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Vascular Plants without Seeds (Concepts (Transformation Theory (Postulates…
Vascular Plants without Seeds
Concepts
First, all extant have a
dibiontic life cycle
: each species has a multicelluar gametophyte and sporophyte.
some algae, especially
Coleochaete
are monobiotic, having only one multicellular generation
a zygote undergoes only meiosis, producing more spores that will grow into new gametophytes.
thus in important step in the evolution of embryophytes was the conversion of a monobiontic ancestors into dibiontic plants
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a sporophyte generation had to come into existence.
one hypothesis, the
interpolation hypothesis
, postulates that a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically
the sporophyte generation would have gradually evolved in complexity while the gametophyte generation remained small.
A sporophyte generation was inserted (interpolated) into the monobionticlife cycle.
in this hypothesis, non vascular plants such as some liverworts might be intermediates in the progression from green algae to vascular plants.
Transformation Theory
postulates that after the dibiontic life cycle originated, both gametophyte and sporophyte became larger, more complex,
and vascularized, in a life cycle with an alternation of isomorphic generations
no living plants have gametophyte that look like sporophytees , but many algae do and some fossil plants did:
some fossils of early vascular plants bore gametangia and grew among other similar plants that bore sporangia.
Postulates that these early ancestors diverged into two clades
(1) non vascular plants in which sporophytes became much simpler an dependent on the gametophytes
(2) the rest of the vascular plants in which sporophytes became increasingly elaborate whereas gametophytes very reduced
Early Vascular Plants
Rhyniophytes
the earliest fossils that definitely were vascular land plants belong to
Cooksonia
a genus of extinct plants
these had upright stems that were simple, short cyclinders with no leaves
pic
they had
Equal dichotomous branching,
both branches being of equal size and vigor.
plants of
Cooksonia
had an epidermis with a cuticle, a cortex of parenchyma, and a simple bundle of xylem composed of tracheids with annular secondary walls
the ends of the branches were swollen and contanied large, multicellular masses of sporogenous tissues by several layers of sterile cells.
as in all plants, only the central cells were sporogenous and the sporangium had to open to release the spores
fossils that have these general characteristics are called
Rhyniophytes
Xylem Structures of early vascular plants
Early Vascular plants had two types of xylem organization.
in both, the center is soild mass of xylem with no pith; this is a
portostele
pic
in an
endarch protostele
, the protoxylem is located in the center and metaxylem differentiates on the outer edge of the xylem mass.
Protoxylem is the xylem that differentiateswhile cells are small and narrow
the other type Stele present in early Vascular plants is an
exarch protostele
pic
with metaxylem located in the center of the xylem mass and protoxylem on the edges as several groups next to the phloem
another type of stele, which did not evolve until later is the
siponostele
pic
one in which pith is present in the center, as occurs in the stems of ferns and seeds plants
Zosterophyllophytes
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another group of early vascular plants are the
Zosterophylophytes
, named after the principle genus
Zosterophyllum
pic
they were small herbs without secondary growth
similar to the rhyniophytes but three characteristics make us think they were a distinct group
(1) their sporangia were lateral, not terminal
(2) Sporangia opened transversely along the top edge and their xylem was an exarch protostele
(3) protoxylem on the outer margin and metaxcylem in the center
they grew as small bunches, only about 15 cm high, Upper portions of their stmes had cuticle, ordinary epidermal cells, and stomata
but lower portions did not, presumably because they grew in swampy, marshy areas
the Mircophyll line of evolution : Lycophytes
Lycophytes represent a distinct line of evolution out of the early land plants that resemble Zosterphyllophytes.
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have lateral sporangia and exarch protosteles and, thus they may have come from a
Zosterophyllum
type of ancestor
Morphology
the earliest lycophytes were members of the genera
Drepanophycus
and
Baragwanathia
they were similar to their presumed ancestors, the zosterphyllophytes, with an important difference: their enations
were large up to 4cm long, and they contained a single well-developed trace of vascular tissues
such enations must have increased photosynthesis, and they could be called leaves, but leaf is an ambiguous term
and enations in division lycophyta are called Microphylls for clarity
Heterospory
in many extinct and extant lycophytes sporangia are clustered together in compact grouped called
cones
or
stobili
which protect them
although many species remained homosporous, other became heterosporous, having microspores
and megaspores that germinate to give rise to distinct microgametophytes and megagamophytes, respectively
Heterospory is a necessary preconditions for the evolution of seeds
Extant Genera
Lycopodium
is fairly common in the forest from the tropical regions to the arctic .
all living species app.200 are small herbs with prostrate rhizomes that have true roots and short upright branches
pic
microphylls are spirally arranged on their stems and secondary growth never occurs.
Sporangia may be arranged in cones or distributed along the shoots
all
Lycopodium
species are homosporous, a plesiomorphic trait
spores germinate and grow into bisexual gametophytes that produce both antheridia and archegonia
Selaginella
is less common in temperate North American and it plants are smaller and easily overlooked or mistaken to be mosses
unlike
Lyc
.
Selaginella
has the apomorphic condition of being heterosporous and the megaagametophyte develops inside the mega spore wall
the megaspore wall is not retained on the sporophyte, however, and is not seed like
Mircogametophyte also develop wothin the spore wall and consisite of a single vegetative cell and an antheridium
many flagellate sperms are produced and then released as the spore wall ruptures.
can be distinguished from lyc. by a small flap of tissue, the
ligule
, on the upper surface of Sel. leaves
although ligule are simpler structures and their advantage are unknown; they are ancient and can be used to distinguish early fossil seleginellas from fossil lycopods
isoetes
is the genus of about 60 species of small, unusual plants call quillworts, which grow in wet, muddy areas that occasionally become dry
their body consists of small corm-like stem that has roots attachted below and leaves alone.
is hetersporous like
selaginella
and almost every leaf contains sporangia.
microphylls in this genus also have ligules
The Megaphyll Line of Evolution: Euphyllophytes
Trimerophytes
Overtoping
: Trimerophytes had an unequal branching in which one stem was more vigorous
Pertica displays
Pseudomonopodial branching
that is single man main trunk rather than a series of dichotomies
the plants have small lateral branches, some fertile and bearing sporangia and others sterile and acting leaves
became distinct from Rhyniophytes during the Lower Devonian and existed until the Upper Devonian period
and then came to an end not by going extinct but rather by evolving into the ancestors of ferns and seed plants
Origin of Megaphylls (Euphylls)
at least three distinct types of homoplasic (analogus) structure called leaves occur in plants:
(2). Enations/microphylls of Zosterophyllophytes and lycophytes
(3) megaphylls leaves that evolved from branch systems and are present in all seed plants, ferns and equisetophytes
(1). leaves on gametophytes of nonvascular plants;
Megapylls evolution is summarized by the
telome theory
the ultimate twigs, those of the last dichotomy are are known as
Telomes
we believe that all megaphyllous plants from a monophyllectic now referrred to as the euphyllophytes
Monilophytes
megaphyllus plants are untied by three synapomorphies
2.they have megaphylls
3.they have 30 kilobase inversion in the large single copy region of their DNA
roots have exarch xylem
Lignophytes- the woody plants
equisetum its clade will be treated as sisters to the fern
Equisetyophytes
classified as divison arthrophyta aka spenophyta
they consists of several genera of extinct plants and one genus with 15 extant species known has
horestails
or
scoruring rushes
reproducitive structures that Equisetem are specialized sporangia that always happen is groups of 5 and 10 located on an umbrella shaped sporangiophore
they have true Monopodial growth, a main trunk that have lateral branches, true leaves and true roots
