3o8 Why Living TJmigs Behave As They Do unit v 



paper moist by adding water when needed. Examine the seedhngs regu- 

 larly for several days. What happens to the roots? to the stems? Is there 

 a control? Can any other stimuli have affected these plants? How can the 

 results be explained? 



4. Can you suggest the method of an experiment to find out whether 

 or not roots have positive hydrotropism? 



5. How does the tulip blossom respond to changes in temperature? 

 Place a tulip blossom standing in a glass of water in an ice box. After 

 half an hour or more place it in a warm room. What do you see? In what 

 respects is the method of this experiment faulty? Why can you not draw 

 conclusions from what you have seen? What should you do to make this 

 experiment valid? 



6. Working with synthetic growth hormones. Use young tomato plants 

 or any other young rapidly growing plants. Indol-butyric acid, indol-acetic 

 acid, or naphthalene-acetic acid are growth substances which may be used. 

 They may be obtained from biological supply houses. Prepare a mixture 

 of about 0.1-0.25 gi"ams of hormone to 50 grams of lanolin. Separate the 

 plants into two groups, one group to be the control. To one group apply 

 small amounts of the lanolin mixture to a small area on the side of the 

 stem facing the light. Watch both groups of plants for a week or more. 

 What do you observe? When you write up your notes describe exactly 

 what you did and what happened; diagrams may help. What have you 

 learned about the action of these synthetic growth hormones? 



7. Cut some slices of raw potato about one eighth of an inch in thick- 

 ness. Allow them to lie exposed to the air for two hours and feel them. 

 Explain. Draw a cell as it must look in the potato slices now. Draw the 

 same cell as it must have looked when you started the experiment. Why 

 do stems and leaves of some plants need little supporting tissue to keep up- 

 right? 



Further Activities in Biology 



1. Do plants respond to artificial light as they do to sunlight? Perform 

 an experiment to find out. 



2. By filtering out some of the rays of light you could find out whether 

 a plant is more sensitive to some rays than to others. 



3. Time the speed with which a sensitive plant responds to a stimulus. 

 Use a watch with a second hand. Gently stroke the lower midrib of one 

 leaf and watch closely for the first sign of response in that leaf. How 

 long does it take before some other leaf responds? Measure the distance 

 the message traveled in that time. 



4. The study of the effect of growth hormones on plant cells has had 

 important practical consequences. Commercial preparations of 2-4-D can 

 be obtained in local hardware stores. Spray the preparation on the weedy 

 area of a lawn (follow directions). Describe the effects of the chemical. 



