GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 699 



nature of the external forces which act upon it. Thus, for example, under a 

 sudden blow the apex of a root behaves like a brittle body, and breaks easily, while 

 it is flexible if slowly bent. 



If the form of an extensible body has been changed by pressure, traction, or 

 bending, and if, when then left to itself, it retains the form to which it has been 

 forced, it is called inelasiic ; if, on the other hand, it resumes its original form, it 

 is elastic. If the changes of form produced by external causes are small, they are 

 usually completely reversed when the body is left to itself, and within these limits 

 the body is perfecdy elastic; but if the change of form exceeds certain limits 

 dependent on the nature of the body and the length of time during which the force 

 has been acting, it does not again assume exactly its previous form. The greatest 

 amount of change which yet permits a complete restoration of the original form 

 determines the Limit of Elasticity o{ the body; when this is exceeded, the stretched 

 substance partially retains the form which it has been made to assume, and the less 

 complete the return to its primitive shape the more imperfect is its elasticity. It 

 would appear as if all organised bodies were imperfectly elastic to any long-continued 

 stretching 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 inor- 

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

 reference only to eff'ects visible externally ; the internal changes which bring about 

 the external effect may be very different in different bodies. Rigidity, /. e. 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 consisting mainly of 

 parenchyma. This is at once experimentally proved by the woody cylinder becom- 

 ing 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 celhwalls taking only a sub- 

 ordinate 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 w^hole ; 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 

 (/. <r. p. 405) 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 



