THE NEKVOUS 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 in 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 



