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Gymnosperms
Chapter 18
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Gymnosperm = ‘naked seed’
• Events that led to evolution of an ovule
– Retention of megaspores within the megasporangium.
• Fleshy and called nucellus
• Megasporangium no longer releases spores
– Reduction in the number of megaspore mother cells in each
megasporangium to one.
– Survival of only one of the four megaspores; leaving a single functional
megaspore in
i th
the megasporangium.
i
– Formation of the megagametophyte inside single functional megaspore
• Formation of endosporic megagametophyte—no longer free-living
• Retained within the megasporangium
– Development of the embryo (young sporophyte) within the
megagametophyte retained within the megasporangium.
– Formation of an integument that envelops the megasporangium; except
at its apex; micropyle.
– Modification of the apex of the megasporangium to receive micropores
or pollen grains.
Fossil record of ovule evolution
Elkinsia polymorpha:
nucellus and 4 to 5
integumentary lobes
with little to no fusion
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Evolution of integuments
Lobes separate
Lobes fused
Fusion incomplete
Complete:
micropyle at top
5 phyla of seed plants with living
representatives
•
•
•
•
•
Cycadophyta
Ginkgophyta
Coniferophyta
Gnetophyta
Anthophyta
Gymnosperms
Progymnosperms
Pitted tracheids
Archaeopteris
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Extinct Gymnosperms
• Pteridospermales (seed ferns)
• Cordaitales (primitive conifer-like plants)
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Living Gymnosperms
•
•
•
•
Cycadophyta
Ginkgophyta
Coniferophyta
Gnetophyta
Gymnosperms
Microgametophytes
• Develop as pollen grains
• In SVP’s water is required for motile,
flagellated sperm to reach & fertilize eggs.
• In gymnosperms water is NOT required as a
medium of transport of the sperm to the eggs.
• Partly developed microgametophyte (pollen
grain) is tranferred bodily to the
megagametophyte.
– Pollination
Differences in pollination
• Conifers & Gnetophytes
– Sperm – non-motile
– Pollen tubes convey them directly to archegonia
• Cycads & Gingko
– Fertilization is transitional between the condition
found in ferns and other seedless plants.
– Produce a pollen tube; does not penetrate
archegonium.
– Haustorial; and may grow for several months.
– Pollen grain bursts in vicinity of archegonium,
releasing multiflagellated, swimming sperm cells.
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Phylum Coniferophyta
• Most numerous, widespread, &
ecologically important of gymnosperm
phyla.
• 70 genera 630 species
Pinus (pines)
• Most familiar of all gynmosperms
– Dominate North America & Eurasia
– Widely cultivated in Southern Hemisphere
• Pinus palustris
– Longleaf pine
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• In seedlings, needlelike leaves are spirally
arranged & borne singly on stems
• After 1 or 2 yrs growth; begins to produce leaves in
bundles or fascicles
– Each contains a specific number of needles (leaves) 1 – 8
• Fascicles are short shoots in which activity
of apical meristem is suspended
– Fascicle of needles in a pine is
morphologically determinate (restricted in
growth) branch.
– Unusual circumstances result in a fascicle of
needles reactivated and growing into a new
shoot with indeterminate growth.
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Needles
• Impressively suited for growth under lowwater conditions.
Retain needles for 2 to
4 yrs
Pinus longaeva –
retained ~ 45 yrs
Stems
• Secondary growth begins early.
• Secondary xylem is produced toward
inside of V.C.
• Secondary
S
d
phloem
hl
iis produced
d
d ttoward
d
outside.
• Xylem consists of tracheids.
• Phloem consists of sieve cells.
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Pinus life cycle
• Microsporangia & megasporangia in pines
are borne in separate cones, or strobili, on
the same tree.
• Microsporangiate cones borne on lower
branches.
• Megasporangiate cones borne on upper
branches.
• Microsporangiate
cones ~ 1-2 cm
long
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• Microsporophylls are spirally arranged &
membranous.
• Each has two microsporangia on lower
surface.
• Many microsporocytes (microspore mother
cells).
– In spring undergoes meiosis – produces 4 haploid
microspores.
– Each develops into a winged pollen grain
• Immature microgametophyte
• Microsporophylls
• Pollen grain with microgametophytes
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• Prothalial cells have degenerated
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Ovulate cones
• Not simply megasporophylls
– They are entire modified determinate branch
systems known as:
• Seed – Scale complexes
• Each seed-scale consists of the
ovuliferous scale; which bears two ovules
on its upper surface.
• Each ovule consists of a multicellular
nucellus (megasporangium).
• Each megasporangium contains
– Megasporocyte
M
t (megaspore
(
mother
th cell)
ll)
– Undergoes meiosis; gives rise to four
megaspores.
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Pollination
• Occurs in spring
• As pollen grains settle on the scales
– Many adhere to pollination drops
– Pollen
P ll grains
i are th
then carried
i d th
through
h
micropylar canal into contact with the nucellus
• Month after pollination four megaspores
are produced
– One of which will develop into a
megagametophyte
– Development of megagametophyte is slow
• 6 mo after pollination
• 15 mo after pollination; pollen tube
reaches the egg cell of archegonium.
– One sperm nucleus unites with egg nucleus
and other degenerates.
Ovulate cones
1 yr old ovulate cones
Pollen bearing cones
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Other important conifers
• In most conifers other than pines, the
reproductive cycle takes only a year
– Seeds produced in same season as the
ovules are pollinated
pollinated.
– Pollination – Fertilization
• 3 days
• 3-4 weeks
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Abies - Firs
Larix
Cypress - Cupressus
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Juniperus - Juniper
Taxaceae - Yews
Araucariaceae
• South America
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Other Gymnosperm Phyla
• Cycadophyta
– Palm-like plants; tropical & subtropical
• Ginkgophyta
– Ginkgo
Gi k biloba;
bil b sole
l liliving
i survivor
i
• Gnetophyta
– Has members with Angiosperm-like features
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Cycadophyta
Zamia pumila
Encephalartos ferox
Female plant:
Ovulate Cones
Cycas siamensis
Female plant:
Seeds borne on
edges
d
off
megasporophylls
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Ginkgophyta
• Ginkgo biloba only living member
• Fan-shaped leaves
• Bears ovules and microsporangia on
diff
different
t individuals.
i di id l
• Rotting seed coat is responsible for ‘vile
odor’.
Gnetophyta
• 3 genera
• Gnetum
– 30 spp
• Ephedra
– 35 spp
• Welwitschia
– 1 spp primarily
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Gnetum
• Trees & climbing vines
– Large leathery leaves
Gnetum gnemon
Microsporangiate
inflorescences
Fleshy seeds
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Ephedra – only genera found in U.S.
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Welwitschia
Only two leaves that continually grow
Microsporangiate strobili
• Ovulate strobili
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Summary
• Seeds develop from ovule
• Seed plants likely evolved from
Progymnosperms
• All Gymnosperms have the same basic life
cycle
• Pollination and Pollen Tube formation
eliminate the need for water for the sperm to
reach the egg
• Four phyla of Gymnosperms with living
representatives
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