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 upv\^ards or downwards. 
The organ is not irritable immediately after this has taken place, for the con- 
tracted cells are not sufficiently 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, Brücke, 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. ' r. 
(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 o*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 stifi" 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. — Brücke, in Müller's Archiv für 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.-Phys , 1866, ip. ^"/g et seq. — P. Bert, Recherches sur les mouvements 
de la sensitive, Paris 1867. 
