MECHANISM OF THE MOVEMENTS. 793 



tractile organs — and therefore the movements themselves — to changes in the 

 turgidity of the parenchymatous cells, which can only be caused by water entering 

 or escaping from them ; and the problem is now essentially to show how this influx 

 or efflux of water can be rendered possible by mechanical contrivances, and by what 

 forces it can be brought about. When the phenomenon under consideration is the 

 influx of water, in other words an increase in the turgidity of the parenchyma in the 

 whole or in one side of the organ, the hypothesis may for the time suffice that the 

 cells have a constant tendency to absorb more water by endosmose. Greater 

 difficulties are presented in the solution of the question why a portion of this water 

 which has been taken up with so much force is again given off on slight concussion 

 or on an increase in the intensity of the light and in consequence of unknown 

 internal causes (in the case of spontaneous periodic movements), and is again 

 replaced subsequently by a like quantity. Some knowledge of the mechanism of 

 these movements is essential to the answering of this question^; and we have now 

 to describe the amount of such knowledge that we possess in the case of a few of 

 the contractile organs which have been most carefully examined. 



I. The mechanis7ii of nwcements caused by contact or concussion. 



(a) The Sensitive Plant- {Mimosa pudica'). The leaf when fully developed is bipinnate, 

 and consists of a petiole from 4 to 6 cm. long with two pairs of petiolules 4 to 5 cm. in 

 length, and on each of these from fifteen to twenty-five pairs of leaflets 5 to 10 mm. long 

 and i'5 to 2 mm. broad. All these parts are connected by contractile organs; every 

 leaflet is immediately attached to the rachis by such an organ from 0-4 to 0*6 mm. long, 

 and this again to the primary petiole by another similar organ from 2 to 3 mm. long and 

 about I mm. thick. The base of the petiole itself is transformed into a nearly cylin- 

 drical contractile organ 4 to 5 mm. long and 2 to 2'5 mm. thick, which is furnished, like 

 those of the petiolules, with a number of long stiff" hairs on the under side ; the upper 

 side being only slightly hairy or not at all. 



Each of the contractile organs consists of a comparatively very thick layer of par- 

 enchyma with a feebly developed epidermis without stomata, and penetrated by an 

 axial flexible but only very slightly extensible fibre- vascular bundle, which separates 

 into several bundles where it emerges into the channeled petiole. The parenchyma 

 consists of roundish cells enclosing, in the eight layers which surround the axial 

 bundle, large air-conducting intercellular spaces which become much smaller in the 

 eighteen or twenty outer layers of cells, and are entirely wanting in those immediately 

 beneath the epidermis. These intercellular spaces are in communication with one an- 

 other from the fibro-vascular bundle to the middle layers of tissue ; the very small ones 

 of the outer layers have the appearance of separated triangular internodes, and when 

 cut (therefore in the irritated state) seem to be full of water. The cells of the 

 under side of the 'pulvinus' are thin-walled, those of the upper side have much thicker 

 walls (about three times as thick) of pure cellulose. Together with a moderate quantity 

 of protoplasm (including a nucleus) and small grains of chlorophyll and starch, each of 

 the cells contains in its cavity a large globular drop, consisting, according to Pfeff"er, 



* This view is essentially that already adopted by Briicke in i S48 in the case of Mimosa, and 

 supported by Unger (Anatomie und Physiologie der Pflanzen, p. 410). 



2 Uutrochet, Mem. pour servir, vol. I, p. 545. — Meyen, Neues System der Pflanz.-Phys., vol III, 

 p. 516 et seq. — Briicke, in Miiller's Archiv fiir Anat. und Phys., 1848, p. 434; ditto, in Sitzungs- 

 berichte der kais. Akad. de Wiss. Wien, vol. L, July 14, 1864. — Hofmeister, Flora, 1852, No. 32 et 

 seq. — Sachs, Handb. der Exp.-Phyi., 1866, p. 479 et seq. — P. Bert, Recherches sur les mouvements 

 de la sensitive; Paris, 1867. 



