PHENOMENA DUE TO THE TENSION OF TISSUES. 715 



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 living cells and placing them in water, 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 the water of imbibition is withdrawn from the piece of cell-wall by placing it in a 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 opposite 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 Althaea or 

 the cells of the internodes of Nitella, are well adapted for this experiment. 



The concave curvature evidently depends on the inner layers of the cell-wall 

 absorbing more water in the direction parallel to their surface than the outer layers, 

 and dms stretching more and becoming the convex side of the system. When water 

 is withdrawn the opposite result must ensue. Let us suppose the cell to be closed 

 and entire and not at all or scarcely turgid, /. e. with no hydrostatic pressure between 

 cell-wall and cell-contents. The inner face of the cell-wall will be in contact with 

 the cell-sap, and will absorb more water than the outside; a tension will therefore 

 be produced, the inner layers of the cell-wall having a tendency to stretch, and 

 being partially prevented from so doing by the outer layers. This tension of the 

 tissues will impart to the cell-wall a certain stiffness and rigidity which is quite un- 

 connected with turgidiiy. But since in the normal state and especially when they 

 are growing cells are always turgid, the whole sj^stem of tissues will be distended 

 independendy of this. 



^ The phenomena here described were firbt observed, although somewhat superficially, by 

 Dutrochet (Mem. pour servir a I'hist. des vegct. et des. anim. 1837, vol. II). Hofmeister, in his 

 treatise On the Bending of Succulent Parts of Plants (Berichte der kon. s'ichs. Gesells. der Wissensch. 

 1S59), made some important corrections of the theory. On the direction of the parts of plants 

 caused by gravitation, see ibid, i860; on the ISIechanical Laws of the Sensitive Motions of Parts of 

 Plants, Flora 1S62, No. 32 et seq. A connected account of the phenomena was given in my Experi- 

 mental-Physiologic, p. 465 et ieq. Very minute investigations were published by Kraus in Bot. 

 Zeitg. 1 86 7, No. 14 et seq., where the transverse tension of wood caused by the increase of its 

 diameter was also for the first time described. Niigeli and Schwendener also contributed to the 

 development of the theory in their ' Microskop,' p 406 et seq. In other respects 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. It is not surprising if in so difficult a subject and one so little 

 worked, different investigators follow entirely different methods for arriving at the same end. 



