782 
MECHANICS OF GROWTH, 
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 jR from 20 to 30 cm. 
long, and to raise the level n in proportion. The wide part of 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 n. If the tube 6' 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 K, 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 is bent in 
any way, or when it is compressed on any side ^ 
It is evident that if the upper glass tube 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 
z'üe 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. 
