Gymnosperms

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Gymnosperms
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Evolution of the Seed

• Seed Production is an extreme form of
  heterospory.
• Modification to process a ovule is formed that
  becomes the seed.
• Seed a mature ovule containing an embryo
• Integuments the layers that surround the
  megasporangium of the immature ovule

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Evolution of the Seed

• Events leading to the evolution of ovule
• Megasporangium do not release spores anymore
• The number of megaspore mom cells reduce to one
• Only one megaspore survives from mom cell,
  leaving one functional megaspore

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Events cont.

• Formation of an endosporic megagametophyte, no
  longer free living, retained in the
  megasporangium.
• Development of the embryo w/in the
  megagametopythe.
• Integument forms and surrounds the megasporangium
• Micropyle opening left at the apex when
  integument forms

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Events cont.

• The apex of the megasporangium is modified to
  receive microspores and pollen grains

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The Fossil Record

• Occur fairly early in vascular plant history b/c
  the oldest ovules/seeds are from Late Devorian
  365 mil. yrs. ago.

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The Fossil Record

• Example Elkisia polymorpha
• Cupules dichotomously branched, sterile
  structures that surround the ovules.

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The Fossil Record

• Example Archaeosperma arnoldii
• Apical portion of the integument divided into
  lobes form the rudimentary micropyle.
• The integument of the ovules evolved thru gradual
  fusion of the lobes until micropyle is only
  opening left

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A seed consists of an embryo

• When ready for fertilization, the nucellus
  contains a megagametophyte composed of nutritive
  tissue and archegonia.
• After fertilization, the integuments develop into
  the seed coat and seeds form.
• Embryo forms w/in seed, along with food.

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There are five Phyla

• All seed plants possess megaphylls large leaves
  w/ several veins, modified to needles and scales.
• Five Phyla of seed plants
• Cyacadophyta, Ginkgophyta, Coniferophyta, and
  Gnetophyta gymnosperms
• Anthophyta angiosperms

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There are Five Phyla

• 720 species of living gymnosperms, w/ individual
  gymnosperm species dominant over wide areas.

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Progymnosperms

• Existed in the Late Paleozic Era
• Characteristics intermediate btw seedless
  vascular trimerophytes and seed plants
• Reproduced by means freely dispersed spores
• Produced 2nd xylem similar to living conifers

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Progymnosperms

• The most impt. evolutionary advance is bifacial
  vascular cambium one that produces 2nd xylem and
  2nd phloem.
• Example Aneurophyton-type

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Progymnosperms

• Example Archaropteris
• Eustele arrangement of vasclar tissue in
  discrete strands around pith

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Progymnosperms

• Evidence from recent decades indicates that seed
  plants evolved from progymnosperms
• Common ancestor for all seed plants

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Extinct Gymnosperms

• Two groups of extinct gymnosperms
• Seed ferns and Cordaitales (coniferlike-plants).
• Cycadeoids or Bennettitales

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Extinct Gymnosperms

• Seed ferns very diverse, highly unnatural, range
  from slender branch, ovule plants to
  Carboniferous plants that look like tree ferns.
• Bennettitales- palmlike leaves

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Living Gymnosperms

• Gymnosperms- means naked seed
• Principal characteristic- ovule and seeds exposed
  on the surface of sporophylls and analogous
  structures.

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Living Gymnosperms

• Female gametophyte produce several archegonia.
• As a result of above, more than 1 egg fertilized,
  several embryos developed in single ovule-
  polyembryony
• H2O is not required as a transport medium for the
  sperm to reach the eggs.

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Living Gymnopserms

• Partly developed male gametophyte (pollen grain)
  is transported bodily to the vicinity of the
  female gametophyte in the ovule
• After this process (pollination), the male game.
  Produces a tubular growth known as pollen tube.
  They dont form antheridia.

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Living Gymnosperms

• Conifers gnetophytes- non-motile sperm and
  pollen tubes convey the sperm directly to the
  archegonia
• Cycads Ginkgo fertilization transitional btw
  conditions in ferns and seed plants.

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Living Gymnosperms

• The pollen tube can be seen as a later
  evolutionary modification.

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The End