MOVEMENTS PRODUCED BY MECHANICAL STIMULI 79 



Although we may safely assume that the perception of the stimulus 

 takes place in the protoplasm, nothing further is known concerning it. We 

 may, however, in general conclude that a mechanical stimulus produces in 

 a sensitive plant some explosive disturbance in the protoplasm involving 

 a sudden release of energy, and that the gradual storage of energy required 

 for the restoration of the original labile condition of equilibrium takes place 

 independently of the processes of stimulation and perception. The latter is 

 shown by the fact that the organ returns to its original position even when 

 its irritability is permanently suppressed by chloroforming or continual 

 shaking. We do not, however, know whether the return of irritability is 

 due to the formation of a substance capable of explosive decomposition, or 

 is a matter of structural rearrangement in the protoplasm, or involves other 

 changes *. In many cases the power of movement may be retained, although 

 the irritability has been suppressed, and this appears to occur more readily 

 in the case of seismonic than of other forms of irritability 2 . 



Historical, The cellular mechanism of movement in the pulvini of Mimosa and 

 the stamens of Cynareae was revealed in the manner stated above by Pfeffer 3 , for 

 although Briicke * in his historical researches recognized that the curvature of the 

 pulvinus of Mimosa pudica was connected with the flaccidity of the responsive half of 

 the pulvinus produced by the escape of water, he did not further investigate the cell- 

 mechanism, and left it uncertain where the stimulation induced a change in the cell- 

 walls, in the protoplasm, or in the cell-sap. Cohn, and also Unger 5 , erroneously 

 assumed that the movement of the stamens of Cynareae is produced by a change in 

 shape of the cells of the filament without any escape of water 6 . The former author 

 inclined to the conclusion that the movement was due to an active contraction of the 

 protoplasm, a view adopted at a later date by Vines and Gardiner, but one which is 

 totally incapable of explaining the high energy of contraction. Hofmeister's ' con- 

 clusion that the cell-wall was the responsive part of the cell was also based upon 

 incorrect or nebulous arguments. 



shaken growing shoots while the percentage of acid often decreases. Niklewski, however, working 

 at Pfeffer' s instigation, found no increase in the percentage of sugar under these circumstances. 



1 Cf. Pfeffer, Physiol. Unters., 1873, p. 143; Osmot. Unters., 1877, p. 192. An attempt to 

 stimulate the stamens of Cynareae by sound-waves was without success. 



2 Irritability is not regained by sections of the stamens of Cynareae or of the pulvinus of Mimosa 

 pudica. 



3 Pfeffer, Physiol. Unters., 1873; a few complementary details are given in the Osmot. Unters., 

 1877, p. 188. The older view that the spiral vessels were the contractile parts is given in the former 

 work. Ray (Historia Plantarum, 1686, p. i) was perhaps the first who attempted a mechanical 

 explanation. A few experiments were also performed by Hooke (Micrographia, 1767, p. 119). Cf. 

 also Sach's History of Botany, 1890, p. 535. 



* Briicke, Archiv f. Physiologic, 1848, p. 443. 



5 Cohn, Abhandlg. d. schles. Ges. f. vaterl. Cultur, 1861, Heft I, p. 28. Cohn (Zeitschr. f. wiss. 

 Zoologie von Siebold u. Kb'lliker, 1863, Bd. xil, p. 366) at a later date compared the contractile 

 cells to muscle- fibres. 



6 Unger, Bot. Ztg., 1862, p. 112; 1863, p. 350. 



7 Hofmeister, Pflanzenzelle, 1867, p. 300. Cf. also Flora, 1862, p. 502 and Pfeffer, Physiol. 

 Unters., 1873, p. 6, 128. 



