6^2 LECTURE XXX VIT. 



divided by a longitudinal wall. Between the parench3'ma-cells lie spacious inter- 

 cellular passages; the centre of the parenchyma is traversed by a delicate fibro- 

 vascular strand, which, like the epidermis, is strongly extended by the turgescent 

 parenchyma. 



If in the first-named preparation one of the filaments, curved convexly outwards 

 and fastened to the corolla below and to the anther-tube above, is touched, it becomes 

 straight and thus shorter, and applied to the style; if this happens with all the 

 filaments, their shortening is rendered noticeable by the downward withdrawal of the 

 anther-tube. After a few minutes the filaments again elongate, becoming arched 

 convexly outwards, and are then again irritable. If the second kind of preparation 

 is employed, where the filaments are cut off and freely movable below, it is easy to 

 convince one's self that every time they are touched a rapid movement follows : if 

 the outside is touched this becomes first concave and then convex, if the inside is 

 touched this becomes concave and then occasionally likewise convex. The shorten- 

 ing of the irritated filament begins at the moment of contact, and soon reaches its 

 maximum, whereupon the elongation begins again at once, and this proceeds 

 rapidly at first and then more slowly. 



As to the mechanics of these movements, we have an investigation of Pfeflfer's 

 in which the filaments oi Cynara Scolymus and Centanrca jacca were chiefly employed. 

 The following embraces the more important results. 



The filaments of the species mentioned are 4-6 mm. long : the tangential 

 diameter of those of the Artichoke (Cynara) is about 0-42 mm, the radial diameter 

 o'2 mm.; in Caitnurea about o'24 and o"i4 mm. The axial vascular bundle is thin 

 and delicate, the irritable parenchyma-cells in Cynara 2-3 times, in Cen/aurea 4-6 

 times as long as broad. Their transverse walls are at right-angles to the longitudinal 

 axis: all the cell-walls, even those of the strand, are thin, only the outer walls of the 

 epidermis being thickened to any considerable extent. The very abundant cell-sap 

 of the parenchyma-cells is surrounded by peripheral protoplasm, which is relatively 

 abundant, and in which Hes a nucleus; the protoplasm exhibits rotation. In the cell- 

 sap a little tannin and a good deal of glucose are dissolved. 



The filaments are irritable along their whole length, i. e. they can shorten them- 

 selves in consequence of contact anywhere. By means of special apparatus Pfeffer 

 succeeded in measuring the shortening under powers of 100 or 200 diameters. The 

 shortening may amount to 8-22 % of the length in the non-irritated condition, a 

 thickening of the filament occurring at the same time, which is, however, far too 

 slight to correlate the shortening with mere change of form ; it points rather to a 

 very considerable diminution in volume. This decrease in volume is due to the 

 escape of water from the cells into the intercellular spaces; it wells forth from these 

 spaces on transverse sections of the filament just as in the organs of the Mimosge, as 

 Pfeffer directly observed. If the intercellular spaces are filled with water by injec- 

 tion, the filaments are still irritable, and the expulsion of water at the transver?e 

 section in consequence of a stimulus is then still more evident. 



The filaments are very extensible and at the same time perfectlv elastic ; they 

 may be stretched to double their length, and yet they will contract again to their 

 original dimensions. 



In the irritable condition the axial strand and the epidermis are strongly 



