RAVEN 9/e
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 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, 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
42.1 Plant Reproductive Development
· Describe the general life cycle of a flowering plant.
· Define phase change.
· Identify two Arabidopsis mutants that have been used to study phase change.
42.2 Flower Reproduction
· Name the four genetically regulated flowering pathways.
· Define floral determination.
· Explain the relationship between floral meristem identity genes and floral organ identity genes.
42.3 Structure and Evolution of Flowers
· List the parts of a typical angiosperm flower.
· Distinguish between bilateral symmetry and radial symmetry.
· Differentiate between microgametophytes and megagametophytes.
42.4 Pollination and Fertilization
· Discuss conditions under which self-pollination may be favored.
· Describe three evolutionary strategies that promote outcrossing.
· List the products of double fertilization.
42.5 Asexual Reproduction
· Define apomixis.
· List examples of plant parts involved in vegetative reproduction.
· Outline the steps involved in protoplast regeneration.
42.6 Plant Life Spans
· Distinguish between herbaceous and woody perennials.
· Define perennial and annual plants.
· Describe the life cycle of a biennial plant.
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.
· 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 / · Have students describe petals are important in determining the pollination strategy of a plant.· Ask students to explain how flowering can be affected by chemicals that inhibit gibberellins.
· Ask students to explain why temperature is a factor in flower induction in certain plants.
Analysis / · Have students describe how mutations to flower arrangement genes could affect a plant’s reproductive success.
· Have students explain how pesticide use can affect the plant composition of an area.
· Have students compare the relative effectiveness of insect versus wind pollination.
Synthesis / · Ask students design an experiment to test if UV light is a factor in insect pollination of flowers.
· Have students design an experiment to test if self-pollination is increased under stable environmental conditions.
· Ask the students develop an experiment to determine the role of stamen number in wind pollinated plants.
Evaluation / · Ask students evaluate the benefits and consequences of breeding self-pollinating fruit crops.
· Ask students to evaluate the reasons for propagating agricultural plants by cloning instead of using self-fertilization.
· 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
Introduction
This demonstration shows students flowers differ in appearance under visible light and ultraviolet light.
Materials
· 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
- 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
· 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