794 
MECHANICS OF GROWTH, 
one of them to contract or expand, while the length of the other apparently does 
not change, both layers were nevertheless in a state of tension, only the one which 
remained unchanged in length was but slightly extensible or compressible, while the 
other possessed these properties in a higher degree. When, on the other hand, an 
internode consists of very extensible cortex and very compressible pith, both will 
alter very considerably in length when separated; and yet the tension is not neces- 
sarily as great as in another internode where the cortex is less extensible and the 
pith less compressible, and where both undergo smaller alterations of length when 
separated. Similarly in our system of steel and india-rubber, if the st^el is supposed 
to be replaced by a cylinder of india-rubber, this cylinder would be very strongly 
compressed by the tube of india-rubber which in its turn would be stretched by 
it ; and when the system was broken up a smaller contraction would take place of 
the tube but a much greater elongation of the cylinder than in the case of the steel, 
even if the tension put into action had been the same in amount as in the system of 
steel and india-rubber. 
Sect. 15. — Phenomena due to the Tension of Tissues in the growing 
parts of Plants^ A. Tension of different layers of a cell-wall. By cutting as large 
pieces as possible out of the walls of hving cells and placing them in water, it is 
possible to demonstrate the existence of tensions in them ; it is found that if the 
cell-wall consists of layers of which the outer ones have a less and the inner ones a 
greater capacity of imbibition, the piece of cell-wall will bend so that the outer side 
becomes concave, the inner side convex. If the greater part of the water of imbibition 
is withdrawn from the piece of cell-wall by placing it in a solution of sugar or in 
alcohol or thick glycerine, the bending diminishes or even changes into the oppo- 
site direction, the inner side becoming concave ; this direction being again reversed 
by again placing the object in water. Narrow strips which may be cut at right 
angles to the surface out of pollen-grains of Cucurbita or Älthcea or the cells of 
the internodes of Nitella are well adapted for this experiment. 
The concave curvature outwards evidently depends on the inner layers of the 
layers of tissue as a general measure of the intensity of the tension ; but this, it will be seen from 
what has here been said, is inaccurate. If, for example, the wood and pith of an old internode are 
isolated, the contraction of the former is scarcely perceptible, while the latter elongates considerably ; 
the pith of the internode was therefore, according to this method, in a state of great tension, while 
the wood was not ; although the degree of tension of the two was really the same, differing only in 
sign (positive and negative). On p. 1 1 2 {I. c), Kraus gives a correct account of the behaviour of the 
layers of tissue of growing internodes. 
^ The phenomena here described were first observed, although somewhat superficially, by 
Dutrochet (Mem. pour servir ä I'hist. des veget. et des anim. 1837, vol. II). Hofmeister, in his 
treatise On the Bending of Succulent Parts of Plants (Berichte der kön. sächs. Gesells. der Wissensch. 
1859), made some important corrections of the theory. On the Direction of the Parts of Plants 
caused by Gravitation, see ibid, i860; on the Mechanics of the Movements due to the Stimulation of 
Parts of Plants, Flora, 1862, No 32 et seq. A connected account of the phenomena was given in my 
Experimental-Physiologic, p. 465 et seq. Very minute investigations were published by Kraus in 
Bot. Zeitg. 1867, No. 14 et seq., where the transverse tension of wood caused by the increase of its 
diameter was also for the first time described. Nägeli and Schwendener also contributed to the 
development of the theory in their ' Mikroskop,' p. 396 et seq. Still these phenomena require a much 
more exhaustive examination than has yet been given them ; the account here given will only serve 
to introduce the student to facts which are easy of observation. In explaining the processes in the 
interior I differ greatly from the views of Hofmeister (Lehre von der Pflanzenzelle, p. 272 et seq.). 
The difference in our views is so complete that it would be useless to point out particular points of 
difference. 
