THE NATURE OF IRRITABILITY 13 



recognition of this fact is of great importance, because until recently the part 

 played by stimuli in all vital phenomena has been overlooked ; it must 

 also be remembered, however, that what actually takes place cannot always 

 be definitely referred to the direct action of the operating stimulus. There 

 can be no doubt, for example, but that the changes of form which are 

 exhibited by plants kept in darkness are a manifestation of irritability, and 

 are due to the absence of light acting as a stimulus, which causes the plant to 

 respond in a manner determined by its inherent nature and the means which 

 it has at its disposal. Every stimulating action of light upon the plant is 

 similar in character. In the same way, when a plant, whose growth has 

 been checked owing to the lack of potassium compounds, is awakened to 

 renewed activity by a supply of the deficient salt, the action is always 

 a stimulating one, whatever may be the nature or equivalency of the 

 changes which the salt at once undergoes in the metabolism of the plant. 



Our knowledge of the physiology of irritability is very incomplete. 

 It is, however, possible to realize the general principles which govern it, 

 by reference to the laws ruling mechanical contrivances made by the hand 

 of man. In every machine it is the particular structure and properties 

 w r hich determine whether or not a reaction will be produced, and what 

 will be its nature. A finger pressure produces an effect only when it is 

 applied to a particular part of a musical-box, and the same finger pressure 

 may start a steam engine, ring an electric bell, fire a mine, or cause 

 a musical-box to play a tune. In the same way, all plants do not 

 necessarily respond to a given stimulus, nor do they react in a similar 

 manner to the same stimulus. Thus a momentary touch causes the leaflets 

 of Mimosa to fold together, while prolonged contact causes Cuscnta to form 

 haustoria, and the tendrils of A mpelopsis to form suckers. 



That different plants should respond differently to the same stimulus 

 as, for example, that one plant should behave geotropically and another 

 ageotropically under the influence of gravity is not more wonderful or 

 mysterious than that the same propulsive force should drive a locomotive 

 either forwards or backwards, according to the way in which it operates 

 upon the working mechanism. 



It is not difficult to realize that there is no essential relation between 

 the stimulus and the result, either in character, or in the amount of energy 

 which each represents. The smallest spark, by igniting a mass of powder, 

 produces an enormous mechanical effect. Similarly, the opening of the 

 steam-pipe supplying an engine always causes the latter to convert the 

 same amount of the energy of the steam into the same kind of motion, and 

 to do relatively the same amount of work. This would still be the case 

 even if the work done in opening the steam-cock were greater than the 

 work which the amount of steam supplied to the engine would enable 

 it to perform. 



