1 4 INTRODUCTION 



Nor is it necessary that a stimulus should always invoke either an 

 immediate response, or a liberation of all the available energy, as when 

 gunpowder explodes, or when the leaflets of a shaken sensitive plant 

 rapidly fold together. On the contrary, an increase in the intensity of the 

 stimulus is commonly followed by an increase in the energy of the response, 

 just as when an engine is driven more and more rapidly as the steam-cock 

 is gradually opened. Similar purposeful relations exist in the plant 

 between the intensity of the stimulus and the amount of response, as is 

 especially well shown by those movements which increase in rapidity and 

 extent with increasing photic, thermal, or chemical stimulation. 



No increase in the response is possible above a certain point, and this 

 is determined by the capabilities of the responding mechanism. Machines 

 may be constructed, which can regulate themselves as plants do, or which 

 may go more and more slowly as the driving force becomes stronger and 

 stronger, and may finally stop when a certain limit is passed. Probably 

 it is a similar action which causes the leaflets of the sensitive plant to close 

 up again if exposed to full sunlight, although when brought from darkness 

 into diffuse daylight they pass from the nyctitropic to the fully expanded 

 condition. The fact that in many cases the paraheliotropic and nyctitropic 

 positions are markedly different, shows that the character as well as the 

 amount of the response may be changed, as soon as the stimulus reaches 

 the necessary intensity. As a general rule, the graphic curve, representing 

 the response to increasing stimulation, rises to a maximum or point of 

 optimal stimulation, beyond which it descends. 



Every stimulus must reach a certain minimal intensity in order to pro- 

 duce a perceptible result. Above this point, the effect which the stimulus 

 produces may become noticeable at once, or only after a latent period. 

 Either a rapid or a slow response may accordingly be produced, which 

 may then slowly or quickly pass away again. An analogy is afforded by 

 a clock, which set going by a shake (stimulus), after a given time (latent 

 period) rings an alarm (result). The clock stops as soon as the energy of 

 the coiled mainspring is exhausted. A frozen plant, on the other hand, 

 when restored to vital activity by being thawed, develops its own propulsive 

 energy in a self-regulatory manner, if supplied with nutriment. 



It is possible to construct machines in which the same stimulus can 

 produce several different results, directly or indirectly, simultaneously or 

 successively. For the maintenance of the regulatory mechanism of the 

 plant the transmission of stimuli is an absolute necessity, and the electric 

 telegraph affords an illustration of how the effect produced by a stimulus 

 may take place at a distant point. 



In every manifestation of irritability, the stimulus and the response 

 must always be clearly distinguished from one another. An irritability, 

 i. e. a power of reacting in a specific manner, is made evident only by the 



