The regularity of the leaf-scar series on a Sigillarian stem may be interrupted by the occurrence of oval scars with a central scar and surrounding groove (fig. 193, E); these occur in zones at more or less regular intervals on the stem, as seen in the partially decorticated cast represented in fig. 199. Zeiller has pointed out that the rows of oval or circular scars, which mark the position of caducous stalked strobili, may occur between the leaf-scars in vertical series, each of which may include as many as 20 scars, while in other cases a single series of such cone-scars may encircle the stem[482]. The zones are usually of uneven breadth, as in _S. Brardi_, and their occurrence produces some deformation of the adjacent leaf-scars. By the earlier writers _Sigillaria_ was compared with succulent Euphorbias, Cacti, and Palms; Brongniart[483] at first included undoubted Sigillarian stems among Ferns, but after investigating an agatized stem from Autun, he referred _Sigillaria_ to the Gymnosperms[484] on the ground that it had the power of producing secondary wood. It was then supposed that _Lepidodendron_ possessed only primary xylem, and that the presence of a vascular meristem in _Sigillaria_ necessitated its separation from the lycopodiaceous genus _Lepidodendron_ and its inclusion in the higher plants. By slow degrees it was recognised, as in the parallel case of the genus _Calamites_, that the presence or absence of secondary vascular tissue is a character of small importance. Williamson, whose anatomical researches played the most important part in ridding the minds of palaeobotanists of the superstition that secondary growth in thickness is a monopoly of the Phanerogams, spoke in 1883 of the conflict as to the affinities of _Lepidodendron_ and _Sigillaria_ as virtually over but leaving here and there “the ground-swell of a stormy past[485].” In 1872 the same author had written: “If then I am correct in thus bringing the Lepidodendra and Sigillariae into such close affinity, there is an end of M. Brongniart’s theory, that the latter were gymnospermous exogens, because the cryptogamic character of the former is disputed by no one; we must rather conclude as I have done that the entire series represents, along with the Calamites, an exogenous group of Cryptogams in which the woody zone separated a medullary from a cortical portion[486].” In 1879 Renault[487] expressed the opinion that Brongniart by his investigation of the anatomy of _Sigillaria elegans_ had established in a manner “presque irréfutable” that _Sigillaria_ must be classed as a Gymnosperm showing affinity with the Cycads.