20 INTRODUCTION 



intense concentration of energy directed towards a particular aim, may 

 produce a marked decrease in sensibility to other stimuli. 



The change to the new conditions of irritability takes a longer or shorter 

 time ; that is to say, a latent period of response always intervenes between 

 a stimulus and the reaction. Accommodation to a change of temperature 

 or to the influence of certain chemical reagents is very rapid, but in 

 many cases, from the length of time which elapses before the end result 

 is produced, it may be concluded that the induction of the new irritable 

 condition takes place but slowly. During this period the after-effect of 

 the original external conditions is being gradually overcome. In case 

 of a reversion to the original conditions, a longer or shorter after-effect 

 will always be perceptible, provided that the new condition of irritability 

 is capable of retrogressive modification, and has not become inherently 

 fixed. An example of the latter is afforded by the permanent bilaterality 

 which is induced in the thallus of Marchantia when the developing 

 gemmae are subjected to unilateral illumination. As a matter of fact, 

 a modification of the inherent characteristics is induced whenever either 

 an internal or an external agency operating upon an organism causes 

 it to assume any properties, however unimportant, which were formerly 

 absent, so that it attains a power of responding differently to special 

 stimuli. 



When a plant is again exposed to the conditions under which it 

 previously existed, it may, or may not, revert to exactly the same condition 

 of irritability as it previously exhibited. Such a return to the original 

 condition does actually take place after the leaflets of Mimosa have 

 been caused to close by mechanical stimulation, and the same is also 

 the case when growing seedlings are stimulated, provided that no per- 

 manent fixation or alteration has been produced by subsequent growth. 

 When the normal position of the dorsal and ventral surfaces of fern 

 prothalli or of branches of Thuja is reversed, a corresponding change in 

 the bilaterality is induced, but only in the new growths formed under the 

 changed conditions. 



From what has already been said, it follows that the result produced 

 by two stimuli acting simultaneously is not simply the sum of the 

 results which they would produce if they acted separately. This is not 

 the case even when the nature of the response is such as to permit of 

 a summation being possible. Whether two stimuli act simultaneously or 

 successively may be of great importance in determining the precise nature 

 of the final result. 



We shall be pursuing, therefore, a correct method of investigation 

 if we study the effects produced by a single or a few stimuli under 

 otherwise constant conditions. In the study of irritability we can dis- 

 tinguish between simple and complex stimulation (Induction), or may 



