Immediately above the centre of the upper boundary of a Sigillarian leaf-scar a ligule pit may often be detected, as shown in [fig. 195], l, and in some cases, e.g. a specimen figured by Germar[463] ([fig. 196], A) as Sigillaria spinulosa (identical with S. Brardi), some circular scars with a central pit surrounded by a raised rim occur on the surface of the stem, either singly or in pairs, near the leaf-scars; these, it is suggested, may represent the position of adventitious roots or, as Germar thought, of some deciduous spinous processes. The leaf-scars are frequently hexagonal in shape, with the lateral angles either rounded ([fig. 200], F) or sharply pointed ([fig. 200], G, H); each scar bears three smaller scars as in Lepidodendron, a central circular, oval or crescentic leaf-trace scar and larger oval or slightly curved scars formed by the two parichnos arms ([fig. 195], p). The larger size of the parichnos arms, the individual cells of which may often be detected as a fine punctation, is a distinguishing feature of the genus, but otherwise the structure is very similar to that in Lepidodendron. As shown in figs. [195], [200], F, G, the three scars may occur nearer the upper than the lower margin of the leaf-base area.
Lepidodendron Wortheni[464] ([fig. 196], E), described from North America by Lesquereux[465], by Zeiller[466] from France, and by Kidston[467] from the Upper and Middle Coal-Measures of England, may be quoted as a Lepidodendron bearing a close resemblance to Sigillaria. The shoots bear cushions two or three times as long as broad and without the usual median division, but with numerous irregular and discontinuous transverse wrinklings. Lepidodendron Peachii Kidston[468] affords another example of a form agreeing both with Sigillaria and with Lepidodendron. An Upper Devonian type described by White[469] as Archaeosigillaria primaeva affords a striking instance of the combination on one stem of Sigillarian and Lepidodendroid leaf-cushions.
The difference between the original surface of a Sigillaria stem and that of partially decorticated specimens is seen in [figs. 196], C and D; in fig. C the bark of Sigillaria Brardi shows the characteristic wrinklings of the superficial tissue, while at a slightly lower level the leaf-scars are replaced by the parichnos casts, a, and fine longitudinal striations represent the elongated phelloderm cells laid bare by the exfoliation of the surface-layers. Similarly, in the rib of Sigillaria laevigata ([fig. 196], D) the parichnos arms, p, and the longitudinal striations are exposed at the lower level, while the surface is smooth and bears rows of widely separated leaf-scars.
Fig. 197. Carica sp. From the Royal Gardens, Kew. (Much reduced.) M.S.
The older part of a Sigillarian stem may present an appearance very different from that of the younger shoots. The leaf-cushions may be stretched apart as the result of elongation and increase in girth, while in some cases the arrangement of the leaf-scars may vary on the same axis as the result of inequalities in growth or changing climatic conditions. The contiguous arrangement of the leaf-scars and narrow cushions characteristic of the Clathrarian form of stem, as was first demonstrated by Weiss[470], and afterwards illustrated by Zeiller[471] and Kidston, may be gradually replaced (on the same specimen) by a more distant disposition of the leaf-scars separated by a smooth intervening surface of bark. The specimen of S. Brardi reproduced in part in [fig. 203], and first figured by Kidston, affords an example of three “species” on one piece of stem, S. Brardi Brongn., S. denudata Goepp. and S. rhomboidea Brongn.[472]
The piece of Carica stem, represented in [fig. 197], illustrates the danger of trusting to the disposition of leaves as a specific criterion.
Similarly, in the ribbed forms the degree of separation of the leaf-scars is by no means uniform in a single species[473]. Some authors have adopted a two-fold classification of Sigillarian stems proposed by the late Prof. Weiss[474] of Berlin, who divided the Sigillariae into (A) Sub-Sigillariae, comprising Leiodermariae and Cancellatae, and (B) Eu-Sigillariae, including Favulariae and Rhytidolepis. Grand’Eury[475] adopts the terms Rhytidolepis and Leiodermaria for ribbed and smooth stems respectively, the type to which the name Clathraria was applied by Brongniart being in some cases at least the young form of Leiodermarian stems. While recognising the artificial distinction implied by such terms as Rhytidolepis, Leiodermaria, and other sub-generic titles, we may conveniently speak of the two main types of Sigillaria stems as ribbed and smooth.
Still older stems of Sigillaria are not uncommon from which the leaf-scars and other superficial tissues have been exfoliated, leaving exposed a longitudinally fissured surface of secondary cortex characterised by pairs of considerably enlarged parichnos strands ([fig. 198]) which are sometimes partially or wholly fused into one (Syringodendron state of Sigillaria). The single or double nature of the elliptical or circular parichnos areas is doubtless due to the degree of exfoliation, which may extend sufficiently deep into the cortex to reach the level of the parichnos before the single strand has bifurcated (cf. Lepidodendron, p. 100). In the Museums of Manchester, Newcastle, and other places casts of large Sigillaria stems may be seen, which illustrate the differences in breadth and regularity of the vertical ribs, and in the size and shape of the parichnos areas in different regions of a partially decorticated stem. A cast of a ribbed species in the Manchester Museum, having a length of 185 cm. and a breadth of 56 cm., shows in the upper portion straight vertical grooves and broad ribs bearing pairs of parichnos scars 11 mm. long; in the lower portion the ribs tend to become obliterated and the parichnos scars, 2 cm. in length, may be partially fused and arranged in much less regular vertical series. A feature of these older ribbed Sigillarian stems is the increase in the number of the ribs from below upwards. Kidston[476] has described a specimen in the Sunderland Museum, 6 feet 6 inches long, with a circumference at the slightly bottle-shaped base of 5 feet. On the lower portion of the stem there are 29 broad ribs; about one-third the height many of these bifurcate, producing as many as 40 ribs in the upper part where the cast has a circumference of 3 feet. The increase in number of the ribs is due in part to bifurcation, but also to the intercalation of new ones. As Kidston points out, this example shows that as a stem grew in length additional leaves were developed at the apex. A similar stem, which illustrates very clearly the increase in the number of ribs from below upwards, may be seen in the Newcastle Museum.
Grand’Eury[477] has described an example of an old stem of a ribless species of Sigillaria, Syringodendron bioculatum, bearing single and double parichnos areas of nearly circular form and with a diameter of 1–2 cm. In a specimen figured by Renault and Roche[478] (Syringodendron esnostense) from the Culm strata in France, the parichnos scars reach a length of 3 cm. As seen in the fragment of a ribbed Sigillaria represented in [fig. 198], the large parichnos areas exhibit a distinct surface pitting in contrast to the fine longitudinal striation of the rib; the difference in surface-appearance is due to the nature of the tissue, which in the parichnos consists of fairly large parenchymatous elements with groups of secretory cells[479], and in the exposed cortex of elongated elements. The vertical line in the middle of [fig. 198], which occurs in the middle of the rib, has probably been formed by splitting of the bark.