138 PLANT RESPONSE 



power of responsive contraction. Others produce the 

 opposite effect, thus protracting the natural period of 

 recovery. 



H3'drogen gas produces a gradual diminution of response, 

 which is restored to its original value on the readmission 

 of air. 



Carbonic acid causes depression, but the restoration to 

 normal conditions is generally followed by temporary exalta- 

 tion above the normal. 



Vapour of alcohol causes gradual, and solution of alcohol 

 rapid, depression. This action may be preceded by tem- 

 porary exaltation. The recovery-period is very much pro- 

 tracted. 



As in the animal, so also in the plant-tissue, ether causes 

 marked relaxation. The depression of response increases 

 progressively with the exposure ; on blowing off the vapour, 

 response is not only restored but may even show an exalta- 

 tion. The opening of the Mimosa leaflets under ether is not 

 indicative of true recovery but of over-relaxation. 



A poisonous reagent, like hydrochloric acid or chlorine, 

 permanently abolishes the response. 



Reagents which individually abolish response may, by 

 their antagonistic character, act as antidotes to one another. 



