Gymnosperms

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The Seed Plants

• Gymnosperms

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Seeds are reproductively superior to spores

• Seeds contain a multicellular embryonic
  sporophyte plant
• Seeds contain a food supply to nourish the embryo
  through growth
• Seeds are protected by a tough seed coat

• A spore is a single cell
• There are few, if any food reserves available to
  a plant developing from a spore
• Spores do not contain a seed coat (although they
  are adapted to survive through harsh conditions)

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Seeds have long been a source of food for
humans. They are easy to store and will remain
viable for extended periods of time (as long as
they are kept dry) A seed develops from an ovule,
which is a megasporangium and its enclosed
structures, following the fertilization of the
egg with a sperm The two groups of seed plants
are the gymnosperms and and the angiosperms The
gymnosperms bear naked seeds, often in
cones The angiosperms are flowering plants that
produce their seeds within a fruit Both
gymnosperms and angiosperms contain vascular
tissue, a life cycle with an alternation of
generations, and are heterosporus
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The Gymnosperms

• Division Coniferophyta

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• General characteristics of the conifers. . .
• This division includes the pines, spruces, firs
  and hemlocks
• They are woody trees or shrubs which are
  evergreen
• The leaves are modified into needles
• Most conifers are monoecious producing male
  and female cones
• Conifers are found worldwide and are an important
  part of forest ecosystems
• Humans use conifers for their wood, turpentine
  and resins. Christmas trees are usually
  conifers

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• A Pine life cycle is typical of conifers. . .
• The tree is the sporophyte generation and so
  forms spores
• Microspores and megaspores are produced in
  different cones

Male cones are smaller than female cones and are
usually produced on lower branches in the
Spring Each male cone is composed of sporophylls
- leaflike structures that bear sporangia At the
base of each sporophyll are two microsporangia,
which contain many microspore mother cells.
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• The microspore mother cells undergo meiosis to
  produce haploid microspores
• The microspores then develop into extremely small
  male gametophytes (pollen grains)
• Pollen grains are shed from the male cone in
  great numbers and some are carried by the wind to
  the immature female cones

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• The female cones are usually found on the upper
  branches of the tree
• The female cones are larger and woody
• The woody scales of the female cone have
  megasporangia at their bases

• Within each megasporangium, meiosis of a
  megaspore mother cell occurs - producing 4
  haploid megaspores
• Three of the megaspores degenerate and one
  develops into the female gametophyte
• The scales of the developing female cone open at
  the time pollen is released

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• Pollen grains, carried to the female cones by air
  currents, drift between the scales and adhere to
  a drop of sticky fluid
• As the fluid evaporates and the scales of the
  cone close, the pollen grain is drawn to the
  megasporangium
• Each pollen grain grows a pollen tube that
  digests its way to the egg through the female
  gametophyte tissue
• A cell within the pollen grain divides to form
  two nonflagellated sperm which travel through the
  pollen tube
• One sperm fuses with the egg to form a zygote
  this will develop into the embryo sporophyte in
  the seed
• The other sperm degenerates
• The female gametophyte tissue becomes nutritive
  tissue to support the embryo
• The seed will also contain a papery wing to aid
  dispersal

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archegonium
Female cone long section
2nd year immature female pine cones
1st year female cone
2nd year mature cones
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• There is a significant time lapse between the
  appearance of cones on the tree and the
  maturation of the seeds
• Pollination - the transfer of pollen grains to
  the female cone occurs in the Spring at this
  time the megaspore mother cells have not yet gone
  through meiosis (there are no eggs)
• Through the Summer months, the female tissue
  matures, and eggs are formed within the
  archegonia during this time the pollen grain is
  growing a pollen tube
• Fertilization - the union of the egg and sperm
  occurs during the Spring of the year following
  pollination
• Seed maturation takes several more months,
  although some seeds remain in the female cones
  years before being shed

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• In the pine life cycle, the sporophyte is
  dominant
• The gametophyte generation consists of
  microscopic structures in the male and female
  cones
• The female gametophyte does produce an archegonia
  where the egg is produced
• The male gametophyte is so reduced that it does
  not produce an antheridia
• A major adaptation in the gymnosperm life cycle
  is the elimination of the need for water to
  transport the sperm (this is done with the pollen
  grain)
• Gymnosperms were the first plants to have a mode
  of reproduction totally adapted to life on land

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Division Cycadophyta

• Cycads are gymnosperms with compound leaves and
  simple seed cones

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• General information about the Cycads. . .
• The Cycads were extremely important during the
  Triassic period
• Most species are now extinct (only about 100
  species remain)
• They are slow growing evergreens
• Considered to be very primitive
• They grow in the tropics and subtropics
• They have compound leaves that give them a
  fernlike or palmlike appearance
• Cycad reproduction is similar to ferns, except
  cycads are dioecious
• They have several primitive features including
  motile sperm the flagella are not necessary for
  fertilization, however. Pollen grains are
  airborne to the female plants. The flagellated
  sperm are considered to be a vestige

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Phylum Ginkgophyta

• Ginkgo is the only living species in its phylum

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• General information about Ginkgo. . .
• There is only one species in the Division
  Ginkophyta The maidenhair tree - Ginko biloba
• It is native to southeastern China, where it is
  cultivated
• 200 million year old fossils of ginkos are nearly
  identical to the ginko found today
• It has flagellated sperm a vestige
• It is dioecious commonly male trees are planted
  as ornamentals because the female bears seeds
  that smell like rancid butter
• It has become popular as a nutritional
  supplement, because it is supposed to enhance
  memory

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Division Gnetophyta

• The gnetophytes have three genera that are
  remarkably diverse

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• General information about the gnetophytes. . .
• The gnetophytes are probably the most unusual
  gymnosperms
• They have some angiosperm-like features including
  very efficient water conduction cells in their
  xylem that resemble vessel elements
• The cone clusters produced in some gnetophytes
  resemble flower clusters

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The genus Gnetum contains tropical vines and
trees with leaves similar to angiosperms The
genus Ephedra includes shrubs found in deserts
and other dry regions. They resemble horsetails
in appearance and are commonly called joint
firs The genus Welwitschia contains a single
species found in the deserts of southwestern
Africa the majority of its body grows
underground. A short wide stem produces two
ribbon-like leaves which grow throughout the
plants life