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Seed Plants I: Seed Plants Without Flowers (“Gymnosperms”) (Division…
Seed Plants I: Seed Plants Without Flowers (“Gymnosperms”)
Division Progymnospermophyta: Progymnosperms
Aneurophytales
The order Aneurophytales contains the more relictual progymnosperms, such as:
Aneurophyton, Protopteridium, Proteokalon
Tetraxylopteris, Triloboxylon, & Eospermatopteris
They varied in stature from shrubs
to large trees, up to 12 m tall
They all had a vascular cambium and secondary growth
Aneurophytes further resembled trimerophytes
in having little webbing between their ultimate branches
Archaeopteridales
These were trees up to 8.4 m tall with
abundant wood & secondary phloem
Stems of Archaeopteris had a siphonostele,
pith surrounded by a ring of primary xylem bundles
Reproduction in archaeopterids was heterosporous
Evolution of Seeds
In free-sporing species, spores can be identified with sporophytes
if some spores were trapped in a sporangium attached to
leaves or wood during fossilization
spores cannot be identified with gametophytes except when
the gametophyte is microscopic and develops within the spore wall
The megasporangium was surrounded by a layer of tissue,
an integument, that projected upward
There was a large micropyle,
a hole in the integument that permitted
the sperm cells to swim to the egg after the
megaspore had developed into a megagametophyte and had produced eggs
Division Coniferophyta: Conifers
they are diverse and all are trees of moderate to gigantic size
Conifers are never vines, herbs, or annuals,
they never have bulbs or rhizomes
Conifer leaves are always simple needles or scales
Leaves of most conifers are perennial, persisting for many years
The venation of conifer leaves is simple
with just one or two long veins running down
center of a needle-shaped leaf or several parallel
veins in scale-shaped leaves
pines have both pollen cones and seed cones
Pollen cones are simple cones with a single short
unbranched axis that bears microsporophylls
Seed cones are more complex than pollen cones
They are compound cones, each consisting of a shoot w/ axillary buds
Conifer eggs are gigantic cells loaded w/ carbs and protein
Division Pteridospermophyta: Seed Ferns
The earliest seed ferns appeared in the Upper Devonian Period
Seed ferns were any woody plant with fern-like foliage
that bore seeds instead of sori on its leaves
Many resembled modern tree ferns (except they had wood), others were vines
Seed ferns had a long-lived vascular cambium
that produced both xylem and phloem
Leaves of seed ferns were similar to those of true ferns
large, compound, and planar
the foliage leaves of seed ferns bore seeds
Division Cycadophyta: Cycads
Most cycads are short plants less than 1 or 2 m tall,
but Macrozamia can reach heights of 18 m
Internally, cycad stems are similar to those of seed ferns
a thick cortex containing secretory ducts
surrounds a small amount of manoxylic wood
cycads produce seed cones and pollen cones,
each on separate plants: cycads are always dioecious
Cycad ovules are like those of seed ferns, having a large,
vascularized megasporangium & a loosely attached, vascularized integument.
Division Ginkgophyta: Maidenhair Tree
This division contains a single living species, Ginkgo biloba
It looks very much like a large dicot tree
with a stout trunk and many branches,
but its wood is like that of conifers:
lacks vessels and axial parenchyma
Ginkgos have both short shoots,
which bear most of the leaves, and long shoots
Reproduction in Ginkgo is dioecious and gymnospermous,
cones are not produced
ovules occur in pairs at the ends of a short stalk
and are completely unprotected at maturity
Division Gnetophyta
contains three groups of enigmatic plants:
Gnetum with 30 species
mostly vines or small shrubs with broad leaves similar to those of dicots
Ephedra with about 40 species
tough shrubs and bushes that are very common in desert regions
Welwitschia mirabilis, the only species in the genus
exist only in deserts of South Africa or in cultivation
short, wide stem and only two leaves
Unlike the pollen cones of all other gymnosperms,
those of gnetophytes are compound and contain small bracts
Seed cones are also compound and contain
extra layers of tissue around the ovules
Division Cycadeoidophyta: Cycadeoids
The cycadeoids (all extinct) had vegetative features almost identical to those of cycads
The two groups differ only in subtle details of the differentiation of
stomatal complexes and in leaf trace organization
individual cones of cycadeoids contained both
microsporophylls and megasporophylls
Each ovule had a stalk, and the megasporangium was
surrounded by an integument that extended out into a long micropyle
Each microsporophyll was cup shaped
and contained numerous microsporangia
