SgZ PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 
therefore that a portion of the water passes from the contractile organ into the tissue 
of the petiole and lamina. The depression of the leaves of 0. Acetosella and stricta 
when sunlight falls suddenly upon them is, like the irritable movements, attended with 
flaccidity, and according to Pfeffer, is of the same nature. 
(b) The external features of the phenomena of irritability exhibited by the stamens 
of Cynaracese^ in the normal condition have already been described. For a close 
examination of them it is necessary to remove single flowers from the capitulum, and to 
cut away the corolla from below as far as the point of insertion of the filaments, or to 
cut across the corolla-tube, stamens, and style above the insertion of the filaments, and 
to fix the reproductive orgfäns which are thus isolated by means of a pin in damp air. 
When the filaments have recovered from the irritation caused by this operation, they are 
convex outwards. The filaments are flat and strap-shaped ; they consist of three or four 
layers of long cylindrical parenchymatous cells, separated by thin straight walls, and sur- 
rounded by a layer of epidermal cells of similar form, strongly cuticularised and growing 
out in many places to hairs, each of which is cut off by a longitudinal wall. Intercellular 
spaces of considerable size lie between the parenchymatous cells ; through the middle of 
the parenchyma passes a delicate fibro-vascular bundle, which, like the epidermis, is 
strongly stretched by the turgid parenchyma. 
If the flower has been dissected according to the plan first described, and one of the 
filaments, curved convexly outwards and fixed below to the corolla, above to the anther- 
tube, is touched, it becomes straight and therefore shorter and in contact along its 
whole length with the style. If all the filaments are touched, it is seen that they have 
considerably decreased in length so as to draw down the anther-tube. After a few 
minutes they resume their original length and curvature, and are then again irritable. 
If the corolla has been dissected according to the second mode, where the filaments are 
cut away and can move freely below, it is easy to see that every time they are touched 
a curvature immediately ensues ; if the outer side is touched, it becomes at first concave, 
then convex ; if the inner side is touched, it becomes concave, and sometimes after- 
wards convex. The contraction of the stimulated filament begins at the moment of 
contact, after some time reaches its maximum, and the organ then at once begins again 
to lengthen, at first quickly, then more slowly. 
With regard to the mechanism of these movements, we are in possession of Pfeffer's 
most acute observations made for the most part upon the filaments of Cynara Scolymus 
and Centaurea jacea. The following is a summary of his most important results. 
The filaments are from 4 to 6 mm. long in these species : the tangential diameter 
of those of Cynara is 0*42 mm., the radial 0*2 mm.; in Centaurea the measurements are 
0*24 and 0*14 mm. The axial fibro-vascular bundle is thin and delicate. The irritable 
parenchymatous cells are in Cynara two or three times, in Centaurea from four to six 
times as long as they are broad, and their transverse walls are at right angles to the long 
axis. All the walls of the cells, even of those forming the bundle, are thin : only the 
external walls of the cells of the epidermis are somewhat thickened. The abundant 
cell-sap of the parenchymatous cells is surrounded by a parietal layer of protoplasm 
of moderate thickness, in which lies a nucleus. The protoplasm exhibits rotation. Some 
tannin and a considerable quantity of glucose is dissolved in the cell-sap. 
The filaments are irritable throughout their whole length, that is, they will contract 
at any point if touched. Pfeffer succeeded by especial contrivances to magnify the 
contractions one or two hundred times. The contraction may amount to from eight 
to twenty-two per cent, of the length of the filament when at rest. It is accompanied 
by a thickening of the filament, which is, however, too slight to suggest that the 
contraction produces merely a change of form ; it rather indicates a considerable 
^ Cohn, Contractile Gewebe im Pflanzenreich, Breslau 1861 ; ditto, Zeitschrift für wiss. Zoologie, 
vol. XII. Heft 3.— Kabsch, Bot. Zeit. 1861, No. 4.— Unger, Bot. Zeit. 1862, No. 15, and 1863, 
No. 46. — Pfeffer, Physiologische Untersuchungen, p. 80. 
