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 three distinct parts : one by means 

 of which external stimulation is received and appreciated ; 

 another whereby movements, &c. are caused ; and a third 

 which is a 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 

 to 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. Each nerve-fibre ends in a 

 sense-organ or a motor mechanism, such as a muscle or a 

 gland-cell, and in a nerve-cell belonging to the co-ordinating 

 apparatus. 



We can easily recognise in plants certain structures 



