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SEED PLANTS: GYMNOSPERMS
Chapter 22
“By the late Devonian and early Carboniferous (385-290 Ma), plants had developed the ability to produce
secondary wood through the development of a cambium layer. Hence, vascular plants were no longer restricted to
heights of less than 1 meter and forests developed with trees as tall as 35 meters. An ideal climate helped the spread
of the flora of this era, creating the Great Coal Forests. There are two parts to the story of the Carboniferous: the
climate at the plants.
The mid to late Carboniferous and early Permian saw the decline of the Giant Clubmoss Flora and the expansion
and proliferation of the seed ferns. The climate was no longer so favorable, and many parts of the world
experienced 'Ice Ages'.
Amongst the lycopsids, progymnosperms, filicopsids and horsetails during the late Carboniferous (354-290Ma)
grew two other significant groups of plants; the pteridosperms and cordaites. These were significant because these
were seed plants which did not use spores for reproduction.
It was during the late Devonian between 395-286Ma that seeds developed (to replace spores). This was a major
evolutionary advance, because it eliminated the requirement for external sources of water for sexual reproduction of
plants…
Some significant groups of plants evolved during the Permian in the climatic conditions which became progressively
drier; these being the Bennettites, Cycads, Ginkgos and Glossopterids. ”
Source: Adam Dimech at http://www.adonline.id.au/plantevol/tour/tour8.htm
http://www.adonline.id.au/plantevol/tour/tour9.htm
http://www.adonline.id.au/plantevol/tour
EVOLUTION OF THE SEED
Seed plants dominate the modern flora.
Seeds have a great survival value.
SEED
SPORE
1. Multicellular embryo
Single cell
2. Food supplied by tissue
Food only in the cell
3. Multicellular seed coat
Covering not cellular
4. Diploid sporophyte
Haploid cell
5. Product of fertilization
Product of meiosis
The immature ovule consists of the megasporangium surrounded by one or two layers of tissue called
integuments.
The seed is a mature ovule containing an embryo.
Events in the evolution of the ovule.
1. Retention of the megaspores within the megasporangium.
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The megasporangium is fleshy is seed plants and is called the nucellus.
2. Reduction in the number of megaspore mother cells (megasporocytes) in each megasporangium to
one.
3. Survival of only one of the four megaspores produced by the spore mother cell.
4. Formation of a highly reduced megagametophyte inside the single functional megaspore.
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Endosporic megagametophyte that is no longer free living and is retained within the
megasporangium.
5. Development of the young sporophyte within the megagametophyte retained within the
megasporangium.
6. Formation of an integument that completely envelops the megasporangium except for an opening at
the apex called the micropyle.
7. Modification of the apex of the megagametophyte to receive the microspores or pollen grains.
Fossil record
The oldest ovules or seeds are from the Late Devonian (365 million years old).
The oldest seed-bearing plant is Elkinsia polymorpha
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The earliest seed plants produced their seeds along their branches without specialized structures, such
as cones or flowers, unlike most living seed plants.
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The seeds were produced singly or in pairs, and were surrounded by a loose cupule.
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This small cup-like structure was lobed and branched dichotomously in the earliest seeds, producing a
somewhat sheltered chamber at one end of the seed.
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Within this cupule, the seed was enclosed by a more tightly adpressed tissue called the integument.
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The integument is a layer of tissue found in all seeds; it is produced by the parent plant, and develops
into the seed coat.
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As the integument evolved to enclose the seed more tightly, an opening was left at one end, called the
micropyle, which permitted pollen to enter and provide sperm to fertilize the egg cell.
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Both the integuments and cupule are believed to be the result of reduced and fused branches or
leaves.
There are five phyla of seed plants with living relatives: Cycadophyta, Ginkgophyta, Coniferophyta,
Gnetophyta and Anthophyta.
Gymnosperm means "naked seed".
In 1825 the Scottish botanist Robert Brown distinguished gymnosperms from the other major group of
seed plants, the angiosperms, whose seeds are surrounded by an ovary wall.
There are four phyla of extant gymnosperms: Cycadophyta, Ginkgophyta, Coniferophyta and Gnetophyta.
These phyla have in common
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Ovules and seeds exposed on the surface of the sporophylls.
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The female gametophyte produces several archegonia (there are some exceptions). Polyembryony
may result from multiple fertilizations.
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The seeds of many gymnosperms are borne in cones and are not visible. The cones, however, are
not the same as fruits.
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Furthermore, at maturity, the cone expands to reveal the naked seeds.
Pollination is the transfer of pollen, the microgametophyte, to the vicinity of the megagametophyte.
During pollination, the immature male gametes, or pollen grains, sift among the cone scales and land
directly on the ovules (which contain the immature female gametes) rather than on elements of a flower
(the stigma and carpel) as in angiosperms.
The endosporic microgametophyte produces a pollen tube.
Cycads and Ginkgo have a motile sperm and fertilization is transitional between seedless plants and seed
plants.
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The pollen of cycads and Ginkgo produces a haustorial pollen tube that penetrates the nucellus
and may grow for several months.
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Eventually the multiflagellated sperm is released near the archegonium
Conifers and gnetophytes have nonmotile sperms.
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In conifers, gnetophytes and angiosperms the pollen tube bring the sperm to egg cell.
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This is considered a later development to haustorial pollen tube.
Some of the divisions of gymnosperms are not closely related to others, having been distinct groups for
hundreds of millions of years.
Currently, about 60-70 genera are recognized, with a total of 700-800 species.
Gymnosperms are distributed throughout the world, with extensive latitudinal and longitudinal ranges.
DIVISION CONIPHEROPHYTA
This is the most numerous, most widespread and most ecologically important of gymnosperm taxa.
It comprises about 50 genera and 550 species.
Conifers probably dominated the upland forests since the late Carboniferous continued to be
important components of the vegetation to this day.
In the wake of dramatic cooling and drying near the close of the Paleozoic, conifers expanded their range
to become dominant elements of tropical and temperate plant communities.
Conifers remained ecosystem dominants through most of the Mesozoic, only yielding their position to the
diversifying angiosperms in the Cretaceous.
CHARACTERISTICS
1. Almost all known conifers (extinct and living) are woody.
2. Most are trees. The most common growth form for conifers is a single erect trunk with branches
emerging at nearly right angles to the main axis
3. Most conifers are evergreen but a few, Larix and Metasequoia, are deciduous.
4. The leaves of extant conifers may be needle-like, awl-shaped or flattened.
5. Conifer leaves have parallel veins.
6. Abundant secondary growth occurs from a bifacial vascular cambium.
7. Xylem is composed entirely of tracheids and wood is generally parenchyma poor and thus pycnoxylic
8. Tracheids of many conifers have a characteristic circular bordered pitting on element walls.
9. They have cones. Cone is a colloquial term for a woody strobilus. Not all conifers produce woody
strobili. In some taxa, ovule-bearing structures are reduced to fleshy, berry-like forms (e.g., Juniper,
Taxus, Podocarpus).
10. Ovules are attached to a scale in various orientations. A bract subtends the scale, in turn.
11. In pollen cones, microsporangia are borne on a single bract without a trace of a second, subtending
structure.
Family Pinaceae
The family Pinaceae is known in the fossil record since the Cretaceous.
A clearly monophyletic group united by characters seen in the mature seed cones:
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bract-scale complexes consisting of well-developed scales that are free from the subtending bracts
for most of their length;
two inverted ovules on the adaxial face of each scale;
a (usually obvious) seed wing developed from the cone scale.
The Pinaceae are resinous trees or rarely shrubs comprising about 9 genera and 225 species found mostly
in temperate regions of the Northern Hemisphere.
The leaves are spirally disposed and are linear and needlelike.
The male or microsporangiate strobili are small, terminal, or more often clustered along the stem axis, and
consist of many papery microsporophylls, each with two microsporangia on the lower surface.
The pollen grains typically have two, bladderlike wings.
The female cones or megasporangiate strobili are woody and often large, consisting of many ovuliferous
scales, each with a pair of adaxial ovules and a more or less distinct subtending bract.
Pines
1. Pinus is the largest genus of conifers with over 100 species.
2. Trees or shrubs, aromatic, evergreen; crown usually conic when young, often rounded or flat-topped
with age. During pollination, the immature male gametes, or pollen grains, sift among the cone scales
and land directly on the ovules (which contain the immature female gametes)
3. Branches usually in pseudowhorls at right angles with the trunk.
4. Shoots dimorphic with long shoots and dwarf shoots.
5. Dwarf shoots borne in close spirals from axils of scaly bracts and bearing fascicles of leaves
(needles); shoot with determinate growth.
6. Long shoots have indeterminate growth.
7. Leaves dimorphic, spirally arranged:
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Foliage leaves (needles) (1)2-5(6) per fascicle, persisting 2-12 or more years, sessile, sheathed at
base by 12-15 overlapping scale leaves.
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Scale leaves usually persisting for life of fascicle or shed after first season
8. Plants monoecious.
9. Staminate cones numerous and small, in a dense, spikelike cluster around base of current year's
growth.
10. Ovulate cones solitary to few, maturing in 1.5-2 or 3 years.
11. The cones have spirally arranged woody scales, each being subtended by a short or long bract and
bearing 2 adaxial ovules.
12. Two seeds at the base of the cone scale, winged.
13. Native to all continents and some oceanic islands of the northern hemisphere, chiefly in boreal,
temperate, or mountainous tropical regions; reaching its southernmost distribution shortly below the
Equator in southeast Asia (Sumatra).
14. Introduced as ornamental and timber trees in much of the Southern Hemisphere.
Pine Life Cycle
A. Ovulate cone.
Ovulate cones are larger and more complex than pollen-bearing cones.
The ovuliferous scales bear the ovules.
Ovuliferous scales are determinate branch systems known as seed-scale complex.
Each seed-scale complex consists of an ovuliferous scale, two ovules born on its upper surface and a
sterile bract.
The scales are arranged spirally around the axis of the cone.
B. Structure of the ovule.
Each ovule consists of a megasporangium also known as the multicellular nucellus.
A massive integument surrounds the multicellular nucellus.
Each megasporangium contains a single megasporocyte or spore mother cell, which immediately
undergoes meiosis.
Meiosis produces four linear megaspores but only one survives and the other degenerate.
The functional megaspore slowly develops into a multinucleated gametophyte. It may take 13 months.
After a year of development the female gametophyte, cell wall formation begins around the free nuclei.
Archegonia (2 to 6) develop at the micropylar end of the ovule.
Megasporangium (2n) → single megasporocyte (2n) → megaspore (n) → multinucleated
gametophyte (n) → archegonia (n) → egg (n)
C. Structure of the pollen grain – male gametophyte
Pollen cones, male strobili, are usually produced in the spring.
The strobilus consists of a central axis with more or less thin scales, microsporophylls, arranged in a
spiral or in whorls around the central axis.
They are usually produced at the end of a branch, and are about 4 cm (1.5 in) long.
The microsporophylls contain two microsporangia in which the microsporocytes, 2N, develop.
Microsposrocytes undergo meiosis and produce haploid microspores, N.
Microspores develop into a much reduced microgametophyte that consists of a generative cell that will
divide to produce two sperm nuclei, and a tube cell that will form the pollen tube.
The mature microgametophyte, pollen grain, is protected by a thick wall; two air bladders project out of
the wall and aid in wind dispersal.
Microsporangium (2n) → microsporocytes (2n) → microspores (n) → microgametophyte (n) →
generative cell (n) → sperm nuclei (n).
D. Pollination
Pollination occurs in the spring.
The scales of the ovulate cone are widely separated.
Pollen may adhere to pollination drops secreted by the micropylar canal at the open end of the ovule.
As the pollination drops evaporate, they pull the pollen in contact with the nucellus.
Shortly after coming in contact with the nucellus, the pollen grain germinates and produces a pollen tube.
15 months after pollination, pollen tube reaches the egg in the archegonium.
The life cycle extends over a period of two years.
YEAR
MICROSTROBILUS
MALE GAMETOPHYTE
MEGASTROBILUS
FEMALE GAMETOPHYTE
ZYGOTE
I
initiation of microstrobilus
sporogenous tissue
initiation of megastrobilus
II
meiosis; shedding of pollen
pollen germination; stalk and
body cell formed.
ovule development; megasporocyte
formed; pollination; meiosis; free
nuclear gametophyte.
III
stalk and body cells move to
lower end of pollen tube; two
sperms produced; rapid growth
of pollen tube through megasporangium.
additional free nuclei; cellular gametophyte; archegonia formed; fertilization;
zygote and embryo development;
maturation of seed; shedding of seed.
Other conifers.
The family Araucariaceae occurs in the Southern Hemisphere and lands that were part of Gondwana.
It has three living genera, Araucaria, Agathis and Wollemia.
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Araucaria is an important ornamental.
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Wollemia was discovered in 1994 is southern Australia.
The family Taxodiaceae includes Sequoia, Sequoiadendron, the bald cypress Taxodium, and the dawn
redwood Metasequoia, discovered in 1944 in China.
Other important conifers are Abies (firs), Picea (spruce), Larix (larch), hemlocks (Tsuga), Juniperus
(junipers) and Cupressus (cypresses).
Podocarpus is a southern conifer widely planted as ornamental in mild climates.
Podocarpus and yews produce their ovules partially surrounded by a fleshy cup called the aril.
DIVISION GINKGOPHYTA
Ginkgo biloba is the last surviving member of a lineage that originated in the late Paleozoic and became
quite diverse during the Mesozoic.
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The name ginkgo is derived from a Chinese language meaning “silver apricot”.
The family Ginkgoaceae was widely distributed in the Northern hemisphere.
Although probably extinct in the wild, the plant was domesticated thousands of years ago in China.
The living plant has distinctive, fan-shaped leaves, produced in clusters at the ends of short, lateral shoots,
and it is very easy to recognize.
Much-branched deciduous trees with long shoots and spur (or short) shoots.
Secondary growth well developed.
Leaves alternate but often appearing whorled on slow-growing short shoots, simple, fan-shaped with
dichotomous venation, often with 2 lobes
Plants dioecious
Microsporangia in pairs on slender microsporophylls (modified leaves) which are loosely arranged in
small pendent strobili borne on spur shoots.
Male gametophyte (pollen) forms an extensive haustorium in between the cells of the nucellus.
Male gametophyte producing 2 large swimming sperm (multiflagellate) within the pollen tube.
Ovules borne in pairs on slender stalks on spur shoots.
Seeds with seed coat differentiated into 3 layers (an outer fleshy layer--fetid with odor of rancid butter
when seed is mature, a middle bony layer and a thin inner fleshy layer).
DIVISION CYCADOPHYTA
Cycads are an ancient group of seed plants with a crown of large compound leaves and a stout trunk.
Cycads are a unique assemblage of plants although grouped with the gymnosperms they are unrelated to
any other group of living plants.
They are a minor component of the flora in tropical and subtropical regions today, but during the Jurassic
Period, they were a common sight in many parts of the world. For this reason, the Jurassic is often
referred to as the "Age of Cycads".
Today cycads may be regarded as relicts consisting of small populations distributed disjunctly on many
continents. There are about 100 living species.
The Cycadaceae are woody, unbranched or sparsely branched, palmlike, seed-bearing trees or shrubs with
thick, pithy stems.
The main roots of cycads are thickened and fleshy and may have storage capacities.
Vascular tissue has tracheids only and lacks vessels.
The leaves are frondlike, pinnately compound, usually stiff, and often with sharply pointed leaflets that
exhibit circinnate vernation.
Leaves are alternate, and spirally arranged in a cluster at the summit of the stem.
Cycads are dioecious.
The ovules and seeds are born naked on the petioles of reduced leaves that are loosely clustered or more
highly modified and compacted into female or ovulate cones.
Male plants produce male or microsporangiate cones that bear many scales, each with an abundance of
microsporangia scattered over the lower surface.
Pollination is usually by insects, mostly beetles.
The pollen grains or microgametophytes that develop from the microspores eventually produce two
multiflagellated motile sperm cells.
Pollen tube is usually unbranched and grows into the female gametophyte.
The developing embryo derives its nutrients from the female gametophyte.
Seeds are typically large, sometimes up to the size of goose eggs.
DIVISION GNETOPHYTA
The phylum Gnetophyta consists of three families, three genera and about 70 species.
Plants are trees, shrubs, lianas, or stumpy turnip-like.
Leaves are simple, opposite or whorled, strap-like in Welwitschiaceae, angiosperm-like in Gnetaceae, or
scale-like in Ephedraceae.
Secondary xylem contains tracheids and vessels, as in angiosperms, although the homology of the vessels
in the two phyla is in question.
Plants are usually dioecious with "flowers" unisexual in compound strobili or "inflorescences".
Ovulate and microsporangiate cones on separate plants except in some species of Ephedra.
Double fertilization is recorded in Ephedra, as in angiosperms, although the two are not exactly
homologous.
The second fertilization produces extra embryos rather than nutritive tissue. Extra embryos degenerate.
Gnetum and Ephedra produce a large female gametophyte that nourishes the embryo.
Wind pollination is the common method of pollen transfer but insects also pollinate Ephedra.
Vessels and double fertilization are angiosperm-like features that place the Gnetophyta closer to the
Anthophyta, flowering plants.
HUMAN RELEVANCE OF GYMNOSPERMS
Some of the most important uses of conifer wood are:
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Douglas fir is possible the most popular timber tree in the world today.
Spruce is the major supplier of newsprint.
Spruce wood is also used in the making of musical instruments and aircrafts.
Pines are used in housing and the construction of many structures.
Pines produce turpentine and rosin.
Dye and tannins are obtained from the eastern hemlock.
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Larch and junipers are used for fence posts.
Cedars are used for cedar chests, cigar boxes and pencils.
Most species have an ornamental value.
Ephedra is the source of the drug ephedrine; an alkaloid used to raise blood pressure, dilate bronchial
tubes in asthma and contracts the nasal mucosa in hay fever.