Chapter 24 Reproduction of Seed Plants

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Chapter 24Reproduction of Seed Plants

• 24.1
• Reproduction with Cones and Flowers

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Essential Questions

• What are the reproductive structures of
  gymnosperms and angiosperms?
• How does pollination differ between angiosperms
  and gymnosperms?

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Alternation of Generations

• All plants have a life cycle in which a diploid
  sporophyte generation alternates with a haploid
  gametophyte generation
• Gametophyte plants produce male and female
  gametes
• When the gametes join, they form a zygote that
  begins the next sporophyte generation
• In seed plants the diploid sporophyte is the
  recognizable plant.
• The gametophyte is hidden inside of the cones
  (gymnosperms) and flowers (angiosperms).

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Figure 24-4
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Figure 24-1
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Life Cycle of Gymnosperms

• Takes place in cones, which are produced by a
  mature sporophyte plant.
• Pollen cones are male cones and produce male
  gametophytes, which are called pollen grains.
• One pollen grain will divide to produce two sperm
  nuclei.
• Seed cones are female and produce female
  gametophytes.
• Ovules in which the female gametophyte develops
  are found near the base of each scale.
• When mature, the ovules contain a few large egg
  cells, each ready for fertilization by the sperm
  nuclei.

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Male and Female Pine Cones
Male Cone
Female Cone
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Pollination

• The gymnosperm life cycle takes two years to
  complete.
• The cycle begins in the spring as the male cones
  release enormous numbers of pollen grains.
• The wind carries the pollen to the female cones
  and are caught in a sticky secretion called the
  pollination drop.

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Fertilization and Development

• If a pollen grain lands near an ovule, the grain
  splits open and begins to grow a pollen tube,
  which contains two haploid sperm.
• Once the pollen tube reaches the female
  gametophyte, one sperm disintegrates and the
  other fertilizes the egg contained within the
  female gametophyte.
• More than one egg cell can be fertilized but only
  one embryo will develop.
• Fertilization produces a diploid zygote the new
  sporophyte plant.

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Structure of Flowers

• Flowers are reproductive organs that are composed
  of four kinds of specialized leaves sepals,
  petals, stamens, and carpels.

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Sepals and Petals

• Outermost circle of floral parts contains the
  sepals, which are green and resemble leaves.
• Sepals enclose the bud before it opens and
  protects the flower while it is developing.
• Petals are found just inside the sepals and are
  often brightly colored to attract insects and
  other pollinators to the flower.
• Sepals and petals are sometimes called sterile
  leaves because they do not produce reproductive
  cells.

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Stamens

• Stamen is the male part and produces the haploid
  male gametophyte
• consists of an anther and a filament
• filament is a long, thin stalk that supports the
  anther
• anther is an oval sac found at tip of filament
  and is where meiosis takes place, producing make
  gametophytes i.e. pollen grains

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Carpels

• Carpel is the female part and produces the
  haploid female gametophyte
• innermost flower parts, also called pistils
• Each carpel has a base that forms an ovary
  containing ovules that produce female
  gametophytes
• A narrow stalk called the style attaches the
  ovary to the stigma
• Stigma is a sticky portion on the end of the
  style where pollen grains land

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Life Cycle of Angiosperms

• Begins when mature sporophyte produces flowers
• Each flower contains anthers and an ovary
• Inside anthers, each cell undergoes meiosis and
  produces four haploid spore cells
• Each of these cells becomes a pollen grain with a
  thickened cell wall to protect it when it is
  released from the anther
• Nucleus undergoes one mitotic division to produce
  two haploid nuclei

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Contd

• Ovary contain the ovules in which female
  gametophyte develops
• A single diploid cell goes through meiosis to
  produce four haploid cells, three of which
  disintegrate
• Remaining cell undergoes mitosis to produce eight
  nuclei
• Eight nuclei and surrounding membrane are called
  embryo sac

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Contd

• Embryo sac is contained within ovule and is the
  female gametophyte
• The egg nucleus, near the base of the
  gametophyte, is the gamete and will become the
  zygote if fertilization takes place
• The zygote grows into a new sporophyte plant also
  called a seedling.

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Figure 24-7
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Pollination

• Once gametophytes have developed inside the
  flower, pollination takes place.
• Most gymnosperms and some angiosperms are wind
  pollinated
• Most angiosperms are pollinated by animals
• Animals are mainly insects, birds, and bats that
  carry pollen from one flower to another
• Insect pollination is more efficient than wind
  pollination