24.3 Plant Hormones. Lesson Objectives. Lesson Summary

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1 24.3 Plant Hormones Lesson Objectives Describe the effects of hormones on plant growth and development. Identify three tropisms exhibited in plants. Describe how plants respond to seasonal change. Lesson Summary Hormones Living organisms produce chemical signals that affect the growth, activity, and development of cells and tissues. Such a chemical is called a hormone. A hormone affects particular target cells that have receptors to which a particular hormone can bind. There are five major classes of plant hormones. Auxins are produced in the apical meristems and cause cell elongation and the growth of new roots. They also inhibit the growth of lateral buds, which produces apical dominance. Snipping off the tip of a stem breaks apical dominance and enables branches to develop. Cytokinins stimulate cell division and are produced in growing roots and developing fruits and seeds. Gibberellins stimulate the growth of stems and fruits. They also stimulate seed germination. Abscisic acid inhibits cell division and causes seed dormancy. Ethylene is a gas that stimulates fruit ripening and causes plants to seal off and drop organs such as leaves and fruits that are no longer needed. Tropisms and Rapid Movements Tropisms are growth responses to environmental stimuli, which cause elongating stems and roots to bend. Phototropism is a response to light. Gravitropism is a response to gravity. Thigmotropism is a response to touch. Rapid movements such as the closing of leaves when touched are caused by changes in cell walls and in osmotic pressure in certain cells. Response to Seasons Plants have regular cycles in their patterns of growth, development, and flowering that are tied to seasonal changes. One environmental stimulus that changes with the seasons is the photoperiod, the relative length of the light and dark periods in a day. A plant pigment called phytochrome causes a plant s response to the photoperiod. The timing of flowering is one plant response to the photoperiod. Preparations for winter dormancy, which include leaf loss and the formation of scales around terminal buds, are also responses to the photoperiod. Lesson 24.3 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 384

2 Hormones 1. What is a hormone? A hormone is a chemical signal produced by a living organism that affects the growth, activity, and development of cells and tissues. 2. What are the functions of hormones in plants? Plant hormones control the development of cells, tissues, and organs. They also coordinate a plant s response to the environment. 3. What is a target cell? A target cell is one that has receptors for and is affected by a hormone. 4. Briefly describe the experiments that Charles and Francis Darwin performed on grass seedlings. SAMPLE ANSWER: Charles and Francis Darwin studied the mechanisms behind a seedling s tendency to bend toward light. They cut off the tips of some grass seedlings and blocked light from reaching the tips of other grass seedlings. Only the seedlings with tips that were exposed to light grew toward the source of the light. For Questions 5 19, match the action with the plant hormone that produces it. Hormones may be used more than once. Action C B D E A C E B D C D E E B A 5. May oppose the effects of auxins 6. Promotes cell elongation 7. Causes petals and leaves to drop 8. Promotes seed germination 9. Promotes seed dormancy 10. Stimulates cell division 11. Causes the enlargement of fruits 12. Causes apical dominance 13. Stimulates fruit ripening 14. Forms in growing roots 15. Forms in aging leaves and flowers 16. Opposes the effects of abscisic acid 17. Stimulates dramatic stem growth 18. Stimulates new root growth 19. Inhibits cell division Plant Hormone A. abscisic acid B. auxin C. cytokinins D. ethylene E. gibberellin Lesson 24.3 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 385

3 Tropisms and Rapid Movements 20. What is a tropism? A tropism is a growth response of a plant to a stimulus from the environment. 21. What causes the bending of stems and roots in tropisms? Give an example. The bending of stems and roots is caused by the difference in concentration of auxins on one side of the stem or root, which causes that side to grow longer than the other side. For example, auxin moves toward the dark side of a stem, causing the cells on that side to elongate more than the cells on the lighted side and making the stem bend in the direction of the light. 22. Complete the table about plant tropisms. Plant Tropisms Tropism Definition Example Gravitropism The response of a plant to gravity SAMPLE ANSWER: Roots growing downward Phototropism The response of a plant to light A stem bending toward a light source Thigmotropism The response of a plant to touch SAMPLE ANSWER: A tendril of a vine coiling around a trellis 23. How are the speed and causes of rapid movements of plants different from the speed and causes of tropisms? Rapid movements happen in a few seconds and are caused by changes in cell walls and the osmotic pressure of certain cells. Tropisms take much longer to appear because they involve the growth of a plant. 24. List two examples of rapid movements that occur in plants. The closing of the leaves of sensitive plants and the closing of a Venus flytrap leaf around an insect are examples of rapid movements. Lesson 24.3 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 386

4 Response to Seasons For Questions 25 28, complete each statement by writing the correct word or words. 25. Plants that flower when nights are longer than days are called short-day plants. 26. Irises that flower in summer when nights are short are called long-day plants. 27. The relative lengths of the light and dark times in a day are a stimulus called the photoperiod. 28. The chemical that causes the seasonal responses of plants is a type of light-sensitive chemical called a(n) pigment. 29. Complete the concept map that summarizes the role of phytochrome in plants. Phytochrome is responsible for plant responses to photoperiod Flowering Leaf loss and fall colors Changes to meristems 30. You have been asked to suggest a flowering plant that would bloom and beautify a lighted parking lot. The parking lot is only dark between midnight and sunrise. What information should you research about the plants you consider? Explain your answer. SAMPLE ANSWER: It will be crucial to know what kind of photoperiod stimulates the plants to flower. The plants will have to be long-day plants (or day-neutral), otherwise flowering will be inhibited by the short dark period. Lesson 24.3 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 387

5 24.4 Plants and Humans Lesson Objectives Identify the major food-supply crops for humans. Describe how humans benefit from plants. Lesson Summary Agriculture The systematic cultivation of useful plants is called agriculture. The beginning of human civilization is linked to the development of agriculture. Worldwide, a few crop plants such as rice, wheat, soybeans, and corn provide the bulk of the food supply for humans and livestock. Through selective breeding, humans have developed a wide variety of fruit and vegetable crops and improved staples such as corn and wheat. Improved farming techniques, such as the use of fertilizers and pesticides, have further increased the food supply from plants. The efforts made to improve crop yields between 1950 and 1970 are referred to as the green revolution. This greatly increased the world s food supply. Fiber, Wood, and Medicine Plants are the source of many raw materials besides food. Fibers such as cotton are used in cloth, bandages, and carpeting. Wood is used for making many objects, such as homes, and for making paper. Many medicines first came from plants. Agriculture For Questions 1 8, complete each statement by writing the correct word or words. 1. Agriculture is the systematic cultivation of plants. 2. The foundation on which human society is built is modern farming. 3. Evidence suggests that agriculture developed between 10,000 and 12,000 years ago. 4. Nearly all agricultural plants belong to the group of plants called angiosperms. 5. The seeds produced by crop plants such as wheat, rice, and corn are the source of most of the food humans eat. 6. Humans changed wild food plants into productive crops through the practice of selective breeding. 7. Cabbage, broccoli, and Brussels sprouts were all developed from the wild plant. 8. The green revolution greatly increased crop yields. mustard Lesson 24.4 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 388

6 9. Complete the concept map. The Green Revolution resulted from use of High-yield seeds Fertilizers Pesticides Fiber, Wood, and Medicine For Questions 10 13, write the letter of the correct answer on the line at the left. C 10. Which is an example of a plant that is a source of food? A. Aloe vera C. maize B. cotton D. Sitka spruce A 11. Cotton fibers are outgrowths of the A. seed coat epidermis. C. vascular cambium. B. ripened ovary wall. D. vascular bundles. D 12. The wood used to make chairs and musical instruments is made up of which plant tissue? A. the cortex C. the mesophyll B. the cambium D. the xylem A 13. Which plant contains chemicals that can help burns and other wounds? A. Aloe vera C. maize B. cotton D. Sitka spruce 14. What effect is global climate change likely to have on agriculture? SAMPLE ANSWER: Global climate change is likely to change the types of crops that are grown in particular areas of the world. It is also likely to cause the development of new crop varieties and methods of farming that could enable people to grow crops in less than ideal conditions. Lesson 24.4 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 389

7 Chapter Vocabulary Review For Questions 1 3, on the lines provided, label the structure that corresponds to each number in the illustration Ovary Style Ovules For Questions 4 15, complete each statement by writing the correct word or words. 4. The female structure of a flower that can be made up of a single carpel or two or more fused carpels is called a pistil. 5. A plant hormone that causes stem and fruit growth as well as seed germination is called gibberellin. 6. A target cell has receptors for a particular hormone. 7. The female gametophyte of an angiosperm is a small structure called the embryo sac. 8. The food storage tissue that results from double fertilization in angiosperms is called endosperm. 9. Seed dormancy is a period during which a plant embryo stops growing. 10. Grafting is the method of propagating many identical woody plants using the buds of a desirable plant. 11. The ripening of fruits is one effect of the plant hormone called ethylene. 12. Identical offspring produced by a single parent plant result from vegetative reproduction. 13. Apical dominance is a major effect of the plant hormone auxin. 14. Pollination occurs when pollen grains land on the stigma of a flower. 15. The anthers of flowers are the tiny sacs that produce the male gametophytes of angiosperms. Chapter 24 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 390

8 THE GREEN LEMONS In the Chapter Mystery, you learned how one factor could disrupt a plant s ripening schedule. Many factors affect a plant s growth and reproduction, as well as its taste and nutrition. Today s consumers are becoming increasingly conscious of the factors that affect the food they eat. Learning Organic Food Farming and Labeling Organic farming is a type of agricultural production system that uses natural methods, such as compost instead of synthetic fertilizers, biological pest control instead of synthetic pesticides, and crop rotation instead of plant growth regulators. Organic farmers are not allowed to use certain products that conventional farmers use. All organic food products must be certified by independent state or private agencies accredited by the U.S. Department of Agriculture. Many consumers think that organic food tastes better and is more nutritious and healthful. Therefore, they are willing to pay more for food labeled organic. Scientists, however, have not proven that organic food is more nutritious. Rules for Labeling Food as Organic 1. Animals producing meat, poultry, eggs, and dairy products must not be given antibiotics or growth hormones. They also cannot be fed animal byproducts, such as feathers and ground-up chicken parts. 2. Cows on organic farms must graze on grass that has not been treated with pesticides. 3. Vegetables must be grown without using conventional pesticides. 4. The use of conventional pesticides, petroleum-based fertilizers, bioengineering, and ionizing radiation is prohibited from all organic food production. Before any food receives an organic label, a government-approved certifier must inspect the farm where the food is produced as well as the companies that process and transport the food. Themes Science and Global Awareness 1. Why are organic food products considered more natural than conventional food products? Organic farming produces food using natural methods. It does not use synthetic, or human-made, fertilizers, pesticides, and other chemicals. 2. Why are many consumers willing to pay more for food items labeled organic? They think the food tastes better and is more nutritious and healthful. Continued on next page Chapter 24 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 391

9 3. Why do you think the U.S. Department of Agriculture deems it important to regulate which foods get the organic label? SAMPLE ANSWER: Because stores can get more money for items labeled organic, they may be willing to put that label on food that is not organic. Food that is not organic is often cheaper for the stores to buy. Therefore, stores would make more money by mislabeling food, but the consumer would be cheated. 4. The first rule states that animals producing organic foods cannot be fed animal byproducts. Why do you think this rule is included? Give an example to support your reasoning. SAMPLE ANSWER: Animal byproducts could include the very chemicals not allowed for animals producing organic food. For example, a chicken could have been raised on a farm using synthetic pesticides. If byproducts from this chicken are fed to an animal on an organic farm, those pesticides could be passed on in the food. 5. Why is important to inspect companies transporting the food as well as those producing the food? SAMPLE ANSWER: A transportation company might transport non-organic food one week and organic the next. Traces of synthetic pesticides and fertilizers may be left in the truck from the non-organic food. These chemicals could then get on the organic food. Inspectors have to inspect everything that comes in contact with organic food before that food can be labeled as truly organic. History of Organic Food The skills used in this activity include information and media literacy, communication skills, critical thinking and systems thinking, creativity and intellectual curiosity, and interpersonal and collaborative skills. Work in a small group to find out about the history of organic food. Decide on a graphic to present to the class. For example, your group may want to make a graph to show how the amount of organic food produced in the U.S. has changed over the past few decades. Or, your group might choose to make a poster or a timeline that shows the evolution of what food is considered organic. Then, present your graphic to the class. Evaluate students work by whether the presentation uses current data and refers to current processes and regulations. Chapter 24 Workbook A Copyright by Pearson Education, Inc., or its affiliates. All Rights Reserved. 392