620 SECONDARY CHANGES. 



described below. During the development of the latter, as may be here remarked, 

 there are extensions of the elements to many times the length of the cambial cells, 

 while the transverse section remains the same or increases, and distortions and 

 mutual displacements result, which have not been thoroughly studied in the cases 

 before us, but a general opinion may be formed upon them from the points of view 

 laid down in Sect. 137. 



The secondary vascular bundles are directly connected, at least at the nodes, 

 with the primary bundles of the leaf-trace, at the point where the latter curve 

 outwards. They are connected one with another in their longitudinal course by 

 numerous anastomoses both radially and tangentially, so that they form a net 

 branched on all sides, the meshes of which are filled up by the interfascicular tissue. 

 In the Aloineae and Dracseneae investigated, the meshes of' the net are elongated, 

 pointed and narrow, only as broad as a few interfascicular cells, and the bundles have 

 a similar undulated course to that of the bundles in the Dicotyledonous stem, and 

 the wood has a corresponding construction. In the stem of Beaucarnea, on the other 

 hand, at least in the tuberous swollen base, the meshes are elongated and polygonal, 

 attaining a width of over im™, and forming a beautiful network, through which at the 

 broad base of the stem the bundles belonging to the older roots of former years run 

 in a radial direction. In radial longitudinal section and in transverse section the 

 bundles in the two cases form more or less regular concentric zones. In the 

 Dracaenese these are irregularly pectinated, since the bundles of one alternate with 

 those of the next inner zone, and project partially into the spaces between them ; in 

 the other investigated cases, however — Yucca, Aloe spec, Beaucarnea — they are 

 more regular, and separated one from another by broad interfascicular spaces. The 

 longitudinal course of the bundles diverges, as has been said above, from the vertical 

 even in dense narrow-meshed woods. In the Dracaenas, however, it is in the main 

 perpendicular, if we neglect the undulations. In Yucca aloifolia, on the other hand, 

 Millardet found the main direction, with many irregularities it is true, to be inclined 

 strongly to the vertical, as much as 45°, and the direction of the inclination changes 

 in successive layers; it is usually, but not constantly, directed to one side in each 

 case for two successive zones, and in the two following zones to the opposite side. 



TAe structure of the secondary vascular bundles is known in the Dracaenas 

 with some, but not with exhaustive accuracy. While the bundles of the leaf-trace 

 have the collateral composition and the sheathing usual for Monocotyledons ^, and 

 contain, according to Caspary '^, only tracheides, there being no vascular perforations 

 even in the spiral primitive tracheides, the secondary strand is composed of a small 

 phloem, which occupies about the middle of the bundle, and is surrounded by, on 

 the average, 2-3 layers of tracheides. The phloem consists, as first pointed out by 

 Wossidlo, of a small number of sieve-tubes with simple narrow-pored, callous 

 transverse plates (p. 175); the tubes are accompanied by delicate cambiform cells. 

 In transverse section the whole number of the thin-walled elements of the phloem is 

 often very small, hardly as many as six, in other cases certainly larger : its general 

 outline is accordingly very variable. At its periphery, that is, bordering on the 

 tracheides, there are thick-walled cells, which have large round non-bordered pits on 

 thin longitudinal walls. All the tracheides are, as far as known, of the same structure; 



• Compare p. 322. 2 /. c., compare p. 165. 



