THE LEAVES 73 



The full effect of each factor may be seen if the 

 arrangement of the leaves of the ivy or some plant 

 which clings closely to a wall is examined. Here 

 each leaf is of such form, size, and length of stalk 

 that it does not seriously overlap or shut the light 

 from any of its neighbors. 



90. Getting in the proper position. — In addition 

 to alterations in the form and size of the stalks and 

 blades in order to get better exposure to light, the 

 protoplasm has the power of moving the petiole and 

 blade in such manner that it will receive the light in 

 the best manner, or the one suited to its capacity. 

 This is done by twisting or bending the stalk or peti- 

 ole, or by bending the stem to which it is attached. 



91. Heliotropic movements. — The action of the 

 leaf or stem for placing the blades in the proper posi- 

 tion for receiving light in response to the sensitive- 

 ness of the plant to light is termed heliotropism. 

 This may be observed in house plants grown by a 

 window. Here all of the leaves have moved in such 

 manner as to make their blades face the window. 

 If such a plant is turned halfway round so that the 

 leaves face away from the window, they will quickly 

 regain their former position. Perform this experi- 



