ELASTICITY AND COHESION OF TISSUES 59 



a stick than a turgid one, and the new position may be rendered permanent 

 by growth when turgidity is restored. 



Rigidity and tension. The tension of a cell-wall due to turgor renders the 

 cell more or less rigid, in the same way as when air or water is forced into a bladder 

 or balloon. There is, however, no exact relation between the tension of the 

 wall and the rigidity of the cell. Still less is this the case in tissues, for here 

 the tissue-strains are variously modified by changes of turgidity, and the arrange- 

 ment of the component cells is also of importance. The fundamental mechanical 

 principles involved are dealt with by Nageli and Schwendener, and the latter also 

 discusses the mechanics of systems of tied girders 1 . Hofmeister 2 ignored the 

 importance of turgidity, and his discussions are by no means clear. 



The amount of stretching is always dependent upon the elasticity and thickness 

 of the cell-wall, as well as upon the osmotic pressure. The amount of tangential 

 stretching varies inversely as the square of the radius of curvature, and hence 

 small thin-walled cells are able to resist a high internal osmotic pressure, since 

 the free portions of the walls are always strongly curved. In cylindrical cells 

 the force of longitudinal extension due to the internal hydrostatic pressure is 

 dependent upon the sectional area, and decreases with it 3 . Thick and highly 

 elastic walls experience a very slight and often hardly perceptible shortening 

 when turgor is removed by plasmolysis or death, but in growing cells, and in 

 the staminal filaments of Cynareae, a pronounced shortening can be produced in 

 this manner. By attaching weights until the tissue has regained its original 

 length, it is possible in some cases to approximately calculate the original osmotic 

 pressure from these data and from the sectional area of the tissue. 



A plasmolysed cylindrical cell, when forcibly stretched, decreases in diameter 

 for the same reasons as in the case of a stretched india-rubber tube. The latter 

 becomes distended when water is forced into it, and to produce elongation 

 without any lateral distension would require very special arrangements. The 

 necessary conditions are, however, fulfilled in the cells of the staminal filaments 

 of Cynareae, for these may be elongated 20 to 30 per cent, of their length by 

 increasing turgor, without their diameters being altered 4 . This is possibly partly 

 due to the unequal elasticities of the different portions of the cell-wall, and it 

 is in fact possible in such cases that partial plasmolysis may cause a cell to 

 elongate slightly. 



A cylinder of metal or india-rubber, when strongly stretched, experiences 

 a slight increase of volume 5 , and this presumably applies also to cylindrical 

 cells when under considerable tension. The fall of hydrostatic pressure produced 

 thereby is of little importance, for the absorption of water will rapidly restore 

 the original osmotic pressure. 



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

 Princip im Bau der Monocotylen, 1874, p. 109. 



2 Hofmeister, Pflanzenzelle, 1867, PP- z68 > 2 73- Cf. Pfeffer, Physiol. Unters., 1873, p. 147. 



3 Pfeffer, Period. Beweg. d. Blattorgane, 1875, p. 114; Nageli and Schwendener, l.c., p. 412. 



4 Pfeffer, Physiol. Unters., 1873, p. 103. 

 8 Nageli and Schwendener, 1. c., p. 399. 



