700 MECHANICAL LAWS OF GROWTH. 



similar forces do not perceptibly change the arrangement of the atoms in the crystalline 

 molecules, but that only the distance of the molecules themselves from one another 

 is increased or diminished. On the other hand it is known that water is retained 

 in the moist cell-walls with great force ; and microscopic examination has shown 

 that it cannot be forced out by the bending compression of the part. No other 

 hypothesis is therefore possible, except that the amount of water in a distended cell- 

 wall is the same as in one in a neutral condition. The particles of water are there- 

 fore merely displaced by external forces, but are not forced out ; they move, for 

 example, with the bending of the part from the concave to the convex side, but 

 afterwards fill up as completely as before the molecular interstices of the substance ; 

 and, since the sum of their tensions is but slightly altered, also occupy nearly the 

 same space. If the same reasoning is applied to tissues without intercellular spaces 

 and filled with sap, it is perfectly obvious that the cell-walls are not susceptible 

 of change of volume any more than in the previous case. The same is the case 

 also with the fluid contained in the cells. The only question now remaining is 

 whether the changes of tension which are caused by external forces modify the 

 permeability of the cell-walls at least in places. If this were the case, then when 

 a tissue is compressed — since the hydrostatic pressure (turgidity) is in no case 

 decreased by it, but the resistance of the cell-wall weakened' — a part of the cell- 

 fluid must obviously be forced out, until the hydrostatic pressure has again reached 

 an equilibrium with the diminished resistance of the cell-walls. In the same 

 manner the eftect of traction on a tissue must be to cause an influx of water 

 through it, or, if this is prevented, the formation of an empty space^. If, on the 

 other hand, the changes of tension which occur in plants have no perceptible 

 influence on permeability, the tissues simply possess the properties of moist cell- 

 walls; in any condition of tension"' they always occupy the same space ^.' 



In order to understand many of the phenomena now to be described, it is 

 necessary to have a clear conception of the changes which a cell filled with sap 

 undergoes in reference to its turgidity when it is compressed or stretched or 

 simply bent by external forces. By Turgidity we understand the hydrostatic pres- 

 sure which the water absorbed by endosmose exercises ecjually on all sides on the 

 cell-wall, and which reacts on the contents in consequence of the elasticity of the 

 cell-wall ; so that in a turo-id cell, while the cell-wall is stretched, the contents 

 are compressed. A clear conception of this state of mutual tension of the cell- 

 wall and cell-contents may be obtained by closing a short wide glass tube at one 

 end with a firm fresh bladder free from holes, pouring in a concentrated solution of 

 sugar or gum, and finally closing also the other end with a thick bladder. This 

 artificial cell, placed in water, absorbs it by endosmose with great force ; the pieces 



* These words are not clearly intelligible. Turgidity or the tension of the cell-wall is always 

 increased, as we shall see directly, by pressure from without on a turgid cell; its resistance to 

 infiltration may in this manner be at length entirely overcome. 



^ Of course only when the cell-wall does not become folded. 



^ By tension is here clearly meant bending, stretching, or pressure from external forces. 



* The discussion given on p. 373 of the work quoted with respect to the alteration of the mole- 

 cular structure of cell-walls by violent mechanical and chemical forces is of no importance for our 

 present purpose. 



