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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 i th imegaspore in the megasporangium.

– 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 microporesor 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 fused

Lobes separate

Fusion incompleteComplete:

micropyle at top

5 phyla of seed plants with living representatives

• Cycadophyta• Ginkgophyta• Coniferophyta Gymnosperms

• Gnetophyta• Anthophyta

ProgymnospermsPitted 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 Gymnosperms

• Gnetophyta

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• 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– 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• 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 inmorphologically 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 low-

water conditions.

Retain needles for 2 to 44 yrs

Pinus longaeva –retained ~ 45 yrs

Stems

• Secondary growth begins early.• Secondary xylem is produced toward

inside of V.C. S d hl i d d t d• Secondary phloem is produced toward 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• Microsporangiate cones borne on lower branches.

• Megasporangiate cones borne on upper branches.

• Microsporangiatecones ~ 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 complexesSeed 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 containsM t ( th ll)– Megasporocyte (megaspore mother cell)

– 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 dropsP ll i th i d th h– Pollen grains are then carried through 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 pollinatedovules are 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

• GinkgophytaGi k bil b l li i i– Ginkgo biloba; sole living survivor

• Gnetophyta– Has members with Angiosperm-like features

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CycadophytaZamia pumila

Encephalartos ferox

Female plant: Ovulate Cones

Cycas siamensis

Female plant: Seeds borne on d fedges of

megasporophylls

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Ginkgophyta

• Ginkgo biloba only living member• Fan-shaped leaves• Bears ovules and microsporangia on

diff t i di id ldifferent individuals.• 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

Microsporangiateinflorescences

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

cyclecycle• Pollination and Pollen Tube formation

eliminate the need for water for the sperm to reach the egg

• Four phyla of Gymnosperms with living representatives