SUMMARY. 127 



drawn together and their main petioles will suddenly bend 

 downward. 



If the same plant be observed during the night, it will be 

 seen that its leaves take just the same form as in the case 

 when touched by a foreign body. 



This sensitiveness to the action of various external 

 stimuli is known as Irritability. 



1. THE IRRITABILITY OF YOUNG ORGANS. 



The organs of plants are most sensitive in their elongat- 

 ing stage to various external stimuli. Thus, young tendrils 

 are soon caused to curve by a slight touch ; and young stems 

 may soon incline towards sunshine 



2. THE IRRITABILITY OF MATURE ORGANS. 



Fully-developed organs are. mostly much weakened in 

 their irritability, but a few organs of some plants can per- 

 form movements even after they are fully matured. Thus, 

 the leaves of the Nemurigusa change their position day and 

 night, and also when touched; the stamens of the Megi 

 move quickly when they are touched ; and the hair-like ap- 

 pendages on the leaves of the Mosengoke curve inwards 

 when touched by a foreign body. 



SUMMARY. 



Nutrition is a', general terni for all the processes of absorption, respira- 

 tion, transpiration, assimilation, and metabolism. 



Transpiration is a process of evaporating water from the surface of the 

 plant-body. 



Respiration is a process of absorbing oxygen and evolving carbonic acid. 



Absorption is a process of diffusing water and substances in solution 

 through the cell-walls of the epidermis into the interior of the plant-body. 



Assimilation is a process in which starch grains are formod in chloro- 

 phyll-corpuscles. 



Metabolism is a general name for all the processes accompanying the 

 construction and the destruction of various tissues. 



