MECHANISM OF THE MOVEMENTS, 887 



introduced into an atmosphere 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 

 Mimoseae 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° G.) 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 Mimosa pudica^ in the small motile organs of the 

 leaflets of Oxalis Aceiosella, and in the filaments of the Cynarese. 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 aff'ects, according to Pfeff"er'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. 



