THE MANIFESTATIONS OF LIFE 



35 



Irritability was first recognized and is most easily 

 demonstrated and best known in its most exaggerated 

 forms, where the response to the stimulation appears 

 to be disproportionate to its intensity. 



This has led to the erroneous impression that mani- 



Fia. 3. Venus' Fly-trap (Dionaea muscipula). An insectivorous plant. 

 The traps for catching the insects are at the tips of the leaves and consist of 

 two valves with spinous edges. At the centre of each valve are several small 

 spines which act as triggers. When an insect disturbs several of these the trap 

 springs and it is caught and compressed between the valves. An enzymic se- 

 cretion is soon poured out by glands in the valves and the insect is slowly dis- 

 solved, its juices being utilized by the plant in its nutrition. (Kerner and Oliver.) 



festations of irritability imply an expenditure of energy 

 disproportionate to the force of the irritating agent. 

 The finger touching the trigger of a gun exerts a very 

 slight pressure, the force of which has no relation to the 

 amazing explosive force that follows it; a small effort 

 turns the throttle of a locomotive by which a heavy train 

 may be set in motion. 



