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Seed Plants I: seed Plants Without Flowers ("gymnosperms")…
Seed Plants I: seed Plants Without Flowers ("gymnosperms")
Concepts
the life cycle of vascular cryptogams is an alternation of independent, heteromorphic generations.
A disadvantage of this life cycle is that the new sporophyte,
while developing from the zygote is temporarily dependent on a tiny gametophyte for its start in life
many new sporophytes perish
all of the genes of each gametophyte and half the genes of each new sporophyte embryo are identical to those of the maternal sporophyte and thus any mutation
that improves the survival of the gametophytes or embryos contributes to the reproductive success of the maternal sporophytes
it would advantageous if the embryo could use the photosynthetic and absorptive capacity of the leaves and roots of the previous sporophyte.
in order for this to happen, the megagametophyte and embryo must be retained inside the maternal sporophyte; this is accomplished
by retaining the megaspore and allowing the megagametophyte to develop within the sporangium
evolution of seeds was preceded by evolution of a vascular cambium
V.C. had arisen independently in some lycophytes and equisetophytes, but cells in this new cambium could undergo radial longitudinal divisions
thus allowing the cambium to grow in circumference as wood accumulated
Lignophytes
we believe that is cambium arose just once in one group of woody plants,
Spermatophytes
shortly after, seeds originated, establishing the seed plants
the plants that existed after the origin of wood but before that of seeds were trees that reproduced with spores, the way ferns reproduced
manoxylic wood
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some plants produce a small amount of very soft, spongy, parenchymatous wood
pycnoxylic wood
the second suite consists of hard, strong wood with little parenchyma and flattened seeds
gymnosperms
are those plants with naked ovules that is ovules located on flat sporophylls for example pines cones
Angiosperms
are the flowering plants, those with carpels, which are believed to be sporophylls that forms a tube-like closed structure; fruits mature carpels.
the divisions of living seed plants commonly accepted now used in this book are
(1). Division Cyncadophyta
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(2). Division Coniferophyta
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(3) Division Ginkgophyta
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(4) division Gnetophyta ( there four are gymnosperms)
(5) division Magnoliophyta (flowering plants)
numerous extinct groups exist, the most significant for us being pro gymnosperms and seeds ferns.
Division progymnospermophyta: Progymnosperms
Trimerophytes were an important group of plants that evolved from rhyniophtes.
one evolutionary trend initiated in trimerophytes was pseudomonopodial branching and the first steps in the evolution of megaphylls (euphylls)
a third group to evolve from trimerophtes was the now extinct progymnosperms so named because some gave rise later to conifers, cycads and the other gymnosperms
as early as 360 million years ago, the Vascular cambium that evolved in progymnosperms was capable of undergoing radial longitudinal divisions
Aneurophytales
contains the more relictual progymnosperms
they varied in stature from shrubs, to large trees up to growth, but the primary xylem of their stems was a protostele like that of rhyniophytes and trimerophytes
further resemble Trimerophytes in having little webbing between their ultimate branches;
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these could not yet be called leaves
Archaeopteridales
these were trees up to 8.4 m tall with abundant wood and secondary phloem
stems of
Archaeopteris
had a siphonstele, pith surrounded by a ring of primary xylem bundles, much like modern conifers and dicots
although the "fronds" of archaeopterids resembled fern leaves, close examination reveals that they were actually
planated branch systems and that the ultimate" leaflets" were really spirally arranged simple leaves
Webbing was only partial in
A. halliana
and
A. obtusa
; these can be considered full-fledge megaphylls
reproduction in archaeopterids was heterosporus
megaspores measured up to 300 in diameter and microspores only 30 in diameter
and each type was produced in its own distinctive sporangium-broad megasporangia and slender microsporangia
sporangia were terminal on short branches mixed with sterile, leaf like branch systems.
megaspores were released from the sporangia, not retained. seeds were not produced
Evolution of seeds
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Investigating the life cycle of extinct plants based on fossil material is difficult, but not impossible.
in free-sporing species, spores can be identified with sporophytes if some spores were trapped in a sporangium attached attached to leaves or wood during fossilization
Unfortunately, spores cannot be identified with gametophytes except when gametophytes is microscopic and develops within the spore wall
if megaspores are retained in the megasporangium, at least some of the investigation is easier
currently, the earliest known progymnosperm species with heterospory is
Chauleria
from the Middle Devonian period approx. 390 million years ago
the megasporangium was surrounded by a layer of tissue, an integument that permitted the sperm cells to swim to the egg after the megaspore had developed into
a megagametophyte and had produced eggs
this is similar to angiosperm ovules and some fossils help us understand the early stages
the sheath of sterile branches must have been important in trapping wind-blown microspores, but at first it would have allowed them to steel anywhere on the megasporangium
as they fused to each other and to the megasporangium, the space at the top of the megasporangium became the place where microspores settled, acting as a
pollen chamber
or holding area
Division Pteridospermophyta: seed ferns
porgymnosperms gave arise to another line of gymnospermous plants in addition to conifers: the cycadophytes
these are classified as three divisions: Pteridospermophtya, Cycadophyta and cycadeoidophyta
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the earliest
seed fern
appeared in the Upper Devonian Period-others appeared later.
seed ferns were any woody plant with fern-like foliage that bore seeds instead of sori on its leaves
seed fern wood was manoxylic, much softer and less dense than wood of conifers and progymnosperms and manoxylic wood also occurs in cycads
not all are closely related to each other, they form a grade rather than a clade
many resembled modern tree ferns, others were vines
Pteridosperms are thought to have evolved from the Aneurophytales because the earliest seed ferns, such as
stenomyelon
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they had a ring of vascular bundles surrounding a pith, an arrangement that also occurs in the stems of all gymnosperms and angiosperms
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unlike ferns however, the foliage leaves of seed ferns bore seeds
within the seed fern ovule, the megasporangium was large and bundles of vascular tissue ran into and through it
Division Coniferophyta: conifers
they are diverse and all are trees of moderate to gigantic size, the giant redwoods of California reach 90m in height and 10m in diameter
Conifers are never vines,herbs or annuals and they never have bulbs or rhizomes
Conifers leaves are always simple needles or scales.
leaves of most conifers are perennial , persisting for many years leaves
Agathis
and
Araucaria
remain even on very old trunks
the venation of conifer leaves is often simple, with just one or two long veins running down the center of a needle-shaped leaf or several parallel veins in scale-shaped leaves
leaf veins have an endodermis and tissue called transfusion tissue, consisting of transfusion parenchyma cells and transfusion tracheids
just as in their progymnosperm ancestor
Archaeopteris
wood of modern conifers lacks vessels
pines are good representatives for closer examination.
the trees are monopodial, with one main trunk bearing many branches
pines like several other conifers have two types of shoot, each with a characteristic type of leaf
tiny papery leaves occur on long shoots and in their axils are short shoots that produced the familiar long needle leaves
like all conifers, pines have both pollen cones and seeds cones
pollen cones are simple cones with a single short unbranched axis that bears microsporphylls
Division Cycadophyta:Cycads
modern cycads are frequently confused with either ferns or young palm trees because they have stout trunks with pinnately compound leaves
most cycads are short plants less than 1 or 2 m tall, but
Macrozamia
can reach heights of 18 m
the trunk is covered with bark and persistent leaves bases that reamin on the plant even after the lamina and petiole have abscised
internally, stem are similar to those of seed ferns
a thick cortex containing secretory ducts surrounding a small amount of
manoxylic
wood.
tracheids are long and wide and rays are massive
even very old stems have only a small amount of wood;
moat support is provided by the tough leaf bases
a prominent pith contains secretory canals
unlike seed ferns, their foliage leaves do not bear ovules
instead they produce seed cones and pollen cones each on separate plants; cycads are always dioecious
Although Cycadophyta was a larger group with many more species in earlier times, currently, it contains 9 or 10 genera and approx. 100 species
Modern ones are highly prized ornamentals in the warmest parts of the USA
the cycadeoids ( all gone) had vegetative features almost identical to those of cycads
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the two groups differ only in the subtle details of the differentiation of stomatal complexes and in leaf trace organization.
on such characters alone, cycadeoids would never be considered distinct from cycads;
however, 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
Division Ginkgophyta maidenhair tree
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this division contains a single living species
Ginkgo biloba
it may seem unusual to erect an entire division for a single species, but
G.biloba
is it self unusual
it looks very much like a large dicot tree with a stout trunk and many branches, but its wood is like that of conifers
a ginkgo tree itself is beautiful and is a popular ornamental because the leaves turn brilliant yellow in autumn but microsporgiate
Division Gnetophyta
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contains three groups of enigmatic plants
Gnetum with 30 species Ephedra with about 40 species and welwithschia mirabilis the only species in the genus
Gnetumns are mostly vines or small shrubs with broad leaves simiar to those of dicots.
they are native to southeast Asia, tropical Africa and the Amazon basin.
plants of ephedra are tough shrubs and bushes that are very common in desert regions in northern Mexico and southwestern USA
All genera are unusual in being gymnosperms with vessels in their wood
this had been thought to show that they might be related to primitive angiosperms, were derived
