m 



GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 781 



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

 rmeability 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 effect of traction on a tissue must be to cause an influx of water 

 into it, or, if this is prevented, the formation of an empty spaced 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 equally 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 turgid 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 

 of bladder which were previously stiff and tense arch into a hemispherical form 

 and offer great resistance to pressure. If a hole is punctured by a fine needle in 

 the bladder, a jet of fluid several feet in height springs from it. The force which 

 drives out the fluid with such violence is the elasticity of the stretched bladder ; 

 but the cause which brings this elasticity into play is the endosmotic attraction for 

 water of the fluid contained in the cell. 



If we suppose in the case of a vegetable cell enclosed on all sides a degree of 

 turgidity sufficient to stretch the cell-wall perceptibly, but leaving it still capable 

 of further tension without bursting, and if this cell-wall is supposed to be extensible 

 and elastic — as is especially the nature of growing and non-lignified cell-walls — the 

 question presents itself : — What changes does the turgidity of the cell undergo when 

 it is stretched or compressed by external forces or otherwise altered in form ? This 

 question can be sufficiently answered for our purpose by the simple contrivance 

 represented in Fig. 478. AT is a wide and thick india-rubber tube to which the glass 

 tube iS, closed at g^ acts as a stopper. After filling K with water, the glass-tube i?, 

 open below at 0, is fixed in and firmly fastened, the level of the water standing 



^ 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 

 filtration 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. 354 of the work quoted with respect to the alteration . of the 

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

 present purpose. 



