THE WORK OF STEMS 



275 



excluding it altogether. Can you now explain why 

 the tips of the stems press themselves to the wall! 



How do the main stem and branches behave when 

 they reach the top of a wall? How do they behave 

 when unable to attach themselves? 



The English Ivy climbs in the same manner as 

 the Boston Ivy, but attaches itself by roots ('Fig. 153) 

 instead of by tendrils. Study its behavior 

 carefully. 



The most interesting adaptations for 

 climbing are found in the twining plants. 

 The Morning-glory, Bindweed, Hop, 

 Scarlet Runner, Pole Bean (or String 

 Bean), Lima Bean or Sweet Potato may 

 be studied as examples of this class of 

 plants. How does the stem grow at first? 

 When does the tip begin to droop ? Does 

 it appear to swing around in a circle as 

 if seeking a support ? As you look down 

 on the stem from above, does the tip 

 move like the hands of a watch (clock- 

 wise) or in the opposite direction (counter- 

 clockwise)? How large is the circle de- 

 scribed by the tip ? How long does it require to make 

 a revolution ? (Does the temperature aft'ect the rate of 

 the revolution?) How does the stem behave when it 

 meets a support ? What is the size of the largest and the 

 smallest support it can twine about ? Is it more advan- 



153. Branch of 

 English Ivy. 



