It is remarkable, in view of the abundance of vegetative shoots, how rarely undoubted Sigillarian strobili have been found; this may, however, be in part due to a confusion with Lepidostrobi which so far as we know do not differ in important respects from Sigillariostrobi[494].
There can be no doubt that Sigillaria usually produced its cones on slender pedicels which bore a few leaves or bracts in irregular verticils, or in short vertical series on comparatively stout stems, an arrangement reminding us of the occurrence of flowers on old stems of Theobroma and other recent Dicotyledons. As Renault[495] pointed out the fertile shoots are axillary in origin.
Dr Kidston[496] is of opinion that certain species of Sigillaria bore cones sessile on large vegetative shoots characterised by two opposite rows of cup-like depressions like those in the Ulodendron form of Lepidodendron Veltheimianum ([fig. 157]). He has described the Ulodendron condition of two species, Sigillaria discophora (König) and S. Taylori (Carr.); the cup-like depressions may have a diameter of several centimetres and are distinguished from those of Bothrodendron by the almost central position of the umbilicus. The specimens which he figures as S. discophora are identified by him with the stem figured by König as Lepidodendron discophorum and by Lindley and Hutton[497] as Ulodendron minus. We have already dealt with the nature of Ulodendron shoots, expressing the opinion that in spite of the often quoted specimen described by D’Arcy Thompson[498], in which a supposed cone occurs in one of the cups, there is no satisfactory case of any undoubted cone having been found attached to the large Ulodendron scars. It is more probable that the Ulodendron depressions represent the scars of branches, either elongated axes, or possibly in some cases deciduous tuberous shoots which served as organs of vegetative reproduction. A specimen figured by Kidston as Sigillaria Taylori from the Calciferous sandstone of Scotland[499] bears a row of slightly projecting “appendicular organs” attached to a Ulodendron axis; but these furnish no proof of their strobiloid nature. The main question is, are these Ulodendron shoots correctly identified by Kidston as Sigillarian? The surface of the specimens shows crowded rhomboidal scars surrounded in some cases by a very narrow border or cushion; the general appearance is, as Kidston maintains, like that of Sigillaria Brardi in which the leaf-scars are contiguous (e.g. [fig. 203], upper part). None of the leaf-scars exhibit the three characteristic features, the leaf-trace and parichnos scars, but only one small scar appears on each leaf-base area. In a more recent paper Kidston figures a small piece of a stem from Kilmarnock, which he identifies as Sigillaria discophora, showing the three characteristic scars on the leaf-base area. There is no doubt as to the Sigillarian nature of this specimen, but it is not clear if the piece figured is part of a Ulodendron shoot[500].
Prof. Zeiller[501] retains the older name Ulodendron minus Lind. and Hutt. in place of König’s specific designation and dissents from Kidston’s identification of Ulodendron minus and U. majus of Lindley and Hutton as one species; he is also inclined to refer these Ulodendron axes to Lepidodendron. In spite of the superficial resemblance to Sigillaria of the specimens described by Kidston, and which I have had an opportunity of examining, I venture to regard their reference to that genus as by no means definitely established. We must recognise the difficulty in certain cases of drawing any satisfactory distinction between Sigillaria and Lepidodendron based on external features, and while giving due weight to the conclusions of so experienced a palaeobotanist as my friend Dr Kidston, I venture to think we are not in a position to state with confidence that Sigillaria possessed Ulodendron shoots.
ii. Leaves.
The leaves of Sigillaria agree closely with those of Lepidodendron; they are either acicular ([fig. 200], D) like Pine needles or broader and flatter like the leaves of Podocarpus. Their attachment to comparatively thick branches[502] shows that they persisted, in some cases at least, for several years as in Araucaria imbricata. The lower surface of the lamina was characterised by a prominent keel ([fig. 142], A and C) which dies out towards the apex; on either side of it are well-defined stomatal grooves (figs. [142], g, g; [143], A; [200], D, g). The upper face may be characterised by another groove ([fig. 142], B) but without stomata. The occurrence of the stomatal grooves, the abundance of transfusion tracheae ([fig. 142], t) surrounding the vascular bundle, and the presence of strengthening hypodermal tissue suggest that the leaves of Sigillaria were of a more or less pronounced xerophilous type and had a fairly strong and leathery lamina. The mesophyll tissue consists either of short parenchymatous cells or of radially elongated palisade-like elements and has the loose or lacunar arrangement characteristic of the aerating system in recent leaves; the slight development or absence of palisade-tissue may indicate exposure to diffuse light of no great intensity.
In most species there is a single vein, but in others the xylem forms a double strand ([fig. 142], B). Sections of the lamina near the apical region present a more circular form, owing to the gradual obliteration of the upper groove and lower keel and to the dying out of the stomatal grooves.
The transverse section of the leaf diagrammatically represented in [fig. 142], A, A′, shows the two stomatal grooves, g, and a prominent keel; the single vein consists of a small group of primary tracheae, x, some delicate parenchyma, and a brown patch of imperfectly preserved tissues, a, resembling the secretory zone tissue of a Lepidodendron. The whole is surrounded by a sheath of rather wide and short thinner-walled spiral or reticulate tracheids, which may be spoken of as transfusion tracheae, t, and compared with similar elements in the leaves of many recent Conifers. To this tissue Renault applies the epithet “water-bearing” and it is very likely that this may have been its function. The shaded portions of the lamina, in [fig. 142], A, represent the distribution of thicker-walled hypodermal tissue. The section of a leaf 3 mm. wide shown in [fig. 142], C, shows an almost identical structure; the transfusion tracheae are richly developed especially on the sides and lower surface of the vascular strand. This leaf occurs in association with a petrified stem of Sigillaria scutellata[503].
Fig. 200.