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Vascular Plants Without Seeds (The Megaphyll Line of Evolution:…
Vascular Plants Without Seeds
Early Vascular Plants
Rhyniophytes
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.
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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.
Zosterophyllophytes
Named after the principle genus Zosterophyllum
Small herbs without secondary growth.
Many of their features resembled rhyniophytes, except:
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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
Morphology
Earliest lycophytes were members of the genera
Drepanophycus
Baragwanathia
Similar to presumed ancestors, Zosterophyllophytes, but important difference.
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Their enations were large, up to 4 cm long, and they contained a single well-developed trace of vascular tissue.
Lycophytes are called microphylls for clarity
Heterospory
Many extinct and extant lycophytes sporangia are clustered together in compact groups called cones or strobili
Necessary precondition for the evolution of seeds.
Extant Genera
Selaginella
Lycopodium
Isoetes
The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes
Division Trimerophytophyta was proposed in 1968 for three genera of extinct plants
Psilophyton
Their fossils strongly resemble those of rhyniophytes
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Pertica
Displays pseudomonopodial branching
Trimerophyton
Most important difference Overtopping
Origin of Megaphylls(Euphylls)
Three distinct types of homoplasic structures called leaves occur in plants:
Enations/Microphylls of zosterophyllophytes and lycophytes
Megaphylls, leaves that evolved from branch systems and are present in all seed plants, ferns, and equisetophytes
Leaves on gametophytes of nonvascular plants
Monilophytes
Several current studies suggest that megaphyllous plants are united by three synapomorphies:
They have megaphylls.
They have a 30-kilobase inversion in the large single-copy region of their plastid DNA
Their roots have exarch xylem.
Equisetophytes
Equisetophytes have been classified as division Arthrophyta.
They consist of several genera of extinct plants and one genus
Equisetum
With 15 extant species known as horsetails or scouring rushes.
Ferns
Early ferns appeared in the Devonian Period and diversified greatly.
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
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Ferns can be found in almost any habitat
The Term "Vascular Cryptogams"
Traditionally the plants of this chapter have been referred to informally as Vascular cryptogams.
The name indicates that they have vascular tissue and that because they lack seeds their reproduction is hidden.
Shared ancestral features and thus do not indicate they are closely related.
