Chapter 22: Seed Plants I: Seed Plants Without Flowers ("…
Chapter 22: Seed Plants I: Seed Plants Without Flowers ("Gymnosperms")
Division Coniferophyta: Conifers
: a cone with several lateral axes attached to the main axis; conifer seed cones are compound
: leaves that the short axis bears
: pollen cones; a single short unbranched axis that bears microsporophylls
: the scale that bears the ovule in gymnosperm seed cones
: produce familiar long needle leaves
: in seed plant embryos, the stalk of cells that pushes the embryo into the endosperm
: tiny papery leaves occur on
: an early stage of embryo development, usually considered to encompass the stages between the zygote and the initiation of the cotyledon primordia
Division Cycadeoidophyta: Cycadeoids
Cycadeoids (all extinct) have 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.
On such characters alone, cycadeoids would never be considered distinct from cycads.
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 micropyle.
Between the ovules were thick, fleshy scales.
Microsporophylls were located below the cluster of megasporophylls and curved upward, enveloping the megasporophylls.
Each microsporophyll was cup shaped and contained numerous microsporangia.
Division Cycadophyta: Cycads
Confusion between cycads and ferns or palm trees
Cycads have stout trunks with pinnately compound leaves.
Most cycads short (less than 1 or 2 m tall)
can reach heights of 18 m.
Cycads trunk is covered with bark and persistent leaf bases.
Leaf bases remain on the plant even after lamina and petiole have abscised.
Cycads stems similar to those of seed ferns.
Thick cortex containing secretory ducts surrounds a small amount of manoxylic (parenchyma) wood.
Tracheids are long and wide, and rays are massive.
Even very old stems have only a small amount of wood
Most support provided by tough leaf bases.
Cycads have a prominent pith that contains secretory canals.
Division Pteridospermophyta: Seed Ferns
Progymnosperms gave rise to another line of gymnospermous plants in addition to the conifers: the cycadophytes.
These are classified as three divisions:
Cycadophyta (cycads, extant)
Cycadeoidophyta (cycadeoids, all extinct)
Pteridospermophyta (seed ferns, all extinct)
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 (clusters of sporangia) on its leaves.
Many resembled modern tree ferns (except they had wood), others were vines.
Pteridosperms are thought to have evolved from the Aneurophytales because the earliest seed ferns had a three-ribbed protostele as in the aneurophytes.
Earliest seed fern such as,
Division Ginkgophyta: Maidenhair Tree
Contains a single living species,
(the "maidenhair tree")
Is itself unusual...
Began to die out early in Tertiary Period, but two species,
The latter species became extinct during the Pliocene Epoch, approximately 10 to 12 million years ago.
Exact ancestors unknown.
Fossilized remains of leaves and wood can be found in almost all areas of the world.
Especially in high latitudes such as Alaska, Canada, and Siberia near the North Pole, and Patagonia, South Africa, and New Zealand near the South Pole.
Became abundant during the Mesozoic Era, especially in the mid Jurassic Period (approximately 170 million years ago).
Must have been one of the seed ferns or a closely related group.
Microsporangiate ("male") trees preferred.
When the megasporangiate ("female") trees produce seeds, the outer fleshy layer of the seed emits butyric acid, which has a putrid odor that is difficult to tolerate.
Pollen is produced in an organ that resembles a catkin, having a stalk and several sporangiophores that each have two microsporangia.
Like ovules of pteridosperms and cycads, the ovules of Ginkgo are large (1.5 to 2.0 m in diameter) and develop a three-layered seed coat.
Looks very much like a large dicot tree with a stout trunk and many branches.
Wood like conifers.
Lacks vessels and axial parenchyma.
Has "broad leaves," but they have dichotomously branched veins like seed ferns, not reticulate venation like dicots.
Ginkgos have both short shoots, which bear most of the leaves, and long shoots.
Reproduction in Ginkgo is dioecious and gymnospermous, but cones are not produced.
Instead, ovules occur in pairs at the ends of a short stalk and are completely unprotected at maturity.
The life cycle of vascular cryptograms is an alteration of independent, heteromorphic generations.
A disadvantage of this life cycle is that new sporophyte, while developing from the zygote, is temporarily dependent on a tiny gametophyte for its start in life.
: synonym fora plant that develops wood; this is used as an informal name for the clade that contains the woody plants
: plants that produce seeds
: wood that contains significant amounts of axial parenchyma, such as that of cycads
: wood with little or no axial parenchyma, such as that of gymnosperms and progymnosperms
: common name for plants with naked seeds; all seed plants that are not angiosperms
Ex: conifers and cycads
: informal term for flowering plants, members of division Magnoliophyta; their seeds develop within a fruit
Division Progymnospermophyta: Progymnosperms
A more derived progymnosperm was
Stems had siphonostele, pith surrounded by a ring of primary xylem bundles, much like modern conifers and dicots.
"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.
Can be considered full-fledged megaphylls.
Webbing was only partial in
Trees up to 8.4 m tall with abundant wood and secondary phloem.
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.
Unfortunately, spores cannot be identified with gametophytes except when the gametophyte is microscopic and develops within the spore wall.
Currently, the earliest known progymnosperm species with heterospory is
from the Middle Devonian Period, approximately 390 million years ago.
In an Upper Devonian fossil, Archaeosperma arnoldii, the megasporangium produced only one megaspore mother cell, and this produced only one large, viable megaspore and three small, aborted cells.
: in flowers, the covering layer over the nucellus of an ovule; usually two integuments (inner and outer) are present
: a hole in the integument that permitted the sperm cells to swim to the egg after the megaspore had developed into a megagametophyte and produced eggs
, the megasporangium was closely surrounded by sterile telomes.
, the telomes were fused at the base, and in
, they were similar to
, fused into one structure except at the tip.
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
or holding area.
Contains the more relictual progymnosperms
Protopteridium, Proteokalon, Tetraxylopteris, Triloboxylon, and Eospermatopteris
They varied in stature from shrubs (
) to large trees, up to 12 m tall.
They all had a vascular cambium and secondary growth, but the primary xylem of their stems was a protostele like that of rhyniophytes and trimerophytes.
Aneurophytes further resembled trimerophytes in having little webbing between their ultimate branches.
Could not yet be called leaves.
: a group of extinct plants believed to have been the ancestors of gymnosperms
Few fossils of gnetophyte organs or tissues are only several million years old.
Pollen distinctive, being spindle-shaped and having narrow ridges.
Certain aspects of their anatomy and reproduction have been interpreted as indicating that gnetophytes and flowering plants constitute two sister clades with a common ancestor.
Too few data to be confident of skepticism; search for more data about phylogenetic relationships.
If so, the two would form the group "Division Anthophyta" or "
Easy to recognize,and fossil pollen of this type occurs as far back as late Triassic Period.
Has not helped reveal their origins.
Too recent to be of much help in understanding evolution and ancestry of group.
Pollen cones of gnetophytes
Seed cones also compound and contain extra layers of tissue around ovules.
The tissue is variously interpreted as an extra integument, bract, or sporophyll.
Compound and contain small bracts.
All three genera are unusual in being gymnosperms with vessels in their wood.
Their vessel elements evolved from tracheids with circular bordered pits, whereas those of angiosperms were derived from scalariform tracheids.
Angiosperms are thought to have evolved from vessel-less ancestors.
The vessels evolving after flowers, not before them.
Gnetophyta contains three groups of enigmatic plants.
with about 40 species
are tough shrubs and bushes.
Very common in desert regions in northern Mexico and southwestern United States and dry mountains in South America.
, the only species in the genus
Have a short, wide stem and only two leaves.
Leaves grow perennially from a basal meristem, becoming increasingly longer.
The few living plants exist only in deserts of South Africa or in cultivation.
with 30 species
are mostly vines or small shrubs with broad leaves similar to those of dicots.
Native to southeast Asia, tropical Africa, and the Amazon Basin.