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 steel 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 is withdrawn from the piece of cell-wall by placing it in a solution of sugar or in 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 Cucurhita or Althcea or the cells of 

 the internodes of Niiella 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. 112 (/. 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 k I'hist. des veget. et des anim. 1837, vol. II). Hofmeister, in his 

 treatise On the Bending of Succulent Parts of Plants (Berichte der kon. sachs. 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 ^2 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, Nageli 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. 



