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Vascular Plants Without Seeds (The Megaphyll Line of Evolution:…
Vascular Plants Without Seeds
Early Vascular Plants
Rhyniophytes
have equal dichotomous branching
upright stems, short cylinders with no leaves
earliest fossils considered to be similar to genus Rhynia
stomata and guard cells occurred in the epidermis
Xylem Structure of Early Vascular Plants
Protostele
solid mass of xylem with no pith
two types
Endarch Protostele
protoxylem is located in the center
differentiates while cells are small and narrow
metaxylem on the outer edge of xylem mass
differentiates after the cells have expanded for a few hours or days
Exarch Protostele
protoxylem on the edges as several groups next to the phloem
metaxylem located in the center of the xylem mass
Siphonostele
pith is present in the center
occurs in the stems of ferns and seed plants
two types of xylem organization
Zosterophyllophytes
small herbs without secondary growth
three distinct characteristics
sporangia opened transversely along the top edge
xylem was an exarch protostele
sporangia were lateral, not terminal
named after the principle genus Zosterophyllum
The Microphyll Line of Evolution: Lycophytes
Heterospory
sporangia are clustered in compact groups called cones or strobili
microspores and megaspores give rise to distinct microgametophytes and megagametophytes respectively
Extant Genera
sporangia arranged in cones or distributed along shoots
spores germinate and produce both antheridia and archegonia
microphylls arranged spirally on their stems
gametophytes are green and photosynthetic
Morphology
single well developed trace of vascular tissue
microphylls later called lycophylls
members of the genera Drepanophycus and Baragwanathia
The Megaphyll Line of Evolution: Euphyllophytes
origin of Megaphylls (Euphylls)
three distinct homoplasic structures called leaves
enations/microphylls of zosterophyllophytes
megaphylls, leaves that evolved from branch systems
leaves on gametophytes of nonvascular plants
megaphyll evolution is summarized by the telome theory
ultimate twigs of last dichotomy known as telomes
megaphyllous plants form a monophyletic clade referred to as euphyllophytes
Monilophytes
euphyllophytes united by three synapomorphies
roots have exarch xylem
have megaphylls
30-kilobase inversion of plastid DNA
all fern and fern allies
Trimerophytes
trend of overtopping
have unequal branching
distinct advancement out of rhyniophytes
some display pseudomonopodial branching
Equisetophytes
Equisetum, the only living genus in this division
The cortex of Equisetum composed of large cells, the outer ones chlorophyllous
horsetails or scouring rushes
Reproductive structures
sporangia occur in groups of five, sporangiophore
Sporangiophores arranged in compact spirals, strobilus
Ferns
most monilophyte species called leptosporangiate ferns
leaf trace diverges from siphonostele
perennial and herbaceous, none is woody
leaves small segment of vascular cylinder known as leaf gap
first ferns appeared in the Devonian period
Leaves may be leathery or delicate
mostly found in shady forests and lakesides
Fern spores usually bears both antheridia and archegonia
Eusporangia and Leptosporangia
Eusporangium initiated when surface cells undergo periclinal divisions
Leptosporangia initiated when single surface cell divides periclinally
The term "Vascular Cryptogams"
reproduction is hidden
have vascular tissue and lack seeds, flowers, fruits
often referres to as "ferns and fern allies"
