THE NERVOUS MECHANISM OF PLANTS 415 
the tentacle, would respond first. The stimulus, apparently, 
has to travel up the gland, and a disturbance has to originate 
at its apex in response, this disturbance travelling down the 
tentacle in the direction of its base. Darwin has pointed 
out that this recalls in a measure the reflex action of the 
animal organism. 
But though this co-ordinating power is very feebly 
developed we cannot deny that there is a power or property 
of protoplasm which represents it, even if in only rudi- 
mentary form. We have already alluded to the purposeful 
character of the responses to stimulation. There must be 
some means by which an appreciation of the character of 
the stimulus is communicated to the protoplasm, which 
suggests a certain possibility of perception which must be 
the antecedent of co-ordination. We do not know whether 
the fact that the response is localised depends upon the 
possession of particular properties by the responding organ, 
so that while the impulses set up in the sense-organ travel 
in all directions through the plant, only certain cells can 
be excited to change in response to them, or whether the 
paths of the conduction of the impulses only take them 
to the responding organ. But the fact remains that the 
response bears a definite relationship to the stimulus, par- 
ticularly to its locality, and m a less degree perhaps to its 
intensity. If a root-tip is brought into contact with an 
obstacle, the bending is invariably in such a direction as 
to enable the root to pass it. When one is allowed to 
impinge upon a small stone at right angles to its direction 
of growth, the curvature continues till the root has turned 
through a right angle, and can for a short distance, at 
any rate, grow parallel to the opposing surface, till, passing 
it, it can again respond to the influence of gravitation 
and grow vertically downwards. The stimulus causing the 
movement of hydrotropism serves to bring the root-hairs 
into contact with the moist surface, thus enabling them to 
discharge their appropriate function. 
The behaviour of the tentacles of Drosera rotundifolia 
