THE NERVOUS MECHANISM OF PLANTS 399 
of the parallelism which it may be expected to show with the 
corresponding system in the animal kingdom. 
The latter, in the most completely organised beings, 
can be shown to possess certain distinct parts : one by means 
of which external stimulation is received and appreciated ; 
another whereby movements, &c. are caused; and a third 
which isa regulating and controlling part, and which can co- 
ordinate the responses to stimulation, or can initiate move- 
ments, &c. in its absence. There are also definite paths or 
channels by which the three are brought into connection with 
each other, generally by impulses passing along such paths 
in definite directions. In the higher animals these are well 
differentiated from each other; we have the sense-organs, 
each devoted to and fitted for the appreciation of particular 
stimuli. We have various motor mechanisms, usually con- 
sisting of muscles or glands which are thrown into activity 
in consequence of the reception of impulses by sense- 
organs. It may appear to be straining matters somewhat 
éo class these as part of the nervous system, but it does not 
appear wrong to do so in the sense that they are the means 
by which alone the working of the more particularly 
nervous elements of that system can be detected. The 
nervous and motor systems are indeed so closely connected 
that for the purposes of this discussion no inconvenience 
will result from classing them together. In the animal 
we have nerve-cells occurring singly or in groups, forming 
very large aggregations such as the brain, or smaller ones, 
the nerve ganglia. All such aggregations, or even single 
cells, are concerned in the task of co-ordinating stimuli and 
responses, or regulating the general life of the organism. 
Lastly, we have well-differentiated nerves which serve as 
the means of communicating between the three other 
factors already mentioned. Hach nerve-fibre ends in one 
direction in a sense-organ or a motor mechanism, such as 
a muscle or a gland-cell, and in the other in a nerve-cell 
belonging to the co-ordinating apparatus. 
We can easily recognise in plants certain structures 
