783 



MECHANICS OF GROWTH, 



R 



somewhere about n in the thin drawn-out upper end of the tube. In order to give 

 to the india-rubber tube, which here represents the cell-wall, a sufficient tension from 

 the outset, it is convenient to make the thin end of the tube H from 20 to 30 cm. 

 long, and to raise the level n in proportion. The wide part of 7? 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 hydrostatic pressure which 

 can be compared with the turgidity of the vegetable cell ; and in this condition the 

 water-level stands at «. If the tube S is now pulled downwards, the elastic tube 

 is lengthened and at the same time 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 X, the space enclosed by the tube K is 

 diminished, as is shown by the rising of the water-level n. 

 The same thing takes place when the tube J^ is bent in 

 any way, or when it is compressed on any side ^ 



It is evident that if the upper glass tube R 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 would 

 now occasion an increase of the hydrostatic pressure, and 

 vice versa. It may 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 diminishes it. If we imagine a straight succulent 

 stem or a growing root to be bent, the cells on the con- 

 vex side will be stretched, those on the concave side com- 

 pressed, and the turgidity will be diminished in proportion 

 in the former and increased in the latter. This result is 

 very clearly confirmed if a very succulent rapidly growing 

 internode of the Grape-Vine is slowly but firmly bent till 

 it describes about a semicircle. It will be observed that 

 during the bending a number of small drops of water 

 escape in rows from the epidermis on the concave com- 

 pressed and shortened side. It is indifferent whether they 

 escape through fissures or are forced out through the cell- 

 walls; in either case they show that the cells display a 

 higher degree of turgidity on the concave compressed side than when the internode 

 was straight. 



In the present state of our knowledge, if we would keep clear of uncertain 

 speculations, the considerations now given must be considered as by no means 

 complete ; but they are sufficient to draw the attention to conditions which must be 

 taken into account as existing in the interior of the growing parts of plants when 

 they are subject to pressure, traction, bending, and so forth, from external forces. 



\ 



Fig. 478.— Apparatus for illustrating 

 the change in the turgidity of cells 

 caused by external distension or com- 

 pression. 



See also Pfeffer, Physiol. Unters. 1873, p. 121. 



