ANOMALOUS THICKENING IN DICOrVLEDONS AND GYMNOSPERMS. 6l7 



must suffer a marginal elongation, and that, especially also in the case of the bundles 

 of the stratum of the leaf-trace, which always extend to the insertion of the leaf, 

 a seat of intercalary growth must be situated in the latter, in which they undergo 

 a permanent growth in length. Further, as the base of the leaf and margin of the 

 stem increase in breadth,, the number of vascular bundles lying side by side in it and 

 in the stratum of the leaf-trace is increased; new ones must therefore arise suc- 

 cessively in the latter, which apparently are connected as branches with those 

 previously present. The mode in which these processes go on remains to be 

 investigated. 



If we start from the well-founded assumption that the mature Welwitschia plant, 

 with the exception of the successively appearing flowering branches, is simply derived 

 from the growth of the embryo known as Dicotyledonous, and retains throughout 

 life its original conformation, the following may with all probability be stated, in 

 accordance with the known anatomical facts, for the early stages of growth. From 

 the broad surfaces of insertion of the Cotyledons a large number of original bundles 

 of the trace converge towards the end of the root, which grows out to the tap root; 

 and here unite into the axile bundle of the root. No bundles are added to the Cotyle- 

 donary trace from higher leaves, since there is no leaf-forming pundum vegetationis 

 at all developed on the axis of the embryo. The ring-like ' margin ' of the axis of the 

 embryo, which bears the surfaces of insertion of the Cotyledons, increases together 

 with the latter in breadth by permanent intercalary growth, proceeding in a centri- 

 fugal direction, so that the apex of the stem attains a discoid form ; its tissue 

 remains, with the exception of a thin corticar layer, permanently in a half-meriste- 

 matic condition. The bundles of the leaf-lrace . undergo an intercalary growth in 

 length at the insertion of the leaf, according as the marginal expansion progresses, 

 and new ones are added to those first present; all those which are successively 

 added arrange themselves in the double stratum of the leaf-trace. Simultaneously 

 with the beginning of this phenomenon an extrafascicular cambium appears, in a 

 manner which cannot be exactly stated, both round the whole stratum of the 

 leaf-trace, and round the axile radial bundle, .which is continued into the half- 

 meristeraatic tissue of the margin, and remains permanently active in the whole 

 remaining circumference of the stem and root as a distinct layer, producing secondary 

 cortex in centripetal succession, and the secondary wood in centrifugal succession. 

 The latter consists of the collateral vascular bundles, which alternate with the unlike 

 tissue, maintaining the arrangement above described. 



As regards the structure of the vascular bundles it may be added that they 

 resemble closely those above described (p. 334) for the leaf, and are accompanied 

 also, like them, by similar strands of fibres, but are not exactly similar to them in 

 other points. Those of the stratum of the leaf-trace and of the inner circle of the 

 root are, with their accompanying fibrous strands, much larger than those of the leaf, 

 and have the form of narrow plates, the margins of which correspond to the outer 

 and inner margins of the bundle. Their structure remains to be investigated in 

 detail. According td Hooker's statements, irregularities in the course of the bundles, 

 and also of the stratum of the leaf-trace, appear in old specimens. It is uncertain 

 whether we have here to do with fresh intercalary formations in the parenchyma. 



