20 PLANT REPRODUCTION AND GROWTH
                                             EXTENDED LECTURE OUTLINE

Flowering Plant Reproduction (p. 403)
20.1   Angiosperm Reproduction (p. 403; Fig. 20.1, 20.2, 20.3)
       A. Asexual Reproduction
          1. The forms of vegetative reproduction are runners, rhizomes, suckers, adventitious
       B. Sexual Reproduction
          1. Angiosperm life cycles involve alternation of generations, with the sporophyte as the
              dominant generation and the gametophyte developing within the confines of the
          2. Both microgametophytes (male gametophytes) and megagametophytes (female
              gametophytes) are housed within the same structure, the flower.
          3. Flower production is seasonal and not a permanent feature of the mature sporophyte.
          4. Pollen grains are the male gametophytes while embryo sacs are the female gametophytes.
          5. Structure of the Flower
              a. Flowers are arranged in whorls.
              b. The outermost whorl, the calyx, is made up of the protective sepals.
              c. The second whorl is the petals, which collectively are called the corolla, and
                  functions to attract pollinators.
              d. The third whorl is the male androecium, made up of stamens and anthers; pollen
                  grains develop within the anthers.
              e. The innermost whorl is the female gynoecium made up of carpels; each carpel is
                  composed of a stigma that collects pollen, a style, and an ovary that houses the
                  ovules in which embryo sacs develop.
          6. Pollen Formation
              a. Pollen grains develop from microspores that form in the pollen sacs.
              b. Within the pollen sacs, the microspore mother cells undergo meiosis to form
                  microspores; microspores undergo mitosis to form pollen grains.
          7. Pollination
              a. Pollination is the process by which pollen arrives at the stigma.
              b. Pollinators include various insects or birds that are attracted to the flowers by scents
                  and colors, and are often rewarded for their efforts by sugary nectar.
              c. Coevolution of flowers and pollinators can be seen in a wide variety of plants and
              d. Wind-pollinated flowers are not fragrant or colorful, and typically produce very large
                  amounts of pollen to ensure that at least some of it arrives at the female stigma.
          8. Egg Formation
              a. Eggs develop in the ovules of the angiosperm flower; within each ovule is a
                  megaspore mother cell.
              b. Each megaspore mother cell undergoes meiosis to produce four haploid megaspores,
                  only one of which survives.
              c. The lone megaspore undergoes mitotic divisions and produces 8 haploid nuclei; one
                  will function as the egg nuclei and two are polar nuclei.
          9. Fertilization
              a. In angiosperms, fertilization occurs in a unique process called double fertilization.
              b. Once pollen grains reach the stigma, a pollen tube grows down through the style until
                  it reaches the ovule.

                c.  One of the cells of the pollen within the pollen tube becomes two sperm cells; one of
                    the sperm cells unites with the egg cell to produce the zygote, and the other sperm
                    cell fuses with two polar nuclei that were produced during meiosis of the egg cell.
                d. The two polar nuclei and the sperm cell fuse and divide rapidly, becoming nutritious
20.2   Seeds (p. 407; Fig. 20.4)
       A. The entire series of events that occur between fertilization and maturity is called development.
       B. In many plants, embryo development is arrested soon after apical meristems and the first
           leaves (cotyledons) are differentiated.
       C. The integuments surrounding the embryo develop into an impervious seed coat that protects
           the embryo throughout its period of dormancy.
       D. Resumption of metabolic activity within the seed leads to germination.
20.3   Fruit (p. 408; Fig. 20.5)
       A. In some plants, the ovary surrounding the ovule develops into fruit.
       B. Fruits appear to have evolved to help in seed dispersal, since animals eat the fruit and defecate
           or drop the seeds at some distance away from the parent plant.
20.4   Germination (p. 409; Fig. 20.6)
       A. When the time is right for the embryo to resume its growth, the seed coat absorbs water, and
           the embryo becomes metabolically active once again.
       B. Oxygen is needed during germination for most seeds, although some seem able to germinate
           in anoxic conditions.
Regulating Plant Growth (p. 410)
20.5   Plant Hormones (p. 410; Figs. 20.7, 20.8; Table 20.1)
       A. After germination, how rapidly a plant grows depends on its meristematic tissues.
       B. As plants grow, cells differentiate.
       C. But one critical difference remains between plant cells and animal cells: once animal cells
           differentiate, they cannot be anything other than what they are in their differentiated form.
       D. Plant cells, on the other hand, can be induced to form whole new plants.
       E. Plant cells retain all the genetic information needed for each type of plant cell in the plant.
       F. Plant growth is regulated by chemical messengers, known as hormones.
       G. At least five major hormones have been found; others also exist but have yet to be chemically
           isolated and described.
20.6   Auxin (p. 412; Figs. 20.9, 20.10, 20.11)
       A. The hormone auxin is a primary growth hormone that is more abundant in shaded portions of
       B. Phototropism, or seeming to grow toward the light, is a commonly observed phenomenon in
           many plants.
           1. The shaded side of the stem elongates faster, causing the plant to lean toward the light.
       C. Synthetic auxins are used as herbicides (such as 2, 4-D) because the plants can be induced to
           literally growth themselves to death.
20.7   Other Plant Hormones (p. 414; Figs. 20.12, 20.13, 20.14, 20.15)
       A. Gibberellins
           1. Gibberellins are a third type of hormone that is made in the apical areas of stems and
           2. Gibberellins appear to cause elongation of internode areas, and also affect flowering and
                seed germination.
       B. Cytokinins
           1. Cytokinins also induce cell division.
           2. Cytokinins are produced in the roots and activate the proteins required for mitosis.
       C. Ethylene
           1. Ethylene is a chemical produced during certain times of fruit ripening.
           2. Ethylene also plays an important ecological role and increases in production when the
                plant is stressed by a variety of factors.

           D. Abscisic Acid
              1. Abscisic acid stimulates leaf drop and suppresses growth during periods when the plant
                 must become dormant.
              2. This hormone also probably induces the formation of winter buds.
Plant Responses to Environmental Stimuli (p. 416)
20.8       Photoperiodism and Dormancy (p. 416; Figs. 20.16)
           A. Plants respond to environmental stimuli in a variety of ways.
           B. Photoperiodism
              1. Plants also respond to patterns of light and dark as the seasons progress, a phenomenon
                  referred to as photoperiodism.
              2. Day length cues plants to flower.
              3. Some plants flower when days are shorter, others as the days become longer.
              4. Still other plants are day-neutral and produce flowers when conditions are favorable,
                  regardless of day length.
           C. The Chemical Basis of Photoperiodism
              1. One class of chemicals, the phytochromes, are pigment molecules that influence
                  flowering in plants.
           D. Dormancy
              1. When environmental conditions are unfavorable to growth, the capacity to become
                  dormant is critical to the survival of certain plants.
              2. Annual plants overwinter as seeds.
20.9        Tropisms (p. 418; Fig. 20.18)
           A. Tropisms are growth responses of plants to external stimuli and control patterns of growth and
           B. Gravitropism
              1. Stems grow downward and shoots upward as a response to gravity, or gravitropism.
           C. Thigmotropism
              1. Thigmotropism occurs when plants respond to being touched

      Be able to characterize the flowering plant life cycle.
      Diagram the basic structure of a flower and indicate the location of its four whorls.
      Describe the steps leading to pollination and fertilization.
      Define “double fertilization”.
      Characterize the nature and function of a seed.
      Discuss the purpose and design of fruit.
      Describe the process of germination.
      Explain how the growth of the plant body is adjusted by hormones.
      Define phototropism and tell what hormone is responsible for this response.
      Discuss the functions of cytokinins, gibberellins, ethylene, and abscisic acid.
      Explain how plant growth and development are related to photoperiod.
      List the three tropisms and what causes them.

      Asexual reproduction (p. 404)
      Pollen grains (p. 404)
      Embryo (p. 404)
      ovules (p. 406) Within the ovules, megasporangia produce haploid megaspores which develop into
       embryo sacs containing the eggs.

    pollination (p. 405) Pollination is the process by which pollen is placed on the stigma. It may arrive
     there by wind or by animals, unless it is self-pollinated.
    double fertilization (p. 406) Double fertilization involves two sperms cells, one of which fertilizes the
     egg cell, and the other of which fuses with the two haploid polar nuclei to become triploid endosperm.
    germination (p. 407) This is the resumption of metabolic activities in the seed, often in response to
    phototropism (p. 412) a phenomenon in which plants bend toward the light
    auxin (p. 412) A plant hormone that regulates cell growth in plants.
    gibberellins (p. 414) Gibberellins are important for stem elongation.
    cytokinins (p. 414) These hormones stimulate cell division and determine the course of differentiation.
    ethylene (p. 414) Ethylene is produced during fruit ripening.
    abscisic acid (p. 415) This hormone stimulates leaves to age rapidly and fall, and plays a role in
     causing dormancy.
    photoperiodism (p. 416) Responses of plants are timed to changes in day length.
    phytochromes (p. 417) The pigment, phytochrome, is involved in many plant growth responses,
     especially those involving day length.
    gravitropism (p. 418) This response to gravity causes stems to grow upward and roots downward.
    thigmotropism (p. 418) Thigmotropism is the response of plants to touch.

1.   Prepare several examples of tropisms ahead of your lecture time. An easy way to show gravitropism is
     to grow corn seedlings in the dark. The etiolated seedlings are white and cannot be growing in response
     to light. A house plant positioned in the same direction in reference to a lateral light source for several
     weeks shows phototropism, as its leaves and stems are oriented toward light. A mimosa, venus flytrap,
     or similar plant is a good example of thigmotropism.
2.   Choose one of the videos listed below to show students the relationship between flowers and their
     pollinators. A number of plants and pollinators have coevolved a species-specific relationship, and
     showing students this provides an excellent and obvious example of evolution.
3.   If your class is small enough, bring in different types of flowers for students to pull apart and locate
     structures as you discuss them. Usually dissecting tools are not necessary, but paper towels under the
     flowers aid in fast cleanup.

Refer to the Johnson instructor web site at http://www.mhhe.com/biosci/genbio/tlw4 for a complete list of
changes to this edition.

1.   Why would ethylene production increase rapidly when a plant is exposed to ozone, toxic chemicals,
     temperature extremes, attacks by predators, or other stresses?
2.   In what directions do roots and stems grow when there is no gravity, such as in a spaceship outside the
     pull of earth?
3.   In Chapter 10 of this text, genetic engineering in agriculture was discussed. Might it be feasible to
     genetically engineer a plant to respond to touch (become thigmotropic) as a means to deter certain
     flying insect predators?

(Telephone and fax numbers and/or web sites for the sources of the following materials are listed in the

Angiosperms. Highlights of this video include the importance of angiosperms, the differences between
monocots and dicots, annuals versus biennials and perennials, and animated sequences showing the life
cycle of a flowering plant, pollen and egg formation, and double fertilization. 31 minutes.
CLEARVUE/eav, WW5VH 1615

Sexual Encounters of the Floral Kind: Intriguing Methods of Pollination. Some of the most unusual
methods for reproduction in the plant world are the focus of this video. Some of the relationships that have
coevolved between flowering plants and their pollinators are described, along with flowers that smell like
rotting flesh to lure pollinators. 54 minutes.
CLEARVUE/eav, WW5VH 1037
(Also offered by Carolina Biological Supply, CE-49-5932-V)

The Private Life of Plants Series. This 6 video set explores the plant kingdom from exotic settings. 1995.
50 minutes each.
Carolina Biological Supply, CE-49-4976


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