MOVEMENTS PRODUCED BY MECHANICAL STIMULI 77 



whereas the elongating upper half slightly decreases in volume 1 . The 

 displacement of the intercellular air by water is also shown by the sudden 

 darkening following stimulation, just as occurs when the pulvinus is injected 

 with water, and as is also shown in the under half of the pulvinus when 

 the movement is mechanically arrested 2 . The presence of intercellular 

 spaces in the inner layers of parenchyma facilitates the rapid extension 

 and removal of water, but nevertheless the outer layers may also give 

 off water with sufficient rapidity, although no system of communicating 

 air-spaces exists between them 3 . The anatomy of the tissues does not 

 therefore enable us to conclude that the outer layers of parenchyma are 

 inactive or less active than the inner layers. 



Additional and important evidence to show that the movement is pro- 

 duced by a fall of turgor is given by the fact that the stimulated pulvinus 

 is more flaccid and less rigid than the unstimulated one. This can be 

 shown by determining in each case the deviation of the angles between the 

 stem and petiole in the normal and inverted positions. Briicke * observed 

 the angles of deviation in the stimulated pulvinus to be two or three 

 times greater than in unstimulated ones. Similar relationships were deter- 

 mined by Hofmeister 5 to exist in the case of stimulated and unstimulated 

 stamens of Cynareae. These facts show that the water is not pressed out 

 by an increase in the elasticity of the cell-wall increasing the pressure on 

 the cell-sap, for in that case the rigidity of the cells and tissues would be 

 increased. From the load required to prevent movement it can be cal- 

 culated that the energy of movement in the pulvinus represents a fall of 

 turgor of two to five atmospheres 6 . Hence it is obvious that the movement 

 cannot be due to an active contraction of the protoplast. 



The fact that the rigidity of a stimulated pulvinus of Oxalis acetosella 

 decreases 7 and that water escapes under favourable circumstances from the 

 stamens of Berberis milgaris when a curvature is produced by irritation 8 , 



1 Pfeffer, 1. c., p. 23, 



3 Pfeffer, 1. c., p. 35. The fact that this change of coloration, first observed by Lindsay in 1 82 7, 

 should not always be distinctly shown probably depends upon the fact that the air which is always 

 only partially displaced may in some cases be displaced but little or not at all. It is therefore quite 

 possible that Schwendener (I.e., p. 212) worked with plants which did not show any change of 

 colour, but the latter has been recently observed by Macfarlane (Biological lectures, 1894, p. 205) in 

 various species of Mimosa^ and more especially in Mimosa stnsitiva. 



8 Pfeffer, I.e., p. n ; Schwendener, 1. c., p. 212. 



* Briicke, Miillers Archiv f. Physiologie, 1848, p. 40. It has not yet been determined why the 

 rigidity rises after chloroforming and also when the irritability is suppressed by repeated shaking. 

 Pfeffer, Physiol. Unters., 1873, p. 65. 



5 Hofmeister, Pflanzenzelle, 1867, p. 311 ; Pfeffer, I.e., p. 145. 



6 Pfeffer, Periodische Bewegungen, 1875, p. in. 



7 Pfeffer, Physiol. Unters., 1873, p. 74. 



8 Pfeffer, 1. c., p. 158. Intercellular spaces are usually present in the stamens of Berberis. Cf. 

 Pfeffer, Zur Kenntniss d. Plasmahaut u. d. Vacuolen, 1890, p. 326, footnote 2. 



