89^ 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 O. 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 Gynaraceae^ 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 organs 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 ceils 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 oi 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 fiir wiss. Zoologie, 

 vol. XII. Heft 3.— Kabsch, Bot. Zeit. 1861, No. 4.— Unger, Bot. Zeit. 1862, No. 15, and 1S63, 

 No. 46. — Pfeffer, Physiologische Untersuchungen, p. 80. 



