PHENOMENA DUE TO THE TENSION OF TISSUES. 795 



cell-wall absorbing more water in the direction parallel to their surface than the 

 outer layers, and thus stretching more and becoming the convex side of the system. 

 When water is withdrawn the opposite result must ensue. Let us suppose the cell 

 to be closed and entire and not at all or scarcely turgid, i. e. with no hydrostatic 

 pressure between cell-wall and cell-contents : the inner face of the cell-wall will then 

 be in contact with the cell-sap, and will absorb more water than the outside ; a 

 tension will therefore be produced, the inner layers of the cell-wall having a tendency 

 to stretch, and being partially prevented from so doing by the outer layers. This 

 tension of its layers will impart to the cell-wall a certain stiffness and rigidity which 

 is quite unconnected with turgidity. But since in the normal state, and especially 

 when they are growing, cells are always turgid, the whole system of tissues will be 

 distended independently of this. 



If narrow strips are cut out of large succulent cells, or very thin slices of tissue 

 are made so as not to contain any perfect cells, a concave outward curvature is 

 obtained at the moment of making the section. This is at once explained by recol- 

 lecting that the outer layer, especially when cuticularised, was in a state of passive 

 tension before the section was made; while the inner layer, which was in an 

 absorbent condition, was swelled up from contact with the cell-sap. At the moment 

 of division this inner layer retains its water of imbibition ; but the outer layer, which 

 was in a state of greater tension, obeys its elasticity, and in consequence of its 

 contraction becomes the concave, the inner the convex surface of the section. It 

 is clear however that these phenomena must also occur when water is removed or 

 absorbed. It is only in this way that it seems to me possible for the cell-walls to 

 take any part in the tension of the tissues, a part which however must always be 

 subordinate in the closed living cell to the influence of turgidity, since this stretches 

 both the inner and outer layers, and every change in the degree of turgidity must 

 cause contraction or distension of the entire cell- wall. 



It is a question not without importance in what relation the imbibition and 

 swelling of the cell-wall stand to the turgidity of the whole cell. If we imagine a 

 single turgid cell, and suppose that from any cause the cell- wall (whether the layers 

 are in a state of tension or not) is able to absorb more water from its contents 

 than it had before, the question arises whether the turgidity is thus increased or 

 diminished. By the increased amount of water absorbed from the contents by the 

 cell-wall, they must be diminished, as also must the hydrostatic pressure on the 

 cell-wall, and the more so when the size of the cell is increased by the imbibition. 

 But since the cell-wall may also increase in thickness, the pressure on the contents 

 may be supposed to increase from this cause. If however we take the simplest and 

 least favourable case, viz. that the size of the cell remains unaltered but the thickness 

 of the wall increases, and therefore that it distends inwardly, this will nevertheless 

 not cause any increased pressure between cell-wall and contents, because the water 

 which was the sole cause of the thickening of the cell-wall and diminution of the 

 cell-cavity was withdrawn from the cavity. The swelling of the cell-wall can at the 

 most diminish the size of the cell-cavity ^ by the volume occupied by the water with- 



^ When an amount of water v penetrates into an organised body, and increases its volume, the 

 increase of volume can never be greater than v, but at the most as large. The evolution of heat 



