1. End Show Slide 1 of 32 Copyright Pearson Prentice Hall Biology

2. End Show Slide 2 of 32 Copyright Pearson Prentice Hall 25–2 Plant Responses

3. End Show 25-2 Plant Responses Slide 3 of 32 Copyright Pearson Prentice Hall Tropisms What are plant tropisms?

4. End Show 25-2 Plant Responses Slide 4 of 32 Copyright Pearson Prentice Hall Tropisms Plants change their patterns and directions of growth in response to a multitude of cues. The responses of plants to external stimuli are called tropisms.

5. End Show 25-2 Plant Responses Slide 5 of 32 Copyright Pearson Prentice Hall Tropisms Plant tropisms include: gravitropism, phototropism, and thigmotropism. Each of these responses demonstrates the ability of plants to respond effectively to external stimuli, such as gravity, light, and touch.

6. End Show 25-2 Plant Responses Slide 6 of 32 Copyright Pearson Prentice Hall Tropisms Gravitropism Gravitropism, the response of a plant to gravity, is controlled by auxins. Gravitropism causes the shoot of a germinating seed to grow out of the soil—against the force of gravity. It also causes the roots of a plant to grow with the force of gravity and into the soil.

7. End Show 25-2 Plant Responses Slide 7 of 32 Copyright Pearson Prentice Hall Tropisms Phototropism Phototropism, the response of a plant to light, is also controlled by auxins. This response can be so quick that young seedlings reorient themselves in a matter of hours.

8. End Show 25-2 Plant Responses Slide 8 of 32 Copyright Pearson Prentice Hall Tropisms Thigmotropism Thigmotropism is the response of plants to touch. An example of thigmotropism is the growth of vines and climbing plants.

9. End Show 25-2 Plant Responses Slide 9 of 32 Copyright Pearson Prentice Hall Tropisms The stems of these plants do not grow straight up. The growing tip of each stem points sideways and twists in circles as the shoot grows. When the tip encounters an object, it quickly wraps around it.

10. End Show 25-2 Plant Responses Slide 10 of 32 Copyright Pearson Prentice Hall Tropisms Some climbing plants have long, twisting leaf tips or petioles that wrap tightly around small objects. Other plants, such as grapes, have extra growths called tendrils that emerge near the base of the leaf and wrap tightly around any object they encounter.

11. End Show 25-2 Plant Responses Slide 11 of 32 Copyright Pearson Prentice Hall Rapid Responses Not all plant responses involve growth. One example is the rapid closing of leaflets that occurs in the Mimosa pudica. If you touch the leaves of a mimosa plant, within seconds, the leaves snap shut.

12. End Show 25-2 Plant Responses Slide 12 of 32 Copyright Pearson Prentice Hall Rapid Responses The secret to this movement is changes in osmotic pressure. The leaves are held apart due to osmotic pressure where the two leaflets join. When the leaf is touched, cells near the center of the leaflet pump out ions and lose water due to osmosis. Pressure from cells on the underside of the leaf, which do not lose water, forces the leaflets together.

13. End Show 25-2 Plant Responses Slide 13 of 32 Copyright Pearson Prentice Hall Photoperiodism What is photoperiodism?

14. End Show 25-2 Plant Responses Slide 14 of 32 Copyright Pearson Prentice Hall Photoperiodism Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants. Spinach and irises flower when days are long and are therefore known as long-day plants. Photoperiodism is the response to periods of light and darkness.

15. End Show 25-2 Plant Responses Slide 15 of 32 Copyright Pearson Prentice Hall Photoperiodism Photoperiodism in plants is responsible for the timing of seasonal activities such as flowering and growth.

16. End Show 25-2 Plant Responses Slide 16 of 32 Copyright Pearson Prentice Hall Photoperiodism

17. End Show 25-2 Plant Responses Slide 17 of 32 Copyright Pearson Prentice Hall Photoperiodism It was later discovered that a plant pigment called phytochrome is responsible for photoperiodism. Phytochrome absorbs red light and activates a number of signaling pathways within plant cells. Plants respond to regular changes in these pathways and these changes determine the patterns of a variety of plant responses.

18. End Show 25-2 Plant Responses Slide 18 of 32 Copyright Pearson Prentice Hall Winter Dormancy Phytochrome also regulates the changes in activity that prepares many plants for dormancy as winter approaches. Dormancy is the period during which an organism's growth and activity decreases or stops.

19. End Show 25-2 Plant Responses Slide 19 of 32 Copyright Pearson Prentice Hall Winter Dormancy How do deciduous plants prepare for winter?

20. End Show 25-2 Plant Responses Slide 20 of 32 Copyright Pearson Prentice Hall Winter Dormancy As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport materials from leaves to roots, and seal leaves off from the rest of the plant.

21. End Show 25-2 Plant Responses Slide 21 of 32 Copyright Pearson Prentice Hall Winter Dormancy Leaf Abscission At summer’s end, the phytochrome in leaves absorbs less light as days shorten and nights become longer. Auxin production drops, but the production of ethylene increases.

22. End Show 25-2 Plant Responses Slide 22 of 32 Copyright Pearson Prentice Hall Winter Dormancy The change in the relative amounts of auxin and ethylene hormones starts a series of events that gradually shut down the leaf. First chlorophyll synthesis stops. Light destroys the remaining green pigment. Other pigments—including yellow and orange carotenoids—become visible for the first time.

23. End Show 25-2 Plant Responses Slide 23 of 32 Copyright Pearson Prentice Hall Winter Dormancy Production of new plant pigments—the reddish anthocyanins—begins in the autumn. Every available carbohydrate is transported out of the leaf, and much of the leaf’s water is extracted.

24. End Show 25-2 Plant Responses Slide 24 of 32 Copyright Pearson Prentice Hall Winter Dormancy Finally, an abscission layer of cells at the petiole seals the leaf off from the plant’s vascular system. Before long, the leaf falls to the ground, a sign that the tree is fully prepared for winter.

25. End Show 25-2 Plant Responses Slide 25 of 32 Copyright Pearson Prentice Hall Winter Dormancy Overwintering of Meristems Hormones also produce important changes in apical meristems. Instead of continuing to produce leaves, meristems produce thick, waxy scales that form a protective layer around new leaf buds. Enclosed in its coat of scales, a terminal bud can survive the coldest winter days.

26. End Show 25-2 Plant Responses Slide 26 of 32 Copyright Pearson Prentice Hall Winter Dormancy At the onset of winter, xylem and phloem tissues pump themselves full of ions and organic compounds. These molecules act like antifreeze in a car, preventing the tree’s sap from freezing, thus making it possible to survive the bitter cold.

27. End Show - or - Continue to: Click to Launch: Slide 27 of 32 Copyright Pearson Prentice Hall 25–2

28. End Show Slide 28 of 32 Copyright Pearson Prentice Hall 25–2 The rapid responses of plants such as the Venus flytrap are caused by changes in a.light levels. b.osmotic pressure. c.temperature. d.moisture.

29. End Show Slide 29 of 32 Copyright Pearson Prentice Hall 25–2 The timing of seasonal activities in plants, such as flowering and growth, depends on a.gravitropism. b.thigmotropism. c.photoperiodism. d.phototropism.

30. End Show Slide 30 of 32 Copyright Pearson Prentice Hall 25–2 The hormone primarily responsible for causing plants to flower depending on the length of the day is a.auxin. b.phytochrome. c.cytokinin. d.ethylene.

31. End Show Slide 31 of 32 Copyright Pearson Prentice Hall 25–2 In most flowering plants, the coming of autumn is marked by a.a decrease in ethylene production. b.an increase in auxin production. c.the production of new plant pigments. d.a rapid growth spurt.

32. End Show Slide 32 of 32 Copyright Pearson Prentice Hall 25–2 In deciduous plants, the process of dormancy involves a.maintaining photosynthesis and storing material in the roots. b.turning photosynthesis off and storing material in the roots. c.maintaining photosynthesis and bringing material up from the roots. d.turning photosynthesis off and bringing material up from the roots.

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