22- Seedless Vascular Plants

Background

• phylum rhinophyta are known as the oldest seedless vascular plants

• monobiontic: plants that have only one multicellular form

• dibiontic: plants that have multicellular form in both sporophyte and gametophyte generations

• all living and most fossil plants are dibiontic with an alternation of heteromorphic sporophytes and gametophytes

Homospory vs. Heterospory

• homospory

  • production of one kind of spore (bryophytes)

  • spores give rise to gametophyte plants that produce both egg and sperm cells

  • typically bisexual with sporangium in sporophyll

• heterospory

  • production of two kinds of spores

    • microspore: give rise to male gametophytes that produce sperm cells

    • megaspore: give rise to female gametophytes that produce egg cells

  • occurs in: certain club mosses, certain ferns, and all seed plants

Early Vascular Plants

• rhyniophytes

  • earliest fossils that were vascular plants belong to Cooksonia, a genus of extinct plants

    • plants were homosporous

  • fossils with these general characteristics are called rhyniophytes

• zosterophyllophytes

  • grew as small bunched with cuticle, ordinary epidermal cells, and stomata on upper portion of naked stems

Microphyll Line of Evolution: Lycophytes

• represent a distinct line of evolution out of early land plants

• Leaves of Vascular Plants

  • there are two type of leaves that may be found on seedless vascular plants

    • microphyll: small and have singular strand

    • club mosses

    • megaphylls: larger and have more than one vascular strand

    • ferns, horsetails, and seed plants

• evolution of true roots

• allowed lycophytes sporophytes to:

  • anchor firmly

  • absorb efficiently

  • grow to tremendous size

• sporangia are clustered together in compact groups called cones or strobili for protection

• although many remained homosporous, others became heterosporous

• Club Mosses

  • small plants with rhizomes and short erect branches

  • extant species have true roots and leaves are microphylls

Megaphyll Line of Evolution: Euphyllophytes

• positioning of branches became more regular and controlled

• if a branch system produces sporangia, the resulting structure is not a leaf but a sporophyll

• all megaphyllous plants form a monophyletic clade known as the euphyllophytes

Polypodiophyta (Ferns)

• can be found in almost any habitat

• all ferns are perennial and herbaceous

• leaf primordia have a distinct apical cell, as it grows it curves inward produces fiddlehead

• largest and most diverse group of vascular seedless plants

  • have xylem and phloem

• most have true roots

• almost all species are homosporous and all have megaphylls

• found primarily in moist tropical habitats

  • few are aquatic

• sori: clusters of sporangia where meiosis occurs

• when they germinate, they grow into small simple heart-shaped or ribbon-shaped photosynthetic gametophytes with rhizoids

• display alternation of generation and have a dominant sporophyte generation

• bodies consist of a rhizome (underground stem, roots, and leaves

• key differences between ferns and mosses:

  • ferns can live in drier places

  • ferns have a highly developed vascular system with vessels that are reinenforced with lignin

  • in ferns, sporophyte is much bigger and longer-lived than gametophyte

  • ferns can grow tall

• lack seeds, flowers, fruits, etc…