THE CAUSES OF MOVEMENT 5 



We must, as far as possible, endeavour to distinguish the processes 

 of sensation, induction, and movement from one another, and to resolve 

 these further into their component factors. At present, this is possible 

 to a very limited extent and only in a few cases, for the best know- 

 ledge of the conditions for stimulation and reaction, as well as of the 

 position, shape, and structure of the percipient organs and of the conducting 

 paths, affords no insight into the processes which underlie perception 

 and induction. Since perception and induction are usually so closely 

 connected that they cannot be separately considered, we shall discuss both 

 at the same time. 



If we restrict the term ' perception ' to the first physiological inter- 

 action involved in stimulation, we must not apply the same term to the 

 whole series of sensory processes, when these cannot be resolved in detail. 

 Preparatory processes may take place which render possible or initiate 

 perception and response. This is the case, for instance, when cutting the 

 stem of Mimosa produces a movement of water which calls forth a response 

 in the pulvini, or when the sinking of dense particles in the cell acts as the 

 cause of a geotropic response. Further, the same would be the case when 

 light or some endosmosing substance produced a chemical change in which 

 one of the products acted as a stimulus. 



Just as one speaks generally of the processes of induction, so also may 

 we group all these preparatory processes together as instances of indirect 

 stimulation although they may be varied and complicated in character. 

 In any case the introduction of special terms for phenomena which are 

 not yet understood, and for facts which are largely unknown or uncertain, 

 is hardly to be recommended l . 



The movement of zoospores towards light or away from it when 

 intense can be regarded as the result of tropic stimulation. Further, the 

 conversion of a positive into a negative heliotropic curvature involves 

 a change in the sensory processes alone when the change from weak to 

 strong light which produces this alteration acts on the sensitive tip of 

 a seedling leaf of Avena, but not on the responding basal portion. But 

 when light or any other agency directly stimulates the responding region, 

 a change in the response may result either from an alteration of irritability 

 or from some influence upon the course of the reaction. The sensory 

 processes themselves may be of greater or less complexity, and hence may 

 be influenced in a variety of ways so as to lead to changed responses. 

 In such cases a change in the final result affords no indication as to whether 

 it is due to a modification of the primary act of perception or of some one 

 of the resulting stages leading from perception to response. 



1 Cf. Czapek, Jahrb. f. wiss. Bot, 1898, Ed. XXXII, pp. 214, 302; Centralbl. f. Physiol., 

 1900, Ed. XIII, p. 209. 



