GROWTH AND MOVEMENT 



431 



the walls. It is not at all certain that there are not other more me- 

 chanical means of transmitting the disturbance that eventuates in move- 

 ment.' 



Responsive region. • — Corresponding to the perceptive region, the 

 place where the final reaction occurs is called the active or responsive 

 region. Of course it is not more active or responsive than the inter- 

 vening regions; but attention is fixed on it as the seat of the selected 

 reaction. Thus, in the root above referred to, the perceptive region is 

 in the root cap, the excitation is propagated backwards through several 

 millimeters of meristematic cells to those in the phase of enlargement, 

 and the region of most rapid growth is the responsive region, because 

 there the growth rate is unequally affected on the upper and under side, 

 and so a curvature appears in that zone, which turns the tip downward 

 again. 



Mechanism of reaction. — Consideration of even one such curvature 

 shows that the nature of the reaction is in no way determined by the 

 nature of the stimulus, since the same stimulus produces a number of 

 reactions differing entirely from the end reaction, curvature. When 

 many movements are studied, this feature appears most strikingly, for 

 it is seen that the same stimulus may produce curvatures in exactly 

 opposite directions in different parts, such as a root and a shoot, while 

 different stimuli may call forth identical responses. Further, stimuli 

 of the same sort at different intensities may call forth opposite reactions. 

 The mode of action is determined in fact by the mechanism concerned. 

 Just as an electric current may ring a doorbell, start an engine, or ex- 

 plode a mine, according to the mechanism at the end of the wire; so an 

 electric current may shorten a stamen, drop a leaf, or curve a tendril, 

 according to the mechanism set into operation in the plant. Yet prob- 

 ably there is some effect, fundamentally similar in each case, which works 

 out to a different final result, just as, in the comparison, the magnetizing 

 of an iron bar underlies the varied results. 



Tropic, nastic, taxic movements. — In some cases, however, the stimu- 

 lus in a measure controls the reaction. A stimulus that acts upon plants 

 from a definite direction, and consequently from one side, may deter- 

 mine by that fact the plane of the consequent curvature, provided the 

 organ be physiologically radial, i.e. capable of response in any plane. 



» The "nerves" of leaves are so called only by analogy, as the correlative terms, "veins" 

 and "ribs," indicate. They probably have nothing to do with transmitting an excitation 

 in ordinary cases, though some recent observations allege the contrary. 



