PHENOMENA DUE TO THE TENSION OF TISSUES. 72/ 



rapidly in the tangential direction ; the cells which have been thus altered in form 

 are divided by radial septa. But at length the epidermis and primary cortex are 

 no longer able to obey the peripheral traction; longitudinal fissures occur in the 

 cortical tissue, generally after the commencement of the formation of cork. When 

 the periderm and cork have been formed on the older parts of stems, these 

 secondary epidermal tissues undergo a continuous strain in the peripheral direction, 

 and exert in turn a radial pressure on the living phloem, cambium, and xylem. The 

 first result of this pressure exerted by the growing inner tissues is the splitting of 

 the layers of bark, especially longitudinally. The form of the fissures depends, 

 however, on the course of the bundles of bast which take part in the formation of 

 the bark, and on other relations of the tissues to one another. If a stem does not 

 in its growth take the form of a cylinder or slender cone but of a spherical tuber, 

 as in Beaucarnea and Testudinaria, the layers of periderm split apart in the form 

 of tolerably regular polygons which cover the spherical surface of the stem like 

 shields. These examples show at the same time that even in Monocotyledons 

 tensions are produced by the subsequent increase of the stem in thickness similar 

 to those caused by the activity of the true cambium-ring ; for in this case it is re- 

 placed by a thickening-mantle, in which new layers of fibro-vascular bundles and 

 intermediate parenchyma are constantly being produced. (See Fig. 91, p. 107.) 



It is evident that before the bark splits or fissures already in existence become 

 wider and penetrate inwards, the transverse tension must attain a certain intensity, 

 which, from the great firmness of the bark, cannot be inconsiderable. At the 

 moment when the splitting takes place at least a portion of the tension must, how- 

 ever, be destroyed. This is clearly the reason why the transverse tension attains 

 its maximum (measured in the way described above), as Kraus has pointed out, 

 above the part of the stem where the scaling-off of the bark begins. But even in 

 annual stems which increase rapidly in thickness, as HeHanthus, Dahlia, &c., the 

 transverse tension does not progressively increase from the apex to the root, but 

 attains its maximum at an intermediate height, below which it diminishes. An 

 explanation of this phenomenon is afi"orded by the fact that the limit of the elas- 

 ticity of the bark is gradually exceeded by the long-continued pressure to which 

 it is subject from within, and that the cell-walls which are strained grow at the same 

 time by intussusception, and thus a portion of their tension becomes neutralised. 



While we may consider the turgidity of the pith and its enormous endosmotic 

 power as the principal cause of the longitudinal tension of growing internodes and 

 leaf-stalks before they become lignified, it is on the other hand probable that the 

 imbibition and swelling of the cell-walls are the chief cause of the transverse tension. 

 The wood, where the transverse tension chiefly originates, is, when mature, scarcely 

 adapted for any distension by turgidity; while at all events in cells or vessels with 

 bordered pits it is altogether impossible. Closed wood-cells, when turgidity is 

 possible in them, cannot however distend greatly ; since their own wall, and the 

 woody substance which surround them are far too inextensible to stretch to any 

 considerable extent under the influence of hydrostatic pressure. It has, on the other 

 hand, been already shown (Sect. 13) what considerable alterations of dimension the 

 wood experiences especially in the peripheral and radial direction simply by imbi- 

 bition. Every layer of wood freshly formed on the inside of the cambium-ring has 



