408 VEGETABLE PHYSIOLOGY 



removal of the stimulus which has originated it a so- 

 called after-effect may be seen by allowing a stimulus 

 to operate for some time and then reversing its direction. 

 This can be done by fastening a root horizontally in a damp 

 atmosphere and, as soon as the curvature commences, 

 inverting it so that the side showing the slight convexity 

 is downwards. The curvature will continue in the original 

 direction for some time and will only slowly cease and be 

 replaced by one in the opposite direction. 



We can distinguish between the general condition of 

 irritability or the state of tone, and these special forms of 

 sensitiveness which we have examined. So long as the 

 conditions remain favourable the general sensitiveness of 

 the plant is maintained, but the power of responding to 

 particular impressions may disappear from various causes 

 without any disturbance of its sensibility to others. The 

 power of appreciating differences in the environment varies 

 with the age of the plant, disappearing in some cases from 

 an organ while it still retains its power of circumnutating. 

 The effect of a prolonged stimulation is sometimes failure 

 to induce a movement. In the case of Dioncea this is 

 very marked. If a leaf is for a time mechanically pre- 

 vented from closing, repeated touching of one of the sensi- 

 tive hairs brings about an exhaustion of its power to receive 

 a stimulus, so that if the leaf is released a disturbance 

 of that particular hair evokes no response. At first it may 

 seem doubtful whether or no the interference with the 

 free response of the leaf may have so injured the motor 

 mechanism as to make it incapable of acting. The exhaus- 

 tion, however, is shown to be that of the hair and not of 

 the blade by the fact that touching another of the hairs at 

 once causes closure. 



The nervous sensitiveness is shown by this and many 

 other similar experiments to be capable of fatigue. A 

 similar suspension of power may be demonstrated by 

 exposing the sensitive parts to anaesthetics, such as the 

 vapour of chloroform or ether. The effect of these drugs 



