Chapter 25 Reproduction in Seed Plants

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Chapter 25Reproduction in Seed Plants

• Charles Page High School
• Stephen L. Cotton

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Section 25-1Cones and Flowers

• OBJECTIVES
• Explain reproduction in gymnosperms.

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Section 25-1Cones and Flowers

• OBJECTIVES
• Describe the structure of a flower.

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Section 25-1Cones and Flowers

• OBJECTIVES
• Discuss reproduction in flowering plants.

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Section 25-1Cones and Flowers

• The cones of gymnosperms and the flowers of
  angiosperms are plant structures specialized for
  the purpose of sexual reproduction
• these are vital in the process that ensures that
  continuation of the species takes place

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Section 25-1Cones and Flowers

• Thus, cones and flowers are as important to
  survival of plant species as roots, stems, and
  leaves are to the individual plant
• Plants alternate between the diploid sporophyte
  generation and the haploid gametophyte generation

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Section 25-1Cones and Flowers

• Gametophyte plants produce male and female
  gametes (from the process of mitosis)
• when male and female gametes join, they form a
  zygote (fertilized egg)
• the zygote develops into the next sporophyte
  generation

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Section 25-1Cones and Flowers

• In seed plants, the sporophyte generation is
  large and obvious
• the gametophyte generation is small and often
  hidden within the cones or flowers

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Section 25-1Cones and Flowers

• Remember the development of cones and flowers,
  and the production of seeds, have enabled seed
  plants to reproduce without being dependent upon
  standing water
• helps them survive the dry conditions of life on
  land

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Section 25-1Cones and Flowers

• Life Cycle of Gymnosperms
• Familiar gymnosperms such as pine trees are
  diploid sporophytes, each of which has grown from
  a zygote contained within a seed
• there are male and female cones

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Section 25-1Cones and Flowers

• Life Cycle of Gymnosperms
• male cones carry structures called
  microsporangia- these produce male gametophytes
  called pollen grains

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Section 25-1Cones and Flowers

• Life Cycle of Gymnosperms
• female cones carry megasporangia- these produce
  the female gametophytes
• the female gametophytes in turn produce ovules-
  the structures in which egg cells form

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Section 25-1Cones and Flowers

• Life Cycle of Gymnosperms (9 steps)
• 1. pollen released from the male cones is carried
  by the wind to the female cones
• 2. a sticky substance is present to catch the
  pollen

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Section 25-1Cones and Flowers

• Life Cycle of Gymnosperms
• 3. the pollen splits, and grow a pollen tube
• 4. two haploid sperm are located within the
  pollen tube
• 5. the tube grows into the ovule, and the two
  sperm break out

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Section 25-1Cones and Flowers

• Life Cycle of Gymnosperms
• 6. One sperm fertilizes the egg
• 7. other sperm disintegrates
• 8. The zygote that is formed grows into an embryo
  encased within what later is the seed

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Section 25-1Cones and Flowers

• Life Cycle of Gymnosperms
• 9. The seed is a neat package, it contains the
  embryo plant as well as a supply of food for the
  embryo when it begins to grow
• Figure 25-2, page 534

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Section 25-1Cones and Flowers

• Life Cycle of Angiosperms
• Angiosperms (flowering plants) are the dominant
  form of plant life on Earth today
• have evolved a life cycle that liberates the
  reproduction from standing water- thus they are
  widespread over the Earth

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Section 25-1Cones and Flowers

• Life Cycle of Angiosperms
• flowers are more than just decorative items
• each flower represents proof of a plants
  survival and offers assurance that a plant
  species will produce more of its own kind

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Section 25-1Cones and Flowers

• A typical flower (Fig. 25-4, page 535) produces
  both male and female gametes
• others may have them produced in different
  flowers on the same plant (such as corn)
• or in totally separate plants (such as willow)

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Section 25-1Cones and Flowers

• Corn- the male gametophytes in flowers at the top
  of the plant, and the female gametophytes in
  flowers located along the stem below the male
  flowers

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Section 25-1Cones and Flowers

• Flowers are actually miniature stems that produce
  4 kinds of specialized leaves
• 1. Sepals
• 2. Petals
• 3. Stamens
• 4. Carpels

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Section 25-1Cones and Flowers

• These are arranged in circles, and have been
  modified to serve different purposes related to
  reproduction
• Sepals- outermost circle may be green and
  resemble leaves enclose the flower bud before
  opening protect the flower

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Section 25-1Cones and Flowers

• All the sepals form the calyx
• Petals- second circle of flower parts often
  brightly colored just inside the sepals all of
  the petals form the corolla
• act as a flower advertisement, attracting
  pollinators to the flower

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Section 25-1Cones and Flowers

• Sepals and Petals are called sterile leaves,
  because they do not produce any gametophytes
• Fertile leaves are located inside the petals
• the fertile leaves contain structures that
  produce male and female gametophytes

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Section 25-1Cones and Flowers

• Stamens- the first circle of fertile leaves
• each stamen has a long filament that supports the
  anther- inside the anther are microsporangia in
  which the male gametophytes (the microspores) are
  formed

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Section 25-1Cones and Flowers

• Carpels- the centermost circle of flower parts
  produced from fertile leaves that have rolled up
• the rolling produces megasporangia (structures in
  which female gametophytes are produced) inside
  the female leaves (rather than on a cone as in
  gymnosperms)

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Section 25-1Cones and Flowers

• One or more carpels forms a pistil
• consists of a base (ovary), a stalk (style), and
  the top of the style (stigma)
• some plants have a short style others like corn
  are quite long, called corn silk

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Section 25-1Cones and Flowers

• The stigma is the surface upon which the pollen
  is deposited by wind or animal pollinators
• In many plants, the stigma is sticky to help
  catch pollen

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Typical Flower(Fig. 25-4, page 535)
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Section 25-1Cones and Flowers

• Female Gametophyte
• located inside each ovary is one or more
  megasporangia, called ovules
• A single diploid cell called the mother cell
  produces a female gametophyte in a series of steps

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Section 25-1Cones and Flowers

• Female Gametophyte (8 steps)
• 1. The megaspore mother cell undergoes meiosis,
  producing four haploid cells, three of which die
• 2. The remaining haploid cell divides by mitosis
  to produce eight nuclei

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Section 25-1Cones and Flowers

• Female Gametophyte
• 3. these 8 nuclei and the membrane that surrounds
  them are called the embryo sac
• the embryo sac is the entire female gametophyte

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Section 25-1Cones and Flowers

• Female Gametophyte
• 4. two of the 8 nuclei locate themselves in the
  center of the sac, and 3 nuclei clump at each end
• 5. The nuclei in the center are called the polar
  nuclei

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Section 25-1Cones and Flowers

• Female Gametophyte
• 6. One of the 3 nuclei in the group closest to
  the opening in the ovule enlarges to become the
  egg nucleus
• 7. The 2 other nuclei flank the egg nucleus

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Section 25-1Cones and Flowers

• Female Gametophyte
• 8. The 3 nuclei at the opposite end of the embryo
  sac die
• The female gametophyte now contains a female
  gamete (egg nucleus) ready to be fertilized

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Section 25-1Cones and Flowers

• Male Gametophyte
• the male gametophyte is even smaller than the
  female
• inside the anthers, microsporangia called pollen
  chambers produce many diploid microspore mother
  cells

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Section 25-1Cones and Flowers

• Male Gametophyte
• each microspore mother cell divides by meiosis to
  produce 4 haploid microspores
• each microspore ultimately becomes a single
  pollen grain

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Section 25-1Cones and Flowers

• Male Gametophyte
• the wall of the pollen grain thickens to protect
  the pollen grains contents from dryness and
  physical damage when it is released from the
  anther

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Section 25-1Cones and Flowers

• Male Gametophyte
• the nucleus of the pollen grain undergoes one
  mitotic division, producing 2 haploid nuclei
• one of these is called the tube nucleus the
  other is the generative nucleus

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Section 25-1Cones and Flowers

• Male Gametophyte
• the tube nucleus disintegrates
• the generative nucleus divides to form 2 sperm
  cells
• the pollen grain, which is the entire male
  gametophyte generation, stops growing until
  deposited on a stigma

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Section 25-1Cones and Flowers

• Male Gametophyte
• eventually the anther dries out, its pollen
  chambers split open, and mature pollen grains are
  released- as dust like mature pollen
• Figure 25-5, page 536

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Section 25-1Cones and Flowers

• Pollination- the transfer of pollen from anther
  to stigma
• 2 types
• 1. self-pollination pollen from anther to stigma
  of same the flower
• most plants do not self-pollinate

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Section 25-1Cones and Flowers

• 2. cross-pollination transfer of pollen from one
  flower to a flower on another plant
• Why is self-pollination uncommon in many plant
  species?
• sexual reproduction produces more variety good!

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Section 25-1Cones and Flowers

• Now lets discuss fertilization.
• Once the pollen grain has landed on the stigma of
  an appropriate flower, it grows a pollen tube
• the generative nucleus divides and forms 2 sperm
  nuclei

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Section 25-1Cones and Flowers

• The pollen tube now contain a tube nucleus, and 2
  sperm nuclei
• By following a chemical trail, the pollen tube
  grows down the style, eventually reaching the
  ovary and enters the ovule through a small hole

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Section 25-1Cones and Flowers

• Note Figure 25-8, top page 539
• When the pollen tube reaches the embryo sac, the
  sperm nuclei enter
• both nuclei participate in a process called
  double fertilization
• this occurs only in angiosperms

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Section 25-1Cones and Flowers

• During double fertilization,
• one sperm nucleus fuses with the egg nucleus to
  form the zygote
• other sperm nucleus fuses with the two polar
  nuclei, forming the triploid (3N)
  endosperm-provides food for the embryo

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Section 25-1Cones and Flowers

• Many animals eat the endosperm, because it is so
  rich in important nutrients
• Indeed, most of the food supply of humans is the
  endosperm of grasses- such as corn, wheat, and
  rice
• Note Figure 25-9, page 539

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Section 25-1Cones and Flowers

• Fertilization causes rapid changes to occur in
  the ovule, ovary, and other structures
• parts of the ovule toughen to form a seed coat-
  protects the delicate embryo and its tiny food
  supply

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Section 25-1Cones and Flowers

• The ovary wall thickens and joins with other
  parts of the flower stem to become the fruit that
  holds the seeds
• Note Figure 25-7, page 538

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Section 25-1Cones and Flowers

• A fertilized flower produces hormones that induce
  the plant to pour energy into the developing
  fruits and seeds
• If a flower is NOT fertilized, these hormones are
  not produced, and the flower withers and falls
  away

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Section 25-1Cones and Flowers

• The development of SEEDS was a major factor in
  the success of angiosperms on land
• seeds provide nourishment and protection for
  delicate embryos

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Section 25-1Cones and Flowers

• Angiosperm seeds have either one or two seed
  leaves called cotyledons
• cotyledons contain stored food
• Monocots, such as corn, have one cotyledon.
  Dicots, such as beans, have 2 cotyledons
• Figure 25-9, page 539

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Section 25-1Cones and Flowers

• The various part of the embryo are named
  according to their point of attachment to the
  cotyledon(s)
• 1. Epicotyl- the length of the stem above the
  cotyledons
• this develops into the plants stem has apical
  meristem

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Section 25-1Cones and Flowers

• 2. Hypocotyl- length of stem below the
  cotyledon(s)
• 3. Radicle- region at the very base of the
  hypocotyl
• contains the apical meristem of the root, and
  will develop into the primary root of the plant

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Section 25-1Cones and Flowers

• In many plants, the food stored in the endosperm
  is almost completely used up by the time the seed
  is mature
• in these seeds, the food used by the embryo
  during germination is stored in large cotyledons

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Section 25-1Cones and Flowers

• Seed coats can be either thin and fragile, or
  thick and woody
• thick seed coats protect seeds from dryness, salt
  water, and other adverse environmental conditions
• also protect when eaten by animals (teeth,
  digestion, etc.)

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Section 25-1Cones and Flowers

• The seeds that are eaten usually pass out of the
  animal some distance away from where the fruit
  was eaten
• in fact, the digestive waste also provides a bit
  of natural fertilizer the plant can use as it
  begins to grow

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Section 25-2Seed Development

• OBJECTIVES
• Explain the germination of seeds.

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Section 25-2Seed Development

• OBJECTIVES
• Discuss dormancy in seeds.

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Section 25-2Seed Development

• Germination is the process by which a seed begins
  to grow when favorable conditions occur
• Figures 25-10 25-11, page 541
• when they germinate, they absorb water this
  causes the endosperm and cotyledons to swell,
  cracking the seed coat

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Section 25-2Seed Development

• Through the cracked seed coat, the radicle
  emerges, and grows into the primary root
• In most monocots, the single cotyledon remains
  within the seed
• the growing shoot emerges protected by a sheath

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Section 25-2Seed Development

• In some dicots, the hypocotyl starts growing soon
  after the primary root starts growing
• the hypocotyl forms an arch that pushes up
  through the soil
• the cells in the hypocotyl are much tougher than
  the cells in the fragile apical meristem

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Section 25-2Seed Development

• Some seeds germinate so rapidly that they are
  practically instant plants
• but, many seeds will not grow when they first
  mature
• instead, these seeds will enter a period of
  dormancy the embryo is alive, but not growing

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Section 25-2Seed Development

• The length of dormancy varies in different plant
  species
• a number of environmental factors can cause a
  seed to end dormancy, and then to begin to
  germinate

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Section 25-2Seed Development

• Seed dormancy serves several purposes for plants
• 1. They may move great distances away from the
  parent plant (earlier we called this seed
  dispersal). An example is the cocoanut Fig.
  25-11, p.541

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Section 25-2Seed Development

• Seed dormancy serves several purposes for plants
• 2. It may allow the seeds a period of time until
  environmental conditions support plant growth
• best if they do not germinate during hot or cold
  weather

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Section 25-2Seed Development

• Other environmental conditions can end seed
  dormancy
• some pine trees would begin to grow when a forest
  fire kills the mature trees, but the heat
  stimulates the seeds to grow!

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Section 25-3Vegetative Reproduction

• OBJECTIVES
• Define vegetative reproduction.

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Section 25-3Vegetative Reproduction

• OBJECTIVES
• Describe how plants are reproduced by cuttings,
  layering, and grafting.

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Section 25-3Vegetative Reproduction

• Sexual reproduction contributes to the genetic
  diversity
• However, many species of flowering plants
  reproduce asexually by vegetative reproduction-
  thus a well-adapted plant can produce offspring
  genetically identical to itself

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Section 25-3Vegetative Reproduction

• Strawberries- send out long trailing stems called
  stolons, that produce roots when they touch the
  ground
• Bamboo plants grow long underground stems that
  can send up new shoots in several places

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Section 25-3Vegetative Reproduction

• Several species of angiosperms produce tiny
  plants on their leaves, or along their stems
• if the parent plant is knocked over, these
  plantlets can grow into new plants
• Figure 25-12, page 543

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Section 25-3Vegetative Reproduction

• New plants can also grow from the leaves of a
  parent plant, if the leaves fall to the ground
  under conditions that allow them to root
• the African violet and certain Begonias often
  grow from leaves

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Section 25-3Vegetative Reproduction

• Sometimes, the characteristics of a particular
  plant are so attractive or beneficial that
  horticulturists want to make many exact copies of
  that plant, while avoiding the variations that
  result from sexual reproduction
• Artificial vegetative reproduction!

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Section 25-3Vegetative Reproduction

• Cuttings- one of the easiest ways Figure
  25-13, page 543
• a grower cuts a length of stem that includes
  lateral buds partially bury it in soil, water,
  or special rooting mixture
• may also need to add rooting powders containing
  stimulants

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Section 25-3Vegetative Reproduction

• Layering- used with plants that take a long time
  to root as cuttings
• stem is cut partway through, and the cut area is
  dusted with rooting powder wrapped in moisture
  or bent to the ground and buried

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Section 25-3Vegetative Reproduction

• In this way, the treated stem receives water and
  nutrients from the parent plant, and also
  develops its own roots
• when completed, the rooted stem is separated from
  the parent plant, and allowed to grow on its own
  (Rubber plants often done this way)

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Section 25-3Vegetative Reproduction

• Grafting and budding are used to reproduce
  seedless plants and varieties of woody plants
  that do not produce strong root systems
• both methods have new plants grown on plants that
  have strong root systems

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Section 25-3Vegetative Reproduction

• 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 the stock

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Section 25-3Vegetative Reproduction

• When stems are used as scions, the process is
  called grafting
• When buds are used as scions, the process is
  called bud grafting or budding
• Then, the cut piece (scion) is attached to the
  stock, and wrapped Figure 25-14, p.544