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Vascular Plants Without Seeds (Concepts (a small sporophyte came into…
Vascular Plants Without Seeds
Early Vascular Plants
Rhyniophytes
this plant had equal dichotomous branching
earliest fossils were vascular plant belong to
Cooksonia
extinct plant
fossils of plant that have
no megaspores
no microspores
Rhyniophytes
& are homosporous
Xylem Structure of Early Vascular Plants
2 types of xylem organization
both had no pith that is a protostele
Endarch protostele
protoxylem in center
meta-xylem differentiates on outer edge of xylem mass
cells were small & narrow
meta-xylem differentiates after cells expanded after few hours or days
Exarch protostele
protaoxylem on the edges several groups next to the phloem
meta-xylem in the center of xylem mass
another type of stele
siphonostele- pith present in center as occurs in stem of fern & seed plants
Zosterophyllophytes
they were small herbs without secondary growth
named after the principal genus
Zosterophyllum
sporangia were lateral, not terminal
sporangia were naked, branched dichotomously
small amount of xylem consist of tracheas with secondary wall
inn
Rebuchia
sporangia occurred together on the end of specialized branches
in
Crenaticaulis
some branching was pseudomonopodial
The Microphyll Line of Evolution
Lycophytes
Exant Genera
spores germinate & produce both antheridia & archegonia
gametophytes are green & photosynthetic
sporangia arranged in cones
Heterospory
sporangia are clustered in one group
microspores and megaspores give rise to distinct micro-gametophytes & mega-gametophytes
Morphology
single well developed vascular tissue
microphylls later called lycophylls
Euphyllophytes
Origin of Megaphylls (Euphylls)
three districts homoplastic structure called leaves
megaphylls evolution is summarized by the telome theory
megophjyllaous plant form a monophyletic clade referred to as euphyllophytes
Monilophytes
roots have exarch xylem
have megaphylls
Trimerophytes
have equal branching
distinct advancement out of rhynophytes
Equisetophytes
horsetail or scouring rushes
Equisetum only living genus
sporangia occur in group of 5
sporangiosphores arranged in compact spiral strobilus
Ferns
Leptosporangia iniated when single surface cells divides periclinally
mostly found in shady forests & lakesides
Eusporangium initiated when surface cells undergo periclinal division
perennial & herbaceous none woody
spores have both antheridia & archegonia
The Term "Vascular Cryptogams"
vascular tissue
examples ferns,
they lack seeds, flower, fruits
Concepts
a small sporophyte came into existence
when a zygote germinated mitotically instead of meiotically
The interpolation hypothesis postulate
each species have multicellular gametophyte & also mutlicellular sporophyte
All exact plants have dibiontic life cycle
Alternative hypothesis
the transformation theory postulates that
after dibiontic life cycle originated, both gametophyte & sporophyte became larger,
more complex & vascularized in the life cycle with alternation of isomorphic generation
