CAUSES OF THE CONDITION OF TENSION IN PLANTS. 793 



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 tissues which are passively distended may be said to be in a state of 

 negati've tension, those which are compressed or hindered in their distension to be in 

 a state of positi've 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 ; /. e. the work which the part in a state of positive tension would perform 

 is equal to the work which would be performed by means of its elasticity by the part in 

 a state of negative tension if the two were disconnected; 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 1000 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 ; /. 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 tension 

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

 perience 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'l 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'imm. ; 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 

 indicates only the amount of stretching to which the india-rubber, and of compres- 

 sion to which the steel was subjected. If therefore the layers of the tissue of an mter- 

 node are separated from one another, the alterations of dimension which then ensue 

 depend on the extensibility and compressibility of the layers as well as on the 

 amount of tension. There is only one case in which the amount of tension can 

 be inferred from the changes in dimension of the tissues when freed from a state of 

 tension, t;/2. when their extensibility and compressibility are the same, and when 

 perfect elasticity also exists in both. But the case is quite different with growing 

 internodes ; the extensibility of the tissues when in a state of tension is constantly 

 changing in consequence of growth. In a young internode the epidermis and wood 

 are very extensible; if they are separated from the pith this latter Only lengthens 

 slightly, because it was only slightly compressed, but the epidermis and the wood 

 contract very considerably because they are very extensible and were stretched by 

 the pith. On the other hand the alterations of dimension in layers of an older though 

 not mature internode will be the reverse. The pith, when freed from the tension, 

 elongates considerably, but the wood contracts only slightly, because its extensibility 

 is now but small and it was but slightly stretched by the pith ; the pith on the contrary 

 being very compressible, was prevented from lengthening by the resistance of the wood. 

 The intensity of the tension cannot by any means be determined in either case from the 

 changes of dimension ; these only show that there are tensions, and indicate also what 

 parts are extensible and compressible, and which are in a state of positive and negative 

 tension^. It may be laid down as a rule that when the separation of two tissues causes 



^ In his treatise On the Tension of the Tissue of the Stem and its Results (Bot. Zeitg. 1867, 

 No. 109) Kraus has employed the differences of length between the entire internode and its isolated 



