As considerable stress has been laid on the anatomical features of the cycadean foliar bundles in discussions on the affinities and phylogeny of certain Palaeozoic genera, it is important to consider the facts more closely[85]. French anatomists described the cycadean bundle as diploxylic on the ground that the centripetal and centrifugal xylems are distinctly different things, the centripetal xylem being primary—a relic of a former organisation—and the centrifugal xylem secondary and homologous with the normal wood of the cauline bundle. The term mesarch has in recent years been applied to the cycadean type of bundle. A mesarch bundle is, however, one in which centripetal and centrifugal xylem are alike in origin, both being primary structures derived from a desmogen strand. Typical mesarch bundles occur in several recent ferns; in the stele of the Osmundaceae, _Gleichenia_, and other genera; but in these plants the xylem is all produced directly from one primary desmogen region and there is no question of ‘primary’ and ‘secondary’ as in the two portions of the xylem of a cycadean bundle. Recent researches into the development of cycadean foliar bundles show that they do not conform to the mesarch type as generally understood. A leaf-trace at the base of a petiole (fig. 399, A) comprises centrifugal xylem only, and this consists of regular rows of tracheids separated by medullary rays: in the lower part of the petiole the structure is gradually modified, the centrifugal xylem is reduced and the formation of centripetal xylem is initiated. At a higher level (fig. 399, B) the centripetal xylem is in excess of the centrifugal and the latter, for a time connected with the former, eventually becomes separated by a few parenchymatous cells from the protoxylem and persists as a small strand or arc of tracheids. Fig. 399 illustrates stages in the transformation of a typical collateral bundle, at the base of a _Stangeria_ petiole, into one in which the xylem is almost wholly centripetal at a higher level in the axis of the frond. A cambium is present in all: in B the centrifugal xylem is more or less clearly differentiated into two portions, loosely arranged tracheids near the phloem, and the more compact groups abutting on the centripetal xylem: figs. C–E show a further reduction in the centrifugal tracheids. The conclusion drawn from developmental study is that the two xylem portions of the bundle are independent in origin[86]. Marsh has, however, shown that in _Stangeria_ bundles near the base of the petiole the centrifugal xylem consists of rows of secondary tracheids and an inner portion not in rows which connects the centrifugal with the centripetal elements; this connecting portion, he adds, is ‘probably primary and connects up the Cycadean foliar bundle with the truly mesarch bundle of the Cycadofilices.’