PROBLEM 4. H01V Plants Respond to Stimuli ^07 



Questions 



1. Define tropisni. What is meant by negative and positive phototropism? 

 Give an example of an animal tropism. 



2. Name other kinds of plant tropisms and the stimulus in each case. 



3. Name seven environmental factors which may cause bending, turn- 

 ing, or twisting responses in plants. Name a response which is not a 

 tropism. 



4. Why are plant responses slower than animal responses? 



5. How does light affect the location of the growth hormones in stems? 

 How does gravity affect its distribution in stems? Explain how un- 

 equal growth causes bending of a stem or root. 



6. What are some of the uses of auxin? 



7. What else besides unequal growth enables parts of plants to turn or 

 bend? Give an example. Define turgid. 



8. Name three insect-eating plants. Explain how two of them catch 

 small insects. What becomes of the insects? 



9. Why is it that both animals and plants respond to their environments 

 Sum up the differences between plant and animal behavior. 



Exercises 



1. To demonstrate bending toward the light, positive phototropism. 

 Obtain two similar small boxes with sliding or hinged covers. Stand the 

 boxes on end with the door facing you. Cut a window in the top of one. 

 In the other, cut a window of the same size on one side about halfway 

 up. Place moist sawdust at the bottom of each box. Plant some young 

 pea seedlings in each. Close the cover and permit the boxes to stand in 

 a warm room. Keep seedlings moist. Observe after some days. Record 

 your observations, and explain. 



2. How do roots respond to light? Cover a tumbler of water with 

 gauze. Suspend several mustard seedlings with straight roots from one 

 to two inches long on the gauze so that the roots are wholly in water. 

 Place a wooden box with a narrow window along one side over the 

 tumbler. Examine after some days. Describe. Does this experiment need 

 a control? Why or why not? What kind of phototropism is shown by 

 mustard roots? 



3. How can you show the positive geotropism of roots and the nega- 

 tive geotropism of stems? Line a jar with heavy, wet blotting paper, 

 making sure that the light is excluded. Cut a sheet of cork so that it can 

 stand up in the jar. Cover the cork with wet blotting paper. Pin pea seed- 

 lings to the cork, making sure that the pins go through the cotyledons 

 and that the peas do not touch the cork. Pin the seedlings so that the 

 tiny roots and stems point in several different directions. Cover the jar 

 with a sheet of glass, lined with wet blotting paper. Keep the blotting 



