GENERAL PROPERTIES OF GROWING 
PARTS 
OF PLANTS. 
781 
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 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 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 equally on all sides on the 
cell-wall, and which reacts on the contents in consequence of the elasticity of the 
cell-wafl; so that in a turgid cell, while the cell-waU is stretched, the contents 
are compressed. A clear conception of this state of miUtual tension of the cell- 
wafl 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 cefl, 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 cefl enclosed on afl sides a degree of 
turgidity sufficient to stretch the cefl-wall perceptibly, but leaving it still capable 
of further tension without bursting, and if this cefl-wall is supposed to be extensible 
and elastic — as is especially the nature of growing and non-lignified cell-wafls — 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. is a wide and thick india-rubber tube to which the glass 
tube closed at^, acts as a stopper. After filling ^ with water, the glass-tube i?, 
open below at 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 alwa}S 
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. 
3 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 
micellar structure of cell-walls by violent mechanical and chemical forces is of no importance for our 
present purpose. 
