408 VEGETABLE PHYSIOLOGY 



If, then, we adroit that there is even a rudimentary 

 nervous system in plants, we may proceed with an inquiry 

 into the degree of its differentiation, and the completeness 

 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 is a regulating and controlling part, and which can 



co-ordinate the responses to stimulation, or can initiate 



movements, &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 consisting 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 



one direction in a sense-organ or a motor mechanism, such 



