Seedless Vascular Plants

Origin of Vascular Plants

• fossils date back about 425 million years ago

• early fossils show intermediate characteristics between bryophytes and unambiguous vascular plants

  • branching sporophytes

  • sporophytes not reliant on gametophytes for nutrients

  • no true leaf, root, or vascular tissue, which appeared first in subsequent fossil record

Presence of vascular tissues

• Develop exclusively in sporophytes of vascular plants, gametophytes lack vascular tissues

• provide rigidity for vertical growth

• have xylem and phloem for transporting and distributing water and nutrients

Sporophytes occupy dominant part of life cycle

• sporophytes don’t continuously rely on gametophytes for nutrients

• sporophytes dominant in size, complexity, and longevity, dominance of life cycle in vascular plants leads to angiosperms

Well developed true roots and leaves

True Roots: complex multicellular structures with internal, intercellular spaces

• anchor and stabilize vascular sporophytes

• facilitate absorption and distribution water and nutrients from the soil

True leaves: complex multicellular structures

• increase surface area of sporophytes for solar energy used in photosynthesis

Two types of leaves in extant vascular plants

Microphylls: small leaves, single veins, likely evolved as outgrowth of stems

Megaphylls: large leaves, highly branched vascular systems likely evolved from webbing between flattened branches

Sporophytes: modified leaves bearing sporangia

• sori are clusters of sporangia on undersides of sporophylls

• strobili are cone-like structured formed by grouping of sporophytes

Homosporous and Heterosporous plants

Homosporous: single type of spore that develops into bisexual gametophytes, most seedless vascular plants are homosporous

Heterosporous: megaspores develop into female gametophytes, microspores develop into male gametophytes, all seed plants and few seedless vascular plants are heterosporous

Characters shared between seedless vascular plants and bryophytes

Flagellated sperms:

• water required for flagellated sperms to fertilize eggs in archegonia

• seedless vascular plants mostly found in moist habitats

Characters shared between seedless vascular plants and other vascular plants

• vascular tissues help attain height

• sporophyte dominant stage of life cycle

• general process of life cycle:

  • haploid spores germinate into gametophytes that mature as males with antheridia and females with archegonium, sporophyte is developed from archegonium (vascular tissues present at this stage), gametophyte dies as sporophyte becomes independent

Phylogeny of seedless vascular plants

Classified into paraphyletic group consisting of two phyla:

• Phylum Lycophyta: approximately 1,200 species such as club mosses, pike mosses, and quillwort

• Phylum Monilophyta: approximately 12,000 species such as ferns, horsetails

Phylum Lycophyta

An ancient division of seedless vascular plants once dominated Carboniferous swamps, has microphyll leaves, strobili containing sporangia, and reproduces via spores rather then seeds

• Club mosses and spike mosses have vascular tissues and are classified under Lycophyta.

• Fossil ancestors of extant lycophytes formed the first forest in Carboniferous and Devonian periods.

• Parts of Carboniferous forest became coal in rock strata

Phylum Monilophyta

Diverse group of seedless vascular plants, have megaphylls leaves which grow by uncoiling, have sporangia with spring-like structures for catapulting spores, most ferns are homosporous

• most diverse in tropical regions but also thrive in temperate forests

• some ferns can grow to tree size