CAUSES OF THE CONDITION OF TENSION IN PLANTS. J I ^ 



these differences in the mode of intussusception even in the different layers of the 

 same passively distended cell-wall may depend on a variety of circumstances ; as, for 

 instance, on the degree of proximity of the layers to the protoplasm, on whether 

 they are in contact externally with the air, &c. But growth by intussusception may 

 also vary according to tlie nature of the tissue of which the cell forms a part, or the 

 chemical properties of the cell-contents, and according as the cells are passively 

 distended or compressed by other cells. All these considerations are however merely 

 hypothetical, and simply indicate the nature of the relations between growth by 

 intussusception and the tensions caused direcdy by imbibition and turgidity. It may 

 in any case be regarded as certain that intussusception is only possible as the result 

 of imbibition and turgidity ; but that these properties, as well as extensibility and 

 elasticity, must, or at least may be, in their turn modified by it. The volume of the 

 growing part increases ; and since this takes place in different degrees in different 

 layers of the same cell-wall, and in different layers of the tissue of the same organ, 

 tensions varying in degree must be produced between these different layers. 



It may not be superfluous to add some explanatory observations relative to what 

 we understand by Tension. 



Corresponding to every tension is an opposite tension. If a tissue which has a 

 tendency to become distended is prevented from doing so by its connection with 

 surrounding tissues, both are in a state of tension, the one negative, the other 

 positive. The tii-sues which are passively distended may be said to be in a state of 

 ncgafive tension, those which are compressed or hindered in their distension to be in 

 a slate of positive tension. ' In a turgid cell, the cell-wall is therefore in a state of 

 negative, the contents in a state of positive tension. 



As long as there is no movement or change of form, the two opposing tensions 

 must be equal ; i. e. the work which the part in a state of positive tension performs 

 is equ^l to the work performed by means of its elasticity by the part in a state of 

 negative tension ; or the elastic forces set in action must perform the same amount 

 of work in two layers with opposite tensions and in equilibrium with one another. 

 If, for example, a steel cylinder looo mm. long is supposed to be placed in an 

 india-rubber tube 500 mm. long and closed below, and if the tube is stretched so 

 that it can be fastened above the upper end of the steel cylinder, we have a system 

 in a state of tension, the india-rubber negative, the steel positive; and since the 

 system is at rest, the opposing tensions must be equal ; i. e. all the particles of 

 the india-rubber tend to contract with the same force as that with which those of 

 the steel, which are now compressed, tend to separate from one another. 



This example shows at the same time that the amount or intensity of the ten- 

 sion can by no means be measured by the changes in dimension which the layers 

 experience at the moment when they are set free from it. Let us, for example, 

 suppose, in our system of steel and india-rubber, that the steel cylinder is shortened 

 o*i mm. out of 1000 by the india-rubber, while the india-rubber tube must be 

 stretched 500 mm. out of 1000 in order to produce an equilibrium. If the tube is 

 now opened above, it at once contracts 500 mm. (supposing it to be perfectly elastic), 

 while the steel cylinder elongates only o'l mm. ; the change of dimension is therefore 

 5000 times greater in the case of the india-rubber than in that of the steel, although 

 the actual tension of the two was the same. But the alteration of dimension 



