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Vascular Plants Without Seeds (Early Vascular Plants (Earliest fossils of…
Vascular Plants Without Seeds
Concepts
All extant plants have a dibiontic life cycle
Each species has a multicellular gametophyte and also a multicellular sporophyte
Some algae are monobiontic
Having only one multicellular generation
Zygote undergoes only meiosis
Interpolation hypothesis
Postulates that a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically
Transformation theory
Postulates that after the dibiontic life cycle originated, both gametophyte and sporophyte became
Larger
More complex
Vascularized
Early Vascular Plants
Earliest fossils of vascular land plants belong to Cooksonia
Genus of extinct plants
Had an epidermis with a cuticle, a cortex of parenchyma, and a simple bundle of xylem composed of tracheids with annular secondary walls
Had equal dichotomous branching
Were homosporous
Rhyniophytes
Rhynia and Aglaophyton were other rhyniophytes
Had two types of xylem organization
Protostele
Exarch protostele
Siphonostele
Endarch protostele
Protoxylem
Metaxylem
Differentiates after the cells have expanded for a few more hours or days and are larger
Xylem that differentiates while cells are small and narrow
Zosterophyllophytes
Small herbs without secondary growth
Outgrowth called enations
The Microphyll Line of Evolution: Lycophytes
Morphology
Earliest lycophytes were members of the genera Drepanophycus and Baragwanathia
Similar to zosterophyllophytes
Microphylls
Enations in the division Lycophyta
Heterospory
In many extinct and extant lycophytes sporangia are clustered together in compact groups
Cones
Strobili
Extant Genera
Lycopodium
Fairly common on forests from tropical regions to the artic
Approximately 200 species
Small herbs with prostrate rhizomes that have true roots and short upright branches
Selaginellas can be distinguished from lycopodiums by a small flap of tissue called the ligule
On the upper surface of Selaginella leaves
The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes
Overtoping
Pseudomonopodial branching
Origin of Megaphylls (Euphylls)
At least three distinct types of homoplasic structures called leaves occur in plants
Leaves on gametophytes of nonvascular plants
Enations/ nicrophylls of zosterophyllophytes
Megaphylls
Evolution of these are is summarized by the telome theory
The ultimate twigs, those of the last dichotomy
Telomes
We believe that all megaphyllous plants form a monophyleyic clade
Euphyllophytes
Are united by three synapomorphies
Their roots have exarch xylem
They have megaphylls
They have a 30-kilobase inversion in the large single-copy region of their plastid DNA
Monilophytes
Lignophytes
Equisetophytes
Classified as division Arthrophyta
Equisetum
15 exact species known as horsetails or scouring rushes
Monopodial growth
Ferns contain two types of sporangia that differ in fundamental aspects of their development
Eusporangium
Initiated when several surface cells undergo periclinal divisions
Leptosporangia
Initiated when a single surface cell divides periclinally and forms a small outward protrusion
Vascualr cryptogams
Often referred to as ferns and fern allies
Indicates that they have vascular tissue and that because they lack seeds their reproduction is hidden
