894 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 
it serves to prove that an alteration of the permeability of protoplasm is possible. 
Moreover it is known that the protoplasm of many cells contracts when they are 
subjected to pressure from without, and this is only possible if an escape of water takes 
place. If, under these circumstances, the cell-wall remains fixed, it is because it was but 
slightly stretched and comparatively inextensible before. If the wall of a cell which has 
been so treated were to contract as considerably as the protoplasm, we should have 
a result similar to that which was assumed above with reference to a stimulus. These 
considerations suggest that the irritability, in its narrowest sense, is a property of the 
protoplasm only, and that it is essential to the existence of irritable organs that the 
irritable protoplasm should be surrounded by cell-walls which are tense in consequence 
of the turgidity of the cells and which can follow the contractions of the protoplasm in 
virtue of their perfect elasticity. 
(c) The irritable stamens of Berberis^ differ considerably in the mechanism of their 
movements from those of Cynarese, especially in that they are irritable not upon the outer 
side but upon the inner side only ; further, the irritable parenchyma includes no inter- 
cellular spaces, but between the thin-walled cells there is a quantity of ' intercellular 
substance ' which is capable of swelling. If the inner side of the filament be touched, 
it curves throughout its whole length. Pfeffer was able to observe in this case also that 
when the filament is cut across, stimulation causes the escape of a drop of water from the 
cut surface. 
(d) Too little is at present known of the mechanism of the irritable gynostemium of 
Stylidium, of the leaves of Dioncea muscipula, of the glands upon the leaves of Drosera, of 
the segments of the stigma of Mimulus, &c., to admit of brief yet satisfactory 
treatment ^. 
Sect. 30.— Mechanism of the Movements produced by Variations of 
Temperature and of Light ^ If plants with motile leaves, like Papilionacese and 
Oxalideae, after having remained in the light, are suddenly placed in the dark, the 
leaves after some time take up their nocturnal position, closing upwards or down- 
wards according to the species (Sect. 28). If light is now let in upon the plant 
in the state of sleep, the leaves again open and assume their diurnal position. 
Placing them in the shade has the same effect as complete darkness, but not so 
strongly. 
These facts show that fluctuations in the intensity of the light cause curvatures 
of the motile parts of plants. If these parts are also irritable to concussion, as in 
Mimosa and Oxalis acetosella, darkness causes a similar position of the leaves to 
concussion. But the internal conditions are, as has been mentioned, very different 
in the two cases ; for the folding up caused by darkness is associated with an increase 
^ Unger, Anat. und Physiol, der Pflanze, 1855, p. 419. — Kabsch, Bot. Zeit., 1861, p. 26. — Pfeffer, 
Physiol. Unters., p. 157. 
^ See also Unger, Anat. u. Phys,, 1855, P-4I9- — Suringar (on Drosera), Vereenigung voor de 
Flora van Nederland eng. den 15 Juli, 1853. — Nitschke (on Drosera), Bot. Zeit. i860. — Schnetzler (on 
Berberis), in Bulletin de la Societe vaudoise des Sei. Nat. X, 1869. — Kabsch (on Berheris, Mimulus, 
See), Bot. Zeit. 1861.— Kabsch (on Stylidium), Bot. Zeit. 1861.— A. W. Bennett, The Movements 
of the Glands oi Drosera, Quart. Journ. Micr. Sei. 1873. 
[See also Kurz, Anatomie des Blattes der Dioncea muscipula ; Du Bois Reymond's Archiv, 1876 : 
also Münk, Die elektrischen und Bevi^egungs-Erscheinungen am Blatte der Dioncea Muscipula, ibid.; 
further, Darwin, Insectivorous Plants, 1875.] 
^ Dutrochet, Mem. pour servir, vol. I. p. 509. — Meyen, Neues Syst. der Pflanz.-Phys. vol. III. 
p. 487. — Sachs, Bot. Zeit. 1857, Nos. 46, 47. — Bert, Recherches sur les mouvements de la sensitive, 
Paris 1867. — Millardet, Nouvelles recherches sur la periodicite de la sensitive, Marburg 1869. — 
Pfeffer, Physiol. Unters., Leipzig 1873. 
