7l6 MECHANICAL LAWS OF GROWTH, 



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

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

 of division this inner layer retains its water of imbibition; but the outer layer, w^iich 

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

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

 is clear however that these phenomena must also occur when w^ater 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 disicnsion 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 w^hole 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 dimin- 

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

 cell-wall, the former 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-w^all 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- 

 drawn from it. No increase of turgidity can therefore take place in this case, and 

 still less when the cell also increases in size. The same argument of course applies 

 also to a multicellular mass of tissue. But the case is different when the Avater with- 

 drawn from the cell-contents by the cell-w^all is replaced by means of endosmose,- 

 and the turgidity thus again increased ; in this case in proportion as water is absorbed 

 by the cell-wall the turgidity and volume of the whole cell must also increase. 



B. Mutual Tension of the layers of tissue of an organ, (i) Tension in the direction 

 of length; i.e. parallel to the axis of growth of the organ. In the internodes of 

 upright stems some idea may be ob:ained, if not of the intensity of the tension, at 

 least of its kind (whether negative or positive), and of its variation in the different 

 layers of tissue, by measuring the length of the internodes, separating the layers 



* 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 development of heat 

 during imbibition indicates that a decrease of volume is taking place, and therefore that although v 

 is the amount of water absorbed by imbibition, the increase of volume is only v — d. 



