MOVEMENTS RESULTING FROM SHOCK 



513 



have figured prominently in the history of that department of physiology which 

 deals with sensitivity. 



Apart from general habit, we may compare the stimulus-movements just 

 discussed in Mimosa, but which are manifested by other plants as well, with 

 nyctitropic movement, inasmuch as we have to deal in this case with the assump- 

 tion of a new position of equilibrium as a result of stimulus, which, however, is 

 temporary only ; for after a short interval we find that the stimulated leaf regains 

 its normal day position, and that too without being again stimulated, just as 

 the darkened leaf re-erects itself of its own accord in daylight. Further, just 

 as the day position may be again assumed in continued darkness, so also a leaf 

 of Mimosa may once more attain the normal day position after continuous 

 small shocks rapidly applied. There are other and fundamental differences, 

 however, between response to shock and to changes in illumination. As 

 already mentioned, there is a difference both in the rapidity as well as in the 

 mechanics of the movement in the two cases ; to both of these points we shall 



I'ig- '59- Mimosa pudica. On the left an unstimulated twi 

 having been shaken. Reduced. (From the Bonn Textbook.) 



in the da}' position ; on the right the same after 



recur presently. A further point of distinction lies in the fact that the shock 

 stimulus under natural conditions is not periodic and that no after-affects have 

 as yet been determined. In all probabihty, even if the shock stimulus be 

 applied for a long time at regular intervals, no oscillations would manifest 

 themselves after the cessation of the stimulation. The movements at present 

 under consideration differ from those of nyctitropism in yet other respects, viz. 

 in their significance in the plant economy, and, in connexion with that, in the 

 nature of the releasing stimulus, and finally in their occurrence. The releasing 

 stimulus may be not merely a shock, but also the mechanical and chemical 

 influence of different bodies and the biological significance of movements due to 

 shock is apparently quite different in different plants. In foliage-leaves, such 

 as those of Mimosa, the movement is apparently intended to frighten away any 

 animal that is calculated to injure the plant. There are special difficulties in 

 proving the exact truth of this theory in this special instance, because it is only 

 when it immediately precedes the visit of the animal that the movement can 

 make any impression on the visitor ; larger animals will be puzzled by the altered 



losr L 1 



