888 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



irritable tissue and finds its way, for the most part, out of the organ into other 

 tissues. As a consequence the volume of the stimulated tissue diminishes. The 

 tissue of the other side of the organ, which has not been stimulated, is turgid and 

 expands at the time when the tense epidermis of the stimulated side is contracting 

 elastically : the latter side becomes concave, the former convex, and the stimulated 

 organ curves so that the parts which are connected with it are passively raised or 

 lowered accordingly as the concavity of the organ, which is always on the stimu- 

 lated side, faces upwards or downwards. 



The organ is not irritable immediately after this has taken place, for the con- 

 tracted cells are not sufficiendy turgid to admit of a further escape of water. After 

 some time the contracted cells absorb water, their turgidity increases, their walls, 

 become tense, the volume of the individual cells and therefore that also of the whole 

 irritable tissue becomes greater, the epidermis is stretched, and a curvature is effected 

 in an opposite direction to that which was effected by stimulation, the stimulated 

 side of the organ becomes convex, and then the organ is again irritable. 



From what has been said it is evident that the volume and the turgidity (rigidity) 

 of the whole organ diminishes during the movement produced by stimulation, that 

 the restoration of irritability is associated with increased turgidity and volume, and 

 finally, that the irritability and the amplitude of the movement must be greater, 

 ceteris paribus, the more turgid the organ is. 



This account of the nature of the irritability and of the phenomena of the 

 movement is derived from Pfeffer's acute observations. He completed the investiga- 

 tions of his predecessors, Lindsay, Briicke, Hofmeister, Sachs, Cohn, Unger and 

 others, by demonstrating the escape of the water from the stimulated cells, a fact 

 which had not been clearly ascertained before. 



(a) The Sensithe Plant^ {Mimosa pudicd). 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 

 parenchyma with a feebly developed epidermis without stomata, and penetrated by 

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

 into several bundles where it emerges into the channelled 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 



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

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

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

 seq. — Sachs, Handb, der Exp,-rhys , 1866, p. 479 «/ s^y.— P. Bert, Recherches sur les mouvements 

 de la seusitive, Paiis 1867. 



