792 MECHANICS OF GROWTH. 



modification of the miccllar forces caused by these conditions which render possible 

 the intercalation of new solid particles among those already in existence. If, for 

 example, a cell-wall is stretched by turgidity, the distance of its micellae increased, 

 and possibly a different arrangement of them brought about, this state may be re- 

 versed on the cessation of the turgidity, by the elasticity of the cell-wall. But if, 

 during the condition of tension, growth takes place by the intercalation of new 

 solid micellae, the tension of the cell-wall is altered and in general diminished. If 

 now the turgidity ceases as before, a new condition of equilibrium occurs in the cell- 

 wall ; a permanent change has been effected by growth, which was rendered possible 

 by hydrostatic pressure and imbibition. 



The share taken by growth in the tension of the tissues amounts to this : new 

 solid micellae are intercalated, and the tension due to imbibition and turgidity is 

 thereby partially neutralised. This is however only momentary; for after the inter- 

 calation of new micellae the turgidity again increases, the degree of imbibition is 

 modified, new tensions are again caused, which on their part are partially neutralised 

 by the intercalation of fresh solid micellae. It is probably near the truth to suppose 

 that the limit of the elasticity of the growing cell-walls is constantly nearly reached 

 by turgidity and imbibition as well as by the secondary tensions produced by them, 

 and that on the other hand the tension is constantly being diminished by the inter- 

 calation of new micellae. Growth may therefore be described as a constant over- 

 stepping of the limit of elasticity of the growing cell-wall which is constantly 

 neutralised by the intercalation of additional solid micellae. 



It will of course be understood that in the brief description now given we do 

 not mean to state a theory of growth, but only to indicate in general terms the 

 mechanical effect exercised by growth on the tension of tissues, and conversely. 

 It would be easy to deduce the explanation in particular cases. If, for example, 

 a cell-wall is imagined distended by turgescence or by traction exerted by the sur- 

 rounding tissue, the intercalation of solid particles in the layers of cellulose already 

 present may take place to a greater or less extent, causing a differentiation in their 

 extensibility, elasticity, and power of imbibition, and thus leading to mutual tensions 

 of the layers, as may be seen almost invariably in thin transverse sections of the 

 cells of plants, and especially in the outer walls of those of the epidermis. But 

 these differences in the mode of intercalation in the different layers of the same 

 passively distended cell-wall may depend on a variety of circumstances; as, for 

 instance, on the degree of proximity of the layers to the protoplasm, on whether 

 they are in contact externally with the air, &c. But growth by intercalation may 

 also vary according to the nature of the tissue of which the cell forms a part, or the 

 chemical properties of the cell-contents, and according as the cells are passively 

 distended or compressed by other cells. All these considerations are however merely 

 hypothetical, and simply indicate the nature of the relations between growth by 

 intercalation and the tensions caused directly by imbibition and turgidity. It may 

 in any case be regarded as certain that intercalation is only possible as the result 

 of imbibition and turgidity ; but that these properties, as well as extensibility and 

 elasticity, must, or at least may be, in their turn modified by it. The volume of the 

 growing part increases; and since this takes place in different degrees in different 

 layers of the same cell-wall, and in different layers of the tissue of the same organ, 

 tensions varying in degree must be produced betv^^een these different layers. 



It may not be superfluous to add some explanatory observations relative to what 

 we understand by Tension. 



