THE VEGETABLE CELL. J 55 



of the lower half of the articulation, resulting from irritation, 

 depends — ^whether, as Brucke a&sumes, but as appears to be highly- 

 improbable, a portion of the cell-sap issues out into the intercel- 

 lular passages, or a relaxation of the cell-walls takes place — ^we 

 know simply nothing, 



Ohserv. Dutrochet subsequently {^^ MemoiireaP i., 537) retracted the 

 theory aboYe-mentioned, concerning the depression of an irritated Mimosa 

 leaf, and set up the opinion that this movement downwards does not 

 depend upon the expansion of the cellular tissue of the upper side of the 

 articulation, but upon curvature of the younger layers of wood, which, 

 in consequence of the irritation, absorb oxygen from their vicinity in a 

 Avay not further explicable, and are thereby caused to curve downwards. 

 After a short time, and from an equally imknown cause, this oxidation 

 of the fibrous tissue ceases, the power of curving belonging to the cellu- 

 lar tibsue again acts, and causes the elevation of the leaf. Leaving the 

 arbitrary character of the whole of this explanation out of the question, 

 there is evidence against Dutrochet's theoiy in the experiments of Brucke, 

 which indicate that the depression of the leaf is connected with relaxation 

 of the articulation, for if the articulation were bent by the curvatuxe of 

 the woody bundle, the cellular tissue of the mider side would be strongly 

 compressed, which must rather increase than diminish the tension of the 

 joint. 



The leaf of Mimosa is sensitive to stimuli of every kind ; 

 shaking, wounding, burning, contact of irritating fluids, electric 

 shocks, &c., all act in the same manner. Quick repetition of an 

 irritation exhausts the sensitiveness towards it tolerably soon. 

 The more vigorous the vegetation of the plant is, the higher the 

 temperature in which it is placed, the more sensitive it is found 

 to be. 



It has already been observed that the direct irritation of the 

 articulation of the leaf is not necessarily required to produce the 

 movement of irritability, but that the effect of a stimulus acting 

 on a distant part is conducted to the ax^ticulation, under which cii- 

 cumstances, it depends on the irritability of the whole plant and 

 the strength of the stimulus, how far the manifestation of irrita- 

 bility becomes difiused from the irritated point. In reference to 

 this conduction of the stimulus, the researches of Dutrochet and 

 Meyen led to the conclusion that it was conveyed by the vascular 

 bundles, and not by the parenchymatous cellular tissue; for on 

 .the one side the conduction of the stimulus was interrupted by 

 cutting away the vascular bundles, and on the other side wounds 

 on the rind, when the incision did not penetrate the wood, were 

 not followed by movements of the leaves. This conduction is 

 comparatively slow; according to Dutrochet's measurements, it 

 amounted to 8 — 1-^ millimeters in a second in the petiole, to only 

 2 — 3 millimeters in the stem. 



xJXQii/ 



Investigation of the other irritable plants, the leaves of Dimwsa^ 

 nils and liohmia, the stamens of Berberis, Oaciiis^ &c., fur- 



