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 hag 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 Dionwa 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 
27 
