8o6 
MECHANICS OF GROWTH. 
by Kraus's very complete experiments ; i. 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 Heltanthus, Ricinus, or Ribes, 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 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 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 intensit}^ 
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 Helianlhus, 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 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 
