712 MECHANICAL LAWS OF GROWTH. 



3. Growth itself must cause states of tension in the layers of a cell-wall or of 

 the tissue of which an organ is composed, if the layers, although firmly united to 

 one another, grow unequally. It is however much more difficult to understand 

 the modifications of tension due to growth than those due to turgidity and imbi- 

 bition, as the former cannot be altered artificially without a material change being 

 caused also in the latter. Since the growth of every organised structure, such as 

 a cell-wall, can only proceed so long as it is permeated with water, and since 

 moreover the growth of the entire cell requires it to be in a turgid condition, and 

 this condition itself has an influence on growth, it is extremely difficult to decide 

 how far each of these phenomena is the cause of the other. If by growth we under- 

 stand, according to the definition already given, only permanent and irreversible 

 changes of organisation, affecting in the first place the molecular structure of the 

 organism, it may be assumed, in accordance with the present state of our know- 

 ledge, that growth is always preceded by imbibition and turgidity, and that it is the 

 tensions of the molecular 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 molecules 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 molecules, 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 is caused by growth, which was however rendered 

 possible by hydrostatic pressure and imbibition. 



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

 new molecules are intercalated in the original substance, and the tension due to 

 imbibition and turgidity is thereby partially neutralised. This is however only 

 momentary; for after the intercalation of new molecules 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 molecules. It is pro- 

 bably 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. But on the other hand the tension is con- 

 stantly being diminished by the intercalation of new molecules. Growth may there- 

 fore be described as a constant overstepping of the limit of elasticity of the growing 

 cell-wall constantly neutralised by intussusception. 



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 between the layers of cellulose 

 already in existence may take place to a greater or less extent, causing a differentiation 

 in their extensibility, elasticity, and power of imbibition, g,nd 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 



