714 MECHANICAL LAWS OF GROWTH, 



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 inter- 

 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, viz. when their extensibility and compressibility are the same, and when 

 perfect elasticity also exists in both. Under this condition in our system of steel 

 and india-rubber, the tension would have been only half as great if the india-rubber 

 tube had been at first 750 instead of 500 mm. long ; it would only have been re- 

 quired in this case to be stretched 250 instead of 500 mm., and the alteration of 

 dimension would show us that the tension was only half as intense as before. 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 w^as only slightly com- 

 pressed, 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 alter- 

 ations 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 ten- 

 sion \ It may be laid down as a rule that when the separation of two tissues causes 

 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 



^ In his treatise On the tension of the tissue of the stem and its resuUs (Bot. Zeitg. 1867, 

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

 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 older internode are 

 isolated, the contraction of the former is scarcely perceptible, while the latter elongates considerably; 

 the pith of the internode was therefore in a state of great tension, while the wood was not; although 

 the degree of tension of the two was the same, differing only in sign (positive and negative). On 

 p. 112 {I.e.), Kraus gives a correct account of the behaviour of the layers of tissue of growing 

 internodes. 



