CHAPTER VII 



EFFECTS OF DIFFERENT GASES ON EXCITABILITY OF MIMOSA 



Induced change of excitability under sudden variation of light — 

 Abolition of excitability by absorption of water — Restoration 

 of excitability by application of glycerin — Stimulating, depressing, 

 and toxic agents — Phenomenon of accommodation — Stimulating 

 action of ozone — Effects of : carbonic-acid gas, vapour of 

 alcohol, ether, carbon disulphide, coal gas, chloroform, ammonia, 

 sulphuretted hydrogen, laughing-gas, nitrogen dioxide, and sulphur 

 dioxide. 



In order to investigate the effects of various gases in modi- 

 fying the excitabiUty of Mimosa, a series of responses, more 

 or less uniform, is first obtained under uniform stimuH, 

 at intervals of 15 minutes. The given gas is now introduced 

 into the plant-chamber, and another series of responses are 

 once more obtained by the action of the same stimuli as 

 before. The variation of amplitude of responses then 

 gives an indication of the excitatory or depressing action 

 of the agent. 



In carrying out the experimental investigation in this 

 manner, we proceed on the assumption that the stimuli 

 apphed are invariable, and that the external conditions are 

 maintained constant, with the sole exception of the change 

 induced by the introduction of the given gas. In order to 

 complete a single investigation a period of nearly two hours 

 is often necessary, which is the time required to take eight 

 responses at intervals of 15 minutes. Of these, the first two 

 give the normal responses, the next four the modified 

 responses under the influence of the gas, and the last two 



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