CHAPTER XIL 

 SENSORY SYSTEM. 



I. GENERAL CONSIDERATIONS. 



All organisms, plants and animals alike, are possessed of the property 

 of irritability, by virtue of which they react or respond to external or 

 internal stimuli. The energy necessary for the carrying out of the 

 response is not derived from the stimulus, but is furnished by the 

 organism itself. The nature and final result of the reaction are, of 

 course, dependent upon the structure of the particular organism con- 

 cerned. Responses- to stimuli generally tend to satisfy definite 

 physiological or ecological requirements, and their importance to the 

 organism depends upon this adaptive quality. Pfeffer terms the series 

 of physiological processes which begins with the perception of a 

 stimulus and ends with the final response, a " chain of stimulation " 

 (Reiz-Kette). In the present chapter we shall deal with the first link in 

 that chain, namely, the perception of stimuli, and with the anatomical 

 features that are correlated with the perceptive function. 



The perception of a variety of external stimuli by a plant pre- 

 supposes the existence of different forms of irritability, and of cor- 

 respondingly differentiated perceptive faculties. This sensitiveness 

 towards external stimuli is vested in the living substance itself. It is 

 obvious that there must be a structural basis underlying this sensitive- 

 ness ; but this protoplasmic structure appears to lie beyond the limits 

 of visibility. 



Relatively unspecialised plants possess no arrangements for the 

 perception of stimuli apart from this " irritable structure " of the proto- 

 plasm. In these circumstances, most if not all the living cells and 

 tissues of an organ or, it may be, of the whole plant-body are 

 sensitive. The structure of such unspecialised sensitive cells is 

 primarily correlated with the performance of various other functions, 

 and is only in a minor degree adapted for the perception of stimuli ; 



