414 VEGETABLE PHYSIOLOGY 
The co-ordination of these factors we have seen is one 
of the most marked features of a highly differentiated 
nervous system. In this respect we cannot note anything 
in the plant which in its elaboration or in its peculiar 
efficiency can be compared with the co-ordinating mechanism 
of animals. Certain responses to stimulation can be effected, 
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. 
Nor can we observe any structural differentiation in the 
direction of such co-ordination, though very complex move- 
ments in some cases point to its existence. We may 
instance the assumption of the nyctitropic position by the 
leaves of Nicotiana and of certain Leguminose. 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 conse- 
quence the resultmg 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 im 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. 
Ii the aggregation were the direct effect of the stimulus, 
those which it reached first, 7.e. those nearest the base of 
