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, ¢.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- 
