MECHANISM OF THE MOVEMENTS. 89 1 



parenchyma has been removed, water may sometimes be seen to escape also from the 

 horizontal cut surface of the parenchyma. It is therefore certain that during the 

 movement produced by stimulation water escapes from the lower parenchyma ; it gives 

 off a small portion of it to the upper parenchyma (as is shown by the measurements 

 that have been quoted), a larger portion flows off at the sides through the intercellular 

 spaces, and a smaller portion apparently enters the central fibro-vascular bundle. The 

 whole amount of water that escapes from the lower parenchyma is so small that it 

 is no doubt at once absorbed by these parts at the moment of irritation. 



Since water escapes from the parenchymatous cells of the under side when stimulated, 

 and passes into the intercellular spaces, the air must be at least partially expelled from 

 the latter ; and this is evidently the cause of the darker colour of the irritated parts 

 already observed by Lindsay. PfefFer fixed a petiole in the normal condition so that the 

 contractile organ could not bend when irritated ; when he touched a point of the irrit- 

 able side he saw the darker colour spread instantaneously from the point of contact. No 

 other explanation of this phenomenon is possible than that the air is expelled from the 

 intercellular spaces and replaced by water, which would cause a smaller amount of light 

 to be reflected from the interior. The expelled air will collect, in consequence of the 

 laws of capillarity, in the larger intercellular spaces round the central fibro-vascular 

 bundle, from which it will easily reach the petiole. 



In the diurnal position of the organ slight transverse folds are seen to run along 

 both sides which after stimulation become more shallow on the upper but deeper on the 

 under side, showing that the consequent curvature causes a slight passive compression of 

 the under side. This side first of all contracts in consequence of its loss of water and of 

 the elasticity of its cell-walls, and then becomes still further compressed by the down- 

 ward curvature of the upper side. 



How it comes about that a slight touch or concussion should cause an escape of 

 water from the strongly turgid cells of the lower side, followed by an energetic 

 reabsorption, cannot for the present be explained^. Pfeffer's observations on the 

 stamens of Cynareae seem to warrant the assumption that the protoplasm of the irritable 

 cells undergoes a change, in consequence of a touch or of concussion, of such a nature 

 that it becomes more permeable to water, and that the water which has passed through 

 the protoplasmic layer simply filters through the cell-wall, which then contracts in 

 virtue of its elasticity. 



The propagation of the stimulus in Mimosa, to which frequent reference has been 

 made above, has been shown by Pfeffer (Jahrb. f. wiss. Bot. IX. p. 308), in confirmation 

 of Dutrochet's results and of my own, to be efl^ected by the fibro-vascular bundles. 

 Since each movement of a leaf produced by stimulation is accompanied by an escape 

 of water from its parenchyma, the water of the axial bundle and of the bundles 

 connected with it is set in motion. If, when an incision is made into the wood of a 

 stem, a drop of water exudes, a movement of the water in the fibro-vascular system 

 is set up, which aff'ects also that of the axial bundle of the contractile organ and of 

 the irritable parenchyma. 



In the contractile organs of the leaflets of Oxalis Acetosella'- , where the anatomical and 

 mechanical contrivances are similar to those of Mimosa, this compression is much 

 stronger, and the under side contracts when the organ is irritated. Pfeffer states 

 that a decrease in volume also takes place, and since a very considerable elongation of 

 the upper parenchyma is required for the movements, there must be a more con- 

 siderable transference of water from the under side. The organs of Oxalis differ 

 from those of Mimosa in remaining irritable when the intercellular spaces are injected 

 with water; but when in this state they become flaccid on irritation; it is probable 



* [For a discussion of this subject see Vines, The Influence of Light on the Growth of Uni- 

 cellular Organs, Arb. d. bot. Inst, in Wurzburg, II. i, 1878.] 

 2 See Sachs, Bot. Zeit. 1857, pi. XIII. 



