42 STIGMARIA FICOIDES. 



of the extinct forms, of a true, exogenously developed, vascular cylinder. This 

 zone with its radial laminae of vessels, its true medullary rays, and its meri- 

 stemic cambium zone, encloses the vascular, axial strand which supplied the 

 vascular bundles to the leaves. These leaf -bundles were all fully developed, and 

 extended continuously through the bark, from the axial vascular strand from which 

 they originated to the leaves on the surface of each twig, before (in many cases 

 very long before) the exogenous cylinder made its appearance. Hence when the 

 first-formed vessels of that cylinder arranged themselves longitudinally round 

 the central axis from which the leaf-bundles emerged, they had to bend round 

 each of the many foliar-bundles that stood in their way, coming together again when 

 they had passed the obstruction. This was also done by each successive exogenous 

 growth without any contribution being made by the latter to the foliar-bundle. Each 

 additional exogenous layer pursued the course followed by that upon which it rested. 

 Hence each foliar-bundle passed outwards through the exogenous cylinder, along 

 horizontal lenticular passages, the vessels enclosing the peripheral portions of which 

 passages were successively superimposed upon the pre-existent bundles. These 

 relations of consecutive, not coeval, origin explain what observation demonstrates 

 to be a fact, viz. that the exogenous cylinder was a vascular network, through the 

 meshes of which the foliar-bundles continued to reach the bark, but without 

 receiving any additional vascular contributions. 



This exogenous zone made its first appearance in the various Lepidodendroid 

 trees at very different stages of their growth. In the L. selaginoides of Halifax 

 we find it existing even in very young twigs. In the Burntisland Lepidodendron 

 it appears, not in the twigs, but in young branches. In the Arran Lepidodendron 

 from Laggan Bay, no traces of it are seen until the branches have attained 

 to a large size. It evidently began to be developed at the junction of the stem 

 with the root of each plant; attaining to a greater relative importance in the 

 latter than in the former organs, since we find it, as shown by Plate IX, fig. 18, 

 reaching the extremities of the true roots, which it never does in the twigs. 

 Since every rootlet derived its vascular strand from this layer, and since 

 rootlets were obviously furnished to the plant in its youngest state, this early 

 development of the exogenous zone in the roots was a matter of absolute 

 necessity. Such was not the case with the aerial parts of the plant, the leaves of 

 which, as we have seen, obtained their vascular strands independently of the exo- 

 genous zone. But the latter evidently crept up the stem from below as a succession 

 of investing cones, each newer investment reaching a higher point than those 

 which preceded it. Now if these views are correct, which I believe them to be, 

 we can understand the functions of the vascular cylinder of Stigmaria. The 

 mineral and nitrogenous food-material absorbed by the rootlets was conveyed to 

 the stem through the exogenous zone, whence it was transmitted laterally to the 



