Part 5

Sigillaria (Plate IV. Figs. 1 and 2) is perhaps the most important plant in the coal formation. The name is derived from sigillum, a seal, to indicate the seal-like markings in the stem. It is found in all coal-shales over the world. Schimper mentions 83 species. It occurs in the form of lofty stems, 40-50 feet high, and 5 feet broad (Figs. 33 and 34). Many stems of Sigillaria may be seen near Morpeth, standing erect at right angles to the planes of alternating strata of shale and sandstone (Fig. 33). They vary from 10 to 20 feet in height, and from one to three feet in diameter. Sir W. C. Trevelyan counted 20 portions of these trees within the length of half-a-mile, of which all but four or five were upright. Brongniart mentions similar erect stems as being found near St. Etienne. The stem of Sigillaria is fluted in a longitudinal manner, like a Doric column, and has a succession of single scars, which indicate the points of insertion of the leaves (Figs. 35, 36, and 37). When the outer part of the stem separates like bark, it is found that the markings presented by the inner surface differ from those seen externally. This has sometimes given rise to the erroneous multiplication of species and even of genera. Sigillaria elegans, as figured by Brongniart in Archives du Museum, i. 405, has a stem consisting of a central cellular axis or medulla, surrounded by a vascular cylinder, and this is invested by a thick cellular cortical layer, the outer portion composed of fusiform cells of less diameter than those of the inner portion. What Brongniart calls medullary rays are mere cracks or separations in the wedges traversed by vessels. In its structure it resembles its root Stigmaria, and must be referred to Lycopodiaceæ, along with Lepidodendron, Halonia, Ulodendron, etc. The small round sporangia of Sigillaria are borne in a single patch on the somewhat enlarged bases of some of the leaves. (See Carruthers on Structure and Affinities of Sigillaria, in Journ. Geol. Soc. Aug. 1869.)

[Sidenote: Fig. 38. _Stigmaria ficoides_, root of Sigillaria, giving off rootlets, which have been compressed.

Fig. 39. _Stigmaria ficoides_ (_S. Anabathra_ of Corda), which is the root of a Sigillaria. The markings are the points whence rootlets proceed.]

It has been ascertained by Professor King and Mr. Binney of Manchester, that the plant called Stigmaria (Fig. 38) is not a separate genus, but the root of Sigillaria (Plate IV. Figs. 1 and 2). The name is derived from στίγμα, a mark, indicating the markings on the axis. It is one of the most common productions of the coal-measures, and consists of long rounded or compressed fragments, marked externally by shallow circular, oblong, or lanceolate cavities (Fig. 39) in the centre of slight tubercles, arranged more or less regularly in a quincuncial manner (Plate III. Fig. 7). The cavities occasionally present a radiating appearance. The axis of the fragments is often hollow, and different in texture from the parts around. This axis consists of a vascular cylinder or woody system, penetrated by quincuncially arranged meshes or openings, through which the vascular bundles proceed from the inner surface of the cylinder to the rootlets (Plate III. Figs. 8 and 9). From the scars and tubercles arise long ribbon-shaped processes, which were cylindrical cellular roots, now compressed (Fig. 38). The vascular cylinder of Stigmaria is composed entirely of scalariform tissue, pierced by meshes for the passage, from the inner surface of the cylinder, of the vascular bundles which supply the rootlets. (Carruthers in Geol. Proc., Aug. 1869.) Stigmaria ficoides (Fig. 38) abounds in the under-clay of a coal-seam, sending out numerous roots from its tubercles, and pushing up its aerial stem, in the form of a fluted Sigillaria. On the Bolton and Manchester Railway Mr. Binney discovered Sigillarias standing erect, and evidently connected with Stigmarias which extended 20 feet or more.[9] Stigmaria is regarded by Schimper as roots, not of Sigillaria only, but of Knorria longifolia (one of the Lepidodendreæ). The base of the stem of this species of Knorria is Ancestrophyllum, and the upper part is Didymophyllum Schottini of Goeppert. Professor King and others suppose that the Fern-like frond called Neuropteris is connected with Sigillaria, but this is a mere conjecture, set aside by the discovery of leaves attached to a species allied to Sigillaria elegans, which establishes that the long linear leaves described under the name Cyperites are the foliage of this genus. Goldenberg has figured the fructification, which consists of small sporangia like those of Flemingites, borne on the basis of but slightly modified leaves. This establishes the opinion that Sigillaria was an acrogenous plant belonging to Lycopodiaceæ. Brongniart reckons it as representing an extinct form of Gymnosperms, and King, having erroneously associated the Cyclopteris with it, places it between the Ferns and Cycadaceæ. Mr. Carruthers informs me that he has examined the stem of a true fluted Sigillaria, with the tissues preserved, and that these agree with the structure of Lepidodendron, a position in which he had already placed it from the structure of its fruit.

[Sidenote: Figs. 40 to 44 exhibit the stems and fructification of Lepidodendron. Fig. 40. Bifurcating stem of _Lepidodendron obovatum_ (_elegans_), showing the scale-like scars, and the narrow-pointed leaves, resembling those of Lycopodium, but much larger. Fig. 41. Stem of _Lepidodendron crenatum_, with the scars of its leaves.]

[Sidenote: Fig. 42. Fructification of Lepidodendron, showing its cone-like form and spiral arrangement of scales. It is called _Lepidostrobus Dabadianus_ by Schimper, but it is probably Triplosporites.

Fig. 43. Longitudinal section of the fructification, showing central axis and scales carrying sporangia. The upper sporangium contains microspores, the lower macrospores; hence it has the character of Triplosporites.]

[Sidenote: In woodcut 44 are represented the fruits of Selaginella (one of the Lycopodiums of the present day), Lepidostrobus, Triplosporites, and Flemingites. Fig. 1. _Selaginella spinulosa_, A. Braun (_Lycopodium selaginoides_, Linn.) 2. Scale and sporangium from the upper portion of the cone. 3. Antheridian microspores from the same. 4. Macrospore. 5. Scale and sporangium from the lower part of the cone, containing macrospores. 6. _Lepidostrobus ornatus_, Hooker. 7. Three scales and sporangia of ditto. 8. Microspores from the sporangia of the upper part of the cone of _Triplosporites Brownii_, Brongn. 9. Macrospore from the sporangia of the lower part (drawn from Brongniart's description and measurements). 10. Scales and sporangia of a cone of Flemingites.[10]]

Lepidodendron (Figs. 40 to 44) is another genus of the coal-measures which differs from those of the present day (Plate IV. Fig. 3). Lepidodendrons, or fossil Lycopodiaceæ, had spikes of fructification comparable in size to the cones of firs and cedars, and containing very large sporangia, even larger than those of Isoetes, to which they approach in form and structure. Schimper, in 1870, enumerates 56 species of Lepidodendron, all arborescent and carboniferous. The stem of a Lepidodendron is from 20 to 45 feet high, marked outside by peculiar scale-like scars (Fig. 41), hence the name of the plant (λεπίς, a scale, and δένδρον, a tree). Although the scars on Lepidodendron are usually flattened, yet in some species they occupy the faces of diamond-shaped projections, elevated one-sixth of an inch or more above the surface of the stem, and separated from each other by deep furrows;--the surface bearing the leaf being perforated by a tubular cavity, through which the bundle of vessels that diverged from the vascular axis of the stem to the leaf passed out. The linear or lanceolate leaves are arranged in the same way as those of Lycopodiums or of Coniferæ, and the branches fork like the former. The internal structure of the stem is the same as that of Sigillaria. The fruit of Lepidodendron and allied genera is seen in Lepidostrobus and Triplosporites (Figs. 42, 43; Plate III, Fig. 10). Carruthers, in his lecture to the Royal Institution, in describing the forms of Lepidostrobus, says--"The fruit is a cone composed of imbricated scales arranged spirally on the axis like the true leaves, and bearing the sporangia on their horizontal pedicels. Three different forms of fruit belong to this genus, or it should perhaps rather be called group of plants. The first of these is the cone named by Robert Brown Triplosporites (Figs. 42, 43), and described by him from an exquisitely preserved specimen of an upper portion, in which the parts are exhibited as clearly in the petrified condition as if they belonged to a fresh and living plant. The large sporangia have a double wall, the outer composed of a compact layer of oblong cells placed endwise, or with the long diameter perpendicular to the surface; the inner is a delicate cellular membrane. The sporangium is filled with a great number of very small spores, each composed of three roundish bodies or sporules. Recently Brongniart and Schimper have described a complete specimen of this fruit, in which the minute triple spores are confined to the sporangia of the upper and middle part of the cone, but the lower portion, which was wanting in Brown's specimen, bears sporangia filled with simple spherical spores ten or twelve times larger than the others (woodcut 44, 9).

"The structure of another form of cone (Lepidostrobus) has been expounded by Dr. Hooker. The arrangement of the different parts comprising it is precisely similar to what occurs in Triplosporites; but the sporangia are filled with the minute triple spores throughout the whole cone (woodcut 44, 6 and 8).

"The third form of cone, described by me under the name Flemingites, differs from the other two in having a large number of small sporangia supported on the surface of each scale; and it agrees with Lepidostrobus in the sporangia containing only small spores (woodcut 44, 10).

"In comparing these fossils with the living club-mosses, one is struck with the singular agreement in the organisation of plants so far removed in time, and so different in size, as the recent humble club-mosses and the palæozoic tree Lepidodendrons. The fruit of Triplosporites, like that of Selaginella (woodcut 44, 1), contains large and small spores, the microspores being found in both genera on the middle and upper scales of the cone, and the macrospores on those of the lower portion (Fig. 43).

"On the other hand, the fruits of Lepidostrobus and Flemingites agree with that of Lycopodium in having only microspores. The size of the two kinds of spores also singularly agrees in the two groups. This is of some importance, for among the recent vascular Cryptogams there is a remarkable uniformity in the size of the spores in the members of the different groups, even when there is a great variety in the size of the plants. Thus the spore of our humble wall-rue is as large as that of the giant Alsophila of tropical regions. So also the spores of Equisetum and Calamites agree in size, as may be seen in woodcut 47, Figs. 3, 4, and 9, where the spores of the two genera are magnified to the same extent. And a similar comparison of the macrospore and microspore of Triplosporites with those of Selaginella, and of the microspore of Lepidostrobus with that of Lycopodium, exhibits a similar agreement. This is made apparent by the drawings in woodcut 44 of the two kinds of spores of Selaginella, 3 and 4, with those of Triplosporites, 8 and 9, which are drawn to the same scale."

The genus Sigillaria, as we have already said, has, according to the observation of Hooker, small sporangia exactly agreeing in size and form with those of Flemingites. Most probably the contents of these small sporangia were the same in both genera, so that Sigillaria would be placed with Flemingites and Lepidostrobus as arborescent Lycopodiaceæ having their affinities with Lycopodium, as they have all microspores only in their fructification.

The scales upon the Lepidodendron stems, as well as those in the cones, are arranged in a spiral manner, in the same way as plants of the present day. Professor Alexander Dickson has examined the phyllotaxis of Lepidodendrons, and gives the following results of his observations (Trans. Bot. Soc. Edin. xi. 145). The fossil remains of Lepidodendrons are often so compressed that it is difficult, or even impossible, to trace the secondary spirals round the circumference of the stem. In those cases, however, where there is comparatively little compression, _i.e._ where the stem is more or less cylindrical, the determination of the phyllotaxis is easy. Of such stems he has examined fifteen specimens, which may be classed according to the series of spirals to which the leaf-arrangement belongs:--

A. Ordinary series, ½, ⅓, ⅖, ⅜, 5/13, etc.

(a.) Single spirals (D turning to the right, S to the left).

(1.) _Lepidodendron_ (Possil Ironstone series). Stem about ¾ of an inch in diameter. Secondary spirals 8 D, 13 S, 21 D. Divergence = 13/34 (or possibly 21/55).

(2.) _Lepidodendron_ (Knightswood, near Glasgow, Mr. J. Young). Stem about 1½ inch in diameter. Secondary spirals 13 D, 21 S, 34 D. Divergence = 21/55.

(3.) _Lepidodendron_ (Possil Sandstone series). Trunk about 2 feet long, with an average diameter of 20 inches. Steepest secondary spirals 55 S, 89 D. Divergence = 55/144.

(b.) Conjugate spirals.[11]

(4.) _Lepidostrobus ornatus_ (Bathgate coal-field). About ¾ of an inch in diameter. Secondary spirals 10 D, 16 S, 26 D, 42 S. Divergence = 13/(34×2) (Bijugate arrangement).

(5.) _Lepidostrobus_ (Plean, Stirlingshire, Mr. Mackenzie). About ½ an inch in diameter. Secondary spirals 9 S, 15 D, 24 S, 39 D. Divergence = 8/(21×3) (Trijugate arrangement).

(6.) _Knorria taxina_ (from collection of Dr. Rankin, Carluke). Somewhat compressed, 2-2½ inches[12] in diameter. Secondary spirals 15 D, 24 S. Divergence = 15/(13×3) (Trijugate arrangement).

(7.) _Lepidodendron_ (from Dr. Rankin's collection). About 1¼ inch in diameter. Secondary spirals 10 D, 15 S, 25 D, 40 S. Divergence = 5/(13×5) (Quinquejugate arrangement).

(8.) _Lepidodendron_ (Dowanhill, Glasgow, Possil Sandstone series). Trunk about 1 foot long, and 1 foot in diameter. The upper portion exhibits secondary spirals 35 D, 56 S, 91 D; thus indicating a 7-jugate arrangement, with divergence = 8/(21×7). The arrangement on the middle and lower portion is indistinct and confused; so much so as to render any determination of the arrangement doubtful.

B. Series, ⅓, ¼, 2/7, 3/11, etc.

(9.) _Lepidodendron_ (Messrs Merry and Cunningham's Clayband Iron-Pit, Carluke). Stem 2 inches in diameter. Secondary spirals 18 S, 29 D, 47 S. Divergence = 21/76.

C. Series, ¼, ⅕, 2/9, 3/14, etc.

(10.) _Lepidodendron_ (R. B. Garden, Edinburgh, Museum). Stem somewhat flattened, 1-1½ inch in diameter. Secondary spirals 9 D, 14 S, 23 D, 37 S. Divergence = 13/60.

(11.) _Lepidodendron_ (Redhaugh, near Edinburgh, Mr. Peach). Stem somewhat flattened, ¾ to ½ inch in diameter. Secondary spirals 9 S, 14 D, 23 S, 37 D. Divergence = 13/60.

D. Series, ⅕, ⅙, 2/11, 3/17, 5/28, etc.

(12.) _Knorria taxina_ (Stockbriggs, Lesmahagow,--Hunterian Museum). About 1 inch in diameter. The specimen consists of a main stem and one of the branches into which it has forked. On the main stem the secondary spirals are 6 D, 11 S, 17 D. Divergence = 5/28 (series, ⅕, ⅙, 2/11, 3/17, 5/28, etc.)--On the branch the secondary spirals are 8 S, 13 D. Divergence = 8/21 (ordinary series, ½, ⅓, ⅖, ⅜, etc.)

E. Series, ½, ⅖, 3/7, 5/12, 8/19, 13/31, 21/50, etc.

(13.) _Lepidodendron_ (from Dr. Rankin's collection). About ⅞ inch in diameter. Secondary spirals 12 D, 19 S, 31 D. Divergence = 21/50.

F. Series, ⅓, 3/10, 4/13, 7/23, 11/36, 18/59, etc.

(14.) _Lepidodendron elegans_ (Possil Ironstone). About 1¼ inch in diameter. Secondary spirals 10 S, 13 D, 23 S, 36 D. Divergence = 18/59.

(15.) _Lepidodendron_ (Possil Ironstone). About 2¼ inches in diameter. Secondary spirals 23 S, 36 D, 59 S, 95 D. Divergence = 47/154.

From the above it is evident that the phyllotaxis of Lepidodendron is extremely variable, as much so perhaps as that of those most variable plants, in this respect, the Cacti. It is also clear that what has been enunciated by Professor Haughton (Manual of Geology, Lond. 1866, pp. 243, 245) as the law according to which the leaves of palæozoic plants were arranged--viz. that of alternate whorls--does not apply to these ancient Lycopods. Lepidodendron aculeatum is noted by Naumann as exhibiting an 8/21 arrangement. (Poggendorff, Annalen, 1842, p. 5.) Professor Alexander Braun (Nov. Acta Ac. C. L. C. xv. 1, pp. 558-9), speaking of the excessive deviation from ordinary arrangements in Equisetaceæ (including Calamites), compares them in this respect with Lycopodiaceæ (including Lepidodendron), saying that in these two families "the utmost limits of the domain of all leaf-arrangement appears to be attained."

Lepidophyllum is certainly leaves of Lepidodendron, the different Lepidophylla belonging to different species of the genus. The slender terminal branches are noticed under the name of Lycopodites. In coal from Fordel Mr. Daw has detected innumerable bodies (Plate III. Figs. 1, 2, 3) which have been shown to be sporangia. (Balfour, Trans. Roy. Soc. Ed. xxi. 187.) On their under surface Mr. Carruthers has observed a triradiate ridge (Plate III. Fig. 4). (Geological Magazine, 1865, vol. ii. p. 140.) These sporangia have been found connected with the cone-like fructification called Flemingites, and resembling Lycopodium (woodcut 44, Fig. 4). Many forms of fossil plants, such as Halonia, Lepidophloios, Knorria, and Ulodendron, belong to the Lepidodendron group. Knorria is said to be the internal cast of a Lepidodendron.

Ulodendron minus and U. Taylori (Plate III. Fig. 11), found in ferruginous shale in the Water of Leith, near Colinton, exhibit beautiful sculptured scars, ranged rectilinearly along the stem. The surface is covered with small, sharply relieved obovate scales, most of them furnished with an apparent midrib, and with their edges slightly turned up. The circular or oval scars of this genus are probably impressions made by a rectilinear range of aerial roots placed on either side. When decorticated, the stem is mottled over with minute dottings arranged in a quincuncial manner, and its oval scars are devoid of the ordinary sculpturings. Bothrodendron is a decorticated condition of Ulodendron.

[Sidenote: Fig. 45. _a_, _Calamites Suckovii_, composed of jointed striated fragments having a bark. Fig. 45. _b_, Septum or phragma of a Calamite.]

Calamites (κάλαμος, a reed) is a reed-like fossil, having a sub-cylindrical jointed stem (Fig. 45, _a_ and _b_; Fig. 46; Plate IV. Fig. 4). The stem is often crushed and flattened, and was originally hollow. Calamites is thus defined by Grand d'Eury (Ann. Nat. Hist. ser. 4, vol. iv. p. 124):--Stem articulated, fistular, and septate; outer part comparatively thin, formed of three concentric zones--1, an exterior cortical layer now converted into coal; 2, a thin subjacent zone of vascular tissue, now invariably destroyed; 3, a sort of inner lining epidermis, which is carbonified. Cortical envelope marked interiorly with regular flutings, interrupted and alternate at the articulations. Inner epidermis smooth, or scarcely striated. Vascular cylinder thin; outer surface of bark more fully fluted and articulated than the inner surface.

[Sidenote: Fig. 46. Vertical stems of fossil trees, Calamites chiefly, found in the coal-measures of Treuil, near Saint Etienne.]

Carruthers gives the following description of the structure of a species of Calamite which he examined:--The stem was composed of a central medulla, which disappeared with the growth of the plant, surrounded by a woody cylinder, composed entirely of scalariform vessels, and a thin cortical layer. The medulla penetrated the woody cylinder by a series of regular wedges, which were continued, as delicate laminæ of one or two cells in thickness, to the cortical layer. The cells of those laminæ were not muriform; their longest diameter was in the direction of the axis. The wedges were continuous, and parallel between each node. As the axial appendages were produced in whorls, the only interference with the regularity of the tissues was by the passing out through the stem at the nodes of the vascular bundles which supplied these appendages. As the leaves of each whorl were (with one or two exceptions) opposite to the interspaces of the whorls above and below, there was also at each node a re-arrangement of the wedges of vascular and cellular tissues.

Schimper considers Calamites as having an analogy with Equisetum in its fructification. He looks on them as fossil Equisetaceæ. Annularia and Sphenophyllum are considered as establishing a passage from the Equisetaceæ to the Lycopodiaceæ. Some gigantic fossil Equiseta had a diameter of nearly 5 inches, and a height of 30 or more feet. The branches, which adorned the higher part of them in the form of a crown, are simple, and have at their extremity a spike of the size of a pigeon's egg, and organised exactly like the spikes of living Equiseta. The subterranean rhizomes are well developed, and gave origin, like many Equiseta, to tubercles which had the form and size of a hen's egg.

The characters of Equisetum of the present day and Calamites, are exhibited in woodcut 47. They show a marked resemblance in the fructification. (See also page 31.)

Plants of Calamites have been seen erect by Mr. Binney, and he has determined that what were called leaves or branches by some are in reality roots. Mr. Binney gives a full description of various Calamites, under the name of Calamodendron commune, in his Memoir published by the Palæontographical Society, 1868. There are between 50 and 60 species recorded.[13]

In Spitzbergen, in rocks of the Carboniferous epoch, there have been found Calamites, Sigillaria, Lepidodendron, and ferns, apparently the same as those found in the Carboniferous epoch in Europe--Calamites radiatus, Lepidodendron Veltheimianum, Sigillaria distans, Stigmaria ficoides. Some species--Sigillaria Malmgreni, Lepidodendron Carneggiannum, and L. Wilkianum--seem to be peculiar to Bear Island.

[Sidenote: Fig. 47. Fruits of Equisetum and Calamites. 1. _Equisetum arvense_, L. 2. Portion of sporangium wall. 3, 4. Spores, with the elaters free. 5. Longitudinal section of the part of one side of cone. 6. Transverse section of cone. 7. _Calamites (Volkmannia) Binneyi_, Carr., magnified three times. 8. Portion of the sporangium wall. 9. Two spores. 10. Longitudinal section of the part of one side of cone. 11. Transverse section of cone.]