MECHANISM OF THE MOVEMENTS. 
887 
introduced into an atmospiiere nearly saturated with these vapours, the irritability 
will disappear in a few minutes. If the organs have been previously stimulated, they 
now expand (without being irritable) in consequence of their becoming more rigid. 
The action of the vapour of ether or of chloroform is a purely local one : only those 
organs which are exposed to it lose their irritability^. 
(6) Frequent stimulation at short intervals (vibration) causes the pulvinus of 
Mimosese to become insensible to stimuli, although they expand during the continuance 
of the stimulation and take up a position of rest such as would have been produced 
by the first shock alone. It is not for from five to fifteen minutes after the cessation 
of the shocks that irritability returns^. 
(7) transitory rigidity caused by electrical agency^ was observed by Kabsch in the 
gynostemium of Stylidium. A weak current produced the same result as concussion; 
a stronger current destroyed the irritability, which however returned after half an hour. 
In Desmodium gyrans, on the other hand, the leaflets which had been rendered rigid by 
cold (22° C.) were again made motile by the action of induction-currents. 
Sect. 29. — Mechanism of Movements caused by Contact or Concus- 
sion *. This has been studied with success by many observers in the articulations 
at the bases of the petioles of Mifnosa pudica, in the small motile organs of the 
leaflets of Oxalis Acetoselia, and in the filaments of the Cynareae. There is but little 
doubt that other motile organs are essentially similar to these types. 
Generally the motile organs are nearly cylindrical in form, but somewhat 
flattened horizontally. A tough, but not brittle, flexible lignified fibro- vascular bundle 
lies in the axis of the organ, and is surrounded by several layers of succulent 
parenchyma which is invested by a feebly-developed epidermis. The turgid paren- 
chyma tends to stretch the axial bundle and the epidermis, whereas these, more 
especially the bundle, offer a resistance to the expansion of the parenchyma. 
It is the parenchyma which is irritable. It may be the parenchyma of one side 
only of the organ which is irritable, as in Mimosa (the lower surface only of the large 
articulations), or of both sides (filaments of Cynareae). 
The conditions of the Irritability are the following : i . that the parenchymatous 
cells tend to absorb water continually and thus to stretch their cell-walls, that is, to 
be strongly turgid; and 2. that a slight concussion of the irritable cells causes an 
escape through their walls of a portion of the water which they contain. This 
sudden change produced by a stimulus probably affects, according to Pfeffer's 
reasoning, the protoplasm of the parenchymatous cells: their cell-walls are not 
irritable and contribute to the movement merely in virtue of their elasticity. When 
the movement has ceased, the absorption of water recommences and their turgidity 
and the irritable condition are restored. 
The Movement itself is produced by the action of the elasticity of the stretched 
cell-walls which comes into play at the moment when the turgid cells give off" water. 
The cell-walls contract elastically in proportion to the quantity of water which 
escapes from the cells. The water passes into the intercellular spaces of the 
* Pfeffer, 1. c. pp. 64-66. 
2 Pfeffer, 1. c. p. 58. 
3 Kabsch, Bot. Zeit. 1861, p. 358. 
* The very extensive literature on this subject has been collated by Pfeffer in his 'Physio- 
logische Untersuchungen,' Leipzig 1873. 
