56 THE ELASTICITY AND COHESION OF THE PLANT-BODY 



normally contain little water and are relatively rigid 1 . The possible 

 amount of elastic stretching seems at the same time to increase, and it is 

 well known that completely dry cell-walls, if not impregnated with fat or 

 oil, are brittle and can be pulverized. In the living plant, however, the 

 cell-membranes are nearly or completely saturated with water, and hence 

 we need only discuss their properties in this condition. 



It is worthy of note that, at least in the mechanical cells, rupture 

 occurs almost immediately the limit of elasticity is exceeded, whereas the 

 breaking strain for metals is usually far beyond that at which they begin 

 to undergo permanent elongation or change of shape (best wrought iron = 

 32 and 63, strongest steel wire=5o and 130 kgs. per sq. mm. respectively) 2 . 

 This coincidence between the limits of elasticity and of cohesion (absolute 

 rigidity) in cell-walls is especially well shown in the wood of maples and 

 poplars, strips of which when subjected to longitudinal tension break 

 before the limit of elasticity is reached, and is in harmony with the fact that 

 a complicated structure cannot be strained beyond the limit of elasticity 

 with safety, even although no actual rupture occurs. In accordance with 

 this, the cell-walls of growing cells are not stretched up to their limits of 

 elasticity, although when subjected to increased tension they may undergo 

 a pronounced permanent elongation by plastic stretching. Thus Ambronn 3 

 has shown that the limit of elasticity of collenchyma cells is exceeded by 

 a load of i to 2 kgs. per sq. mm., whereas breaking occurs only when a load 

 of 8 to 12 kgs. is reached. It is worthy of note that the elongation ceases with 

 small loads in a few hours, possibly as the after effect of some such vital 

 action upon the cohesion of the cell-wall as may also influence the growth 

 in surface extent of the cell- wall. In any case an exact determination 

 of the limits of elasticity is rendered very difficult in the case of bodies 

 saturated with imbibed water, owing to the influence of tension and 

 pressure upon the amount of water they contain 4 , and hence upon their 



1 This seems to apply to all organized bodies capable of imbibition, whether derived from 

 animals or plants. According to Weinzierl (I.e., p. 460), the maximal tensile strength is reached 

 when the percentage of water is reduced to a certain low limit. 



2 [In the case of platinum wire, however, the breaking strain (34 kgs. per sq. mm.) is very little 

 greater than that at which permanent stretching begins, whereas strips of the wood of the oak and 

 birch undergo permanent elongation with loads of 2-3 and 1.6 kgs. per sq. mm. respectively, but break 

 only with loads of 5-6 and 4-3 kgs. per sq. mm. See Landolt and Bornstein's Tabellen, p. 275.] 



3 Ambronn, Jahrb. f. wiss. Bot., 1879-81, Bd. xn, p. 521; J. Cohn, ibid., 1892, Bd. xxiv, 

 p. 166 ; C. Miiller, Ber. d. Bot. Ges., 1890, p. 165 ; Haberlandt, 1. c., p. 139. 



4 Reinke, 1. c., p. 17 ; Lehmann, Molekularphysik, 1888, Bd. I, p. 530. [There is in such cases no 

 exact relationship between the amount of elongation and the stretching force, equal increments of the 

 latter producing less elongation beyond a certain limit ; that is, Young's modulus of elasticity increases 

 beyond a certain degree of tension, just as it does in the case of a muscle fibre. Cf. Mackendrick's 

 Physiology, Vol. I, p. 398. On the staminal filaments of Cynareae, cf. Pfeffer, 1. c., p. 108. The 

 modulus of elasticity of strongly stretched india-rubber and of elastic sinews increases progressively 

 as the tension increases, and this although the sectional area decreases and no water is lost.] 



