No. 522] PHYSIOLOGICAL RESPONSE 



329 



Another primary phase of response is the propagation 

 of the excitation. The disturbance called perception 

 propagates itself, that is, it initiates in adjacent material 

 a corresponding change, progressing from point to point, 

 as shown by secondary effects produced at a greater or 

 less distance from the point where the stimulus is ap- 

 plied. The precise character of the change itself has 

 been little studied in plants, though the rate of propaga- 

 tion is known to be of the same order of magnitude as 

 that in the nerves of the lower animals — say 1 to 10 mm. 

 per second. 



The secondary phase of response is manifested either 

 separately by turgor mechanisms and by growth mechan- 

 isms, or by the two conjointly. In turgor mechanisms 

 altered turgor of the cells in a definite region causes dis- 

 placement of parts attached. In growth mechanisms 

 local differences in the rate of growth of the organ dis- 

 places the adjacent parts. When combined the earliest 

 displacement is due to turgor, and this is made permanent 

 by growth. 



Examples are so familiar that they need hardly be 

 cited, and a single one of each will suffice. When the tip 

 of a leaflet of Mimosa is stimulated by burning with a 

 lens, the excitation is propagated to the petiolule of the 

 leaflet, the turgor of its upper and anterior cells is so re- 

 duced that those on the opposite side compress them, 

 bending the petiolule sharply and so carrying the leaflet 

 forward and upward. The disturbance may spread to 

 other leaflets and even to the base of the petiole, with 

 appropriate curvatures in each motor organ. 



When a primary root is placed horizontal, perception 

 occurs mainly in 'the tip, the excitation is propagated 

 backward, and. the secondary response appears as differ- 

 ential growth of the cells chiefly in the third and fourth 

 millimeter from the tip, which * produces the well-known 

 curvature. 



Though much discussed, it is not clear whether the 

 secondary response of tendrils depends on a growth 

 mechanism or a turgor mechanism. I am inclined to be- 



