IRRITABILITY 203 



individual cells of the active organism. This property of proto- 

 plasm is called irritability. The possession of this property enables 

 an organism to make the adjustments necessary for living in cer- 

 tain environments. Some simple experiments with plants will 

 demonstrate tropisms. 



Problem. What response does a plant make to sunlight f 



Prepare a box with a series of shelves arranged alternately on opposite sides 

 so that they overlap each other. The shelves should be several inches apart 

 and extend into the box about three fourths of the distance. Cut a small 

 window in the side of the box near the top. Place a plant in a small flower 

 pot in the bottom of the box. Keep the plant well watered. Place the box 

 so that the window faces the sunlight and leave it for two or three weeks. 



I. A . Describe the growth of the plant, telling how it differs from the normal 



or usual method of growth of a plant. 



B. What was the value of the shelves in the experiment? 



C. The response of an organism to light is called phototropism (a turning 

 to light). Is the response of the shoot of the plant toward or away from 

 sunlight ? 



D. What is the value of phototropism to a plant? 



Problem. What is the response of different parts of a plant to 

 gravity f 



Prepare and fill a pocket garden or Petri dish with moist cotton, and place 

 mustard seeds in a row across the middle of it. Keep the same edge or side 

 of the garden up until the seedlings have grown to be three fourths of an inch. 

 After making definite observations, turn the garden to an angle of forty-five 

 degrees and keep in this position until the plants have grown another three 

 fourths of an inch. Then make a second examination. Repeat this three or 

 four times, in each case waiting the number of days required for three fourths 

 of an inch additional growth before making a definite conclusion and before 

 turning again. Be sure to keep the moisture evenly distributed throughout 

 the cotton. 



I. A. Describe the direction of growth of the root ; of the shoot. 



B. After your first turning what was the result? after the second, third, 

 and fourth ? This response of the plant to gravity is called geotropism. 



C. How does gravity affect the growth of the root and of the shoot ? 



D. What is the value of geotropism in a plant? 



