8o6 MECHANICS OF GROWTH. 



by Kraus's very complete experiments ; /. e. if the rings of tissue in a transverse 

 section of the stem or in a woody branch are separated from one another, by 

 dividing it longitudinally and then separating the rings, they contract the more the 

 nearer they lie to the circumference, and the contraction is the more considerable, 

 compared with the original circumference of the whole, the older the internode from 

 which the section is taken. The traction upon the cells of the epidermis and of the 

 primary cortex caused by the transverse tension is easily observed by the microscope 

 in the transverse section, if young internodes of plants which increase rapidly in 

 thickness, as Helianthus^ Ricinus, or JRibes, are compared with those which have 

 already been forming wood for some weeks or months. The form of the cells 

 shows that they have been stretched in the peripheral direction (see Fig. 56), and 

 have in consequence grown rapidly in that 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 forma- 

 tion 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 peri- 

 pheral 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 w^hich 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 Tesiudinaria, 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 in those 

 Monocotyledons also which grow in thickness tensions are produced by the sub- 

 sequent increase of the stem in thickness similar to those caused by the activity 

 of a true cambium-ring ; for in this case it is replaced by a thickening-ring, in which 

 new layers of fibro-vascular bundles and intermediate parenchyma are constantly 

 being produced. (See Fig. 104.) 



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-ofi" of the bark begins. But even in 

 annual stems which increase rapidly in thickness, as Helianlhus, Dahlia, Sec, 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 afforded 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 



