CHAPTER XIII. 

 STRUCTURES SERVING FOR THE TRANSMISSION OF STIMULI. 



I. GENERAL CONSIDERATIONS 



Both in animals and in plants response does not follow immediately 

 upon stimulation. The two processes are always separated by a 

 number of intermediate changes. As already stated, the whole series 

 may be collectively termed a chain of stimulation {Rciz-Kctte) ; such a 

 chain always begins with an act of perception and ends with the 

 reaction or response induced, the most important of the connecting 

 links being the transmission of the stimulus between the perceptive 

 and the reactive regions of the cell or organ. Transmission of stimuli 

 must occur, even where perception and response take place within 

 the same cell ; for the same part of the protoplasm, in a unicellular 

 organism or in an individual cell of a higher plant, cannot simultaneously 

 perform both these functions. When the sensory- and motor- 

 organs are more widely separated in space, transmission of stimuli takes 

 place over greater distances, and hence becomes a still more conspicuous 

 feature of the chain of stimulation. The tactile sensory epithelium of 

 the stamens of Berberis immediately adjoins the motor-tissue of the 

 filament. Here the transmission of stimuli merely consists in a 

 transference of the excitation aroused in the sensitive protoplasm 

 to the adjacent motor-tissue by mechanical contact. In an ordinary 

 root, the statolith-apparatus in the root-cap is separated by a consider- 

 able amount of meristematic-tissue from the sub-apical region that 

 executes geotropic curvatures. Stimuli have to be transmitted over 

 still greater distances in the leaves and stems of Mimosa pudica, where 

 a single excitation may gradually extend to all the leaves on a 

 shoot, or even over the whole plant. The three preceding examples 

 sufficiently illustrate the diversity that exists in respect of the 

 distances over which external stimuli may be propagated. 



In considering whether special histological arrangements are required 



