THE NERVOUS MECHANISM OF PLANTS 417 



which was downwards during the first exposure. The same 

 curvature will be seen if they are placed in a vertical posi- 

 tion after the amputation. The long delay in the response 

 may no doubt be attributed partly to the disturbance set 

 up by the amputation ; but the fact that the response to the 

 stimulus does eventually take place shows that the delay 

 is due to slowness of changes in the responding protoplasm 

 and not in the part which is sensitive. 



An even more striking instance of action after the 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 sensitivity 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 responce of the leaf may have so injured the motor 



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