SECONDARY THICKENING. NORMAL DICOTYLEDONS. 457 



course is undulated in such a manner, that within the internode they show alternately 

 lateral union and separation, at short vertical intervals, both with one another and 

 with the leaf-trace bundles, and thus form a net with vertically elongated, narrow 

 meshes, which are filled up by the bands of parenchyma. The whole process begins 

 at the lateral margins of the leaf-trace bundles, and is continued from the latter 

 through the interfascicular segments of the original ring. It may therefore be said, 

 that the leaf-trace bundles coalesce, by means of successive increase in breadth at 

 their lateral margins, so as to form a closed ring, which is only traversed by the 

 narrow radial bands of parenchyma, and in which the primary leaf-trace bundles 

 only remain characterised by the fact that they project more deeply into the pith 

 (and by the special structure of this projecting portion). In the typical cases, the 

 whole body thus formed may actually be termed one collateral vascular bundle, with 

 an annular cross-section. The origin and orientation of the cambial zone is, in the 

 cases in question, essentially the same as in those brought forward above. 



This coalescence of the leaf-trace rudiments to form a ring appears most 

 clearly in those internodes which contain only a few trace-bundles, with a simple 

 course. In the internode of Euonymus latifolius ^ the rudiments of the traces of the 

 adjoining pair of leaves, each consisting of a single bundle, appear first at two 

 diametrically opposite points, between the pith and external cortex ; then the traces 

 of the next higher pair appear in the middle of the spaces between the first. From 

 the lateral margins of these four bundles, according to their order of origin, rapidly 

 succeeding divisions extend through the interfascicular bands, so as to form the 

 small-celled rudiment of the closed ring of bundles ; and finally the formation of the 

 definitive tissue takes place in the latter, beginning with new divisions and consecutive 

 differentiation at the points where the formation of the ring started, and advancing 

 towards completion in the same direction as the latter. The whole ring, including 

 the original leaf-trace rudiments, consists finally, especially in the xylem, of alter- 

 nating radial bands of bundles, and of non-equivalent elements, which show the 

 longitudinal course already stated. Fraxinus ^ behaves quite similarly, and most 

 Rubiacese, Asclepiadeae, Apocyneae, &c., having a very regular radially arranged ring 

 of wood, are also connected with this type. Comp, Sects. 61, 63. 



The formation of the closed ring is less evident in the case of internodes 

 possessing numerous leaf-trace bundles, which from the first are separated by very 

 narrow interfascicular bands, e. g. Acer, Sambucus ^ &c. ; the result, however, is es- 

 sentially the same. How far the closing of the ring proceeds exclusively from the 

 coalescent margins of the leaf-trace bundles, or also from small intermediate bundles 

 arising like those of Clematis, remains to be investigated for each particular case. 



{c) The coalescence of the vascular bundles to form a continuous ring may go 

 still further, so that no alternating dissimilar radial bands appear between the 

 original bundles, but the whole of the tissue forming these zones assumes the struc- 

 ture of a vascular bundle, if this expression be allowed for the sake of brevity ; i. e. it 

 consists of the elements of vascular bundles, with similar structure and arrangement 

 to those of the later developed portion, and of the products of secondary growth in 



^ Sanio, Botan. Zeitg. 1S63, p. 360. 

 * Compare Nageli, Beitr. i. p. 95 - ^ Ibid. I.e. 



