96 RESEARCHES ON IRRITABILITY OF PLANTS 



dioxide, on the other hand, is extremely poisonous. Intro- 

 duction of this gas was attended by an immediate excitatory 

 fall, which was repeated twice. After this the plant became 

 perfectly insensitive (fig. 55) ; the gas had in reality killed it. 



Sulphur Dioxide 



Equally fatal is the effect of sulphur dioxide. Intro- 

 duction of the gas was attended by an immediate excitatory 



F1G.55. — ^Toxic effect of 

 nitrogen dioxide : applica- 

 tion of gas at X induced 

 excitation, followed by loss 

 of excitability and death 

 of plant. 



Fig. 56. — Abolition of ex- 

 citability and death of 

 plant by the action of 

 sulphur dioxide. 



fall of the leaf, after which it became quite insensitive 

 (fig. 56). Restoration of fresh air did not revive the 

 plant, which succumbed completely to the toxic action of 

 the gas. 



bUMMARY 



There is in general a temporary depression of excitability 

 in Mimosa under sudden diminution of intensity of light. 

 Absorption of water induces a depression or abolition 



