THE NATURE OF IRRITABILITY 15 



production of some definite result. The process of stimulation involves 

 a series of changes due to interaction between the stimulus applied to an 

 organism, and the percipient part affected. A careful distinction must be 

 made between the reception of the stimulus, the series of changes which 

 then take place, and the final result they produce, regardless of whether 

 we are or are not able at present to directly investigate them. In many 

 organs the sensitive region and the responsive tissue are some distance 

 apart ; thus the root-tip perceives the stimulus which induces a geotropic or 

 hydrotropic curvature in the regions some distance behind it, while in 

 Drosera the stalk of the tentacle curves, but only the head is sensitive 

 to contact. In all such cases the stimulus must be transmitted, and this 

 transmission of stimuli is the means- by which different parts of the plant 

 are linked together, and by which their respective functional activities are 

 correlated with such wonderful accuracy, however widely separated the 

 different parts may be. The conduction of stimuli is probably a very 

 complicated phenomenon, and may take place in a variety of ways. Mimosa 

 pudica, however, affords a case in which the transmission is effected simply 

 by a movement of water. This is probably in correlation with the fact 

 that here the result of the stimulation is a simple physical movement, and 

 it often happens that non-living parts may co-operate with living ones in 

 producing such a manifestation of irritability. 



A child who, by pressing a button, causes a musical-box to play 

 a tune, either immediately or after an interval of time, may be quite in- 

 capable of understanding the different mechanical contrivances by which the 

 result is produced. If the entire mechanism is hidden from him, he is 

 naturally unable to tell whether the pressure he has applied acts directly 

 upon the musical-box, or indirectly, as by closing an electric circuit, or by 

 setting a clock in motion which at a given moment may cause the already 

 wound-up musical-box to play. Whether the mechanism is purely physical, 

 or involves chemical reactions as well, is obviously equally hidden from 

 him, and at the same time the tune which he hears affords not a particle of 

 evidence as to whether the energy which operates on the mechanism of the 

 musical-box is supplied by a falling weight, by hydraulic means, or by 

 steam power. Nor will the most perfect knowledge of the exciting stimulus 

 and the final result which it produces, reveal the complicated series of 

 changes and interactions which may intervene between them. We cannot 

 be certain that the same result is always produced in the same way, and 

 indeed a given stimulus frequently produces different results on different 

 plants, while, on the other hand, a particular response may be produced 

 by different stimuli. 



Hence it is easy to understand why we cannot always tell, when the result 

 is different to what was expected, whether this is due to an alteration in 

 the percipient organ or in the processes subsequent to stimulation. Thus, 



