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Vascular Plants Without Seeds (The Megaphyll Line of Evolution:…
Vascular Plants Without 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
Plants of Cooksonia were homosporous
no separate microspores & megaspores
Xylem Structure of Early Vascular Plants
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
siphonostele
one in which pith is present in the center
Zosterophyllophytes
named after the principal genus Zosterophyllum
small herbs without secondary growth
3 characteristics that make them distinct:
sporangia were lateral
sporangia opened transversely along the top edge
xylem was an exarch protostele
Zosterophyllum plants (all are extinct) grew as small bunches
approx. 15 cm high
Stems of Zosterophyllum were naked (smooth)
branched dichotomously
The Microphyll Line of Evolution: Lycophytes
Morphology
earliest lycophytes were members of the Drepanophycus & Baragwanathia
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
Heterospory
In many extinct and extant lycophytes sporangia
are clustered together in compact groups called
cones or strobili, which protect them
Heterospory is a necessary precondition
for the evolution of seeds
Although many species remained homosporous,
others became heterosporous,
having microspores & megaspores
Extant Genera
Lycopodium (“ground pine” or “club moss”) is fairly common in forests
from tropical regions to the arctic
Microphylls are spirally arranged on their stems,
& secondary growth never occurs
Spores germinate & grow into bisexual gametophytes
that produce both antheridia & archegonia
Selaginellas can be distinguished from lycopodiums
by a small flap of tissue, the ligule,
on the upper surface of Selaginella leaves
The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes
Division Trimerophytophyta was proposed in 1968
for 3 genera of extinct plants:
Trimerophyton, Psilophyton, & Pertica.
Trimerophytes are considered a distinct advancement out of rhyniophytes
because of several special features
pseudomonopodial branching
a single main trunk rather than a series of dichotomies
The plants have small lateral branches,
some fertile & bearing sporangia
& others sterile & acting as leaves
Origin of Megaphylls (Euphylls)
3 distinct types of homoplasic (analogous) structures called leaves occur in plants:
leaves on gametophytes of nonvascular plants
enations/microphylls of zosterophyllophytes & lycophytes
megaphylls, leaves that evolved from branch systems
& are present in all seed plants, ferns, & equisetophytes
Megaphyll evolution is summarized by the telome theory
The ultimate twigs, those of the last dichotomy
are known as telomes
Monilophytes
euphyllophytes contains 2 sister clades,
the monilophytes & the woody plants (lignophytes)
Many monilophytes are plants we know as ferns
others have been called “fern allies”
Equisetophytes
Equisetophytes have been classified as division Arthrophyta
They consist of several genera of extinct plants
& one genus, Equisetum
w/ 15 extant species known as horsetails or scouring rushes
They are all herbs without any secondary growth
Ferns
Early ferns first appeared in the Devonian Period & then diversified greatly
the rest of the monilophytes have more than 12,000 species,
almost all of those are called leptosporangiate ferns
The Term “Vascular Cryptogams”
Traditionally, the plants of this chapter have been referred to informally as vascular cryptogams
(or often as “ferns & fern allies”)
This name indicates that they have vascular tissue
& that because they lack seeds their reproduction is hidden (crypto)
when we use a term such as “vascular cryptogam,”
we are referring to a grade, to basically a level of evolutionary advancement
Similarly, the term “bryophyte” is often used for
all the nonvascular plants, not to indicate that they are related,
but that they are at a level of evolution that does not include the presence of vascular tissue.
