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Name ___________________________________ Date _____________ Class ______________ Plant Responses and Growth Key Concept Summaries What Are Three Stimuli That Produce Plant Responses? Plants respond to stimuli by growing toward or away from the stimulus, a response called a tropism. Growth toward a stimulus is a positive tropism; growth away is a negative tropism. Touch, gravity, and light are three important stimuli that trigger growth responses, or tropisms, in plants. A plant’s response to touch is called thigmotropism. A vine that curls around an object it touches shows a positive thigmotropism. Geotropism is a plant’s response to the force of gravity. As they grow downward, roots show a positive geotropism. Stems growing up and against the force of gravity show a negative geotropism. Phototropism is a plant’s response to light. Leaves, stems, and flowers that grow toward light show a positive phototropism. Plants respond to stimuli by producing hormones, chemicals that affect how a plant grows and develops. Auxin is a hormone that speeds up the rate at which a plant’s cells grow and controls a plant’s response to light. As auxin builds up on the shaded side of a stem, cells on that side grow faster and bend the stem toward the light. How Do Plants Respond to Seasonal Changes? Plants respond to the changing seasons because the amount of light they receive changes. The amount of darkness a plant receives determines the time of flowering in many plants. A plant’s response to seasonal changes in the length of night and day is called photoperiodism. Some plants will only bloom when the night lasts a certain length of time. Plants can be grouped according to critical night length, or the number of hours of darkness that determines whether or not a plant will flower. Short-day plants flower when nights are longer than a critical length. They bloom in fall or winter. Long-day plants flower when nights are shorter than a critical length. They bloom in spring and summer. Day-neutral plants have a flowering cycle that is not sensitive to periods of light and dark. They can bloom year-round, depending on weather. Some plants prepare for winter by going into a state of dormancy, or a period when growth or activity stops. Dormancy helps plants survive freezing temperatures and the lack of liquid water. Cooler weather and shorter days may trigger a plant to prepare to become dormant. Name ___________________________________ Date _____________ Class ______________ Plant Responses and Growth Review and Reinforce Understanding Main Ideas Answer the following questions in your notebook. 1. List three stimuli that produce responses in plants and give the name for each response. 2. What is geotropism? How can a plant show both a positive and a negative geotropism? 3. What is photoperiodism? What is its effect in many plants? 4. What happens when a plant enters a state of dormancy? Building Vocabulary Match each term with its definition by writing the letter of the correct definition in the right column on the line beside the term in the left column. 5. _____ critical night length 6. _____ auxin 7. _____ short-day plant 8. _____ long-day plant 9. _____ hormone 10. _____ day-neutral plant 11. _____ tropism a. a hormone that controls a plant’s response to light b. a plant that flowers when the nights are shorter than a critical length c. a plant’s growth response toward or away from a stimulus d. a chemical that affects the growth and development of a plant e. a plant whose flowering cycle is not sensitive to periods of light and dark f. a plant that flowers when the nights are longer than a critical length g. the number of hours of darkness that determines whether or not a plant will flower Write About It 12. In your notebook, explain three different types of response a plant may have to light—its direction or the length of time it lasts.