MOVEMENTS PRODUCED BY MECHANICAL STIMULI 75 



produced by a fall of turgor when the cell-wall was previously stretched 

 sufficiently, it is possible that in certain cases no response may be shown 

 although the cells react as in the filaments of Cynareae. This special 

 irritability is, however, certainly not a general phenomenon, and the stamens 

 of Helianthus annum, for instance, have no seismonic irritability although 

 the cell-walls undergo a considerable elastic stretching when the cells are 

 fully turgid 1 . 



The protoplast remains closely pressed against the cell-wall of a 

 stimulated cell, and this is still the case, even when a stimulated staminal 

 filament is loaded with a weight sufficient to prevent any contraction. 

 The retraction of the protoplasm from the cell-wall, such as occurs during 

 rejuvenescence, necessitates that the centrally-directed pressure exercised 

 by the protoplasm should be greater than the osmotic pressure of the 

 cell-sap, which cannot therefore be very great. This must also be the 

 case when, as Schlitt and also Benecke found, the protoplast of a Diatom 

 subjected to mechanical and other stimuli contracts away from the cell- 

 wall 2 . It is possible that this stimulatory plasmolysis may be the result 

 of a sudden change of permeability in the plasmatic membranes allowing 

 the osmotic materials in the cell to escape. 



Stimulation also causes a fall of turgor in the under half of the 

 dorsiventral primary pulvinus of Mimosa pudica. The change of inclination 

 of the petiole is so great as to need a pronounced curvature of the pulvinus. 

 This, though aided by the mechanical moment exercised by the leaf- 

 segments, is mainly produced by an active contraction of the cells in 

 the under stimulated side, which cells are compressed by the expansion 

 of the upper turgid half of the pulvinus until equilibrium is restored. 

 The original condition of turgor is then gradually reproduced in the lower 

 half of the pulvinus which expands, raising the leaf and producing the 

 compression of the upper half of the pulvinus which aids in the rapid 

 curvature of the stimulated pulvinus 3 . 



After the upper half of the pulvinus has been carefully removed no 

 movement is produced by stimulation, whereas when the lower half is 



1 Pfeffer, 1. c., p. 107. 



2 Schiitt, Die Peridineen der Planktonexpedition, 1895, p. no; Beneclce, Jahrb. f. wiss. Bot, 

 1901, Bd. xxxv, p. 554. According to Nageli (Tflanzenphysiol. Unters., 1855, Heft i, p. 13) 

 mechanical pressure causes in Spirogyra, and according to Hofmeister (Pflanzenzelle, 1867, p. 303) 

 in Nitella, a withdrawal of the protoplasm from the cell-wall. It remains, however, to be seen 

 whether we are dealing here with stimulatory functions or with the results of mechanical injury, and 

 the observations of Schiitt and Benecke require further proof. 



* For details concerning the structure and mechanics of the pulvinus of Mimosa see Pfeffer, 

 Physiol. Unters., 1873, p. 9; Haberlandt, Das reizleitende Gewebesystem der Sinnpflanze, 1890, 

 p. 23 ; Physiol. Anat., 2. Aufl., 1896, p. 475 ; Sinnesorgane im Pflanzenreich, 1901, p. 38 ; Schwen- 

 dener, 1897, Gesammelte Abhandlungen, Bd. II, p. 211. On the structure and mechanics of the 

 pulvini of the leaflets cf. Schwendener, 1. c., p. 236. 



