporophyte produces a single type of spore that develops into a bisexual photosynthetic gametophyte; characteristic of most seedless vascular plants.

Heterosporous spore production

The condition where a sporophyte produces megaspores (which develop into female gametophytes) and microspores (which develop into male gametophytes); characteristic of all seed plants and a few seedless vascular plants.

Fern gametophyte

A small, independent, bisexual photosynthetic haploid plant growing on or below the soil surface that bears flagellated sperm-producing antheridia and egg-producing archegonia.

Sporopollenin

A durable polymer present in the walls of fern spores that protects them as they are dispersed through the air.

Fern sporangium

A spore-producing capsule organized in sori on fern leaves, often equipped with spring-like structures that catapult spores into the air.

Phylum Lycophyta

A clade of ~1,200 living species of seedless vascular plants including club mosses, spike mosses, and quillworts, all possessing microphylls and being either homosporous or heterosporous.

Phylum Monilophyta

A diverse clade of ~12,000 living species of seedless vascular plants comprising ferns, horsetails, whisk ferns, and their relatives.

Whisk ferns (Psilotum nudum)

Monilophytes that resemble ancestral vascular plants but are closely related to modern ferns; they lack true roots and leaves due to secondary evolutionary loss.

Horsetails (Equisetum)

Monilophytes with brushy stems and single-veined leaves resulting from secondary evolutionary simplification (secondarily microphyllous); today restricted to approximately 15 species in one genus.

Carboniferous global cooling

A climatic cooling event caused by prolific growth and photosynthesis of seedless vascular forests, which produced a 5-fold decrease in atmospheric $CO_2$ and formed modern coal deposits.

Aglaophyton

An early fossil plant dating to $\sim 425\text{ mya}$ that exhibited traits intermediate between bryophytes and vascular plants; it possessed independent, branching sporophytes not continuously reliant on gametophytes, but lacked roots, leaves, and true vascular tissues.

Vascular tissue

Specialized conducting tissue that evolved exclusively in the sporophytes of vascular plants, providing the structural capacity to grow tall and gain an evolutionary advantage over nonvascular plants.

Xylem

Vascular tissue that conducts water and minerals throughout the plant via dead, hollow cells whose cell walls are reinforced by lignin for mechanical support.

Phloem

Vascular tissue composed of living cells that distributes nutrients, sugars, and organic products across the plant body.

Lignin

A durable polymer that reinforces the water-conducting cells of xylem, providing the essential structural support that allows vascular sporophytes to stand upright.

Roots

Complex multicellular organs of vascular plant sporophytes that anchor the plant and absorb and transport water and nutrients from the soil; they may have evolved from underground stems.

Leaves

Multicellular structures that expand the sporophyte surface area to intercept and capture solar energy for photosynthesis.

Microphylls

Small leaves supported by a single unbranched vein, hypothesized to have evolved as stem outgrowths; characteristic of lycophytes.

Megaphylls

Leaves supported by a highly branched vascular system, hypothesized to have evolved through the development of webbing between flattened stem branches.

Sporophylls

Modified leaves in vascular plants that bear sporangia, the organs responsible for spore production.

Sori

Clusters of sporangia situated on the undersides of fertile fern sporophylls.

Strobili

Cone-like structures formed by compact clusters of sporophylls, observed in lycophytes and most gymnosperms.

Homosporous spore production

A reproductive pattern found in most seedless vascular plants where a sporangium produces a single type of spore that subsequently develops into a bisexual gametophyte.

Heterosporous spore production

A reproductive pattern present in all seed plants and a few seedless vascular plants, involving megaspores that form female gametophytes and microspores that form male gametophytes.

Megaspores

Haploid spores produced by megasporangia on megasporophylls in heterosporous plants that develop into female gametophytes bearing eggs.

Microspores

Haploid spores produced by microsporangia on microsporophylls in heterosporous plants that develop into male gametophytes bearing sperm.

Archegonia

Multicellular female gametangia on the gametophyte that produce eggs and secrete a chemical attractant directing flagellated sperm toward the egg.

Antheridia

Multicellular male gametangia on the gametophyte that produce flagellated sperm requiring a thin film of external water to swim and fertilize an egg.

Phylum Lycophyta

A phylum of seedless vascular plants comprising approximately $1{,}200$ living species, including club mosses, spike mosses, and quillworts, all possessing microphylls.

Phylum Monilophyta

A diverse phylum of seedless vascular plants comprising approximately $12{,}000$ living species, encompassing ferns, horsetails, and whisk ferns.

Whisk ferns

Monilophytes closely related to modern ferns that lack true roots and leaves due to secondary evolutionary loss.

Horsetails (Equisetum)

Monilophytes characterized by jointed, brushy stems, currently restricted to the single genus Equisetum ($15$ species), whose leaves are secondarily microphyllous via evolutionary simplification.

Seedless vascular plants (phylogeny)

A paraphyletic assemblage of plants comprising Lycophyta and Monilophyta that share vascular tissue and reproduction by spores, but do not form a complete monophyletic clade.

Carboniferous coal forests

Extensive swamp forests of lycophyte trees, horsetails, and ferns during the Carboniferous period; their high photosynthetic activity drove a $5\text{-fold}$ reduction in atmospheric $CO_2$ (inducing global cooling), and their partially decayed remains accumulated as coal strata.