Fig. 198. Sigillaria with large parichnos areas. (⅓ nat. size.) M.S.
Grand’Eury’s description of fossil forests of Sigillariae in the rocks of the St Étienne[480] district affords a striking picture of these arborescent Pteridophytes; he speaks of the stems of some of the trees as swollen like a bottle at the base, characterised by the Syringodendron features and terminating below in short repeatedly forked roots of the type known as Stigmariopsis. Other specimens of Sigillaria stumps show a marked decrease in girth towards the base; this tapered form is regarded by Grand’Eury as the result of the development of aerial columnar stems from underground rhizomes.
The nature of the root-like organs of Sigillaria is dealt with in the sequel: a brief reference may, however, be made to the occurrence of stumps of vertical trunks which pass downwards into regularly forked and spreading arms. These arms lie almost horizontally in the sand or mud like the underground rhizomes of Phragmites and other recent plants growing in swampy situations where water is abundant and where deeper penetration of the soil would expose them to an insufficient supply of oxygen[481]. It is certain that Sigillaria had no tap-root, but was supported on spreading subterranean organs bearing spirally disposed long and slender rootlets which absorbed water from a swampy soil.
Fig. 199. Partially decorticated stem of Sigillaria showing two zones of cone-scars. From a cast in the Sedgwick Museum, Cambridge. M.S. (⅕ nat. size.)
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.
In 1855 Goldenberg[488] described some strobili which he regarded as those of Sigillaria and recognised their close resemblance to a fertile plant of Isoetes. He was led to the conclusion, which had little influence on contemporary opinion, that Sigillaria is related to Isoetes and must be classed among Pteridophytes. To these long and narrow strobili Schimper gave the name Sigillariostrobus[489]. In 1884 Zeiller[490] supplied confirmation of Goldenberg’s view by the discovery of cones borne on pedicels with Sigillarian leaf-scars, thus demonstrating the generic identity of cones and vegetative shoots, which Goldenberg had connected on the evidence of association. Zeiller’s more recent work[491] and the still later researches of Kidston[492] have added considerably to our knowledge of the morphology of Sigillarian cones. Grand’Eury’s remark made so recently as 1890[493] that opinion in regard to the Gymnospermous nature of Sigillaria is losing ground every day, bears striking testimony to the pertinacity with which old beliefs linger even in the face of overwhelming proof of their falsity.