58 THE ELASTICITY AND COHESION OF THE PLANT-BODY 



side of a turgid shoot or root when forcibly bent, and in some cases, as for 

 example in the motile pulvini of Oxalis leaves, the active bending results in 

 a pronounced compression of the cells on the concave side. 



The escape of water takes a certain time, and hence partly arises 

 the fact that thin-walled cells and tissues behave like a stick of sealing-wax, 

 in that they are plastic under steady pressure and are capable of slow 

 bending, but are brittle to sudden pressure and snap when rapidly bent. 

 This brittlencss is only shown in the turgid condition, but is also largely the 

 result of the properties of the cell-walls. Hence it is not shown by the 

 turgid staminal filaments of Cynareae, whose cell-walls are capable of 

 considerable elastic stretching. Turgidity becomes of less and less im- 

 portance as the cell-walls grow stronger and thicker, and the resistance 

 to longitudinal tension is always independent of the turgor. In fact a 

 turgid shoot is torn asunder by a smaller tensile strain than when it is 

 flaccid, for in the first case the walls are already under considerable osmotic 

 tension. 



The arrangement and mode of union of the component cells are also 

 of considerable importance with regard to the elasticity and power of 

 stretching of tissues in response to tension. Thus in a loose tissue, as in 

 a chain of india-rubber rings, the elongation may largely result from 

 a change in shape of the component cells, coupled with a distortion of the 

 bounding walls of the air-spaces. In tissues composed of cylindrical cells 

 the elongation of the organ is the sum of that of the individual cell-walls, 

 so long as no internal displacement or rupture occurs 1 . The elasticity 

 of the cell-wall usually increases when stretching growth ceases ; growing 

 cells which are stretched considerably are naturally the ones which shorten 

 most when turgor is removed. Again, the growing regions of shoots and 

 roots, owing to the properties of their cell-walls, are more or less plastic, and 

 by steady pressure can be caused to assume abnormal curves or shapes, 

 a fact well known to gardeners. These curvatures become permanent if 

 the growing region is held fixed until adult tissue is formed. 



This plasticity was studied by Sachs and also de Vries 2 , and it is owing 

 to it that a strongly shaken shoot curves in the direction of the greatest 

 bending 3 . A drooping shoot can be more easily bent and wound around 



1 Nageli and Schwendener, Mikroskop, 1877, 2. Aufl., p. 404. 



2 Sachs, Lehrbuch, 1873, 3. Aufl., p. 691; Arbeit, d. Bot. Inst. in Wiirzburg, 1873, Bd. I, 

 P- 393 ; de Vries, I.e., 1874, Bd. I, p. 539. On the plasticity of tendrils cf. Pfeffer, Unters. a. d. 

 Bot. Inst. in Tubingen, 1885, Bd. I, p. 489. 



3 Hofmeister, Jahrb. f. wiss. Bot., 1860, Bd. n, p. 237; Prillieux, Ann. d. sci. nat., 1868, 

 5 e se'r., T. IX, p. 248 ; Sachs, Lehrb., 1873, 3. Aufl., p. 692. Kerner (Schutzmittel d. Pollens, 1871, 

 p. 34) observed similar curvatures in rubbed or shaken flower-stalks. On the decrease in the 

 diameter of shaken shoots cf. Kraus, Sitzungsb. d. naturf. Ges. z. Halle, 1881, p. 27. Shaking may 

 also act as a stimulus. 



