780 MECHANICS OF GROWTH. 



would appear as if all bodies were imperfecdy elastic to any long-continued stretch- 

 ing or alteration of form, and as if there were no limit of elasticity in the case 

 of very long-continued but weak external influence. In all these points organised 

 bodies, especially the growing parts of plants, exhibit the same phenomena as unor- 

 ganised bodies. It must however be remembered that the terms explained above 

 have reference only to effects visible externally ; the internal changes which bring 

 about the same external effect may be very different in different bodies. Rigidity, 

 i. €. resistance to bending, depends, for example, evidently on very different internal 

 conditions in the case of a woody cylinder and of a succulent stem or root consist- 

 ing mainly of parenchyma. This is at once experimentally proved by the woody 

 cylinder becoming less flexible and even brittle from loss of water, while the 

 flexibility of succulent parenchyma is thereby increased. This is readily understood 

 on recollecting that the flexibility of the woody cylinder depends on that of the walls 

 of the wood- cells, which are not closed cavities, and therefore cannot become turgid, 

 while the flexibility of parenchymatous tissue depends on the change of form of the 

 closed turgescent cells, the extensibility and elasticity of the cell-walls taking only 

 a subordinate part. Changes of form take place however more easily the less the 

 turgidity of the cells ; a parenchymatous tissue may be compared to an aggre- 

 gation of bladders each of which is full of water; if they are all turgid with water, 

 each bladder is tense and rigid, as also is the whole ; if, on the contrary, they 

 contain only enough water to fill without distending them, each separate bladder is 

 flaccid, as also is the whole, which can therefore be bent in any direction. A mass 

 of parenchyma may therefore be stiff and rigid even if its cell-walls are thin and 

 very flexible, if only they are firm enough not to give way from the pressure of the 

 water which stretches them, or to allow it to filter through. The flexibility and elas- 

 ticity of the moist cell-wall cannot however be compared directly with these pro- 

 perties in a perfectly dry cell-wall or a strip of metal, as Nageli and Schwendener 

 {I.e. p. 399) have already shown. 'If we consider first of all,' they say, 'a frag- 

 ment of moist cell-wall, say a lamella of the thallus of Caulerpa, a bast-fibre 

 thickened so that the cell-cavity has disappeared, a spiral vessel, and so forth, it is 

 proved by their behaviour to polarised light that stretchings, bendings, and other 

 similar forces do not perceptibly change the arrangement of the atoms in the crys- 

 talline micellae, but that only the distance of the micellae themselves from one 

 another is increased or diminished. On the other hand it is known that water 

 is retained in the moist cell-walls with great force; and microscopic examination 

 has shown that it cannot be forced out by bending or by compression of the part. 

 No other hypothesis is therefore possible, except that the amount of water in a tense 

 cell-wall is the same as in one in a neutral condition. The particles of water are 

 therefore merely displaced by external forces, but are not forced out ; they move, 

 for example, with the bending of the part from the concave to the convex side, but 

 afterwards fill up as completely as before the micellar interstices of the substance ; 

 and, since the sum of their tensions is but slightly altered, also occupy nearly the 

 same space. If the same reasoning is applied to tissues without intercellular spaces 

 and filled with sap, it is perfectly obvious that the cell-walls are not susceptible 

 of change of volume any more than in the previous case. The same is the case 

 also with the fluid contained in the cells. The only question now remaining is 



