406 VEGETABLE PHYSIOLOGY 
mentary form. We have already alluded to the purposeful 
character of the responses to stimulation. There must be 
gome 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 to a less degree perhaps to its 
intensity. Ifa 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 
is very interesting in this connection. The leaf is of some 
size, and can therefore receive stimuli over a fairly large 
area. When the tentacles bend over in response to the 
alighting of an insect, they do not do so irregularly, but 
always place their glandular apices directly upon the 
spot which is the centre of the disturbance. This is very 
definitely purposeful, the invader being captured and 
digested wherever it alights, as all the tentacles are brought 
-to bear upon it. 
