EFFECTS OF DIFFERENT GASES 89 



gations where it is required to make an electrolytic contact 

 with the pulvinus without inducing any change in its motile 

 excitability. 



I shall now proceed to describe the effects of various 

 gases and vapours on the excitability of Mimosa. The 

 plant is enclosed in a small glass chamber, the different 

 gases being made to stream in and out through entrance 

 and exit tubes. The various effects induced may be classi- 

 fied as (1) stimulating, (2) depressing, and (3) toxic. The 

 exaltation of excitability induced by stimulating agents 

 is exhibited by the enhancement of amplitude of response. 

 The effect of depressing agents is seen in the diminution of 

 amplitude of response ; in this class may be included agents 

 which have slight narcotic action. In all these cases the 

 removal of the gas is attended by the restoration of normal 

 excitability of the plant. A curious fact noticeable in this 

 connection is the phenomenon of accommodation. Under 

 the action of a slightly depressing agent, there is induced a 

 diminution of excitability. But the plant may accommo- 

 date itself to the change, in consequence of which the 

 excitability is more or less restored to the original condition. 

 It should also be borne in mind that the character of the 

 reaction is modified to a certain extent by the tonic con- 

 dition of the plant, a plant in a vigorous condition being 

 better able to withstand unfavourable circumstances than 

 one in a weak condition. 



Lastly, there are gaseous agents which are toxic in 

 their action ; their application is attended by rapid loss of 

 excitability and death of the plant. I will now describe 

 in detail the effects of various gases, beginning with those 

 which stimulate and ending with others which cause the 

 death of the plant. 



Ozone 



The stimulating effect of this gas is clearly seen in 

 fig. 46. The particular leaf, before the application of 



