CONCLUSIONS. 41 



or Lepidodendra and of Sigillarias ; and they are equally unanimous in believing 

 that these primaeval Lycopodiaceas, found in the Devonian, Carboniferous, and 

 Permian strata, are the remote ancestors of the modern Lycopodiaceas. The 

 question arises how far does our present knowledge respecting the morphology and 

 histology of these ancient arborescent forms enable us to detect connecting links 

 between them and their degraded living descendants. 



That the dichotomous ramifications of the branches, the structure and arrange- 

 ment of the leaves, and the entire morphology of their reproductive organs, furnish 

 such links is indisputable. But we are now familiar with other morphological 

 features presented by these ancient types, the relations of which to those of living 

 ones are not quite so clear. 



That the axial vascular strand of the living Lycopodium and Selaginella is the 

 homologue of the non-exogenous inner vascular zone of Lepidodendron can scarcely 

 be doubted. In both cases these tissues constitute the only axial vascular elements 

 possessed by the youngest branches on which all the leaves are developed, and to 

 which leaves they supply the only vascular bundles that those leaves ever possess. 1 

 These ancient and modern vascular axes also agree in their mode of growth which 

 is in both cases centripetal. 



There can be equally little doubt that the rootlets of Stigmaria correspond to 

 those of living forms, both in their structure and their acropetal order of develop- 

 ment. The vascular bundle in the centre of the Stigmarian rootlet, as well as the 

 cellular zone which invests it, is almost identical with that of Selaginella, and 

 approximates still more closely to that of Isoetes. In two respects the affinities 

 of the Stigmarian rootlets with those of Isoetes are remarkable. In both orga- 

 nisms these rootlets are given off from the lower part of a downward prolongation 

 of a caulome, which prolongation never develops leaves ; the rootlets, therefore, are 

 produced upon an axis which grew in the opposite direction to that in which the 

 leaf-bearing part of the stem grew. In addition to this, the rootlets of Isoetes and 

 of Stigmaria agree in the circumstance that, in both, they are converted, during life, 

 into fistular cylinders, owing to the disappearance or non-development of the 

 delicate parenchyma, which ought to occupy the space between the outer cortical 

 layer and the investing sheath of the central vascular bundle. But important 

 differences have been produced by the introduction, into both the stems and roots 



1 M. Eenault's idea that in the many extinct forms which possess a diploxyloid vascular axis, i.e. 

 an inner centripetal and an outer exogenous axis, each of these two cylinders contributed to the 

 formation of the leaf-bundles, cannot be accepted. In all cases the leaves and their leaf-bundles 

 were developed before any exogenous zone made its appearance ; and in several known Lepidodendroid 

 plants the branches attained to a large size before any such zone began to grow. To suppose that, in such 

 cases, the leaves had to wait for their complete vascular structures until, having done their chief work, 

 they were ready to be cast off, is impossible. Hence I must reject this assertion that the foliar bundles 

 had a double origin, as it is alike contrary to probability and to observed facts. 



