Vascular Plants Without Seeds (The Megaphyll Line of Evolution:…
Vascular Plants Without Seeds
Early Vascular Plants
Equal Dichotomous Branching
Both branches being of equal size and vigor.
Fossils that have these general characters are called rhyniophytes.
Epidermis with cuticle, cortex of parenchyma, simple bundle of xylem composed of tracheids with annular secondary walls.
Rhynia and Aglaophyton, and Cooksonia
Protostele is when the center is a solid mass of xylem with no pith.
In an endarch protostele protoxylem is located in the center and metaxylem differentiates on the outer edge of the xylem mass.
Named after the principle genus Zosterophyllum
Small herbs without secondary growth.
Many of their features resembled rhyniophytes, except:
Their xylem was an exarch protostele, protoxylem on the outer margin and metaxylem in the center.
Sporangia opened transversely along the top edge.
Their sporangia were lateral, not terminal.
The Microphyll Line of Evolution: Lycophytes
Lycophytes are called microphylls for clarity
Similar to presumed ancestors, Zosterophyllophytes, but important difference.
Their enations were large, up to 4 cm long, and they contained a single well-developed trace of vascular tissue.
Earliest lycophytes were members of the genera
Necessary precondition for the evolution of seeds.
Many extinct and extant lycophytes sporangia are clustered together in compact groups called cones or strobili
The Megaphyll Line of Evolution: Euphyllophytes
Division Trimerophytophyta was proposed in 1968 for three genera of extinct plants
Most important difference Overtopping
Displays pseudomonopodial branching
Their fossils strongly resemble those of rhyniophytes
Origin of Megaphylls(Euphylls)
Three distinct types of homoplasic structures called leaves occur in plants:
Leaves on gametophytes of nonvascular plants
Megaphylls, leaves that evolved from branch systems and are present in all seed plants, ferns, and equisetophytes
Enations/Microphylls of zosterophyllophytes and lycophytes
Several current studies suggest that megaphyllous plants are united by three synapomorphies:
Their roots have exarch xylem.
They have a 30-kilobase inversion in the large single-copy region of their plastid DNA
They have megaphylls.
With 15 extant species known as horsetails or scouring rushes.
They consist of several genera of extinct plants and one genus
Equisetophytes have been classified as division Arthrophyta.
Ferns can be found in almost any habitat
Equisetophytes only have15 species of living plants, the rest of the monilophytes have more than 12,000 species, and almost all of those are what are called leptosporangiate ferns
Early ferns appeared in the Devonian Period and diversified greatly.
The Term "Vascular Cryptogams"
Shared ancestral features and thus do not indicate they are closely related.
The name indicates that they have vascular tissue and that because they lack seeds their reproduction is hidden.
Traditionally the plants of this chapter have been referred to informally as Vascular cryptogams.