112 THE NATURE AND WOUK OF PLANTS 



152. Nodding or circular movements due to un- 

 equal growth. — Select a vigorous specimen of the 

 hop, bean, or morning-glory, and tie all of it to an 

 upright stake except the tip of the stem a foot in 

 length. Now set a second thin stake in the ground 

 so that its top is just below the tip of the stem. 

 Note the position of the tip an hour later. If it has 

 moved, set up another stake. Repeat this process 

 until the tip has moved around in a complete circle. 

 What was the direction of the movement ? What 

 length of time was necessary to complete the circle ? 

 This circular movement of the tip is due to the 

 fact that one side of the stem grows faster than 

 the other sides for a short time, then it slows down 

 and a region next to it grows most rapidly, and 

 so on around the stem. This results in tilting 

 the tip toward every point of the compass in suc- 

 cession. 



A nodding movement, due to the alternate growth 

 of two sides of a flattened organ, may be seen if the 

 growing leaves of narcissus are observed in this 

 manner. 



153. Length of life. — The length of time a 

 single specimen of a plant may live varies enor- 



