THE NERVOUS MECHANISM OF PLANTS 405 



but no definite regulation of any function shows any great 

 completeness. We have seen this particularly in the case 

 of the influence of temperature. Though a certain range 

 of temperature is imperative for the plant's well-being, it 

 has no power, or but little, to co-ordinate its own produc- 

 tion or expenditure of heat with the variations of tempera- 

 ture to which it is exposed. 



Neither anatomically nor physiologically do we find 

 much differentiation in the direction of such co-ordination. 

 The plant shows an almost complete absence of the 

 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 of 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 are 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 corresponds 

 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- 



