MECHANISM OF THE MOVEMENTS. 
891 
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 eflfected 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 affects 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 Würzburg, II. 1, 1878.] 
See Sachs, Bot. Zeit. 1857, pi. XIII. 
