Part 6

According to Carruthers the Equisetaceæ are represented in Britain by the two genera Calamites found in primary beds, and Equisetum found in secondary rocks and living at the present day. The difference in the structure of their fruits is shown in woodcut 47. The fruit of Calamites, called Volkmannia Binneyi (woodcut 47, 7), is a small slender cone composed of alternating whorls of imbricate scales, twelve in each verticil. The scales completely conceal the leaves connected with the fructification. The fruit-bearing leaves are stalked, peltate, and are arranged in whorls of 6. There are four sporangia borne on the under-surface of the peltate leaves. These spore-cases have cellular parietes, and in their interior there is a deposit of cellulose in the form of short truncate processes not unlike imperfect spirals. The spores are spherical, and appear to have thread-like processes proceeding from them, similar to elaters. The fruit-cone bears a marked resemblance to the fruit of Equisetum in its fruit-bearing leaves, sporangia, spores, and elaters (see Figs. 18, 19, 20, 21). In the modern plant all the leaves of the cone are fructiferous, while in the fossil plant some are fruit-bearing, and others are like the ordinary leaves of the plant. It is thought that the fossil may be reckoned as having a somewhat higher position than that possessed by the living genus.

[Sidenote: Fig. 48. Foliage and fruits of Calamites. 1 and 2. Asterophyllites; 3 and 4. Annularia; 5 and 6. Sphenophyllum.]

The different forms of foliage called Asterophyllites, Sphenophyllum, and Annularia, belong to the one genus Calamites, but they may form, perhaps, well-characterised sections when their fruits are better known. In woodcut 48 representations are given of the foliage and fruit of varieties of Calamites. In 1 and 2 we see the simplest form called Asterophyllites. The leaves are linear and slender, with a single rib. The form called Annularia (3 and 4) differs chiefly in having a larger amount of cellular tissue spread out on either side of the midrib. This form has a different aspect in a fossil state from the other, from its whorls of numerous broad leaves spread out on the surface of deposition, while the acicular leaves of Asterophyllites have penetrated the soft mud, and are generally preserved in the position they originally occupied in reference to the supporting branch. The third form (5 and 6) is called Sphenophyllum, and consists of whorls of wedge-shaped leaves, with one or more bifurcating veins. They occur like those of Annularia, spread out on the surface of the shale.

[Sidenote: Fig. 49. _Araucarioxylon Withami_, Krauss (_Pinites Withami_), from the Coal-measures, Craigleith, near Edinburgh, showing pleurenchyma with disks, and medullary rays. An excellent specimen of a stem of this pine may be seen in the Edinburgh Royal Botanic Garden.

Fig. 50. _Trigonocarpum olivæforme_, an ovate, acuminate, three-ribbed, and striated fruit or seed, which some suppose to be a sporangium of a Lepidodendron, others refer it to Cycadaceæ. Hooker refers it to Coniferæ like Salisburia.]

True Exogenous trees exist in the coal-fields both of England and Scotland, as at Lennel Braes and Allan Bank, in Berwickshire; High-Heworth, Fellon, Gateshead, and Wideopen, near Newcastle-upon-Tyne; and in quarries to the west of Durham; also in Craigleith quarry, near Edinburgh, and in the quarry at Granton, now under water. In the latter localities they lay diagonally athwart the sandstone strata, at an angle of about 30°, with the thicker and heavier part of their trunks below, like snags in the Mississippi. From their direction we infer that they have been drifted by a stream which has flowed from nearly north-east to south-west. At Granton, one of the specimens exhibited roots. In other places the specimens are portions of stems, one of them 6 feet in diameter by 61 feet in length, and another 4 feet in diameter by 70 feet in length. These Exogenous trees are Gymnosperms, having woody tissue like that of Coniferæ. We see under the microscope punctated woody tissue, the rows of disks being usually two, three, or more, and alternating. They seem to be allied in these respects to Araucaria and Eutassa (Fig. 61, p. 74) of the present flora. Araucarioxylon or Pinites Withami (Fig. 49) is one of the species found in Craigleith quarry; the concentric layers of the wood are obsolete; there are 2, 3, or 4 rows of disks on the wood, and 2-4 rows of small cells in the medullary rays. Along with it there have also been found Dadoxylon medullare, with inconspicuous zones, 2, 3, and 4 rows of disks, and 2-5 series of rows of cells in the rays. Pissadendron antiquum (Pitus antiqua) having 4-5 series of cells in the medullary rays, and P. primævum (Pitus primæva), with 10-15 series of cells in the medullary rays, occur at Tweedmill and Lennel Braes in Berwickshire; Peuce Withami (Fig. 1, p. 3) at Hilltop, near Durham, and at Craigleith. Sternbergia is considered by Williamson as a Dadoxylon, with a discoid pith like that seen now-a-days in the Walnut, Jasmine, and Cecropia peltata, as well as in some species of Euphorbia.[14] Sternbergia approximata is named by him Dadoxylon approximatum. Hooker believes from the structure of Trigonocarpum (Fig. 50) that it is a coniferous fruit nearly allied to Salisburia (Trans. Roy. Soc. 1854). Several species of Trigonocarpum occur in the Carboniferous rocks, such as T. olivæforme from Bolton (Plate II. Fig. 5), and T. sulcatum from Wardie, near Edinburgh (Plate II. Fig. 6). Noeggerathia and a few other plants, such as Flabellaria and Artisia, are referred by Brongniart to Cycadaceæ. Flabellaria borassifolia, according to Peach, has leaves like Yucca. Noeggerathia has pinnate leaves, cuneiform leaflets, sometimes fan-shaped; the veins arise from the base of the leaflets, are equal in size, and either remain simple or bifurcate, the nervation (venation) being similar to that of some Zamias.

The fossils of this period, referred to as Antholithes,[15] have just been shown by Mr. Carruthers to be the inflorescence of Cardiocarpum (Geol. Mag. Feb. 1872), and he proposes to set aside the former name, confining it to the tertiary fossils to which it was originally given by Brongniart, and to use the latter name. The main axis of the inflorescence is simple, stout, and marked externally with interrupted ridges. The axis bears in a distichous manner sub-opposite or alternate bracts of a linear-lanceolate form and with decurrent bases. In the axils of the bracts were developed flower-like leaf-bearing buds, and from them proceeded three or four linear pedicels, which terminated upwards in a somewhat enlarged trumpet-shaped apex. To this enlarged articulating surface was attached the fruit, to which has been given the generic name Cardiocarpum[16] (Fig. 51). The place of attachment is indicated by the short straight line which separates the cordate lobes at the base of the fruit. The fruit is flattish, broadly ovate, with a cordate base and sub-acute apex. It consists of an outer pericarp, inclosing an ovate-acute seed. That the pericarp was of some thickness, and formed probably a sub-indurated rind, is shown by a specimen preserved in the round, and figured (Fig. 53 _a_). The pericarp is open at the apex; and the elongated tubular apex of the spermoderm passes up to this opening. The seed forms a distinct swelling in the centre of the fruit, and a slight ridge passes up the middle to the base of the apical opening.

[Sidenote: Fig. 51. _Cardiocarpum Lindleyi_, Carr. Fig. 52. Do., Coal-measures, Falkirk.]

These fossils are believed to be an extinct form of Gymnosperms. Two species have been described, of both of which we are able to give figures. The first figure is from the specimens collected by Mr. Peach at Falkirk. It is Cardiocarpum Lindleyi (Figs. 51, 52); it has a primary axis with sub-opposite axillary axes, bearing four to six lanceolate leaves and three or four pedicels. Primary bracts short and arcuate. Fruit ovate-cordate, with an acute bifid apex, and a ridge passing up the middle of the fruit.

[Sidenote: Fig. 53. _Cardiocarpum anomalum_ (Carr.), natural size: with separate fruit (_a_), twice natural size--Coal-measures, Coalbrookdale.]

The second species is Cardiocarpum anomalum (Fig. 53) from Coalbrookdale; it has a primary axis with alternate or sub-opposite axillary axes, slender and elongated, bearing many linear leaves, and several slender pedicels; primary bracts long, slender, and straight; fruits small, margined. The somewhat magnified separate fruit (_a_) shows the thickness of the pericarp and the enclosed seed.

[Sidenote: Fig. 54. _Pothocites Grantoni_, Paterson. _a_, Spike natural size; _b_, Portion of spike magnified; _c_, Perianth, 4-cleft, magnified.]

In the bituminous shale at Granton, near Edinburgh, Dr. Robert Paterson discovered in 1840 a peculiar fossil plant, which he called Pothocites Grantoni (Fig. 54, _a_). It is figured in the Transactions of the Edinburgh Botanical Society, vol. i. March 1840. It is a spike covered by parallel rows of flowers (Fig. 54, _b_), each apparently with a 4-cleft calyx (Fig. 54, _c_). It was supposed to be allied to Potamogeton or Pothos, more probably to the latter. In that case it must be referred to the natural order Araceæ. The original specimen is deposited in the museum at the Royal Botanic Garden, Edinburgh.

Our knowledge of the real state of the vegetation of the earth when coal was formed must be very limited, when we reflect how seldom the fructification of coniferous trees has been met with in the coal-measures. A very doubtful fragment, supposed to be a cone, is given in Lindley and Hutton's work, under the title of Pinus anthracina; but it is believed by Carruthers to be a fragment of a Lepidodendroid branch. Lyell never saw a fossil fir-cone of the Carboniferous epoch, either in the rocks or museums of North America or Europe. Bunbury never heard of any other example than that noticed by Lindley and Hutton. Principal Dawson is disposed to think that the suberin of cork, of epidermis in general, and of spore-cases in particular, is a substance so rich in carbon that it is very near to coal, and so indestructible and impermeable to water, that it contributes more largely than anything else to the mineral. Sir Charles Lyell remarks--"To prevent ourselves, therefore, from hazarding false generalisations, we must ever bear in mind the extreme scantiness of our present information respecting the flora of that peculiar class of stations to which, in the Palæozoic era, the coal-measures probably belonged. I have stated elsewhere my conviction that the plants which produced coal were not drifted from a distance, but nearly all of them grew on the spot where they became fossil. They constituted the vegetation of low regions, chiefly the deltas of large rivers, slightly elevated above the level of the sea, and liable to be submerged beneath the waters of an estuary or sea by the subsidence of the ground to the amount of a few feet. That the areas where the carboniferous deposits accumulated were low, is proved not only by the occasional association of marine remains, but by the enormous thickness of strata of shale and sandstone to which the seams of coal are subordinate. The coal-measures are often thousands of feet, and sometimes two or three miles, in vertical thickness, and they imply that for an indefinite number of ages a great body of water flowed continuously in one direction, carrying down towards a given area the detritus of a large hydrographical basin, draining some large islands or continents, on the margins of which the forests of the coal period grew. If this view be correct, we can know little or nothing of the upland flora of the same era, still less of the contemporaneous plants of the mountainous or alpine regions. If so, this fact may go far to account for the apparent monotony of the vegetation, although its uniform character may doubtless be in part owing to a greater uniformity of climate then prevailing throughout the globe. Mr. Bunbury has successfully pointed out that the peculiarity of the carboniferous climate consisted more in the humidity of the atmosphere and the absence of cold, or rather the equable temperature preserved in the different seasons of the year, than in its tropical heat; but we must still presume that colder climates existed at higher elevations above the sea."

The plants of the coal-measures are evidently terrestrial plants. Brongniart agrees with Lyell in thinking that the layers of coal have in general accumulated in the situation where the plants forming them grew. The remains of these plants covered the soil in the same way as layers of peat, or the vegetable mould of great forests. In a few instances, however, the plants may have been transported from a distance, and drifted into basins. Phillips is disposed to think that this was the general mode of formation of coal-basins. He is led to this conclusion by observing the fragmentary state of the stems and branches, the general absence of roots, and the scattered condition of all the separable organs. Those who support the drift theory, look on the coal plants as having been swept from the land on which they grew by watery currents at different times, and deposited in basins and large sea-estuaries, and sometimes in lakes. The snags in the Mississippi, the St. Lawrence, and other large rivers, are given as instances of a similar drifting process.

The vegetation of the coal epoch seems to resemble most that of islands in the midst of vast oceans, and the prevalence of ferns indicates a climate similar to that of New Zealand in the present day. In speaking of the island vegetation of the coal epoch, Professor Ansted remarks (Ancient World, p. 88)--"The whole of the interior of the islands may have been clothed with thick forests, the dark verdure of which would only be interrupted by the bright green of the swamps in the hollows, or the brown tint of the ferns covering some districts near the coasts. The forests may have been formed by a mixture of several different trees. We would see then, for instance, the lofty and widely-spreading Lepidodendron, its delicate feathery fronds clothing, in rich luxuriance, branches and stems, which are built up, like the trunk of the tree-fern, by successive leafstalks that have one after another dropped away, giving by their decay additional height to the stem, which might at length be mistaken for that of a gigantic pine. There also should we find the Sigillaria, its tapering and elegant form sustained on a large and firm basis--enormous matted roots, almost as large as the trunk itself, being given off in every direction, and shooting out their fibres far into the sand and clay in search of moisture. The stem of this tree would appear like a fluted column, rising simply and gracefully without branches to a great height, and then spreading out a magnificent head of leaves like a noble palm-tree. Other trees, more or less resembling palms, and others like existing firs, also abounded, giving a richness and variety to the scene; while one gigantic species, strikingly resembling the Norfolk Island pine, might be seen towering a hundred feet or more above the rest of the forest, and exhibiting tier after tier of branches richly clothed with its peculiar pointed spear-like leaves, the branches gradually diminishing in size as they approach the apex of a lofty pyramid of vegetation. Tree-ferns also in abundance might there be recognised, occupying a prominent place in the physiognomy of vegetation, and dotted at intervals over the distant plains and valleys, the intermediate spaces being clothed with low vegetation of more humble plants of the same kind. These we may imagine exhibiting their rich crests of numerous fronds, each many feet in length, and produced in such quantity as to rival even the palm-trees in beauty. Besides all these, other lofty trees of that day, whose stems and branches are now called Calamites, existed chiefly in the midst of swamps, and bore their singular branches and leaves aloft with strange and monotonous uniformity. All these trees, and many others that might be associated with them, were, perhaps, girt round with innumerable creepers and parasitic plants, climbing to the topmost branches of the most lofty amongst them, and relieving, in some measure, the dark and gloomy character of the great masses of vegetation."

Hugh Miller remarks--"The sculpturesque character of the nobly-fluted Sigillarias was shared by not a few of its contemporaries. Ulodendrons, with their rectilinear rows of circular scars, and their stems covered with leaf-like carvings, rivalled in effect the ornately relieved torus of a Corinthian column. Favularia, Halonia, many of the Calamites, and all the Lepidodendrons, exhibited the most delicate sculpturing. In walking among the ruins of this ancient flora, the palæontologist almost feels as if he had got among the broken fragments of Italian palaces erected long years ago, when the architecture of Rome was most ornate, and every moulding was roughened with ornament; and in attempting to call up in fancy the old Carboniferous forests, he has to dwell on this peculiar feature as one of the most prominent; and to see in the multitude of trunks darkened above by clouds of foliage that rise upon him in the prospect, the slender columns of an older Alhambra, roughened with arabesque tracery and exquisite filigree work."

_FLORA OF THE PERMIAN EPOCH._

[Sidenote: Figs. 55 and 56. _Walchia piniformis_, Sternb., a common species in the Permian rocks of Europe. Fig. 55. Plant with leaves and fructification. Fig. 56. Fructification, natural size.]

The nature of the vegetation during the Permian period, which is associated with the Carboniferous, under the reign of Acrogens, has been extensively illustrated by Goeppert. Brongniart has enumerated the fossils in three different localities, which he refers doubtfully to this period. 1. The flora of the bituminous slates of Thuringia, composed of Algæ, Ferns, and Coniferæ. 2. Flora of the Permian sandstones of Russia, comprehending Ferns, Equisetaceæ, Lycopodiaceæ, and Noeggerathiæ. 3. Flora of the slaty schists of Lodève, composed of Ferns, Asterophyllites, and Coniferæ. The genera of Ferns here met with are those found in the Carboniferous epoch; the Gymnosperms are chiefly species of Walchia and Noeggerathia (the latter is supposed by Schimper to be a Cycad); Lepidodendron elongatum, Calamites gigas, and Annularia floribunda, are also species of this period. Walchia is a conifer characteristic of the Permian epoch, of which there are eight species described (Figs. 55 and 56). It has a single seed to each scale of the cone, and two kinds of leaves, the one short and imbricated, the other long and spreading. Among the plants of the Permian formation Goeppert enumerates the following:[17]--Equisetites contractus, Calamites Suckowi, C. leioderma, Asterophyllites equisetiformis, A. elatior, Huttonia truncata, H. equisetiformis, many species of Psaronius, one of the filicoid plants, Hymenophyllites complanatus, Sphenopteris crassinervia, Sagenopteris tæniæfolia, Neuropteris imbricata, and many other species of these genera; several species of Odontopteris, Callipteris, Cyclopteris, Dioonopteris, Cyatheites, Alethopteris, Noeggerathia, Cordaites, Anthodiopsis, Dictyothalamus, Calamodendron, Arthropitys; besides species of Sigillaria, Stigmaria, and Lepidodendron. Various fruits are also mentioned, under the names of Rhabdocarpum, Cardiocarpum, Acanthocarpum, Trigonocarpum, and Lepidostrobus.

FOSSIL FLORA OF THE SECONDARY OR MESOZOIC PERIOD.

REIGN OF GYMNOSPERMS.

[Sidenote: Fig. 57. _Pinus sylvestris_, Scotch Fir.

Fig. 58. _Abies excelsa_, common Spruce Fir of northern Europe.

Fig. 59. _Larix Europæa_, the Larch, indigenous on the Alps of middle Europe.

Fig. 60. _Cedrus Libani_, Cedar of Lebanon.]

The Gymnospermous plants of the present day are included in two natural orders, Coniferæ and Cycadaceæ. Under Coniferæ are enumerated the various species of Pine (Fig. 57), Spruce (Fig. 58), Larch (Fig. 59), Cedar (Fig. 60), Eutassa, Araucaria (Fig. 61), Sequoia, Cryptomeria, Taxodium, Cypress, Juniper (Fig. 70), Salisburia, Dacrydium, Yew (Fig. 71), etc.

[Sidenote: Fig. 61. _Araucaria excelsa_, called also _Altingia_ or _Eutassa_ or _Eutacta excelsa_, Norfolk Island Pine.

Fig. 62. Woody tubes of fir, with single rows of discs.

Fig. 63. Woody tubes of fir, with double rows of discs, which are opposite to each other.

Fig. 64. Woody tubes of _Araucaria excelsa_, with double and triple rows of discs, which are alternate.

Fig. 65. Longitudinal section of the stem of a Gymnosperm, showing tubes of wood marked with punctations in one or more rows, and a medullary ray composed of cells running across the pleurenchyma.]

[Sidenote: Fig. 66. Linear leaves of _Pinus Strobus_, Weymouth Pine, in a cluster of five, with scaly sheath at the base.

Fig. 67. Cone of _Pinus sylvestris_, Scotch Fir.

Fig. 68. Cone of _Cupressus sempervirens_, common Cypress.

Fig. 69. Scale, _s_, of mature cone of _Pinus sylvestris_, with two naked winged seeds, _m m_, at its base; _ch_ marks the chalaza, _m_ the micropyle.]

The Coniferæ of the present day are distinguished as resinous trees or shrubs with punctated woody tissue (Figs. 62, 63, 64, 65), linear acerose or lanceolate parallel-veined leaves, sometimes clustered, and having a membranous sheath at the base (Fig. 66). Male flowers in deciduous catkins; female flowers in cones (Figs. 67, 68). The seeds are considered by most botanists as being naked, _i.e._ not contained in a true pistil (Fig. 69). Some of the conifers have a succulent cone, as the juniper (Fig. 70), and the yew (Figs. 71-73) has a succulent mass covering a single naked seed (Fig. 73). The yew also has its pleurenchyma marked both with punctations and spiral fibres. The arrangement of the punctations in the Coniferæ gives characters which enable us to classify the woods into groups that have some relation to the genera established from the reproductive organs (see Figs. 62-65).

[Sidenote: Fig. 70. Fruiting branch of _Juniperus communis_, common Juniper, with linear acerose leaves and succulent cones.

Fig. 71. Branch of _Taxus baccata_, common Yew.

Fig. 72. Male flower of Yew, with bracts at the base.

Fig. 73. Fruit of Yew, consisting of a single naked seed partially covered by a succulent receptacle.]

The natural order Cycadaceæ is not so largely represented at the present day as it was during the Mesozoic epoch. Among the genera of the present day are Cycas (Fig. 74), Zamia, Macrozamia, Encephalartos (Fig. 75), Dion, Stangeria, etc. They are small palm-like trees or shrubs, with unbranched stems, occasionally dichotomous, marked with leaf-scars, and having large medullary rays along with pitted woody tissue. The leaves are pinnate, except in Bowenia, which has a bipinnate leaf. Males in cones. Females consisting of naked ovules on the edges of altered leaves, or on the inferior surface of the peltate apex of scales.[18]

_FLORA OF THE TRIAS AND LIAS EPOCHS._

[Sidenote: Fig. 74. _Cycas revoluta_, one of the false Sago-plants found in Japan.

Fig. 75. _Encephalartos (Zamia) pungens_, another starch-yielding Cycad.]