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 Dionea 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 anesthetics, such as the 
vapour of chloroform or ether. The effect of these drugs 
