THE CAMBIUM-RING. 157 



ness. In this, however, I shall again keep in view only the commonest or typical 

 cases. That in the great variety of the woody plants, innumerable more or less 

 extensive deviations from this type occur, hardly needs mention. What is to follow 

 concerns, in the first place, the growth in thickness of the shoot-axes ; deviations in 

 the roots (always insignificaiit) may be mentioned with them as occasion arises. 



The whole growth in thickness is connected, as stated, with the functional activity 

 of the cambium. The origin of the latter, however, is itself again dependent upon the 

 original nature of the vascular bundles. Here are concerned only those parts of the 

 bundles which run as so-called leaf-traces in the interfoliar parts, the upper ends 

 being cast off in the leaves at their death. In the plants in question, these leaf- 

 traces are seen, on the transverse section of the shoot-axis, to be arranged in a 

 circle; and their longitudinal course is in general parallel to the surface, as 

 represented in Fig. 129. The phloem-portions of these usually not very numerous 

 bundles are all turned towards the surface of the shoot-axis ; the xylem-portions 

 being directed towards the centre of the transverse section. From the very first, 

 even before the origin of the cambium ring, the elements of the vascular bundles 

 are arranged in radial rows. 



The first indication of the commencing growth in thickness consists in that a 

 layer of cells, lying in the vascular bundle between the phloem and xylem, grows in the 

 radial direction, and accordingly becomes divided by tangentially placed partition- 

 walls. Thus originates the fascicular cambium. The cells of this, arising by con- 

 tinually repeated divisions, when they lie towards the phloem side develope into elemepts 

 of the secondary cortex (secondary phloem), and when they arise on the inner side of 

 the cambium constitute new elements of the wood. In many cases, the growth in 

 thickness progresses in such a manner that this cambium layer, lying in each 

 individual vascular bundle, causes the phloSm as well as the xylem of the bundle to 

 increase in the radial direction, so that the primitively rounded transverse section of 

 the bundle becomes gradually elongated radially, and widened outwards into a wedge 

 shape. The fundamental tissue lying between these vascular bundles which are 

 growing in thickness, developes at the same time, by repeated cell-divisions. In this 

 manner, however, a closed mass of wood is not formed ; nor is a continuous layer of 

 secondary cortex formed around the wood (e.g. stem of Gourd). 



In the typical case of growth in thickness, there is formed, after the production 

 of a cambium layer in each leaf-trace, a similar layer also in the fundamental tissue 

 between each two neighbouring bundles (see Fig. 165,^); and this always in such 

 a manner that the parenchyma cells of the fundamental tissue are elongated in the 

 radial direction, and divide by means of tangentially placed longitudinal walls. Thus 

 arises the interfascicular cambium, which becomes joined on to the fascicular cambium, 

 and together with this constitutes a continuous cambium layer. This appears on 

 !the transverse section as a ring; but of course it is really a hollow cylinder, 

 running in the tissue of the shoot-axis. On the inside of this cambium-ring lie 

 the xylem-portions, and on its outside the phloem-portions of the leaf-traces ; and 

 since the cambium-ring produces wood progressively on the whole of its inner 

 side, a woody ring or hollow cylinder of wood is produced, enclosing the pith 

 or inner portion of fundamental tissue. On the outside of the cambium-ring arises 

 in like inanner a hollow cylinder of secondary cortex. It may be mentioned 



