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Vascular Plants Without Seeds (The Megaphyll Line of Evolution:…
Vascular Plants Without Seeds
Concepts
Dibiontic life cycle
All extant plants and all known fossil plants
Have multicellular gametophytes and sporophytes
Transformation theory
Gametophytes and sporophytes grew large and vascularized
Interpopulation hypothesis
Sporophytes came from zygotes dividing
Mitotically rather than meiotically
Hypothesis that explains sporophyte generation
Monobiontic
Have one multicellular generation
Some algae
The Megaphyll Line of Evolution: Euphyllophtes
Origin of megaphylls (euphylls)
3 homoplasic leaf structure
Megaphylls- leaves evolved from branch systems (seed plants)
Leaves on gameteophytes (nonvascular)
Enations/microphylls (zosterophyllophytes and lycophytes)
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Telome theory
If branches produce sporangia, sporophyll would form
Branches align on one plane (planation)
Parenchyma develops between telomes and lower branches (webbing)
Telomes- branched lateral stems
Megaphyllous plants form monphyletic clades (euphyllophytes)
Monilophytes
Euphyllophytes are united by three synapomorphies
Have megaphylls
Roots have exarch xylem
Have 30-kilobase inversion in their plastid DNA
And woody plants (lignophytes)
Euphyllophytes have two sister clades (moilophytes)
Ferns
Leptosporangia
A single surface cell divides periclinally
Sori (singular sorus)
Cluslters of sporangia where meiosis occurs
Leptosporangiate ferns
Modern day ferns
Leaf trace
Leaves a vascular cylinder called a leaf gap
Eusporangium
When surface cells undergo periclinal divisions
Trimerophytes
Pseudomonopodial branching
A single main trunk rather than several dichotomies
Overtopping
Unequal branching where one stem is more vigorous
Equisetophytes
Monopodial growth
A main trunk, lateral branches, true leaves and roots
Reproductive structures (sporangia) form on the sporangiophore
Horsetails/scouring rushes
Vascular Cryptogams
This name indicates that they have vascular tissue
They lack seeds so their reproduction is hidden (crypto)
Similarly, the term “bryophyte” is often used for
All the nonvascular plants
Indicates evolution that doesn't include vascular tissue
Vascular cryptogam
A level of evolutionary advancement
Plants of this chapter have been called vascular cryptogams
Or often as “ferns & fern allies”
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The Microphyll Line of Evolution: Lycophytes
Extant Genera
Spores germinate & grow into bisexual gametophytes
that produce both antheridia & archegonia
Lycopodium (“ground pine” or “club moss”) is fairly common in forests
from tropical regions to the arctic
Selaginellas can be distinguished from lycopodiums
by a small flap of tissue, the ligule,
on the upper surface of Selaginella leaves
Microphylls are spirally arranged on their stems,
& secondary growth never occurs
Morphology
enations were large, up to 4 cm long
contained a single well-developed trace of vascular tissue
enations in the division Lycophyta are called microphylls
earliest lycophytes were members of the Drepanophycus & Baragwanathia
Heterospory
In many extinct and extant lycophytes sporangia
are clustered together in compact groups called
cones or strobili, which protect them
Although many species remained homosporous,
others became heterosporous,
having microspores & megaspores
Heterospory is a necessary precondition
for the evolution of seeds
Early Vascular Plants
Rhyniophytes
The earliest fossils that were vascular land plants belong to Cooksonia
These had upright stems that were simple, short cylinders
w/ no leaves
They had equal dichotomous branching,
both branches being of equal size & vigor
Xylem Structure of Early Vascular Plants
siphonostele
one in which pith is present in the center
two types of xylem organization
In both, the center is a solid mass of xylem w/ no pith
this is a protostele
endarch protostele
protoxylem is located in the center & metaxylem
differentiates on the outer edge of the xylem mass
exarch protostele
w/ metaxylem located in the center of the xylem mass
& protoxylem on the edges as several groups next to the phloem
Plants of Cooksonia were homosporous
no separate microspores & megaspores
Zosterophyllophytes
Named after the principal genus Zosterophyllum
small herbs without secondary growth
3 distinct characteristics:
xylem was an exarch protostele
sporangia were lateral
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sporangia opened transversely along the top edge
Zosterophyllum plants (all are extinct) grew as small bunches
approx. 15 cm high
Stems of Zosterophyllum were naked (smooth)
branched dichotomously
Sporangia
Fern
Zosterophyllophytes