chapter 22-gymnospermsuam-web2.uamont.edu/facultyweb/fawley/botany/ch 22.pdf · of seed-bearing...
TRANSCRIPT
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Chapter 22-Gymnosperms
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Gymnosperms
Key innovations in the evolution of land plants
were pollen and seeds.
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Gymnosperms
Key innovations in the evolution of land plants
were pollen and seeds.
Pollen eliminated the need for water that the
sperm could swim in to reach the archegonium
and fertilize the egg cell.
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In ferns, lycophytes
and bryophytes the
flagellate sperm
have to swim to the
egg cell.
Some seed plants
also produce sperm.
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A pollen grain is an immature
microgametophyte (male
gametophyte).
A pollen grain actually
consists of several nuclei
and sperm cells.
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In association with the development of the ovule, paleobotanical
evidence indicates the evolution of pollen from approximately
365 Ma.
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“Prepollen” which contain morphological features of spores,
such as a trilete scar.
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Gymnosperms
Key innovations in the evolution of land plants
were pollen and seeds.
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The seed replaces spores as the mechanism of
dispersal.
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Packaging the embryo into a seed enables plants
to surround the embryo with nutritive tissue,
providing for the early survival of the embryo
when it is no longer attached to a gametophyte.
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A seed develops from the megasporangium.
Before the megasporangium matures into a seed it is
called an ovule.
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The ovule is composed of tissue of the megasporangium
inside a protective coating called the integument.
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The integument has a small hole, the micropyle, that allows
the entrance of the pollen.
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Within the megasporangium, meiosis produces a
megaspore which gives rise to the female gametophyte
and ultimately the egg cell is produced in the archegonium.
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Within the
megasporangium,
meiosis produces a
megaspore which
gives rise to the
female gametophyte
and ultimately the
egg cell is produced
in the archegonium.
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Fertilization occurs
with the growth of the
male gametophyte,
derived from the
pollen, which
produces either
sperm or special
nuclei that fuse with
the egg.
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After fertilization, the embryo develops, the
integument hardens into a seed coat and the
micropyle closes.
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Key steps in the evolution of seeds.
• Heterospory.
Already found in some fern relatives and also
present in progymnosperms
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Life cycle of Selaginella
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The transition from plants that were homosporous
(one spore size) to heterosporous (two spore sizes) is one
of the most important evolutionary trends in the development
of seed-bearing plants.
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It is postulated that the larger spores of heterosporous plants
were the precursor ovules, and the smaller spores, the
precursor of pollen.
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Key steps in the evolution of seeds.
• Retention of the megaspore in the megasporangium.
Reduction in the number of functional megaspores from
four (typical number of products of meiosis) to one.
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Key steps in the evolution of seeds.
•Integument
The origin of the integument is not known, but it may
have evolved from lobes of leaf-like tissue that
surrounded the megasporangium
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Another innovation in gymnosperms is wood.
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Wood enables gymnosperms and angiosperms to
grow much taller than other plants.
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The presence of wood
and seeds defines a
lineage that includes the
gymnosperms and
angiosperms (flowering
plants)
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The extinct plant
Archeopteris, which lived
about 300 million years
ago, represents a group
that were intermediate
between the ferns and
the gymnosperms.
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Archeopteris was a substantial tree, reaching
perhaps 75 m tall.
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Archeopteris was a substantial tree, reaching
perhaps 75 m tall.
Archeopteris could achieve
this height because of the
presence of true wood, with
lignified secondary growth
very much like modern
gymnosperms.
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Another intermediate
group were the “seed
ferns” (also extinct).
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Another intermediate
group were the “seed
ferns” (also extinct).
These plants were
originally thought to
be tree ferns, but
fossils were found
with seeds attached
to the fronds.
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Botanists now recognize
five living lineages of
seed plants.
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Four of these
lineages are what
we refer to as
gymnosperms.
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The gymnosperms with
the most ancestral
features are the Cycads.
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Cycads were a dominant part of the earth’s
vegetation about 200 million years ago.
Cycas revoluta
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Cycads were a dominant part of the earth’s
vegetation about 200 million years ago.
Only 11 genera and about 125 species survive.
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Cycads were a dominant part of the earth’s
vegetation about 200 million years ago.
Only 11 genera and about 125 species survive.
These species are almost all found in tropical
regions.
Cycas revoluta
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Cycads are dioecious, meaning that they have two
types of sporophytes.
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Cycads are dioecious, meaning that they have two
types of sporophytes.
One sporophyte produces the females structures
(ultimately ovules and egg cells) and the other
sporophyte produces the male structures
(ultimately producing pollen).
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In cycads, the seed producing strobuli (or cones)
are large and often protected by woody plates or
prickles.
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In cycads, the seed producing strobuli (or cones)
are large and often protected by woody plates or
prickles.
The pollen strobuli are also large. The pollen is
transported to the ovules by beetles or by wind.
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Ginkgo
A single species,
Ginkgo biloba,
remains of this once
thriving group of
plants.
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Ginkgo, unlike cycads, is a large tree that
superficially resembles angiosperms more than
most gymnosperms.
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The leaves are deciduous (fall off in winter) and
broad, unlike the leaves of most modern
gymnosperm trees.
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Like cycads, Ginkgo is dioecious.
Pollen is produced in small strobuli and
transported to ovulate trees by the wind.
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Like cycads, Ginkgo is dioecious.
Pollen is produced in small strobuli and
transported to ovulate trees by the wind.
The pollen germinates and, like cycads,
produces sperm that swim to the egg cells in
ovules.
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Hirase Sakugorō
Motorized sperm of Ginkgo biloba
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Conifers
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The conifers
Many of these plants
possess a distinctive
woody seed cone.
The name conifer,
means “cone-bearer”
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The conifers
Some conifers do not
have the woody cone.
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The conifers
The most familiar of these
is the juniper, which produces
a fleshy “berry” that could
easily be confused with an
angiosperm fruit.
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The conifers
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The Pinaceae includes the Pines, Firs, and
Spruces.
These are among the most important forest
trees for wood production.
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The Pinaceae includes the Pines, Firs, and
Spruces.
These are among the most important forest
trees for wood production.
Leaves are needle-like
and single or in groups
called fascicles, which
are borne on special
short shoots.
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Pinus, the pines
93 species; most are large, long-lived trees.
The bristlecone pines includes the oldest living
organisms (Pinus longaeva), nearly 5,000
years old.
http://sonic.net/bristlecone/
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Pinus, the pines
Pines almost always have needles clustered in
groups of 2 to 5 per fascicle.
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Pinus, the pines
The needles are oval to nearly triangular in
cross section.
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Pinus, the pines
The woody cones bear the seeds.
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The Firs, genus Abies.
Symmetrical trees that are generally smaller than
pines.
http://www.cnr.vt.edu/dendro/dendrology/syllabus/factsheet.cfm?ID=94
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The Firs, genus Abies.
The cones are upright and woody.
The cones shatter to release
seeds.
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The Firs, genus Abies.
Needles are borne singly, instead of in fascicles.
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The Firs, genus Abies.
Each years growth produces a new whorl of
branches.
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The Firs, genus Abies.
The age of tree can be
determined by the
number of whorls.
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The Firs, genus Abies.
Firs are restricted to cool regions of the Northern
Hemisphere, where there are about 40 species.
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The Spruces, genus Picea.
Spruces resemble firs, but the needles are angular
in cross section, rather than flat as in firs.
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The Spruces, genus Picea.
Seed cones are pendant and
light and somewhat papery.
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The Spruces, genus Picea.
There are about 40
species of Spruce, also
restricted to the
Northern Hemisphere.
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The hemlocks, genus Tsuga.
Pyramidal trees with slender, horizontal branches
and drooping tops.
http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-
new/tsuga_canadensis.html http://www.urbanext.uiuc.edu/treeselector/index_tree.cfm
?id=117
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The hemlocks, genus Tsuga.
Pyramidal trees with slender, horizontal branches
and drooping tops.
Cones are small and pendant, like spruce.
http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-
new/tsuga_canadensis.html
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The douglas firs, Pseudotsuga
Douglas firs are recognized by their pendant cones
that have distinctive 3-lobed bracts between the
scales.
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Larches and tamaracks, genus Larix.
These are deciduous conifers and they stand out
in the autumn when their needles turn brilliant
yellow.
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Larches and tamaracks, genus Larix.
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The cedars, the genus Cedrus.
Native to North Africa and Asia and used as
ornamentals in North America.
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The Cupressaceae includes the junipers,
cypresses and redwoods.
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The Cupressaceae includes the junipers,
cypresses and redwoods.
The members of this family are highly variable.
The cypresses (genus Cupressus) have woody cones,
whereas junipers (genus Juniperus) have fleshy cones.
juniper cypress
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The Cupressaceae includes the junipers,
cypresses and redwoods.
Redwoods and relatives
are large trees.
The redwoods are the tallest
existing trees, with many
specimens over 300 feet.
Redwood
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Taxaceae includes the yews.
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Taxaceae includes the yews (genus Taxus)
A single seed is covered by a fleshy aril, which is
formed from the ovule.
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Podocarpaceae and Araucariaceae are different
from other gymnosperms in that they have a
single ovule per cone scale.
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Podocarpaceae and Araucariaceae are different from
other gymnosperms in that they have a single ovule
per cone scale.
These are mainly Southern Hemisphere conifers.
Some are major lumber trees.
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Podocarpaceae and Araucariaceae
The so-called Norfolk Island
pine and monkey-puzzle tree
are widely planted ornamentals.
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Pine Life Cycle
Pines, like all seed
plants, are
heterosporous.
The vegetative plant
is a diploid
sporophyte that
produces a male
gametophyte in one
structure and a
female gametophyte
in another.
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Pine Life Cycle
Pollen is produced in
a male strobilus, a
series of densely
packed sporophylls
that are borne on the
lower branches of
the trees.
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Pine Life Cycle
Each strobilus has a large
number of small
microphylls attached
spirally.
Microsporocytes undergo
meiosis and produce
haploid microspores.
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Pine Life Cycle
Each strobilus has a large
number of small
microphylls attached
spirally.
Microsporocytes undergo
meiosis and produce
haploid microspores.
The microspore nuclei
divide several times inside
the spore wall to produce
the pollen grain
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Pine Life Cycle
The ovulate cones are
produced from the top
branches of the tree.
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Pine Life Cycle
Two ovules develop on the
upper surface of each
scale.
The megaspore inside the
ovule develops very slowly
into the female
gametophyte, a process
that takes just over a year
in pines.
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Pine Life Cycle
Immature and mature
female cones are often
seen on the same tree.
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Pine Life Cycle
A sticky pollination drop
exudes from the
micropyle.
This drop traps pollen
grains, which exude a
chemical signal that
causes the rapid
absorption of the liquid by
the ovule.
This pulls the pollen into
the micropylar chamber.
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Pine Life Cycle
The pollen germinates and
slowly generates a pollen
tube that grows toward the
egg.
The nuclei of the pollen
divide and produce 2
sperm.
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Pine Life Cycle
One sperm nucleus
enters the egg cell and
eventually fuses with
the egg nucleus.
The fertilization
produces the diploid
zygote that eventually
becomes the embryo.
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Other Gymnosperms –
the Gnetophytes
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Gnetophytes have some characteristics similar
to flowering plants.
Gnetophytes are represented by three living
genera: Ephedra, Gnetum, and Welwitschia.
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Ephedra is a shrub found in desert areas of the
western U.S.
Stem node with
scale-like leaves.
Seed inside aril.
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Gnetum is tropical, with about 30 species of trees,
shrubs, and vines.
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Welwitschia is a plant of the very dry Namib Desert
of southern Africa.
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It produces long, leather leaves that trail along the
ground, with a short stem and a very long tap root.