312 LIFE MOVEMENTS IN PLANTS 



Rise of temperature induces greater expansion, and 

 enhancement of rate of growth of the more excitable lialf 

 of the organ ; lowering of temperature induces the oppo- 

 site effect. 



Two types of thermonastic movements are met with, 

 the positive exhibiting a movement of opening during rise 

 of temperature ; in these the inner half of the organ is 

 relatively the more excitable. Example of this is seen in 

 the Crocus and in Zephyranthes. 



In the negative type, rise of temperature induces a 

 movement of closure. Here the outer half of the organ 

 is the more excitable. The flower of Nymphcea belongs 

 to this type. 



The effect of stimulus is antagonistic to that of rise 

 of temperature. In positive thermonastic organs stimulus 

 induces a movement of closure ; in the negative type it 

 induces a movement of opening. 



