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CHAPTER 27 C ONTROL OF G ROWTH AND R ESPONSES IN P LANTS Chapter Outline 27.1 Plant Responses A. Organisms Respond to Stimuli 1. One defining characteristic of life is an ability to respond to stimuli. 2. Adaptive organisms respond to environmental stimuli because it leads to longevity and survival of the species. 3. Animals have nerves and muscles; plants respond by growth patterns. B. Tropisms 1. A tropism is plant growth toward or away from a directional stimulus. a. The stimulus comes from only one direction instead of many. b. Growth toward a stimulus is a positive tropism ; growth away from a stimulus is a negative tropism . c. By differential growth, one side elongates faster; the result is a curving toward or away from a stimulus. 2. Three well-known tropisms are named for the stimulus that causes the response. a. Phototropism is growth of plants in response to light; stems show positive phototropism. b. Gravitropism is response to earth’s gravity; roots demonstrate positive gravitropism and stems demonstrate negative gravitropism. c. Thigmotropism is unequal growth due to touch (e.g., coiling of tendrils around a pole). 3. Response to a stimulus first involves reception of the stimulus. 4. The next step is transduction of the stimulus into a form meaningful to the organism. 5. Finally, there is a response by the organism. C. Phototropism 1. Early researchers, including Charles Darwin and his son Francis, observed plants curve toward light. 2. Phototropism occurs because cells on shady side of stems elongate. 3. It is believed that a yellow pigment related to riboflavin acts as a photoreceptor for light. a. Following reception, the plant hormone auxin migrates from the bright side to the shady side of a stem. b. How reception of stimulus couples to production of auxin is not yet known. 4. Auxin is also involved in gravitropism, apical dominance, and root and seed development. D. Gravitropism 1. An upright plant placed on its side displays negative gravitropism ; it grows upward opposite gravity. 2. Charles Darwin and his son were first to show that roots display positive gravitropism . a. If the root cap is removed, roots no longer respond to gravity. b. Later researchers showed root cap cells contain statoliths , starch grains within amyloplasts. c. Due to gravity, amyloplasts settle to the lowest part of the cell. 3. The hormone auxin underlies both positive and negative gravitropisms. a. The two tissues respond differently to auxin, which moves to the lower side of both stems and root. b. Auxin inhibits the growth of root cells; cells of the upper surface elongate and the root curves downward. c. Auxin stimulates the growth of stem cells; cells of the lower surface elongate and the stem curves upward. E. Thigmotropism
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This note was uploaded on 10/24/2009 for the course BIO 172 taught by Professor Clark during the Fall '08 term at University of Michigan.

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