Download CHAPTER 42: PLANT REPRODUCTION

CHAPTER 42: PLANT REPRODUCTION
WHERE DOES IT ALL FIT IN?
Chapter 42 is a follow-up of Chapter 36 and builds up the general information on green plants
provided in Chapter 30. A quick summary of Chapter 30 is essential for success at covering Chapter
42. In addition, students should be encouraged to recall the principles of eukaryotic cell structure and
evolution associated with the particular features of plants.
SYNOPSIS
Flowering plants have been tremendously successful, evolutionarily, because they produce
flowers and true fruit. Their success is an excellent example, perhaps the best-known example, of
co-evolution among plants and animals. The consequence of co-evolution is greater genetic
diversity due to a wider distribution of plant gametes. In stable environments, however, there
may be less need for genetic diversity, and there, asexual reproduction (cloning) may be
advantageous.
Flowers form in response to highly regulated processes including light, temperature, and internal
and external signals, a process analogous to reproductive development in animals. In
angiosperms, internal development changes are called competence, that is, competence to
respond. Competence is followed by phase change, the transition to morphological changes.
These changes may be quite obvious or very subtle. Phase change requires both sufficient signal
and the ability to perceive the signal. The signals determine which flower parts—sepals, petals,
stamens, and carpels—form, and where they form. Environmental cues are important. Genetic
manipulations are possible.
There are three known genetically regulated pathways to flowering. One, the light-dependent
pathway influences how plants respond to seasonal changes in the relative length of day and
night. Plants are classified as short-day, long day, or day-neutral, accordingly. This classification
correlates with the amount of uninterrupted darkness that induces flowering. This also is known
as photoperiodism, or photoperiodic response. Short-day plants flower when daylight becomes
shorter than a critical length. Long day plants flower when daylight becomes longer than a
critical length. Day-neutral plants flower when mature regardless of day length. Several different
forms of phytochrome and a blue-light sensitive molecule called cryptochrome perceive
photoperiod. A conformational change in cryptochrome triggers a cascade of events that leads to
the production of a flower. The temperature-dependent pathway supports the theory that cold
temperatures can either accelerate or permit flowering in many species. Similarly to the effects
of light, the outcomes of this pathway ensure that flowering occurs at optimal times for different
species. Vernalization refers to the phenomenon whereby some plant species require a chilling
period in order to flower. Vernalization my function with autonomous pathways in the promotion
of flowering. The autonomous pathway may have evolved first. It functions independently of
external cues except for basic nutrition. Day-neutral plants depend on this pathway as they
appear to “count and remember” until, at some point, shoots, perhaps in conjunction with
inhibitory signals from roots, are determined to flower. The three flowering pathways lead to an
adult meristem becoming a floral meristem by either activating or repressing the inhibition of
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floral meristem-identity genes that turn on floral organ identity genes. All this activity only leads
to the beginning of flower formation. Flowers contain the haploid generations that will produce
gametes. Flowers also allow for an increase in pollination opportunities, likely allowing for an
increase in genetic diversity. A great variety of floral phenotypes exist, explaining the great
variety of angiosperm species. Two major evolutionary trends lead to this diversity: (1) Separate
floral parts became grouped together, and (2) Floral parts were reduced or lost. Other
evolutionary trends affect flower symmetry. Radial symmetry is believed to be more primitive
than bilateral symmetry. Mutations in either flowers or pollinators may prevent fertilization from
occurring.
Fertilization is the union of gametes, that is, eggs and sperm, from either the same or different
flowers of the same species. In angiosperms, the gametophyte generations are very small. The
female gametophyte is the embryo sac. The male gametophytes are pollen grains whose shapes
are specialized for specific flower species. A complex series of events called double fertilization
occurs uniquely in the angiosperms. In this process, one sperm cell fertilizes the egg while a
second one helps from the endosperm that nourishes the embryo.
Pollination refers to the transfer of pollen that forms within pollen sacs from microspore mother
cells to the stigmas of female flowers. This process may or may not lead to fertilization.
Pollination in early plants occurred passively, by such mechanisms as the wind, a random event.
Subsequently, co-evolution of flowers and animals, especially insects (bees, moths, butterflies,
birds, bats, and other animals) has enhanced diversity and success of both. Wind is still an
important agent for pollination. Wind-pollinated plants are not dependant on live pollinators.
While cross-fertilization or out-crossing contributes to genetic diversity, self-pollination is
ecologically important, as pollinators are not required. Diversity is still present meiosis, and
recombination still occurs in the development of both male and female gametophytes.
Outcrossing is facilitated when stamens and pistils are separated. Monoecious plant species
possess both male and female flowers while dioecious species have either male flowers or female
flowers but not both. In monoecious species, male and female flowers may mature at different
times, further enhancing opportunities for outcrossing. Such plants are referred to as
dichogamous.
Least genetic variability results from asexual reproduction in which only mitotic cell division
occurs. One form, vegetative reproduction, provides for progeny cloned from parents. Examples
of vegetative reproduction are runners, rhizomes, suckers, and adventitious plantlets. Apomixis
is another form that occurs in some plants, for example, some citruses, grasses, and dandelions.
In apomixes embryos are produced asexually from parent plants. Asexual reproduction tends to
occur in marginal or harsh environments, and the progeny are genetically identical to the parent
individual. Tissue culture, a technique that allows for regeneration of whole plants from
individuals cells or tissues, is an artificial type of asexual reproduction.
Flowers are characterized as complete and incomplete, depending on the number of whorls of
parts: calyx (outermost whorl consisting of sepals), corolla (interior to calyx, consisting of
petals), androecium (collective term for the stamens, the male structures), and gynoecium
(collective term for the female parts: single or fused carpel/s or pistil/s that include stigma, style,
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and ovary) Ovules, contained in ovaries, develop into seeds.
Plants live for variable periods of time. There is not a direct correlation between life span and
mode of reproduction. Nonetheless, woody species that incur secondary growth tend to live
longer than herbaceous species that do not form secondary growth. Herbaceous species,
however, may be classified as annuals since they grow, flower, and set seed prior to dying during
one growing period. Biennial species require two seasons during which to accomplish this, and
perennial species continue to grow, indefinitely. Perennial plants include woodland, wetland and
prairie species as well as woody species such as shrubs and trees. The latter are considered
deciduous if their leaves abscise and fall each year, appearing bare during stressful times such as
winter in most temperature areas. Evergreens, on the other hand, drop their leaves throughout the
year, and never appear bare of foliage. Abscission is the process whereby senescent plant parts,
as leaves, respond to hormonal changes and environmental cues, promoting death and drop of
those parts. Fall season color displays of many deciduous species in temperature zones involve
the abscission process.
LEARNING OUTCOMES
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Describe how plants produce flowers in response to environmental cues.
Explain meristems, competence, phase change, photoperiodism, the roles of different hormones,
and how these different factors interact in the flowering process.
Describe how flowers evolved. What are the parts of a complete flower?
Discuss the three known genetically regulated pathways for flowering. What is the role of
phytochrome in these pathways? List the evolutionary advantages provided by angiosperm
flowers.
Describe the two major evolutionary changes that led to the diversity of flower phenotypes.
Differentiate between pollination and fertilization.
Describe the processes of pollen formation and embryo sac formation.
Explain fertilization in angiosperms.
Understand the advantages of asexual reproduction in plants. What is apomixis?
Name and describe the parts of a flower. Which part becomes the seed?
Explain the life span of annual, biennial, and perennial plants.
Differentiate between complete and incomplete flowers, and between monoecious and dioecious
plants. Which arrangement promotes outcrossing?
Understand the advantages of asexual reproduction in plants.
Differentiate among annual, biennial, and perennial plants.
Understand the process of abscission.
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 42 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students are unaware that plants develop environmental adaptations
Students believe that plants lack tissues and organs
Students are unaware of all of the functions of leaves
Students are unaware of the relationship of flowers to other plant parts
Student are unfamiliar with the chemistry of plant defensive chemicals
Students believe that all flowers are insect pollinated
Students do not believe that plants produce eggs
Students equate pollen to sperm
Students believe that all plants produce seeds
Students are confused by the role of gametophytes and sporophytes
Students are not sure of the role of meiosis in plant life cycles
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
Hints regarding flower terminology: staminate flowers have stamens, pistillate flowers have
pistils. Monoecious means “one house”; therefore a monoecious plant has both staminate and
pistillate flowers on a single plant. Dioecious means “two houses,” thus a dioecious plant has
two different kinds of plants (male and female), each with its own type of flower.
Pollination refers only to the transfer of pollen. This event may or may not be followed by
fertilization. What factor/s can impede fertilization?
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 42.
Application
Analysis
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Have students describe petals are important in determining the pollination
strategy of a plant.
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Ask students to explain how flowering can be affected by chemicals that
inhibit gibberellins.
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Ask students to explain why temperature is a factor in flower induction in
certain plants.
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Have students describe how mutations to flower arrangement genes could
affect a plant’s reproductive success.
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Have students explain how pesticide use can affect the plant composition
of an area.
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Have students compare the relative effectiveness of insect versus wind
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pollination.
Synthesis
Evaluation
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Ask students design an experiment to test if UV light is a factor in insect
pollination of flowers.
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Have students design an experiment to test if self-pollination is increased
under stable environmental conditions.
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Ask the students develop an experiment to determine the role of stamen
number in wind pollinated plants.
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Ask students evaluate the benefits and consequences of breeding selfpollinating fruit crops.
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Ask students to evaluate the reasons for propagating agricultural plants by
cloning instead of using self-fertilization.
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Ask students to evaluate the effectiveness of improving flower and fruit
production in agricultural plants by increasing fertilizer levels in the soil.
VISUAL RESOURCES
Three-dimensional models or drawings of flowers are extremely helpful, especially in presenting
the idea of whorls and flower specializations, and double fertilization. The best models are those
that can be taken apart to show the interior of the pistil and anther. Of course, living specimens
are ideal.
Also examine various types of fruit from the outside inward, and then let your students eat the
fruit! Scanning electron micrographs of pollen are very impressive in showing some of the
beautiful, unseen intricacies of nature. Those pollens that cause common allergies are very spiky
while other pollen grains are globular.
Show examples of long-day, short-day, and day-neutral plants. This helps students realize that
photoperiodism is not a remote concept. Discuss the methods used to initiate flowering at certain
seasons. Easter lilies do not bloom in early March without being forced by altering light and
temperature. Certain succulents are similarly forced and produce blooms at Easter,
Thanksgiving, or Christmas. Many temperate plants will not bloom when grown in southern
climates, including lilacs, forsythia, and gladiolus. These all require exposure to cold for a
certain period. However, farther north than the Appalachians, gladiolus freezes if left in the
ground over winter. Most varieties of apples do not flower or fruit when grown beyond certain
latitudes.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. UV Vision
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Introduction
This demonstration shows students flowers differ in appearance under visible light and
ultraviolet light.
Materials
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Computer with Media Player and Internet access
LCD hooked up to computer
Web browser linked to Ultraviolet Flowers website at
http://www.naturfotograf.com/UV_flowers_list.html
Procedure & Inquiry
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Explain to the students that you want show flowers under two lighting conditions.
Tell the students to pay close attention to the flowers images.
Load up the website and click on various flowers.
First show the flower under visible light.
Then scroll down to the flower under UV light.
Ask the students to describe the differences between flowers under visible and UV light.
Also ask them to explain any variation in how flowers appear under UV light.
Then ask them to explain if there is a reason that the flowers appear differently under UV
light.
B. Name the Pollination Strategy
Introduction
This demonstration asks students to guess the pollinator and the pollination strategy by
looking at images of flowers.
Materials
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Computer with Media Player and Internet access
LCD hooked up to computer
Web browser linked to Angiosperm Pollination Syndromes website at
http://www.cas.vanderbilt.edu/bioimages/pages/pollination.htm
Printout of Angiosperm Pollination Syndromes website as a reference for the answers to
the queries
Images from Angiosperm Pollination Syndromes website cut and pasted onto a work
document so that each flower takes up one whole page when projected on a screen
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Procedure & Inquiry
9. Explain to the students that you want to guess the pollinator and pollination strategy of
each plant.
10. Show a flower and ask a student in the class to make a guess and justify their answer.
11. Acknowledge the answer and provide the correct answer if the student was incorrect.
12. Continue until you feel the students understand how to recognize form and function in
flowers.
USEFUL INTERNET RESOURCES
1. Images of various plant flowers and developmental structures are valuable for
supplementing a lecture based on Chapter 42.. The Botanical Society of America has a
useful website for obtaining images for classroom teaching. The site is available at
http://www.botany.org/plantimages/imagemap.php.
2. Invasive plants are a consequence of introducing non-native plants into new regions.
These plants somehow exploit the environment for unusually high reproductive success.
The United States National Arboretum has a Invasive Plants website that can be used as a
source of information for Chapter 42. The website is available at
http://www.usna.usda.gov/Gardens/invasives.
3. Plant and animal agriculture rely on the job of pollinators for a variety of purposes. The
commercial applications of pollination can be share with students by using information
from The Pollination Home Page. This website is located at http://pollinator.com/.
4. The United States Department of Agriculture has a fun website on pollination ecology. It
can be used to get trivia for a lecture on Chapter 42 or for student projects. The website
can be found at http://www.fs.fed.us/wildflowers/pollinators/index.shtml.
LABORATORY IDEAS
A. Floral Morphology
This activity encourages students to identify variations in floral morphology.
a. Explain to students that they investigating the morphology of flower variation.
b. Then explain that they will be using brine shrimp amoebocytes as a model system for
looking at the effectiveness of cell-killing power.
c. Tell students that they will be investigating the conditions needed for fungal spore
germination in two types of fungi.
d. Provide students with the following materials
a. Dissection microscope
b. Sharp scalpel
c. Probes
d. Flowers:
i. Carnation
ii. Lily
iii. Gladiolus
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iv. Aster
v. Delphinium
vi. Orchid
e. Computer with Internet access and the following links:
i. http://www.ndsu.edu/instruct/mcclean/plsc731/flower/flower3.htm
ii. http://www.cc.ndsu.nodak.edu/instruct/mcclean/plsc731/flower/flower2.ht
m
iii. http://wwwbiology.ucsd.edu/labs/yanofsky/flower/intro_to_flower_dev.htm
iv. http://biology.clc.uc.edu/fankhauser/Wildflowers/Plant_categories/Plant_F
amilies.htm
f. Book with flower parts diagram
e. Instruct students to observe the differ flowers and record their observations about:
a. Number and arrangement of petals
b. Number and arrangement of sepals
c. Number of whorls
d. Makeup of whorls
e. Number and arrangement of stamens
f. Organization of the carpels
g. Fusion of floral parts.
f. Have the students use the information from the provided websites to hypothesis about the
mutations that produced the genetic variety of the flowers they observed.
g. Then tell the students to share their hypotheses with the class.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
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Have students tutor high school students learning plant reproduction.
Have students volunteer with a garden club on a community garden project.
Have students give a demonstration on the diversity of flowers elementary students.
Have students volunteer at botanical garden or nature center.
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