Lesson Overview

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Lesson Overview

• 24.1 Reproduction in Flowering Plants

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The Structure of Flowers

• Flowers are reproductive organs that are
  composed of four different kinds of specialized
  leaves sepals, petals, stamens, and carpels.
• This diagram shows the parts of a typical
  angiosperm flower.

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Sepals and Petals

• The outermost circle of floral parts contains
  the sepals.
• Sepals enclose the bud before it opens, and they
  protect the flower while it is developing.

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Sepals and Petals

• Petals, which are often brightly colored, are
  found just inside the sepals.
• The colors, number, and shapes of such petals
  attract insects and other pollinators to the
  flower.

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Stamens

• The stamens are the male parts of the
  flowereach stamen consists of a stalk called a
  filament with an anther at its tip.
• Anthers are the structures in which pollen
  grainsthe male gametophytesare produced.

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Carpels

• The innermost floral parts are the carpels,
  which produce and shelter the female gametophytes
  and, later, seeds.
• Each carpel has a broad base forming an ovary,
  which contains one or more ovules where female
  gametophytes are produced.

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Carpels

• The diameter of the carpel narrows into a stalk
  called the style. At the top of the style is a
  sticky or feathery portion known as the stigma,
  which is specialized to capture pollen.

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Carpels

• Botanists sometimes call a single carpel or
  several fused carpels a pistil.

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The Angiosperm Life Cycle

• Angiosperms have a life cycle that shows an
  alternation of generations between a diploid
  sporophyte phase and a haploid gametophyte stage.
  
• Male and female gametophytes live within the
  tissues of the sporophyte.

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Development of Male Gametophytes

• The male gametophytesthe pollen grainsdevelop
  inside anthers.
• First, meiosis produces four haploid spore cells.

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Development of Male Gametophytes

• Each spore undergoes one mitotic division to
  produce the two haploid nuclei of a single pollen
  grain.
• The two nuclei are surrounded by a thick wall
  that protects the male gametophyte.

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Development of Female Gametophytes

• Female gametophytes develop inside each carpel
  of a flower.
• The ovulesthe future seedsare enveloped in a
  protective ovarythe future fruit.

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Development of Female Gametophytes

• A single diploid cell goes through meiosis to
  produce four haploid cells, three of which
  disintegrate.

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Development of Female Gametophytes

• The remaining cell undergoes mitosis, producing
  eight nuclei. These eight nuclei and the
  surrounding membrane are called the embryo sac.
• The embryo sac, contained within the ovule,
  makes up the female gametophyte of a flowering
  plant.

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Development of Female Gametophytes

• Cell walls form around six of the eight nuclei.
• One of the eight nuclei, near the base of the
  gametophyte, is the nucleus of the eggthe female
  gamete.
• If fertilization takes place, this egg cell will
  fuse with the male gamete to become the zygote
  that grows into a new sporophyte plant.

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Pollination

• Pollination is the transfer of pollen to the
  female portions of the flower.
• Some angiosperms are wind pollinated, but most
  are pollinated by animals.
• Because wind pollination is less efficient than
  animal pollination, wind-pollinated plants, such
  as oak trees, rely on favorable weather and sheer
  numbers of pollen grains to get pollen from one
  plant to another.

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Pollination

• Animal-pollinated plants have a variety of
  adaptations, such as bright colors and sweet
  nectar, to attract and reward animals.
• Animals have evolved body shapes that enable
  them to reach nectar deep within certain flowers.

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Pollination

• Insect pollination is beneficial to insects and
  other animals because it provides a dependable
  source of foodpollen and nectar.
• Plants benefit because the insects take the
  pollen directly from flower to flower.
• Insect pollination is more efficient than wind
  pollination, giving insect-pollinated plants a
  greater chance of reproductive success.

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Fertilization

• If a pollen grain lands on the stigma of a
  flower of the same species, it begins to grow a
  pollen tube.
• Of the pollen grains two cells, one cellthe
  generative celldivides and forms two sperm
  cells. The other cell becomes the pollen tube.

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Fertilization

• The pollen tube contains a tube nucleus and the
  two sperm cells.
• The pollen tube grows into the style, where it
  eventually reaches the ovary and enters an ovule.

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Fertilization

• Inside the embryo sac, two distinct
  fertilizations take placea process called double
  fertilization.
• First, one of the sperm nuclei fuses with the
  egg nucleus to produce a diploid zygote, which
  will grow into the new plant embryo.

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Fertilization

• Second, the other sperm nucleus fuses with two
  polar nuclei in the embryo sac to form a triploid
  (3N) cell.
• This cell will grow into a food-rich tissue
  known as endosperm, which nourishes the seedling
  as it grows.

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Fertilization

• By using endosperm to store food, the flowering
  plant spends very little in the way of food
  resources on producing seeds from ovules until
  double fertilization has actually taken place.
• The resources saved can be used to make many
  more seeds.

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Vegetative Reproduction

• Many flowering plants can reproduce asexually.
  This process, known as vegetative reproduction,
  enables a single plant to produce offspring
  genetically identical to itself by mitosis. It
  does not require gametes, flowers, or
  fertilization.
• This process takes place naturally in many
  plants.

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Types of Vegetative Reproduction

• New plants may grow from roots, leaves, stems,
  or plantlets.
• A potato is an underground stem that can grow
  whole new plants from buds, called eyes.
• Because vegetative reproduction does not involve
  pollination or seed formation, a single plant can
  reproduce quickly.

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Types of Vegetative Reproduction

• Asexual reproduction allows a single plant to
  produce genetically identical offspring, enabling
  well-adapted individuals to rapidly fill a
  favorable environment.
• One drawback of asexual reproduction is that it
  does not produce new combinations of genetic
  traits, which may be valuable if conditions in
  the physical environment change.

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Plant Propagation

• To propagate plants with desirable
  characteristics, horticulturists use cuttings or
  grafting (shown) to make many identical copies of
  a plant or to produce offspring from seedless
  plants.

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Plant Propagation

• One of the simplest ways to reproduce plants
  vegetatively is by cuttings.
• A grower cuts from the plant a length of stem
  that includes a number of buds containing
  meristem tissue.
• That stem is then partially buried in soil or in
  a special mixture of nutrients that encourages
  root formation.

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Plant Propagation

• Grafting is a method of propagation used to
  reproduce seedless plants and varieties of woody
  plants that cannot be propagated from cuttings.
• To graft, a piece of stem or a lateral bud is
  cut from the parent plant and attached to another
  plant, as shown.

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Plant Propagation

• Grafting works only when the two plants are
  closely related, such as when a bud from a lemon
  tree is grafted onto an orange tree.
• Grafting usually works best when plants are
  dormant, which allows the wounds created by the
  cut to heal before new growth starts.