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Chapter 21: Vascular Plants Without Seeds (The Megaphyll Line of Evolution…
Chapter 21: Vascular Plants Without Seeds
Concepts
all extant plants have a dibiontic life cycle
multicellular gametophyte
multicellular sporophyte
some algae are monobiontic
one multicellular generation
interpolation hypothesis
a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically
sporophyte generation gradually evolved in complexity while the gametophyte generation remained small
sporophyte generation was interpolated into the monobiontic life cycle
transformation theory
after the dibiontic life cycle originated, both gametophyte and sporophyte became larger, more complex, and vascularized
life cycle with an alternation of isomorphic generations
Currently-all living and most fossil plants are dibiontic with an alternation of heteromorphic sporophytes and gametophytes
Early Vascular Plants
Rhyniophytes
equal dichotomous branching - both branches being of equal size and vigor
Cooksonia
extinct plants
equal dichotomous branching
homosporous
early vascular plants
two types of xylem organization
in both, center is a solid mass of xylem with no pith - protostele
endarch protostele
protoxylem is located in the center
metaxylem differentiates on the outer edge of the xylem mass
exarch protostele
metaxylem located in the center of the xylem mass
protoxylem on the edges
siphonostele
pith is present in center
ferns
seed plants
Zosterophyllophytes
small herbs without secondary growth
lateral sporangia
xylem was an exarch protostele
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enations
outgrowths
increased photosynthetic surface area of plants
In
Asteroxylon
they contained stomata and a small trace of vascular tissue
The Microphyll Line of Evolution: Lycophytes
Lateral sporangia
Exarch protosteles
Came from a
Zosterophyllum
type of ancestor
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Morphology
Different from zosterophyllophytes in that their enations were large and they have a single well-developed trace of vascular tissue
microphylls-their enations are close to being called leaves
evolution of true roots allowed lycophyte sporophytes to anchor firmly, absorb efficiently, and grow to tremendous size
Major flaw of vascular cambium
cells could not undergo radial longitudinal division
new fusiform initials could not be produced
Heterospory
strobili/cones
sporangia clusted together in compact groups
necessary precondition for the evolution of seeds
Lycophytes represent an ancient line of evolution distinct from seed plants
convergent evolution in several characters: leaves, roots, secondary growth, and almost seeds
most became extinct
Extant genera
Lycopodium
"ground pine"
"club moss"
small herbs with prostrate rhizomes that have true roots and short upright branches
microphylls are spirally arranged
Selaginella
plants are smaller and easily overlooked
resurrection plant
has derived condition of being heterosporous
megaspore is not retained
ligule - flap of tissue on upper tissue of leaves
Isoetes
quillworts
heterosporous
almost every leaf contains sporangia
also have ligules
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The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes
Three genera of extinct plants
Psilophyton
Pertica
pseudomonopodial branching
single main trunk
have terminal sporangia that dehisced laterally, homospory, dichotomous branching, and an endarch vascular cylinder of tracheids
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Trimerophyton
overtopping
unequal branching in which one stem was more vigorous
Origin of Megaphylls (Euphylls)
3 distinct types of homoplasic structures called leaves occur in plants
leaves on gametophytes of nonvascular plants
enations/microphylls of zosterophyllophytes and lycophytes
megaphylls, leaves that evolved from branch systems and are present in all seed plants, ferns, and equisetophytes
telome theory
Monilophytes
euphyllophytes united by three synopomorphies
their roots have exarch xylem
they have megaphylls
they have a 30-kilobase inversion in the large single copy region of their plastid DNA
Contain two sister clades
monilophytes
lignophytes
Equisetophytes
horsetails
scouring rushes
herbs without secondary growth
leaves are all small megaphylls
true roots are present, produced at rhizome's nodes
sporangia always occur in groups of five to ten
Ferns
derived from trimerophytes
leptosporangiate ferns
all are perennial and herbaceous
none is woody
never have secondary xylem
leaf trace diverges from siphonostele at each node leaving a leaf gap
leaf
underside of the leaf are sori where meiosis occurs
most ferns are homosporous
two groups of water ferns are heterosporous
Eusporangia and Leptosporangia
eusporangium is initiated when several surface cells undergo periclinal divisions
leptosporangia are initiated when a single surface cell divides periclinically and forms a small outward protrusion
Psilotum
and
Tmesipteris
simplest of all living vascular plants
The term "Vascular Cryptogams"
"ferns and fern allies"
have vascular tissue
lack seeds
reproduction is hidden
lack seeds, flowers, fruits
