GENERAL SURVEY 347 



Under extreme sub-tonicity the normal negative response 

 may even be converted into abnormal positive. 



The anomalous erection, after a preliminary fall of the 

 leaf of Mimosa under continuous stimulation, is explicable 

 on the common characteristics of response in plant and 

 animal tissue. In both contraction is reversed to relaxa- 

 tion under fatigue (p. 84). 



Variation of Motile Excitability under 

 Changes of External Condition 



The motile excitability of Mimosa undergoes abolition 

 under sudden change from light to darkness. After a 

 certain time the plant accommodates itself to the changed 

 condition, with the restoration of normal excitability. 



There is again a variation of motile excitability depending 

 on the time of the day. The motility is at its maximum at 

 a certain hour, and minimum at a different hour. This 

 peculiarity is probably connected with the question of 

 variation of turgor. The excess of turgor in the pulvinus 

 caused by absorption of water is attended by a reduction 

 or abolition of motile response. The lost motility may, 

 however, be restored by the application of glycerin. 



Very characteristic are the effects exerted by the different 

 gases. Some induce a stimulating action, others give rise 

 to depressing or toxic effects. Ozone enhances the excita- 

 bility. Carbonic-acid gas and vapour of alcohol induce a 

 moderate depression of excitability from which the plant 

 recovers on readmission of fresh air. Coal gas and vapour 

 of carbon disulphide also induce a depressing effect. The 

 vapour of ether exerts a moderate narcotic action. The 

 effect of chloroform is far more pronounced, the loss of 

 excitability under its action being more complete and 

 persistent. Ammonia has marked effect in the abolition 

 of excitability. Sulphuretted hydrogen, nitrogen dioxide, 

 and sulphur dioxide are very toxic in their action ; their 



