U6 



PLANT RESPONSE 



opposite of contraction, and which therefore brings about a 

 position contrary to that effected by stimulus. 



We have seen that ether produces a relaxing effect which 

 is more rapid than the process of relaxation that brings about 

 recovery. And we have seen how, in consequence of this 

 excessive relaxation, excitatory contraction is diminished or 

 abolished. Hence, we see how a stimulated Mimosa leaflet 

 under ether relaxes into an outspread position, which never- 

 theless is indicative of no re- 

 newed sensitiveness such as ac- 

 companies true recovery. 



Effect of vapour of hydro- 

 chloric acid. — I shall now deal 

 with the case of strongly 

 poisonous agents, of which 

 hydrochloric acid may be taken 

 as typical. On passing the 

 vapour of hydrochloric acid into 

 the plant chamber there was 

 produced a great relaxation, 

 and the responses underwent a 

 rapid diminution which ended 

 in abolition. The effect of this 

 poison is so persistent that 

 the blowing-in of fresh air did 

 nothing to revive the response. But the timely application 

 of vapour of ammonia is found to act as an antidote, restoring 

 the response Cfig. 73). 



Effect of chlorine gas. — This gas also produces a marked 

 depression of excitability, which, under long-continued action, 

 brings about the permanent abolition of response. The 

 accompanying photographic record (fig. 74) shows the effect 

 very clearly. The normal responses to the left are seen to 

 be very rapidly diminished after the application of this gas, 

 the response being reduced to one-eighth of its original value 

 in the course of nine minutes. There are other important 

 considerations in connection with this question, of relaxation 



FIG. 73. Effect of HC1 Vapour 



Arrow indicates moment of ap- 

 plication. Depressing effect 

 neutralised by antagonistic 

 action of NH, at x . 



