596 SECONDARY CHANGES. 



variations appear to occur. The leaves are arranged in decussating pairs. The course 

 of the bundles of the leaf-trace, which is certainly very simple, has not been exactly in- 

 vestigated. In the young, and but slightly extended internode they are disposed round 

 a narrow pith in a ring, which is in transverse section somewhat bluntly rectangular. 

 The two shorter sides of the rectangle are occupied by the bundles belonging to the 

 next higher pair of leaves, the two longer ones by those which descend from higher 

 leaves. The bundles are collateral. Their phloem portions are surrounded by a zone 

 of narrow thick-walled coUenchymatous elements, which is several layers of cells thick, 

 and runs round the whole ring : this zone is bordered externally by the thick large-celled 

 outer cortex, traversed by the network of bundles mentioned on p. 297. The innermost 

 limiting layer of this (Plerome-sheath) is developed as a starch layer (comp. p. 415). 

 Before the development of the bundles of the leaf-trace is complete, tangential cambial 

 divisions begin in an outer layer (but not the outermost) of the zone of coUenchyma : thus 

 they are extrafascicular. As far as I could see they begin first on the long sides, then 

 on the short sides of the rectangle, and are then continued from these four starting-points 

 over a completely circumferential layer of cells. The tangential divisions and the 

 arrangement of their products in radial rows are from the first very regular, and remain 

 so, since the latter and the cambium always uniformly follow the slight expansion of 

 the internal parts. Externally to the circle of the leaf-trace, in the first place a zone 

 many layers in thickness is formed, consisting of radially-arranged interfascicular 

 tissue — in M. virens of sclerotic elements. Further outwards the vascular bundles 

 appear in the intermediate tissue, which becomes sclerotic with the exception of the 

 thin-walled portion surrounding the phloem ; in the transverse section these bundles are 

 arranged in interrupted and irregular concentric annular zones, which are often inter- 

 locked one with another. The vessels of the bundles are at least for the most part derived 

 directly from simple cambial tangential divisions. When the xylem of a bundle has 

 been thus formed, an initial strand is added at its outer side, from the active divisions of 

 which the delicate phloem is produced; I am not able to say whether the outermost 

 vessels of the bundle are also thus formed or not. Centripetal divisions of the cambium, 

 the succession of which has not been exactly traced, produce in M. virens a secondary 

 layer of bast composed of radially seriate, elongated parenchymatous cells. In stems, 

 in which the radial rows of the secondary wood numbered over fifty elements, there 

 were only five cells in one radial row of the bast. After the extension of the interno de 

 is complete, and before the first ring of secondary vascular bundles is formed, the thick 

 outer cortex is thrown off by a formation of periderm, which starts from the starch- 

 layer ; the layer of cork cells thus derived is several layers thick over the broad sides of 

 the ring of bundles, being formed by tangential division of the starch-layer ; over the 

 narrow sides, however, it is for certain distances a single layer, and arises apparently 

 directly by suberisation of the starch-layer, without previous divisions. 

 The roots of the Mesembryanthema have not been investigated in detail. 

 2. From each of the leaves arranged in opposite and decussating pairs on the foliage 

 shoots oi Miraiilis Jalapa and longifolia (Figs. 234, 235) three bundles of the trace enter 

 the stem, one median {m), and two lateral (/). They traverse the internode with a 

 radially perpendicular course, and at the next lower node each trace unites to form a single 

 bundle {-v) ; this projects rather deeper into the pith, and passes perpendicularly down 

 the next node, inserting itself at the second lower node on the traces, which there join 

 into a single bundle ; it curves first to one side and is affixed to one of them, and then by 

 a second shank, which appears later, it is joined to the one on the other side. The 

 transverse section of a young internode (see Fig. 235) therefore shows first eight bundles, 

 ranged round a prismatic pith : on each side, corresponding to the next higher pair of 

 leaves, are the three bundles of the trace of the leaf above it, and alternating with these 

 two traces, but rather nearer the centre, are the opposite united bundles from the pair 

 of leaves next but one higher. Somewhat later new bundles appear in the internode 

 outside the eight bundles of the ring, and at the same time a ring of meristem, the 



