276 EXPEBIMENTS WITH PLANTS 



tageous for it to twine up large stems or small ones (re- 

 member that it must, in order to thrive, get its leaves 

 above those of the plant on which it twines)? Does 

 it twine best on a vertical or on an inclined support ? 

 Can you see any advantage in this? Place the sup- 

 port on which it is twining in a horizontal position; 

 how does the plant behave? Does this make it ap- 

 pear as though gravity were a cause of the twining ? 

 Reverse a vertical support on which the plant has 

 made several turns, so that the tip of the plant points 

 downward ; how does the plant behave ? (For this 

 experiment potted plants, with upright sticks set in 

 the soil as supports, are very convenient, since it is 

 only necessary to incline the pot or turn it upside 

 down.) Does light affect the twining? Keep the 

 plants in darkness, and observe the result. 



Are the turns of the stem equally steep at the tip 

 and at the base ? Does this arrangement help to hold 

 the plant in place by causing it to hug the support? 

 Can the plant twine as well on a smooth support 

 (e. g., a glass rod) as on a rough one (a branch or 

 twig) ? How does the plant behave when it reaches 

 the top of the support? 



What causes the plant to twine? The best answer 

 we can give at present to this question is that the 

 twining is due to the influence of gravity, which stimu- 

 lates one side to grow more rapidly than the other. 

 When the tip points north it is the west side which is 



