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Vascular Plants Without Seeds (Concepts (transformation theory postulates…
Vascular Plants Without Seeds
Concepts
some algae are monobiotic having only one multicellular generation, only undergo meiosis once :check:
interpolation hypothesis postulates that a small sporophyte generation came into existence when zygote germinated mitotically :red_flag:
all extant plants have a dibiontic life cycle: each species has a multicellular gametophyte and also a multicellular sporophyte :star:
transformation theory postulates that after dibiotic life cycle originated
gametophyte and sporophyte became larger, more complex and vascularized in a life cycle with an alteration of isomorphic generations
postulates early ancestors diverged into two clades
non vascular plants in which sporophytes became much simpler and dependent on gametophytes
rest of the vascular plants in which sporophytes became increasingly elaborate whereas gametophytes became very reduced
The Megaphyll Line of Evolution: Euphyllophytes
Equistophyte
stems have characteristic jointed structure with whorl of fused leaves at the nodes with small leaves that have little vascular tissue
if branches are present they alternate with the leaves
contain species known as horsetails ot scouring rushes, living plant any herb without secondary growth :check:
true roots are present
internal structure is distinctive as external morphology
stems have a pith and siphonosteles, not prosteles
protoxylem forms next to pith interior to metaxylem and is endarch.
reproductive system is specialized sporangia always occur in groups of 5-10 located on umbrella shaped sporangiophore :
always arranged in compact spirals forming a strobilus. is homosporous
after spores are released, germinate on moist soil and develop into small green gametophytes. No vascular tissue or epidermis/ stomata
Have true monopodial growth a main trunk, lateral branches, true leaves, and true roots
Ferns
can be found in almost any habitat from desert to underwater
fern sporophyte consists of single axis that bears both true roots and megaphyllous leaves :check:
contain 15 living species known as leptosporangiate ferns which are the common ferns we recognize :star:
at each node, a leaf trace diverges from the siphonostele, leaving a small segment of vascular cylinder as parenchyma, region called leaf gap :star:
leaves of ferns may be leathery or delicate only one cell thick in filmy ferns, but many layers thick in most.
upper level of palisade parenchyma and lower region of spongy mesophyll.
on underside of leaf contain sori, clusters of sporangia where meiosis occurs :check:
when spores germinate, they grow into small, simple heart shaped or ribbon shaped photosynthetic gametophytes
eeusporangia and Leptosporangia
the eusporangium is initiated when several surface cells undergo periclinal divisions resulting in a small multilayerd plate of cells :check:
outer cells develop into the sporangium wall and the inner cells proliferate into sporogenous tissue
results in relatively large sporangium with many pores
Leptosporangia are initiated whrn a single surface cell divides perclinally and forms a small outward protrusion :check:
ferns contain two types of sporangia that differ in fundamental aspects of their development :star:
Psilotum and Tmesipteris
Psilotum
constructed and upright stems that branch dichotomously, have epidermis cortex, and simple vascular cylinder with no pith :check:
psilotm
unique that has no ability to make roots or leaves, contain vascular tissue :star:
two small genera contain simplest of all living vascular plants
Origin of Megaphylls (Euphylls)
have two sister clades :star:
monilophytes
known as ferns or fern allies, among other that are apart of different clades of ferns
lignophytes
euphyllophytes are united by three synapomorphies :star:
they have megaphylls
they have a 30-kilobase inversion in the large singlecopy region of the plastid DNA
their roots have exarch xylem
Trimerophytes
fossils strongly resemble those of rhyiophytes having terminal sporangia that dehisced laterally, homospory, dichotomous branching and endarch vascualr cylinder of tracheids :check:
overtoping, trimerophytes had an unequal branching in which one stem was more vigorous. :red_flag:
division trimerophyte proposed for 3 genera of extinct plants
pseudomonopodial branching, single main trunk rather than series of dichotomies :check:
branches would later become more regular and controlled but rhyniophytes dichotomy were irregular and unpredictable :check:
correlated with the evolution of a pseudomonopodial growth habit and the presence of numerous photosynthetic lateral branches
other types of phyllotaxy that occured were alternate,decussate, distichous, tetrastichous and even whorled :star:
Early vascular plants
Rhyniophytes
other species contained a prostrate rhizome, upright naked stems and terminal sporangia. :check:
stomata and guard cells occurred in the epidermis and there was a layer that appears to have been a cuticle
similar to these plants, two were gametophytes, not sporophytes
ends of their stems bore flattened cup-shaped areas that contained gametangia both antheridia and archegonia but not sporangia :green_cross:
plants had upright dichotomously branched stems, vascular tissue, stomata, and cuticle
much larger and more complex than gametophytes of any mosses, liverworts, hornworts living today
possible plants had an alteration of isomorphic generations.
another fossil of the same time and sporophytes with naked axes that branched dichotomously and were 20cm tall.
had stomata and terminal sporangia. Each sporogium had a short columella and tissue surrounded columella on all sides and the top.
similarities between hornworts and these fossil raise possibility that vascular plants may have arisen from hornworts like ancestors
earliest fossils had equal dichotomous branching, branches being of equal size and vigor :check:
ends of branches were swollen and contained large multicellular masses of sporogenous tissue surrounded by sterile cells
only central cells were sporogenous and sporangium had to open to release spores.
plants had epidermis with cuticle, cortex of parenchyma, simple bundle of xylem composed of tracheids
were homosporous, no separate microspores and megaspores.
ALL characteristics of rhyniophytes :red_flag:
Xylem structure of early vascular plants :check:
two types of xylem organization, in both center is solid mass of xylem with no pith called protostele :star:
endarch protostele, protoxylem is located in center and metaxylem differentiates on the outer edge of xylem mass :check:
protoxylem is xylem that differentiates while cells are small and narrow and metaxylem differentiates after cells have expanded
exarch protostele with metaxylem located in the center of xylem mass and protoxylem on the edges as several groups next to phloem :star:
a type of stele that did not evolve until later is siphonostele. one in which pith is present in center as in stems of ferns and seed plants :check:
Zosterophyllophytes
small herbs without secondary growth, features similar to those of rhyniophytes minus three characteristics :black_flag:
sporangia opened transversely along the top edge
xylem was an exarch protostele, protoxylem on the outer margin and metaxylem in the center
sporangia were lateral not terminal
only 15cm high, upper portions of stems had cuticle, ordinary epidermal cells, and stomata but lower portion did not
another group of early vascular plants are the zosterophyllophytes :star:
outgrowths called enations ranged from quite small to long thin scales. Increased photosynthetic surface area of the plants :check:
The Microphyll Line of Evolution: Lycophytes
Heterospory
some species remained homosporous and other became heterosporous having microspores and megaspores that germinated into distinct microgametophytes and megagametophytes
is necessary precondition for the evolution of seeds
sporangia are clusterd together in compact groups called cones or strobili which protect them :<3:
megaspore in some species was retained within sporophyll, protected by thick walled cells of the sporangium
represent ancient line of evolution distinct from seed plants but having convergent evolution in several characters, leaves, roots, etc. :check:
extant Genera
all species are small herbs with prostrate rhizomes that have true roots and short upright branches. Microphylls spirally arranged
sporangia may be arranged in cones or distributed along the shoots. all are homosporous :star:
lycopodium is fairly common from tropical regions to the arctic.
spores germinate and grow into bisexual gametophytes that produce both antheridia and archegonia
Selaginella
is less common in temperate North America and its plants are smaller and easily overlooked, best known species is resurrection plant
contain a small flap of tissue called ligule on the upper surface of leaves, simple structure and unknown advantage
Isoetes
unusual plant with corn-like stem and roots attached below and leaves above
are heterosporous and almost every leaf contains sporangia. Microphylls in this genus also have ligules.
contains cortex parenchyma, on exterior, interior has vascular tissue with mixture of tracheids, sieve elements and parenchyma
anatomy and morphology preserved in fossil indicate many instances of convergent evolution with seed plants :check:
in seed plants, megaspores were retained in the megasporangium and evolved into seeds
absorb CO2 from roots in soil and mud
elaborate, efficient leaves wood, bark, and roots evolved. Sporangia separated form reproductive organs, grouped in strobili
Morphology
microphylls are the "leafs" of lycophytes, micro refering to the evolution from small enations, not their actual size. can be up to 78cm long :check:
had true roots to help anchor self firmly absorb efficiently and grow to tremendous sizes
enations were large, up to 4 cm long and contained a single well developed trace of vascular tissue :black_flag:
had vascular cambium and secondary growth, wood resembled secondary xylem of pines and other conifers :check:
cells could not undergo radial longitudinal division and new fusiform initials could not be produced
represent a distinct line of evolution out of the early land plants that resembled zosterophyllophytes :star:
have lateral sporangia and exarch protostels
The Term Vascular Cryptogams
they lack seeds, flowers, fruits however derive cladograms from derived features
vascular cryptogams indicates they have vascular tissue and because they lack seeds their reproduction is hidden
