THE NERVOUS MECHANISM OF PLANTS 405 



differentiation which reaches its highest point in the nerve- 

 cell. There is apparently no co-ordinating mechanism 

 which receives the impulses from the sense-organs, and 

 initiates in consequence the resulting movement. One 

 case only has so far been put on record which even 

 suggests a complexity of this kind. Attention has been 

 called by Darwin to a peculiarity in the behaviour of the 

 tentacles ot Drosera, in which something of this nature is 

 seen. When one of these organs is stimulated, its actual 

 bending is preceded by a curious motility of the protoplasm 

 of the cells of its stalk which has been called aggregation. 

 If a tentacle on the surface of the leaf is excited, the 

 tentacles of the margin ar'e gradually inflected towards the 

 excited spot. If the cells of one of these marginal tentacles 

 are watched during the experiment, their contents are 

 found to undergo this aggregation, but those nearest its 

 apex manifest it first. If the aggregation were the direct 

 effect of the stimulus, those which it reached first, i.e. those 

 nearest the base of 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 corre- 

 sponds in a measure to 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 



