GEXERAL PROPERTIES OF GROWING PARTS OF PI ANTS. 



•OT 



R 



lil 



K 



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 w^ithout bursting, and if this cell- wall 

 is supposed to be extensible and elastic — as is espe- 

 cially the nature of growing and non-lignified cell- 

 w^alls — the question presents itbelf: — What changes 

 does the turgidity of the cell undergo when it is 

 stretched or compressed by external forces or other- 

 wise altered in form? This question can be sufficiently 

 answered for our purpose by the simple contrivance 

 represented in Fig. 448. i^ is a wide and thick 

 india-rubber tube to which the glass tube S, closed 

 at g, acts as a stopper. After filling A' with water, 

 the glass-tube i?, open below at 0, is fixed in and firmly 

 fastened, the level of the water standing somewhere 

 about ;/ in the thin drawn-out upper end of the 

 tube. In order to give to the india-rubber tube, 

 which here represents the cell-w^all, a sufficient ten- 

 sion from the outset, it is convenient to make the 

 thin end of the tube 7? from 20 to 30 cm. long, and 

 to raise the level ;z in proportion. The wide part of 

 R is fixed in a holder, so that the cell hangs down. 

 A condition of equilibrium is thus established between 

 the elasticity of the india-rubber tube and the hydro- 

 static pressure which can be compared with the tur- 

 gidity of the vegetable cell ; and in this condition the 

 water-level stands at 71. If the tube S is now pulled 

 downwards, the elastic tube is lengthened and at the 

 same tinie made narrower, but the amount of space 

 enclosed by it is increased, as may be seen by the 

 falling of the water-level n in the narrow glass tube. If on the other hand the 

 glass tube S is pushed up and the india-rubber tube thus compressed without any 

 bending or creasing taking place in /sT, the space enclosed by the tube I^ is dimin- 

 ished, as is shown by the rising of the water-level n. The same thing takes place 

 when the tube K is bent in any way, or when it is compressed on any side. 



It is evident that if the upper glass tube 7? were closed at n so as to prevent a 

 rise or fall of the water-level, any change which previously caused a rise of the level 

 w^ould now occasion an increase of the hydrostatic pressure, and vice versa. It 

 therefore be stated that in a closed and turgid cell any pressure acting from 

 without or any curvature increases the turgidity, while any stretching of the cell 



i! s 



\ 



Fig. 448.— Apparatus for illustrating 

 the chanjje in the turgidity of cells 

 caused by external distension or com- 

 pression. 



