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Vascular Plants Without Seeds (The Microphyll Line of Evolution:…
Vascular Plants Without Seeds
Concepts
dibiontic life cycle
all extant plants (still in existence)
all known fossil plants
multicellular gametophyte
multicellular sporophyte
interpolation hypothesis
postulates:
small sporophyte came into existence
when a zygote germinated mitotically
instead of meiotically
transformation theory
postulates:
after origination of the dibiontic life cycle
gametophyte and sporophyte became
larger
more complex
vascularized
life cycle with alteration of isomorphic generation
Early Vascular Plants
Rhyniophytes
earliest fossil record of land plants
Cooksonia
a genus of extinct plants
simple upright stems
short cylinders
with no leaves (naked stems)
equal dichotomous branching
both branches
equal size/vigor
epidermis w/ cuticle
cortex of parenchyma
simple bundle of xylem
tracheids w/ annular secondary walls
Xylem Structure of Early Vascular plants
protostele
solid mass of xylem
no pith
endarch protostele
protoxylem located in center
metaxylem differentiates on outer edge
exarch protostele
metaxylem located in center
protoxylem on edges
siphonostele
evolved later
pith is present in center
as occurs in ferns/seed plants
Zosterophyllophytes
small herbs
no secondary growth
lateral sporangia
opened transversely along top edge
exarch protostele
enations
outgrowths
increased photosynthetic surface area
in
Asteroxylon
contained stomata
The Microphyll Line of Evolution: Lycophytes
lateral sporangia
exarch protosteles
may have come from
Zosterophyllum
ancestor
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Morphology
micropylls
micro- evolution from small enations
enations must have increased photosynthesis
lycophyll has been suggested
Heterospory
cones or strobili (singular, strobilus)
compact groups of sporangia
necessary precondition for seed evolution
Extant Genera
Lycopodium
(ground pine or club moss)
fairly common in forests
200 living species
small herbs w/ prostrate rhizomes
true roots
short upright branches
tropical regions to arctic
homosporous
spores germinate bisexual gametophytes
produce:
antheridia
archegonia
Selaginella
resurrection plant, S.
lepidophylla
heterosporous
ligule
small flap of tissue
The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes
several special features:
overtopping
unequal branching
one stem more vigorous
pseudomonopodial branching
main trunk
rather than a series of dichotomies
Origin of Megaphylls (Euphylls)
three distinct types of homoplastic (analogous) structures called leaves occur in plants:
1) leaves of gametophytes of nonvascular plants
2) enations/microphylls of
zosterophyllophytes
and
lycophytes
3) megaphylls-leaves evolved from branch systems
present in all seed plants
ferns
and equisetophytes
telome theory
summarizes megaphyll evolution
telomes
ultimate twigs
last dichotomy
planation
subdivisions of lateral branch align in plane
webbing
parenchyma develops between telomes
Monilophytes
current studies suggest
megaphyllous plants (euphyllophytes)
united by three synapomorphies:
1) their roots have exarch xylem
2) they have megaphylls
3) 30-kilobase inversion in DNA
monilophytes
lignophytes
woody plants
more than 12,000 species
leptosporangiate ferns
leaf trace diverges from siphonostele
leaf gap
small segment of vascular cylinder
just parenchyma
leaves of ferns may be leathery or delicate
once cell thick
upper layer of palisade parenchyma
lower region of spongy mesophyll
sori (singular, sorus)
underside of leaf
cluster of sporangia
site of meiosis
Eusporangia and Leptosporangia
1 more item...
leaves are small (
Trichomanes
)
several meters long (tree ferns)
almost always compound
rachis
leaflets
Equisetophytes
division Arthrophyta (also called Sphenophyta)
15 extant species known
horsetails
scouring rushes
all herbs
no secondary growth
sporangiosphore
sporangia in groups of 5-10
Calamitales
true monopodial growth
The Term "Vascular Cryptogams"
"ferns and fern allies"
name indicates:
they have vascular tissue
reproduction is hidden (crypto)
