Ch24 Reproduction of Seed Plants

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Ch-24 Reproduction of Seed Plants
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24-1 Reproduction with Cones and Flowers

• Objectives
• Know the reproductive structures of angiosperms
  and gymnosperms.
• Know the differences between fertilization in
  gymnosperms and angiosperms.
• Vocabulary
• Pollen cone pollen grain
• Seed cone ovule

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24-1 Reproduction with Cones and Flowers

• Pollen tube sepal petal
• Stamen filament anther
• Carpel ovary style
• Stigma endosperm
• Double fertilization

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241 Reproduction With Cones and Flowers
Section 24-1

• 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-sperm and eggs,
  respectively.

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241 Reproduction With Cones and Flowers

• When the gametes join, they form a zygote that
  begins the next sporophyte generation. In some
  plants, the two phases of the life cycle are
  distinct, independent plants.

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Figure 241 Evolution of the Gametophyte and the
Sporophyte
Section 24-1
Gametophyte (N) Sporophyte (2N)
Bryophytes
Ferns
Seed plants
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241 Reproduction With Cones and Flowers

• B. Life Cycle of Gymnosperms
• Pine trees and other gymnosperms are diploid
  sporophytes. As you will see, this sporophyte
  develops from a zygote that is contained within a
  seed.

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241 Reproduction With Cones and Flowers

• Reproduction in gymnosperms takes place in cones,
  which are produced by a mature sporophyte plant.
• Gymnosperms produce two types of cones pollen
  cones and seed cones.

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241 Reproduction With Cones and Flowers

• 1. Pollen Cones and Seed Cones
• are also called male cones because they produce
  male gametophytes called pollen grains.
• A pollen grain is a four-celled organism that
  contains haploid cells. One of the cells of the
  gametophyte divides later to produce sperm cells.

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241 Reproduction With Cones and Flowers

• The more familiar seed cones, which produce
  female gametophytes, are generally much larger
  than pollen cones. Near the base of each scale
  are two ovules in which the female gametophytes
  develop. Within the ovules, meiosis produces
  haploid cells that grow and divide to produce
  female gametophytes.

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241 Reproduction With Cones and Flowers

• These gametophytes may contain hundreds or
  thousands of cells. When mature, each gametophyte
  contains a few large egg cells, each ready for
  fertilization by sperm nuclei.

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241 Reproduction With Cones and Flowers

• 2. Pollination The gymnosperm life cycle
  typically takes two years to complete. Long
  before the female gametophyte has developed in
  the seed cone, pollen is released from male cones
  during the spring. Many pollen grains are
  released and carried by the wind. Some of these
  pollen grains reach female cones.

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241 Reproduction With Cones and Flowers

• There, some pollen grains are caught in a sticky
  secretion on one of the scales of the female
  cone. This sticky material, known as a
  pollination drop, ensures that pollen grains stay
  on the female cone.

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Figure 244 The Life Cycle of a Gymnosperm
Section 24-1
Haploid (N) Diploid (2N)
MEIOSIS
FERTILIZATION
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241 Reproduction With Cones and Flowers

• 3. Fertilization and development
• If a pollen grain lands near an ovule, the grain
  splits open and begins to grow a structure called
  a pollen tube, which contains two haploid sperm
  cells. Once the pollen tube reaches the female
  gametophyte, one sperm cell disintegrates, and
  the other fertilizes the egg contained within the
  female gametophyte.

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241 Reproduction With Cones and Flowers

• . Fertilization produces a diploid zygote the new
  sporophyte plant. This zygote grows into a small
  embryo. During this time, it is encased within
  what will soon develop into a seed.

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241 Reproduction With Cones and Flowers

• The seed consists of three generations of the
  life cycle. The outer seed coat is part of the
  old sporophyte generation, the haploid cells
  surrounding the embryo are part of the female
  gametophyte, and the embryo is the new sporophyte
  plant.

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241 Reproduction With Cones and Flowers

• Structure of Flowers
• Flowers are reproductive organs that are com-
  posed of four kinds of specialized leaves
  sepals, petals, stamens, and carpels.

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241 Reproduction With Cones and Flowers

• Sepals and Petals
• The outermost circle of floral parts contains the
  sepals, which in many plants are green and
  closely resemble ordinary leaves. Sepals enclose
  the bud before it opens, and they protect the
  flower while it is developing.

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241 Reproduction With Cones and Flowers

• Petals, which are often brightly colored, are
  found just inside the sepals.
• The petals attract insects and other pollinators
  to the flower.
• Because they do not produce reproductive cells
  themselves, the sepals and petals of a flower are
  sometimes called sterile leaves.

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241 Reproduction With Cones and Flowers

• Stamens and Carpels
• Inside the petals are two layers of fertile
  leaves.
• These leaves bear the structures that produce
  male and female gametophytes.

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241 Reproduction With Cones and Flowers

• The male structures consist of an anther and a
  filament, which together make up the stamen. The
  filament is a long, thin structure that supports
  the anthers.

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241 Reproduction With Cones and Flowers

• Anthers are the structures where meiosis takes
  place, producing haploid male gametophytes-pollen
  grains. In most angiosperms, each flower has
  several stamens. If you rub your hand on the
  anthers of a flower, a yellow-orange substance
  may stick to your skin. This subtance is pollen,
  which consists of thousands of male gametophytes.
  

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241 Reproduction With Cones and Flowers

• The innermost floral parts are carpels, which
  produce the female gametophytes. Each carpel has
  a broad base containing an ovary, which contains
  one or more ovules where female gametophytes are
  produced. The diameter of the carpel narrows into
  a stalk called the style. At the top of the style
  is a sticky portion known as the stigma, where
  pollen grains frequently land.

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241 Reproduction With Cones and Flowers

• Some flowers have several carpels fused together
  to form single reproductive structure called a
  pistil.

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241 Reproduction With Cones and Flowers

• Life Cycle of Angiosperms
• Reproduction in angiosperms takes place within
  flower. Following pollination and fertilization,
  the seeds develop inside protective structures.
  The life cycle of angiosperms is shown in Figure
  24-7. Inside the anthers-the male part of the
  flower-each cell undergoes meiosis and produces
  four haploid spore cells.

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241 Reproduction With Cones and Flowers

• Each of these cells becomes a single pollen
  grain. The wall of each pollen grain thickens,
  protecting the contents of the pollen grain from
  dryness and physical damage when it is released
  from the anther.

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241 Reproduction With Cones and Flowers

• The nucleus of each pollen grain undergoes one
  mitotic division to produce two haploid nuclei.
  The pollen grain, is the entire male gametophyte,
  usually stops growing until it is released from
  the anther and deposited on a stigma.

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241 Reproduction With Cones and Flowers
E. Pollination

• Once the gametophytes have developed inside the
  flower, pollination takes place.
• Most gymnosperms and some angiosperms are wind
  pollinated, whereas most angiosperms are
  pollinated by animals. These animals, which
  include insects, birds, and mammals, carry pollen
  from one flower to another.

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241 Reproduction With Cones and Flowers

• . Because wind pollination is less efficient than
  animal pollination, wind-pollinated plants, rely
  on favorable weather and sheer numbers to get
  pollen from one plant to another.

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241 Reproduction With Cones and Flowers

• Animal-pollinated plants have a variety of
  adaptations, such as bright colors and sweet
  nectar, that draw animals to them.
• Animals, have developed patterns of behavior to
  help them find flowers. They have also evolved
  body shapes that enable them to reach nectar deep
  within certain flowers. ,

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241 Reproduction With Cones and Flowers

• Insect pollination has become common because it
  increases, the fitness of both organisms. It is
  beneficial to insects and other animals because
  it provides a dependable source of food.
• The food may take the form of pollen itself or a
  sugar-rich liquid called nectar.

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241 Reproduction With Cones and Flowers

• Plants also benefit because the insects take
  their pollen directly from flower to flower in a
  process known as vector pollination. Vector
  pollination is much more efficient than wind
  pollination, giving insect-pollinated plants a
  higher probability of reproductive success.

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241 Reproduction With Cones and Flowers

• In fact, many plant biologists suggest that the
  angiosperms displaced the gymnosperms thoroughly
  during the last 100 million years in part because
  of pollination by insects.

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Compare/Contrast Table
Section 24-1
Comparing Wind-pollinated and Animal-pollinated
Plants
Characteristics Pollination method Relative
efficiency of pollination method Plant
types Reproductive organs Adaptations that
promote pollination
Wind-pollinatedPlants Wind pollination Less
efficient Mostly gymnosperms and some
angiosperms Cones Pollination drop
Animal-pollinated Plants Vector pollination More
efficient Angiosperms Flowers Bright colors,
sweet nectar
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241 Reproduction With Cones and Flowers

• F. Fertilization in Angiosperms
• If a pollen grain lands on the stigma of an
  appropriate flower, it begins to grow a pollen
  tube. The generative nucleus within the pollen
  grain divides and forms two sperm nuclei.

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241 Reproduction With Cones and Flowers

• The pollen tube now contains a tube nucleus and
  two sperm nuclei. The pollen tube grows into the
  style. There, it eventually reaches the ovary and
  enters the ovule.
• Inside the embryo sac, two distinct
  fertilizations take place. First, one of the
  sperm nuclei fuses with the egg nucleus to
  produce a diploid zygote cell. This cell will
  grow into the new plant embryo.

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241 Reproduction With Cones and Flowers

• Second, the other sperm nucleus does something
  truly remarkable-it fuses with two other 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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241 Reproduction With Cones and Flowers

• Because two fertilization events take place
  between the male and female gametophytes, this
  process is known as double fertilization. Double
  fertilization may be one of the reasons why the
  angiosperms have been so successful. Recall that
  in gymnosperms, the food reserve built up in
  seeds is produced before fertilization takes
  place.

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241 Reproduction With Cones and Flowers

• As a result, if an ovule is not fertilized, those
  resources are wasted. In angiosperms, if an ovule
  is not fertilized, the endosperm does not form,
  and food is not wasted by preparing for a
  nonexistent zygote.

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Figure 245 The Structure of a Flower
Section 24-1
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Figure 247 The Life Cycle of an Angiosperm
Section 24-1
Haploid (N) Diploid (2N)
MEIOSIS
Ovule
FERTILIZATION
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242 Seed Development and Germination

• Objectives
• Know how seeds are dispersed.
• Know what factors influence the dormancy and
  germination of seeds.
• Vocabulary
• Seed coat fruit
• Dormancy germination

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242 Seed Development and Germination
Section 24-2

• Seed and Fruit Development
• Seed coat- is a toughened outer covering that
  surrounds and protects a seed.
• Fruit-any seed that is enclosed within an embryo
  wall.

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242 Seed Development and Germination

• B. Seed Dispersal
• Dispersal by Animals
• Seeds dispersed by animals are typically
  contained in fleshy, nutritious fruits.
• Dispersal by Wind and Water
• Seeds dispersed by wind and water are typically
  light weight, allowing them to be carried in the
  air or float on the surface of the water.

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242 Seed Development and Germination

• Seed Dormancy
• Environmental factors such as temperature and
  moisture can cause a seed to end dormancy and
  germinate.

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242 Seed Development and Germination

• Seed Germination
• Seed germination is the early growth stage of the
  plant embryo. When seeds germinate, they absorb
  water.

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242 Seed Development and Germination

• The absorbed water causes the endosperm and
  cotyledons to swell, cracking open the seed coat.
  Through the cracked seed coat, the radicle
  emerges and grows into the primary root.

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242 Seed Development and Germination

• Recall that monocots have a single cotyledon, or
  seed leaf. In most monocots, the single cotyledon
  remains within the seed. The growing shoot
  emerges while protected by a sheath.

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242 Seed Development and Germination

• In dicots, which have two cotyledons, germination
  takes place in one of two ways. In some species,
  the cotyledons emerge above ground and protect
  the first foliage leaves. They then wither and
  drop off the plant.

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242 Seed Development and Germination

• In other species, the cotyledons remain below the
  soil and provide a food source for the growing
  seedling. The epicotyl grows in an arch that
  protects the delicate shoot tip until it breaks
  through the surface of the soil.

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Section 24-2
SeedGermination
in
have
have
that
that
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24-3 Plant Propagation and Agriculture

• Objectives
• Know what forms of vegetative reproduction occur
  in plants.
• Know what plant propagation is.
• Know which crops are a major food source for
  humans.

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24-3 Plant Propagation and Agriculture

• Vocabulary
• Vegetative reproduction
• Stolon
• Grafting
• budding

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24-3 Plant Propagation and Agriculture

• A. Vegetative Reproduction
• Vegetative reproduction includes the production
  of new plants from horizontal stems, from
  plantlets, and from underground roots.

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24-3 Plant Propagation and Agriculture

• Plant Propagation
• Horticulturists use plant propagation to make
  many identical copies of a plant or to produce
  offspring from seedless plants.

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24-3 Plant Propagation and Agriculture

• 1. Cuttings
• This is the simplest way to reproduce/clone
  plants. A section of plant is cut and then
  rooted.

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24-3 Plant Propagation and Agriculture

• 2. Grafting and budding are used to reproduce
  seedless plants and varieties of woody plants
  that do not produce strong root systems. In both
  of these techniques, new plants are grown on
  plants that have strong root systems.

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24-3 Plant Propagation and Agriculture

• To do this, a piece of stem or a lateral bud is
  cut from the parent plant and attached to another
  plant. The cut piece is called the scion, and the
  plant to which it is attached is called the
  stock.

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24-3 Plant Propagation and Agriculture

• When stems are used as scions, the process is
  called grafting. When buds are used as scions,
  the process is called budding.
• Grafting usually works best when plants are
  dormant because the wounds created can heal
  before new growth starts. In all cases, grafts
  are successful only if the vascular cambiums of
  scion and stock are firmly connected to each
  other.

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24-3 Plant Propagation and Agriculture

• C. Agriculture
• 1. Worldwide Patterns of Agriculture
• Most of the people of the world depend on a few
  crop plants, such as wheat, rice, and corn, for
  the bulk of their food supply.

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24-3 Plant Propagation and Agriculture

• 2. Changes in Agriculture
• The efficiency of agriculture has been improved
  through the selective breeding of crop plants and
  improvements in farming techniques.