i8 INTRODUCTION 



and comprehensive grasp of the phenomena of stimulation and irritability 

 has not as yet been universally attained. 



In the same way potassium salts and compounds of nitrogen are also 

 to be regarded as essential stimuli to vital activity. This stimulating action 

 is especially marked when a plant, impoverished and stunted for want of 

 nitrogen, is incited to rapid development and growth by a supply of saltpetre. 

 Owing to the reciprocity between different parts of the vital mechanism, 

 every vital process involving a chemical change or a transference of 

 energy, radiates kinetic energy in some form or other, which either acts 

 as a stimulus to increased activity or calls fresh activities into existence. 

 Thus, by respiration that is to say, by the stimulus created by the con- 

 sumption of oxygen not only is a supply of energy ensured, but also the 

 irritability of the plant, its power of response to stimuli, is directly or 

 indirectly maintained. In many cases, though not necessarily in all, the 

 stimulating agent may enter into the metabolism of the plant, or take 

 a direct part in the production of the result which it primarily induced l . 



This inductive action of a stimulus is by itself insufficient to produce 

 any result, for in every case the organism must have at its disposal the 

 necessary supplies of constructive material and energy. 



As development proceeds the continued vital activity of the plant may 

 give rise to a series of progressive disturbances of equilibrium which are 

 continually readjusted, and these may finally result in marked changes of 

 character and disposition, even although normal and constant external 

 conditions are maintained. Such automatic variations are to be regarded 

 as the result of internal stimuli, such as arise in every plant even under 

 perfectly normal and constant external conditions. Reactions taking place 

 in relation to changes in the external conditions are to be regarded as the 

 means by which the inception of an adaptive modification is rendered 

 possible. 



In the progress of development a change may take place in the inherent 

 disposition which determines the character of the response to any given 

 stimulus, and this disposition is also liable to modification by the operation 

 of external agencies. Consequently the phenomena to which a particular 

 stimulus will give rise may be affected by antecedent or simultaneous 

 stimulation, influencing or altering the irritability which the first stimulus 

 excites. The intimate correlation of the entire vital mechanism renders it 

 probable that every excitation exercises some effect upon other manifesta- 

 tions of irritability, even though this effect may not always be directly 

 perceptible. 



1 Hiippe (Verhandlungen d. Ges. deutscher Naturf. u. Aerzte, 1893, p. 154) is certainly in 

 error in stating, as a general character of a stimulating action, that the operating stimulus supplies 

 the additional energy necessary for the inception of the result produced. See Pfeffer, Jahrbuch 

 f. wiss. Bot. , 1895, Bel. xxvin, p. 239. 



