Fossil plants, Vol. 2

i. General.

In view of the close resemblance between Lepidodendron and Sigillaria, another lycopodiaceous plant characteristic of Carboniferous and Permian floras, a comparatively brief description of the latter genus must suffice, more particularly as Lepidodendron has received rather an undue share of attention. Sigillaria, though abundantly represented among the relics of Palaeozoic floras, especially those preserved in the Coal-Measures, is rare in a petrified state, and our knowledge of its anatomy is far from complete. In external form as in internal structure the difference between the two genera are not such as enable us to draw in all cases a clearly defined line of separation.

In the Antediluvian Phytology, Artis[448] figured a fossil from the Carboniferous sandstones of Yorkshire which he called Euphorbites vulgaris on account of a superficial resemblance to the stems of existing succulent Euphorbias. Rhode[449] also compared Sigillarian stems with those of recent Cacti. The specimen described by Artis is characterised by regular vertical and slightly convex ribs bearing rows of leaf-scars in spiral series, like those on the cushions of Lepidodendron. A few years earlier Brongniart[450] had instituted the genus Sigillaria[451] for plants with ribbed but not jointed stems bearing “disc-like impressions” (leaf-scars) disposed in quincunx; the type-species named by the author of the genus Sigillaria scutellata is identical, as Kidston[452] points out, with Euphorbites vulgaris of Artis and with the plant afterwards figured by Brongniart as S. pachyderma[453]. Brongniart in 1822 figured another type of stem characterised by the absence of ribs and by prominent spirally arranged cushions bearing relatively large leaf-scars like the upper part of the specimen shown in fig. 203; this he named Clathraria Brardii, a well-known and widely distributed Carboniferous and Permian species now spoken of as Sigillaria Brardi (figs. 196, A–C; 203). A third type of stem figured by Brongniart as Syringodendron striatum[454] agrees with Sigillaria scutellata in having ribs, but differs in the substitution of narrow oval ridges or depressions for leaf-scars; this is now recognised as a partially decorticated Sigillaria, in which the vascular bundle of each leaf is represented by a narrow ridge or depression. The name Syringodendron, originally used by Sternberg, is conveniently applied to certain forms of Sigillarian stems which have lost their superficial tissues. A fourth generic name, Favularia, was instituted by Sternberg[455] for Sigillarian stems with ribs covered with contiguous leaf-scars of hexagonal form and prominent lateral angles (fig. 193, A; fig. 200, G).

The generic or sub-generic title Rhytidolepis, also instituted by Sternberg, is applied to ribbed Sigillarian stems such as S. scutellata, S. rugosa (fig. 193, B), S. mammillaris (fig. 195), or S. laevigata (fig. 196, D). Goldenberg[456] proposed the name Leiodermaria for smooth Sigillarian stems with leaf-scars not in contact with one another (fig. 196, C).

The shoot system of Sigillaria consisted of a stout stem tapering upwards to a height of 100 feet[457] or more as an unbranched column, with its dome-shaped apex[458] covered with linear grass-like leaves or, in some species, such as Sigillaria Brardi[459], S. Eugenii[460], etc., the main trunk was occasionally divided by apparently equal dichotomy. The younger portions of the stem or branches were in some species clothed with leaves separated by a narrow zigzag groove surrounding their hexagonal bases, while in other forms each leaf was seated on a more or less prominent cushion having the form illustrated by Sigillaria McMurtriei (fig. 194) or by the example represented in fig. 200, H; or as in the ribbed species shown in figs. 193, B, and 195, the leaves in vertical series were separated from one another by longer portions of the ribs. As in Lepidodendron the cushions are frequently characterised by irregular transverse wrinklings and other[461] surface-ornamentation which in some instances at least may have been produced as the result of post-mortem shrinkage of superficial tissue. From the rarity of shoots with the foliage attached, it would seem that the leaves persisted for a comparatively short time and were cut off by an absciss-layer leaving behind a well-marked leaf-scar area. The linear leaves, reaching in rare cases a length of one metre (e.g. S. lepidodendrifolia) but usually much shorter, possessed a single median bundle, and the lower face was characterised by two stomatal grooves and a median keel. It is not uncommon to find leaf-bases of Sigillaria detached from the stem and preserved as separate impressions. The term Sigillariophyllum used by Grand’Eury[462] may be applied to detached leaves, though it is by no means easy to distinguish between the foliage of Sigillaria and Lepidodendron. A comparison of a typical species of Sigillaria, such as S. rugosa (fig. 193, B) or S. Brardi (fig. 196, A–C) with a typical Lepidodendron reveals obvious differences in the form of the leaf-cushion, but in some cases the distinction becomes purely arbitrary.

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.

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.

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.

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.

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].

Renault[504] has shown that the leaf-traces of Sigillaria spinulosa (= S. Brardi) are accompanied in the outer cortical region of the stem by a fairly large amount of secondary xylem; in sections of the free lamina which he figures the secondary elements are much less obvious and represented by a few tracheae only. Similarly, in the leaf-base of S. Brardi (fig. 200, E) the xylem consists of both primary and secondary elements (x, x²), but in the lamina the latter is poorly if at all represented. In the lamina of the leaves of S. Brardi the primary xylem forms a narrow slightly curved band with two lateral groups of narrower, presumably protoxylem elements; this is surrounded by delicate parenchyma styled by Renault, on very slender evidence, phloem (“liber”). Some dark cells below the xylem are described as sclerous tissue, and surrounding the bundle is a sheath of transfusion tracheae (dotted area in fig. 200, E). It is possible that the elements spoken of with hesitation by Renault as secondary xylem are transfusion tracheae.

There has probably been some confusion in the minds of authors between sclerous tissue and dark secretory tissue in Sigillarian leaves; the crescentic band, a, shown in fig. 142, B, which corresponds in position with the sclerous tissue of Renault in S. Brardi leaves, appears to be of the nature of secretory tissue.

The diagram shown in fig. 142, B, illustrates a type of leaf very like those already described, except that there are two xylem strands, x. The difference between the double strand and the single bundle seen in figs. 142, A, C and 200 E, is comparatively small, but it is a real distinction. This type of leaf (fig. 142, B) was originally described by Renault[505] under the generic title Sigillariopsis. The genus was founded on a French petrified specimen consisting of part of a ribbed stem possessing a stele of the Sigillarian type and characterised by separate primary xylem strands, like those of S. Brardi described by Brongniart in 1839. Renault considered the presence of two xylem strands in the leaf a sufficient reason for the institution of a new genus and named the specimen Sigillariopsis Decaisnei. Prof. Bertrand of Lille kindly photographed for me Renault’s type-specimen and sent several prints with explanatory notes. The transverse section of the leaves shows very clearly the two xylem strands; each strand consists of a triangular group of primary tracheae with the protoxylem apex pointing towards the lower surface of the lamina. Below each primary strand of centripetal xylem is an arc composed of a few small tracheae which Renault and Bertrand describe as secondary xylem; it is, however, not clear from the photomicrographs that these are of secondary origin, their position and appearance reminding one of the primary centrifugal xylem of a cycadean foliar bundle. Below this centrifugal xylem is another arc of imperfectly preserved elements described by Renault as a protective sheath and by Bertrand as glandular tissue; the latter term is probably the more correct as the tissue may well correspond to the secretory-zone tissue of Lepidodendron stems. Fairly large groups of transfusion tracheids occur on the flanks of the xylem. Prof. Bertrand points out that one of his sections, cut nearer the apex of a leaf than that figured by Renault with a single xylem strand, contains a double strand and thus shows the latter’s description to be an incorrect interpretation of the imperfectly preserved tissues.

The Sigillariopsis type of leaf was recognised by Scott[506] in English material on which he founded the species Sigillariopsis sulcata. In a section which he has recently figured[507] a lacuna below the two xylem strands is described as “representing secretory tissue”; a band of transfusion tracheae almost encircles the pair of bundles.

In a note published in 1907, Kidston[508] demonstrated the association of Sigillariopsis leaves with an undoubted Sigillarian stem of the Rhytidolepis type and expressed his conviction that Renault’s genus is identical with Sigillaria. The correctness of Kidston’s conclusion has been proved by Arber and Thomas[509] who found that the leaf-traces of Sigillaria scutellata bifurcate during their course through the outer region of the cortex and enter the leaf as two distinct strands of primary xylem. In the section from Dr Kidston’s collection shown in fig. 142, B, the lamina, 4 mm. wide, consists mainly of thin-walled assimilating tissue composed of radially elongated cells abutting at the periphery on hypodermal mechanical tissue, except at the edges of the stomatal grooves which are bounded by the small-celled epidermis. A broad sheath of thicker-walled elements, s, surrounds numerous scattered transfusion tracheae, t, and below the two xylem strands, x, which are embedded in delicate parenchyma there is a crescentic band of dark tissue, a, resembling the smaller strand, a, in fig. 142, A′, and the secretory zone tissue of a Lepidodendron stem.

iii. Fertile shoots of Sigillaria.

Reference has already been made to the manner of occurrence of strobili on Sigillarian stems; it remains to describe the structure of these reproductive shoots. Sigillariostrobus, the name given to Sigillarian strobili, may be defined in general terms as follows:

Cylindrical cones, rarely dichotomously branched[510] as in species of Lycopodium and Selaginella, which may reach a length of 30 cm. (e.g. Sigillariostrobus nobilis Zeill.[511]) and a diameter 2–5 cm.; peduncle long and slender, sometimes bearing acicular bracts or, after leaf-fall, characterised by leaf-cushions and leaf-scars like those on vegetative shoots (fig. 201, E). The stalked cones are borne in irregular verticils and in some species in vertical series, the fertile zones being separated by comparatively long sterile portions of the stem (fig. 199). The cones were deciduous and, in certain cases if not in all, the individual sporophylls became detached from the cone-axis on maturity. The slender axis bore spiral or verticillate imbricate sporophylls attached at right angles or more or less obliquely. The basal rhomboidal portion bore spores on its upper surface (fig. 201, F), presumably enclosed in a somewhat radially elongated sporangium (fig. B) and was prolonged distally into a narrow lanceolate free portion, in some species with a ciliate border (fig. D). The sporangia probably produced megaspores and microspores, but such spores as have been recognised appear to belong to the former category. The designation Triletes is applied to isolated spores of Sigillaria or to those of Lepidodendron.

Sigillariostrobus Tieghemi Zeiller[512] (figs. 201, E, F). In this species, from the Coal-field of Valenciennes, the pedicel bore acicular leaves or bracts attached to the upper portion of leaf-cushions arranged in vertical series (fig. E). The cones reached a length of 16 cm. and a breadth of 2·5–5 cm.; the sporophylls are borne in alternating verticils with 8–10 in each whorl. Several megaspores (2 mm. in diameter) appear to have been produced in tetrads in each sporangium.