MECHANICS OF GROWTH. 
modification of the micellar 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 soHd 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 between these different layers. 
It may not be superfluous to add some explanatory observations relative to what 
we understand by Jensmi. 
