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Chapter 21: Vascular Plants w/o Seeds (The Megaphyll Line of Evolution:…
Chapter 21: Vascular Plants w/o Seeds
Early Vascular Plants
Rhyniophytes
simple, short cylinders with no leaves
equal dichotomous branching
equal size and vigor
general characteristics
epidermis with cuticle
cortex of parenchyma
xylem composed of tracheids
central cells sporogenous
homosporous
no separate microspores and megaspores
stomata and guard cells in epidermis
Types
Rhynia
Aglaophyton
Lynonphyton
Sciadophyton
Horneophyton
Xylem Structure
Protostele
solid mass of xylem no pith
Endarch protostele
protoxylem is located in the center
metaxylem on outer edge of xylem
xylem cells are small and narrow
metaxyelm are large and wider
Exarch protostele
metaxylem in center
protoxylem on the edges
Siphonostele
pith present in center
occurs in ferns and seed plants
Zosterophyllophytes
principal genus: Zosterophyllum
small herbs with no secondary growth
similar to Rhyniophytes
different in 3 ways
sporangia is lateral
sporangia opened transversely
xylem is exarch protostele
outgrowths called enations
The Microphyll Line of Evolution: Lycophytes
Morphology
earliest were members of the genera: Drepanophycus
similar to presumed ancestors: Zosterophyllophytes
only difference was enations were large up to 4 cm
enations increased enough to be considered leaves
contained a single well-developed trace of vascular tissue
Lycophyta called microphylls for clarity
evolution of true roots
Heterospory
lycophytes sporangia are clustered together in compact groups called
cones or strobili these protect them
many remained homosporous and others became heterosporous
having microspores and megaspores
germinated into microgametophytes and megagametophytes
Extant Genera
Lycopodium
ground pine or club moss
fairly common in forests from tropical region to arctic
approximately 200 species
homosporous
Selaginella
less common in temperate North America
plants are small and often overlooked as moss
resurrection plant
curls up and turns brown until re-moistened
can be distinguished by a small flap called the ligule
Isoctes
around 60 species
small, unusual plants call quillworts
grow in wet muddy areas
corn-like stems that has roots attached below
heteropsporous
leafs contain sporangia
The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes
created for 3 genera of extinct plants
resemble Rhyniophytes
considered a distinct advancement of Rhyniophytes
trend of overtopping
unequal branching
one stem more vigorous
pseudomonopodial branching
single main trunk
Origin of Megaphylls
3 distinct types of homoplasic structures
1: leaves on gametophytes of nonvascular plants
2: enations/microphylls of zosterophyllophytes and lycophytes
3: megaphylls
leaves that evolved from branches and present in all seed plants
Telome Theory
megaphyll evolution
Monilophytes
united by 3 synapomorphies
1: their roots have exarch xylem
2: have megaphylls
3: have a 30-kilobased inversion in the large single-copy region
euohyllophytes contain two sister clades
Monilophytes
woody plants (lignophytes)
Equisetophytes
classified as division Arthrophyta
15 extant species known as horsetails or scouring rushes
all living herbs with no secondary growth
their aerial stems have characteristics jointed structures
Ferns
first appeared in Devonian Period
almost all called leptosporangiate ferns
consists of a single axis, either a vertical shoot or horizontal rhizome
that bears both true roots and megaphyllous leaves
at each node, a leaf trace diverges from the siphonostele
leaves a small section of vascular cylinders called a leaf gap
most are homosporous
when spores germinate they are small heart shaped or ribbon shaped photosynthetic gametophytes
on lower surface with no vascular tissue or epidermis
Eusporangia and Leptosporangia
Eusporangium
initiated when several surface cells undergo periclinal divisions
resulting in a small multilayered plate of cells
outer cells develop into sporangium wall
inner cells proliferate into sporogenous tissue
results in relatively large sporangium with many spores
Leptosporangium
initiated when a single surface cell divides periclinally
form a small outward protrusion
undergoes more division
which results in a small set of sporogenous cells and a thin covering of sterile cells
The term "Vascular Cryptograms"
indicates that they have vascular tissue
that is because they lack seeds
reproduction is hidden
