Chapter X: I. EQUISETALES (continued) (2)

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Our knowledge of the structure and manner of occurrence of Calamite leaves is very incomplete. There are numerous foliage-shoots among the fossils of the Coal-Measures which are no doubt Calamitean, but as they are nearly always found apart from the main branches and stems, it is generally impossible to do more than speak of them as probably the leaf-bearing branches of a Calamite. The familiar fossils known as _Asterophyllites_, and in recent years often referred to the genus _Calamocladus_, are no doubt Calamitean shoots; but they are usually found as isolated fragments, and it is seldom that we are able to refer them to definite forms of _Calamites_. Another common Coal-Measure genus, _Annularia_, is also Calamitean, and at least some of the species are no doubt leafy shoots of _Calamites_. Although it is generally accepted that the fossils referred to as _Asterophyllites_ or _Calamocladus_ are portions of _Calamites_, and not distinct plants, it is convenient, and indeed necessary, to retain such a term as _Calamocladus_ as a means of recording foliage-shoots, which may possess both a botanical and a geological value.

Some of the Calamite casts, especially those referred to the subgenus _Calamitina_, are occasionally found with leaves attached to the nodes. In some stems the leaves are arranged in a close verticil, and each leaf has a narrow linear form and is traversed by a single median vein. Figures of Calamite stems with verticils of long and narrow leaves may be found in Lindley and Hutton[636], and in the writings of many other authors[637]. In the specimen shown in fig. 85 the leaves are preserved apart from the stem, but from their close association with a Calamite cast, and from the proofs afforded by other specimens, it is quite certain they formed part of a whorl of leaves attached to the node of a true Calamite, and a stem having that particular type known as _Calamitina_[638] (figs. 99, 100). It is probable that in some Calamites, and especially in younger shoots, the leaves had the form of narrow sheaths split up into linear segments. This question has already been referred to in dealing with certain Palaeozoic fossils referred to _Equisetites_[639].

A few years ago the late Thomas Hick[640], of Manchester, described the structure of some leaves which he believed to be those of a Calamite. He found them attached to a slender axis which possessed the characteristics of a young Calamite branch. There can be little doubt that his specimens are true Calamite leaves. The sketches of fig. 86 have been made from the sections originally described by Hick. Fig. 86, 1 shows a leaf in transverse section; on the outside there is a well-defined epidermal layer with a limiting cuticle. Internal to this we have radially elongated parenchymatous cells forming a loose or spongy tissue, the cells being often separated by fairly large spaces (fig. 86, 5), especially in the region of the blunt lateral wings of the leaf. Some of these cells contain a single dark dot, which in all probability is the mineralised nucleus. These pallisade-like cells probably contained chlorophyll and constituted the assimilating tissue of the leaf. In the centre there is a circular strand of cells limited by a layer of larger cells with black contents, enclosing an inner group of small-celled parenchyma and traversed by a few spiral or scalariform tracheids constituting the single median vein. It is hardly possible to recognise any phloem elements in the small vascular bundle; there appear to be a few narrow tracheids surrounded by larger parenchymatous elements (fig. 86, 2). At one point in the epidermis of fig. 86, 1, there appears to be a stoma, but the details are not very clearly shown (fig. 86, 4); the two cells, _s_, _s_, bordering the small aperture are probably guard-cells.

1. Transverse section; _t_, vascular bundle; _x_, sheath of cells.
× 35.
2. Vascular bundle consisting of a few small tracheids, _t_.
3. A tracheid and a few parenchymatous cells, the latter with nuclei.
4. A stoma; _s_, _s_, guard-cells.
5. Pallisade cells and intercellular spaces.
From a section in the Manchester Museum, Owens College.]

The nature of the assimilating tissue, the comparatively thick band of thin-walled cells with intercellular spaces, and the exposed position of the stomata suggest that the plant lived in a fairly damp climate; at least there is nothing to indicate any adaptation to a dry climate.

In the Binney collection of plants in the Woodwardian Museum, Cambridge, there is a species of a very small shoot bearing three or four verticils of leaves which possess the same structure as those of fig. 86. We may probably regard such twigs as the slender terminal branches of Calamitean shoots.

α. _Calamocladus_ (_Asterophyllites_).

The generic name _Asterophyllites_ was proposed by Brongniart[641] in 1822 for a fossil previously named by Schlotheim[642] _Casuarinites_, and afterwards transferred to Sternberg’s genus _Annularia_. In 1828 Brongniart[643] gave the following diagnosis of the fossils which he included under the genus _Asterophyllites_:—“Stems rarely simple, usually branched, with opposite branches, which are always disposed in the same plane; leaves flat, more or less linear, pointed, traversed by a simple median vein, free to the base.” Lindley and Hutton described examples of Brongniart’s genus as species of _Hippurites_[644], and other authors adopted different names for specimens afterwards referred to _Asterophyllites_.

At a later date Ettingshausen[645] and other writers expressed the view that the fossils which Brongniart regarded as a distinct genus were the foliage-shoots of _Calamites_, and Ettingshausen went so far as to include them in that genus. In view of the generally expressed opinion as to the Calamitean nature of _Asterophyllites_, Schimper[646] proposed the convenient generic name _Calamocladus_ for “rami et ramuli foliosi” of _Calamites_. Some recent authors have adopted this genus, but others prefer to retain _Asterophyllites_. In a recent important monograph by Grand’Eury[647] Calamitean foliage-shoots are included under the two names, _Asterophyllites_ and _Calamocladus_; the latter type of foliage-shoots he associates with the stems of the subgenus _Calamodendron_, and the former he connects with those Calamitean stems which belong to the subgenus _Arthropitys_.

It is an almost hopeless task to attempt to connect the various forms of foliage-shoots with their respective stems, and to determine what particular anatomical features characterised the plants bearing these various forms of shoots. We may adopt Schimper’s generic name _Calamocladus_ in the same sense as _Asterophyllites_, but as including such other foliage-shoots as we have reason to believe belonged to _Calamites_. Those leaf-bearing branches which conform to the type known as _Annularia_ are however not included in _Calamocladus_, as we cannot definitely assert that these foliage-shoots belong in all cases to Calamitean stems. Grand’Eury’s use of _Calamocladus_ in a more restricted sense is inadvisable as leading to confusion, seeing that this name was originally defined in a more comprehensive manner as including Calamitean leaf-bearing branches generally. We may define _Calamocladus_ as follows:—

Branched or simple articulated branches bearing whorls of uni-nerved linear leaves at the nodes; the leaves may be either free to the base or fused basally into a cup-like sheath (_e.g._ Grand’Eury’s _Calamocladus_). The several acicular linear leaves or segments which are given off from the nodes spread out radially in an open manner in all directions; they may be either almost at right angles to the axis or inclined at different angles. Each segment is traversed by a single vein and terminates in an acuminate apex.

As a typical example of a Calamitean foliage-shoot the species _Calamocladus equisetiformis_ (Schloth.) may be briefly described. The synonymy of the commoner species of fossil plants is a constant source of confusion and difficulty; in order to illustrate the necessity of careful comparison of specimens and published illustrations, it may be helpful to quote a few synonyms of the species more particularly dealt with. The exhaustive lists drawn up by Kidston in his _Catalogue of Palaeozoic plants in the British Museum_ will be found extremely useful by those concerned with a systematic study of the older plants.

_Calamocladus equisetiformis_ (Schloth.). Fig. 87.

1809. _Phytolithus_, Martin[648].
1820. _Casuarinites equisetiformis_, Schlotheim[649].
1825. _Bornia equisetiformis_, Sternberg[650].
1828. _Asterophyllites equisetiformis_, Brongniart[651].
1836. _Hippurites longifolia_, Lindley and Hutton[652].
1855. _Calamites equisetiformis_, Ettingshausen[653].
1869. _Calamocladus equisetiformis_, Schimper[654].
1869. _Annularia calamitoides_, Schimper[654].

The above synonyms do not exhaust the list[655], but they suffice to illustrate the necessity of a careful comparison in drawing up tables of species, in connection with geographical distribution or for other purposes.

_Calamocladus equisetiformis_ may be briefly defined as follows:—A central axis possessing a hollow pith of Calamitean character, divided externally into well-marked slightly constricted nodes and internodes; from the nodes long narrow and free leaves are borne in whorls; from the axils of some of the leaves lateral branches are given off inclined at a fairly wide angle to the main axis, and bearing crowded verticils of spreading acicular leaves.

The unusually good specimen, 38·5 cm. long, shown on a much reduced scale in fig. 87, illustrates the characteristic habit of this form of _Calamocladus_. It is from the Radstock coal-field of Somersetshire, one of the best English localities for Coal-Measure plants. An exceedingly good collection of Radstock plants has recently been presented to the British Museum by Mr J. McMurtrie; it includes many fine specimens of _Calamites_. A small example—probably of this species—from Coalbrook Dale, near Dudley, in Shropshire, and now in the British Museum, illustrates very well the appearance of a young and partially expanded Calamitean foliage-shoot. The central axis, 6·5 cm. in length, includes about 15 internodes, and terminates in a bud covered by several small leaves. Lateral branches are given off at a wide angle, and small unexpanded buds occur in the axils of several of the leaves.

As an example of the leaf-bearing branches which Grand’Eury has recently described as _Calamocladus_, using the genus in a more restricted sense than is adopted in the present chapter, reference may be made to the fragment shown in fig. 68, _A_. The foliage-shoots of this type bore verticils of linear leaves, coherent basally in the form of a cup, at the ends of branches and not in a succession of whorls on each branch. The association of reproductive organs, in the form of long and narrow strobili, with _Calamocladus_ is referred to in the sequel.

The specimens described by Grand’Eury are in the École des Mines Museum, Paris; some of the shoots which are well preserved bear a resemblance in habit of growth to the genus _Archaeocalamites_.

β. _Annularia._

In 1820 this generic name was applied by Sternberg[656] to some specimens of branches bearing verticils of linear leaves. In 1828 Brongniart[657] thus defined the genus _Annularia_:—“Slender stem, articulated, with opposite branches arising above the leaves. Leaves verticillate, flat, frequently obtuse, traversed by a single vein, fused basally and of unequal length.”

In the works of earlier writers we find frequent illustrations of specimens of _Annularia_, which are compared with Asters and other recent flowering plants. Lehmann[658] contributed a paper to the Royal Academy of Berlin in 1756, in which he referred to certain fossil plants as probable examples of flowers, among them being a specimen of _Annularia_. He refers to the occurrence of fossil ferns and other plants, and asks why we do not find flowers of the rose or tulip; his object being “not to acquire vain glory, but to give occasion for others to look into the matter more clearly.”

The general habit of the fossils which are now included under _Annularia_ agrees closely with that of _Calamocladus_. There is the same spreading form and a similar foliage in the two genera, but in _Annularia_ the members of a whorl are always fused into a basal sheath, and the segments are not of equal length. We may thus summarise the characteristic features of the genus:—

Opposite branches are given off in one plane from the nodes of a main axis; the leaves are in the form of narrow sheaths divided into numerous and unequal linear or narrow lanceolate segments, each with a median vein. The segments in each whorl appear to be spread out in one plane very oblique to the axis of a branch, instead of spreading radially in all directions; the lateral segments are usually longer than the upper and lower members of a whorl. The vegetative branches possess the same type of structure as _Calamites_.

A comparison of _Annularia_ and _Phyllotheca_ has already been made in Chapter IX. (p. 282). Potonié[659] has recently given a detailed account of Annularian leaves; he compares them with those of _Equisetum_, and describes the occurrence on the lamina of each leaf-segment of a broad central band or midrib, with a groove, probably containing stomata, on either side. He shows that in well-preserved specimens of _Annularia_, it is possible to recognise certain minute surface-features, such as the presence of hairs and stomata, which enable one to detect a close resemblance between the leaves of Calamite stems and those of Annularian shoots.

It is not always easy to distinguish between _Annularia_ and _Calamocladus_; the collar-like basal sheath in the leaves of the former is a characteristic feature, but that cannot always be recognised. On the other hand, the leaves of _Calamocladus_ may sometimes be flattened out on the surface of the rock and simulate the deeply cut sheaths of _Annularia_. It is difficult to decide how far the manner of occurrence of Annularian leaves in one plane, which is commonly insisted on as a generic character, is an original feature, or how far it is the result of compression in fossilisation. Probably the leaves of a living _Annularia_ were spread out at right angles to the axis, as in the ‘verticils’ of such a plant as _Galium_.

Dawson[660] has described some fossils from the Devonian rocks of Canada as species of _Asterophyllites_; the figures bear a closer resemblance to the genus _Annularia_. The same author figures some irregularly whorled impressions as _Protannularia_, which appear to be identical with a fossil described by Nicholson[661] from the Skiddaw slates (Ordovician) of Cumberland as _Buthotrephis radiata_, but the specimens are too imperfect to admit of accurate determination.

_Annularia stellata_ (Schloth.). Fig. 88.

1820. _Casuarinites stellatus_, Schlotheim[662].
1826. _Bornia stellata_, Sternberg[663].
1828. _Annularia longifolia_, Brongniart[664].
1834. _Asterophyllites equisetiformis_, Lindley and Hutton[665].
1868. _Asterophyllites longifolius_, Binney[666].
1887. _Annularia Geinitzi_, Stur[667].
1887. _Annularia westphalica_, Stur.

This species was figured by Scheuchzer[668] in his _Herbarium Diluvianum_, and compared by him with a species of _Galium_ (Bedstraw). Brongniart first made use of the generic name _Annularia_ for this common Coal-Measure species, which may be defined as follows:—

Stem reaching a diameter of about 6–8 cm., with internodes 6–12 cm. in length, the surface either smooth or faintly ribbed. Primary branches given off in opposite pairs from the nodes, the lateral branches giving off smaller branches disposed in the same manner. The smaller branches bear verticils of leaves at each node; both leaves and ultimate branches being in one plane. The leaves are narrow, lanceolate-spathulate in form, broadest about the middle, 1–5 cm. in length and 1–3 mm. broad, hairy on the upper surface[669]; each leaf is traversed by a single vein.

Each whorl contains 16–32 segments, which are connected basally into a collar or narrow sheath; the lateral segments are usually longer than the upper and lower. The branches are about 6–20 mm. broad, with finely ribbed internodes 3–7 cm. long, bearing verticils of leaves; the ultimate branches arise in pairs in the axils of the lateral segments of the verticils.

The strobili are of the _Calamostachys_[670] type and are borne on the main branches or possibly on the stem; they have a long and narrow form and are attached in verticils at the nodes. Each strobilus consists of a central axis bearing alternate whorls of linear lanceolate sterile bracts and sporangiophores, about half as numerous as the sterile bracts; each sporangiophore bears four ovoid sporangia.

The anatomical structure of a specimen referred to _Annularia stellata_ has been described by Renault[671]. The cortex consists of parenchyma traversed by lacunae and limited peripherally by a denser hypodermal tissue. In the stele Renault describes 14 xylem strands, each with a large carinal canal. The pith was apparently large and hollow. The same author describes an _Annularia_ strobilus in which the lower sporangiophores bear macrosporangia, and the upper microsporangia.

The references in the footnote should be consulted for figures of this species of _Annularia_; it is from the examination of such specimens as are referred to in the note that the above diagnosis has been compiled[672].

_Annularia sphenophylloides_ (Zenk.). Fig. 89.

1833. _Galium sphenophylloides_, Zenker[673].
1865. _Annularia brevifolia_, Heer[674], Strobilus.
1876. _Calamostachys_ (_Stachannularia_) _calathifera_, Weiss[675].

Principal branches 8–12 mm. wide, with internodes 8–10 cm. in length, giving off two opposite branches at the nodes; from the secondary branches arise smaller branches in opposite pairs. The leaf-verticils and branches are all in one plane. Each verticil consists of 12–18 spathulate segments, 3–10 mm. long, cuneiform at the base and broader above, with an acuminate tip; the lateral segments are slightly longer than the upper and lower members of a whorl.

The small and crowded leaf-whorls give to this species a characteristic appearance, which readily distinguishes it from the larger-leaved forms such as _Annularia stellata_. A fossil figured by Lhwyd[676] in 1699 as _Rubeola mineralis_ is no doubt an example of _Annularia sphenophylloides_.

Annularian branches are occasionally found with cones given off from the axils of some of the leaf-whorls. An interesting specimen, which is now in the Leipzig Museum, was described by Sterzel in 1882[677], showing cones attached to a vegetative shoot of _Annularia sphenophylloides_. The long and narrow strobili—2·5 cm. long and about 6 mm. broad—appear very large in proportion to the size of the vegetative branches. A fertile shoot consists of a central axis bearing whorls of bracts alternating with sporangiophores, to each of which are attached four sporangia. The specimen in fig. 89, _A_, does not show the details clearly; each transverse constriction represents the attachment of a whorl of linear bracts; the whole cone appears to consist of a series of short broad segments. The divisions in the lower half of each segment mark the position of the sterile bracts, while those of the upper half represent the outlines of the upper sporangia of each whorl of sporangiophores, the lower sporangia being hidden by the ring of linear bracts[678]. On some portions of the specimen of fig. 89, _A_, it is possible to recognise the outlines of cells on the coaly surface-film; these probably belong to the sporangium wall. This type of cone is included under the genus _Calamostachys_, a name applied to Calamitean strobili with certain morphological characters, as described on p. 351.

_c. Roots._

In 1871 Williamson[679] described some sections of what he considered to be a distinct variety of a Calamite stem. The chief peculiarity which he noticed lay in the absence of carinal canals, and in the solid pith. Some years later the same observer[680] came to the conclusion that the specimens were probably those of a plant generically distinct from _Calamites_; he accordingly proposed a new name _Astromyelon_. Subsequently Cash and Hick[681] gave an account of some examples of apparently another form of plant, to which they gave the name _Myriophylloides Williamsonis_; and Williamson[682] suggested the term _Helophyton_ as a more suitable generic designation. It was, however, demonstrated by Spencer[683] that the plant described by Cash and Hick was identical with Williamson’s _Astromyelon_. Williamson[684] then gave an account of several specimens of this type illustrating various stages in the growth and development of the _Astromyelon_ ‘stems,’ which he compared with the rhizome of the recent genus _Marsilia_.

In 1885 Renault[685] published an account of _Astromyelon_ in which he brought forward good evidence in favour of regarding it as a Calamitean root. The same author has recently given some excellent figures and a detailed description of certain specific types of these Calamite roots, and Williamson and Scott’s memoir on the roots of _Calamites_ has rendered our knowledge of _Astromyelon_ almost complete. Some of the finest specimens, in which the organic connection between typical Calamite stems and _Astromyelon_ roots is clearly demonstrated, are in the Natural History Museum, Paris. There are several sections also from English material which show the connection between root and stem very clearly.

Casts of the hollow pith of Calamite rhizomes or aerial branches are occasionally found in which slender appendages are given off either singly or in tufts from the nodal regions. Many examples of such casts have been figured by Lindley and Hutton[686], Binney[687], Grand’Eury[688], Weiss[689], Stur, and other writers[690]. The large stem-cast of fig. 90 illustrates the manner of occurrence of long branched roots on the nodes of a Calamite growing in sandy or clay soil. The lower and more darkly shaded portion of the specimen is covered by a layer of coal representing the carbonised wood and cortex, which has been moulded on to the sandstone pith-cast. In fig. 77 (p. 316) a fairly thick root is seen, in organic connection with one of the nodes, _N_ 3, and on _N_ 2 there is a scar of another root.

There are certain external characters by which one may often recognise a Calamitean root. There is no division into nodes and internodes as in stems, and as the pith of the root was usually solid the parallel ribs and grooves of stem-casts are not present. In smaller flattened roots there may sometimes be seen a central or excentric black line representing the stele, and the surface of the root presents a curious wrinkled or shagreen texture, probably due to the shrinkage of the loose lacunar cortex. The occasional excentric position of the stele is no doubt due to the displacement of the vascular cylinder as a result of the rapid decay of the cortical tissues. In the Bergakademie of Berlin there are some unusually good examples of Calamite casts bearing well-preserved root-impressions; these include the original specimens figured by Weiss[691].

No doubt some of the roots figured by various writers under the names _Pinnularia_[692] and _Hydatica_[693] belong to _Calamites_, but it is often impossible to identify detached specimens with any certainty.

The section figured diagrammatically in fig. 91 _A_ shows the characteristic single series of large lacunae, _l_, in the middle cortical region. In the centre there is a wide solid pith surrounded by a ring of vascular tissue, _x_. The appearance of the middle cortex is very like that of the stem of a water-plant such as _Myriophyllum_, the Water Milfoil; it shows that the Calamite roots grew either in water or swampy ground. In fig. 91 _B_, the root characters are clearly seen; the centre of the stele is occupied by large parenchymatous cells which are rather longer than broad in longitudinal view; at the periphery there are four protoxylem groups _px_, alternating with four groups of phloem, _ph_, the latter being situated a little further from the centre of the stele. The structure is therefore that of a typical tetrarch root. In the example represented in the figure secondary thickening has begun, and the cambial cells internal to each phloem group have given rise to a few radially disposed tracheids, _x²_. Beyond the phloem there are two layers of parenchyma representing, as regards position, a pericycle and an endodermis. In the ordinary pericycle and endodermis of the roots of most plants the cells of the two layers are on alternate radii, but in the Calamite root, as in _Equisetum_ roots, the cells of these layers are placed on the same radii, as seen in the neighbourhood of _x²_ in the figure. This correspondence of the radial walls of the endodermal and pericyclic cells points to the development of both layers from one mother-layer, and suggests the ‘double endodermis’ or phloeoterma of _Equisetum_ (p. 254). The cells in the outer of these two layers have slight thickenings on the radial walls recalling the usual character of endodermal cells. The phloeoterma is succeeded by a few layers of parenchyma, constituting the inner cortex, and beyond this we have the large lacunae separated from one another by slender trabeculae of cells. The outer cortex is limited by a well-defined layer of thick-walled cells, which may be spoken of as the _epidermoidal[694] layer_. Roots possessing this superficial layer of thicker cells have no doubt lost the original surface-layer which produced the absorptive root-hairs.

_B._ Central cylinder (stele) of root, _px_, protoxylem; _ph_,
phloem; _x²_, secondary xylem; _l_, phloeoterma. × 75. After
Williamson and Scott.]

The xylem elements have the form of spiral, reticulate and scalariform tracheids.

In roots or rootlets smaller than that shown in fig. 91 _B_, the primary xylem may extend to the centre of the stele, and form a continuous axial strand; in such examples the structure may be diarch, triarch or tetrarch. The origin of the cambium agrees with that in recent roots, the cells immediately external to the protoxylem tracheids become meristematic, as also those internal to the phloem. Another root-character is seen in the endogenous origin of lateral members. Good examples of branching roots are figured by Williamson[695] and by Williamson and Scott[696].

Older roots[697] are usually found in a decorticated condition. A transverse section of root in which secondary thickening has been active for some time presents on a superficial view a close resemblance to a stem of _Calamites_, but a careful comparison at once reveals important points of difference. The specimen diagrammatically sketched in fig. 92 illustrates very clearly the origin of a root from the node of a Calamite stem. The section has passed through a stem in a tangential direction, showing the characteristic arrangement of the vascular bundles _x_, and principal medullary rays _m_. The small leaf-traces, _t_, _t_, afford another feature characteristic of a Calamite stem. The portion of stem to the right of the figure has been slightly displaced, and between this piece and the root _R_, one of the ubiquitous Stigmarian appendages, _s_, has inserted itself. At _R_ a fairly thick and decorticated root is seen in oblique transverse section; at the upper end the root tracheids are seen in direct continuity with the xylem of the stem. In the centre of the root is the large solid pith surrounded by twelve bluntly pointed xylem groups, composed in the main of radially disposed scalariform elements with narrow secondary medullary rays like those in a stem. Between each xylem group there is a broad medullary ray, which tapers rapidly towards the outside, and is soon obliterated by the formation of interfascicular secondary xylem. At _R′_ a portion of another root is seen in transverse section, and _R″_ the inner part of a single xylem group is shown more clearly. The solid pith and the absence of carinal canals are the two most obvious distinguishing features of the roots.

_R_, _R′_. Root (_Astromyelon_) in transverse and oblique section,
_x_, xylem; _m_, primary medullary ray; _t_, leaf-trace; _s_,
Stigmarian appendage.

_R″_, the inner portion of one of the xylem wedges of _R′_ more
highly magnified. Sketched from a section in the Cambridge
Botanical Laboratory Collection.]

As Renault points out, roots of _Calamites_ have been figured by some writers[698] as examples of stems, but it is usually comparatively easy to distinguish between roots and stems. On examining the xylem groups more closely, one notices that the apex of each is occupied by a triangular group of centripetally-developed primary tracheids, the narrow spiral protoxylem elements occupying the outwardly directed apex. The protoxylem apex is usually followed externally by a ray of one or two radially disposed series of parenchymatous cells. This ray is not distinguished in fig. 92 _R″_ from the rows of xylem tracheids. Each xylem group is thus formed partly of centripetal xylem and in part of secondary centrifugal xylem; the latter is associated with secondary medullary rays, as in stems, and contains a broader ray (_fascicular ray_ of Williamson and Scott[699]) immediately opposite each protoxylem strand. In the roots of recent plants (_e.g._ _Cucurbita_, _Phaseolus_, &c.) a broad medullary ray is often found opposite the protoxylem, and such an arrangement is a perfectly normal structure in roots[700].

Renault has recently described several species of Calamite roots which he designates by specific names, some of them belonging to stems with the _Arthropitys_ structure, and others to _Calamodendron_. Some of the roots figured by the French author have an axial strand of xylem with 7–15 projecting angles of protoxylem[701]. These he considers true roots, but the larger specimens with a wide pith he prefers to regard as stolons. In the latter he mentions the union of the primary centripetal with the secondary centrifugal wood as a distinguishing feature. It has been shown, however, that each group of secondary xylem includes a median ray of parenchyma, and that the whole structure is essentially that of a root, and not that of a modified stem or stolon. The organs described by Renault as true roots are probably rootlets, and as Williamson and Scott have demonstrated, there is every gradation between the smaller specimens with a solid xylem axis and those with a large central pith.

It is interesting to note that Renault’s figures of _Calamodendron_ roots show the closest resemblance to those of the subgenus _Arthropitys_.

_d. Cones._

The occurrence of fossil plants in the form of isolated fragments is a constant source of difficulty, and is well illustrated by the numerous examples of strobili which cannot be connected with their parent stems. We are, however, usually able to recognise Calamitean cones if the impressions or petrified specimens are fairly well preserved, but it is seldom possible to correlate particular types of cones with the corresponding species of foliage-shoots or stems. Palaeobotanical literature contains numerous illustrations and descriptions of long and narrow strobili designated by different generic terms such as _Volkmannia_, _Brukmannia_, _Calamostachys_, _Macrostachya_ and others; many of these have since been recognised as the cones of _Calamites_, while some species of _Volkmannia_ have been identified with _Sphenophyllum_ stems. Before further considering the general question of Calamite cones, a few examples may be described in detail as types of fructification which are known to have been borne by _Calamites_. The examples selected are species of the two provisional genera _Calamostachys_ and _Palaeostachya_.

The usual form of a Calamite cone is illustrated in fig. 93, which represents a fertile shoot bearing a few narrow linear leaves of the _Calamocladus_ type; in the axils of some of these are borne the long strobili.

_Calamostachys Binneyana_ (Carr.). Figs. 94 and 95.

In 1867 Carruthers[702] gave an account of the structural features of the species of cones named by him _Volkmannia Ludwigi_ and _V. Binneyi_, the generic term having been originally used by Sternberg[703] for some impressions of Carboniferous strobili. Brongniart[704] in 1849 referred to the various forms of _Volkmannia_ as cones of Asterophyllitean branches, and the latter he regarded as the foliage-shoots of a Calamite stem. In 1868 Binney[705] published a description, with several illustrations, of the cones named by Carruthers _Volkmannia Binneyi_, and referred to them as the fructification of that type of Calamite stem spoken of in a previous section of this chapter (p. 311) as _Calamites_ (_Arthropitys_) _communis_ (Binney). This cone is now usually spoken of as _Calamostachys Binneyana_; the specific name _Binneyana_ being suggested by Schimper[706] in 1869 as more euphonious than that proposed by Carruthers. In recent years our knowledge of both _C. Binneyana_ and _C. Ludwigi_ has been considerably extended. We shall confine our attention in the following account to the former species[707]. Some excellent figures of the latter species may be found in Weiss’ Memoir[708] on Calamarieae.

One of the largest examples of _Calamostachys Binneyana_ so far recorded has a length of 3–4 cm. and a maximum diameter of about 7·5 mm. The axis of the cone bears whorls of sterile leaves or bracts at equal distances; the linear bracts of each whorl are coherent basally as a disc or plate of tissue attached at right angles to the central axis of the cone. The periphery of each of these discs divides up into twelve linear segments, which curve upwards in a direction more or less parallel to the strobilus axis, and at right angles to the coherent portion of each whorl. The manner of occurrence of the whorls is shown in fig. 94, which has been sketched from a large section in the Williamson collection. The segments of the successive sterile verticils alternate with one another, so that in the surface-view of a cone the long and narrow free bracts appear spirally disposed. Midway between these alternating sterile verticils there is a series of fertile appendages, also given off in regular whorls. Each fertile whorl consists of about half as many members as the segments of a sterile whorl, and the members of the several fertile whorls are superposed and not alternate. Each member has the form of a stalk or sporangiophore given off at right angles from the cone axis; this is expanded distally into a peltate disc bearing four sporangia attached to its inner face. In fig. 94 we can only see the basal portions of the sporangiophores, which are shown in the upper part of the sketch as pointed projections, _Sp_, from the cone axis. Each sporangiophore is traversed by a vascular strand which sends off a branch to the base of a sporangium (fig. 95, A, _t_).

_Sp_, sporangiophores; _S_, sporangia.
(From specimen no. 1022 in the Williamson Collection, British
Museum.)]

The axis of the cone is occupied by a single stele, usually triangular in section; the stele consists of a solid pith of elongated cells surrounded by six vascular bundles, two at each corner. A somewhat irregular gap marks the position of the protoxylem of each strand, and portions of spiral or annular tracheids may occasionally be seen in the cavity. These cavities, which may be spoken of as the carinal canals, disappear at the nodes, where there is a mass of short reticulately pitted tracheids, as in a Calamite stem. Vascular bundles pass upwards in an oblique direction from the central stele to supply the bracts, each of which is traversed by a single strand of tracheids. The coherent portion, or disc, of each sterile whorl consists of sclerenchymatous elements towards the upper surface, and of parenchyma below. The pedicel of the sporangiophores consists of fairly thick-walled cells traversed by a single vascular strand, and the peltate distal portions are made up of parenchymatous cells arranged in a palisade-like form at right angles to the free surface of the sporangiophores. The vascular strand of the pedicel forks into two halves just below the peltate head, and these branches again bifurcate to send a branch to each sporangium. The four sporangia of each sporangiophore are attached by a narrow band of tissue to the shield-shaped distal expansion (fig. 95, A).

In a tangential section of a cone, such as the lower portion of fig. 94 and in fig. 95, B, the sporangiophores present the appearance of narrow stalks (fig. 95, B, _a_) in the middle of a cluster of sporangia, and the latter appear more or less square in outline. The wall of a sporangium is made of a single layer of cells (fig. 95, B) which present a characteristic appearance in surface-view (fig. 95, C), the thin walls being crossed at right angles by small vertical plates. In the tangential section of the coherent sterile whorls (fig. 95, B, _b_ and _b_) the vascular strands are occasionally seen in transverse section (fig. 95, B, _t_), as they pass outwards to the several free bracts.

_A._ A sporangiophore and one sporangium. _t_, vascular bundle. × 45.
_B._ Tangential section showing portions of two sterile discs, _b_,
_b_; a sporangiophore, _a_, with its four sporangia, in two of
which are seen the spores; _t_, vascular bundle. × 35.
_C._ Surface-view of cells of a sporangium wall. × 130.
_D._ Spores and remains of mother-cells. × 130.

(After Williamson and Scott.)]

The spores in _Calamostachys Binneyana_ are all of the same size, and no macrospores have ever been seen. In well preserved specimens tetrads of spores may be seen, still enclosed by the wall of the spore-mother-cell (fig. 95, A and D); and the torn remnants of the mother-cell sometimes simulate in appearance the elaters of an _Equisetum_ spore. In surface-view a spore often shows clearly the three-rayed marking, which is a characteristic feature of daughter-cells formed in a tetrad from a mother-cell. The spores of a tetrad are in some cases of unequal size, some having developed more vigorously than others. This unequal growth and nourishment of spores is clearly shown in fig. 96, which represents a sporangium of a heterosporous Calamitean strobilus, _C. Casheana_. Williamson and Scott[709] have described striking examples of spores in different stages of abortion, and these authors draw attention to the importance of the phenomenon from the point of view of the origin of a heterosporous form of cone. The abortion of some of the members of a spore-tetrad and the consequent increased nutrition of the more favoured daughter-cells, might well be the starting-point of a process, which would ultimately lead to the production of well defined macrospores and microspores. The young microsporangia and macrosporangia of recent Vascular Cryptogams such as _Selaginella_, _Salvinia_ and other heterosporous genera are identical in appearance[710]; it is not until the spore-producing tissue begins to differentiate into groups of spores, that the sporangia assume the form of macrosporangia and microsporangia. During the evolution of the various known types of pteridophytic plants heterospory gradually succeeded isospory, and this no doubt occurred several times and in different phyla of the plant kingdom. In the mature sporangia of some of the Calamitean strobili we have in the inequality of the spores in one sporangium an indication of the steps by which heterospory arose; and in the immature sporangia of some recent genera we are carried back to a stage still nearer the starting-point of the substitution of the heterosporous for the isosporous condition.

_Calamostachys Casheana_ Will. Fig. 96.

To Williamson[711] again is largely due the information we possess as to the structure of this type of Calamitean strobilus. Its special interest lies in the occurrence of macrospores and microspores in the same cone.

The strobilus axis agrees in structure with that of _C. Binneyana_, but in _C. Casheana_ a band of secondary xylem forms the peripheral portion of the triangular stele. Were any further proof needed of the now well-established fact that secondary growth in thickness is by no means unknown as an attribute of Vascular Cryptogams, the co-existence in the same cone of a cambium layer producing secondary wood and bark, and cryptogamic macrospores and microspores, affords conclusive evidence[712]. The dogma accepted by many writers for a considerable number of years that the power of secondary thickening is evidence against a cryptogamic affinity, has been responsible for no little confusion in palaeobotanical nomenclature.

On the axis of _Calamostachys Casheana_ there are borne alternate whorls of fertile and sterile appendages similar to those in the homosporous _C. Binneyana_, but they are inclined more obliquely to the axis of the cone. Macrospores and microspores have been found in sporangia borne on the same sporangiophore.

A sporangium with macrospores and abortive spores. × 65.

(After Williamson and Scott.)]

The spore-tetrads in the macrosporangia occasionally include aborted sister-cells like those noticed in _C. Binneyana_; this phenomenon is well illustrated by the unequally nourished spores in the sporangium of fig. 96, but no such starved spores have been found in the microsporangia. In this cone, then, heterospory has become firmly established, but the occurrence of undersized spores in a macrospore-tetrad leads us back to the probable lines of development of heterospory, which are seen in _C. Binneyana_ at their starting-point.

In the two species of strobili which have been described, _Calamostachys Binneyana_ and _C. Casheana_, the sporangiophores or sporophylls are given off at right angles to the axis, and midway between the sterile whorls. These are two of the most important distinguishing features of the Calamitean cones included under the generic term _Calamostachys_. In another form of cone, which also belongs to Calamitean stems, the sporangiophores arise in the axil of the sterile leaves, and are inclined obliquely to the axis of the cone. To this type the generic name _Palaeostachya_ has been applied by the late Prof. Weiss[713] of Berlin. The portion of a cone shown in fig. 97 shows the arrangement of the sterile and fertile appendages characteristic of _Palaeostachya_.

It is practically impossible to distinguish between cones of the _Calamostachys_ and _Palaeostachya_ type in the case of imperfectly preserved impressions; indeed we cannot assume that all long and narrow cones with spirally disposed verticillate bracts are Calamitean. We must have the additional evidence of internal structure or of the direct association of the cones with Calamitean foliage.

_Palaeostachya vera_ sp. nov. Fig. 98.

In 1869 Williamson[714] described a fragment of a strobilus which showed certain anatomical features indicative of a close relationship or even identity with _Calamites_. Some years later[715] a much more perfect example was obtained from the Coal-Measures of Lancashire, and the additional evidence which it afforded definitely confirmed the earlier views of Williamson. The cone was more fully described by Williamson in 1888, as “the true fruit of _Calamites_.” It is clearly a form of Weiss’ genus _Palaeostachya_; Williamson and Scott[716] refer to it in their Memoir as _Calamites pedunculatus_. It is preferable, however, to retain the generic designation _Palaeostachya_ for cones of this type. As the name _P. pedunculata_ has previously been adopted by Weiss[717] for a cone figured by Williamson[718] in 1874, and afterwards referred to by that author in writing as _P. pedunculata_, it is proposed to substitute the specific name _vera_; this specific name being chosen with a view to put on record the fact that it was this type of cone that Williamson first proved to be the _true_ fructification of the Calamite.

The axis of _P. vera_ is practically identical in structure with a Calamitean twig. There is a hollow pith in the centre of the stele surrounded by a ring of 16–20 collateral bundles, each of which is accompanied by a carinal canal as in a vegetative shoot. As the pedicel of the strobilus passes into the cone proper it undergoes some modification in structure, but retains the characteristic features of a Calamite. The diagrammatic longitudinal section of fig. 98, which is copied from a drawing by Williamson[719], shows the broadening of the vascular strands at the nodes, and here and there a carinal canal is seen internal to the wood.

The axis of the cone bears whorls of bracts at right angles to the central column. Each whorl consists of about 30–40 segments coherent basally into a disc of prosenchymatous and parenchymatous tissue. The free linear bracts curve sharply upwards from the periphery of the disc, approximately parallel to the axis of the cone. From each of these sterile whorls there are given off 16–20 long and slender obliquely-inclined sporangiophores, _sp_, which arise from the upper surface of the disc close to the axis. Each sporangiophore no doubt bore four sporangia, _S_, containing spores of one size,—about ·075 mm. in diameter. The specimens of _Palaeostachya vera_ so far obtained do not show the actual manner of attachment of the sporangia, but more complete examples of other species of _Palaeostachya_[720] enable us to assume with certainty that the sporangiophores terminated in a distal peltate expansion bearing four sporangia on its inner face.

A transverse section of the axis of the cone in the region of the sterile and fertile appendages shows the vascular bundles arranged in pairs. In a section through the peduncle of the cone, below the lowest whorl of bracts, the bundles of the stele are situated at equal distances apart. The cortical tissue of the peduncle is traversed by a ring of large canals[721] similar to the vallecular canals of an Equisetum stem.

Isospory is not a constant characteristic of _Palaeostachya_; some forms have been found with macrospores and microspores[722].

_Other Calamitean cones, and examples illustrating the
connection between Cones and Vegetative Shoots._

It would be out of place in an introduction to Palaeobotany to attempt an exhaustive account of the various cones which were probably borne by Calamitean plants, but there are a few general points to which the attention of the student should be directed. The examples dealt with in the foregoing description illustrate the fact, that plants included under the comprehensive genus _Calamites_ bore cones possessing distinct morphological features. There are, however, other types of strobili which have been found in organic connection with _Calamites_; and some of these must be taken into account in dealing with Calamarian plants. The genera _Volkmannia_, _Brukmannia_, _Huttonia_, _Macrostachya_, in addition to _Calamostachys_ and _Palaeostachya_ and others, have been applied by different writers to Calamitean cones. As Solms-Laubach[723] has suggested, it is wiser to discard _Volkmannia_ and _Brukmannia_, as they have been made to do duty for cones of widely different forms. It is better to adhere to the provisional generic names used by Weiss, as they enable us to conveniently systematise the various Calamarian strobili.

The following classification may be given of the better known cones, some of which we are able to describe in considerable detail, while others are still very imperfectly known. We have good evidence that all these strobili were borne by vegetative shoots of the type of _Calamites_, _Calamocladus_ or _Annularia_.

1. _Calamostachys_[724] (including _Paracalamostachys_ and
_Stachannularia_).

Cones long and narrow, consisting of a central axis bearing alternate whorls of sterile and fertile appendages, the latter having the form of sporangiophores attached at right angles to the axis midway between the sterile verticils, and bearing four sporangia on the inner face of a peltate distal expansion.

_Calamostachys Binneyana_ Schimp., _C. Ludwigi_ Carr., _C. Casheana_ Will., may be referred to as examples of this type of cone; also some of the strobili described by different authors as species of _Volkmannia_[725], _Brukmannia_[726], &c.

Although one cannot make out the detailed structure of a Calamite cone in the absence of internal structure, it is often possible to recognise the essential features in specimens preserved in ironstone nodules, such as those from Coalbrook Dale in Shropshire, or by carefully examining the carbonised impressions on shale under a simple microscope.

Weiss applies the term _Paracalamostachys_[727] to cones of the _Calamostachys_ form, but in which the manner of attachment cannot be made out. Such a cone as that of fig. 93 should probably be referred to this sub-type of _Calamostachys_ in the absence of definite evidence as to the position of the sporangia.

Another term _Stachannularia_, originally used by Weiss as a genus[728], was afterwards[729] applied to cones of the same general type as _Calamostachys_, in which the sporangiophores have the form of thorn-like structures bearing on their upper side a lamellar expansion. There is however some doubt as to the correct interpretation of the features associated with cones included in _Stachannularia_; for an account of such forms reference must be made to the writings of Weiss, Renault[730], Solms-Laubach[731] and others[732].

_Calamostachys_ cones have been found in organic union with branches bearing leaves of the _Annularia_ type, also with _Calamocladus_ foliage, and the branches bearing such cones have been found in actual connection with Calamitean stems. The association of cones and vegetative stems and branches is shown in tabular form on p. 363.

2. _Palaeostachya[733]._

In this genus the general habit agrees with that of _Calamostachys_, and in imperfectly preserved specimens it may be impossible to discriminate between _Calamostachys_ and _Palaeostachya_. The latter form is characterised by the attachment of the sporangiophores in the axil of the sterile bracts, or immediately above them, as shown in figs. 97 and 98.

EXAMPLES. _Palaeostachya vera_ sp. nov., _P. pedunculata_ Will. afford examples of this form of strobilus. The genus _Palaeostachya_ includes several species previously described under the genus _Volkmannia_[734].

Strobili of this generic type are known in organic association with Annularian branches, as well as with _Calamocladus_ and _Calamites_.

3. _Macrostachya._

This generic name was originally applied by Schimper[735] to certain forms of Calamitean stems, of the type afterwards referred to the sub-genus _Calamitina_ by Weiss, bearing long and thick cones. The name is, however, more appropriately restricted to strobili, which differ from the two preceding genera in their greater length (14–16 cm.) and in the more crowded and imbricating whorls of bracts. The internodes of the cones are very short, and each whorl of bracts consists of about 20 coherent members separated at the periphery of the disc into short pointed teeth. The internal structure of _Macrostachya_ has not been satisfactorily determined. An account by Renault[736] of a petrified specimen does not present a very clear idea as to the structural features of this form of Calamitean strobilus.

THE ASSOCIATION OF CALAMITEAN VEGETATIVE SHOOTS AND CONES.

[Sidenote: HUTTONIA.]

The generic name _Huttonia_, suggested by Sternberg[747] in 1837, is applied to cones which closely resemble _Macrostachya_ in habit, but differ—so far as our scanty knowledge enables us to judge—in the arrangement of the members. The student must refer to Weiss[748], Solms-Laubach[749] and other writers[750] for a further account of these types, and of another rare and little-known form of cone, called by Weiss Cingularia[751].

Macrostachyan cones have been found attached to stems of _Calamites_ which are included in the sub-genus _Calamitina_ (p. 367). The larger size of _Macrostachya_ as a distinguishing feature is not always a safe test; some cones which belong to _Palaeostachya_ [_e.g._ _P. arborescens_ Sternb.] and _Calamostachys_ (_e.g._ _C. Solmsi_) are much thicker and larger than the majority of species of these two genera.

It would appear from the examples selected to illustrate the connection between strobili and vegetative shoots, that the _Annularia_ type of branch usually bears cones which conform to the genus _Calamostachys_ (_Stachannularia_); while the Asterophyllitean branches—_Calamocladus_—are associated with _Palaeostachya_ and _Macrostachya_. But this rule is not constant, and we are not in a position to speak of cones of a particular type as necessarily characteristic of definite types of Calamitean shoots.

• • • • •

Although it is admitted by the great majority of Palaeobotanists that the Calamites were all true Vascular Cryptogams, the older view that some members of the Calamarieae are gymnospermous has not been given up by Renault[752]. This observer has recently described some seeds which he believes were borne by Calamitean stems; he admits, however, that no undoubted female cones of _Calamodendron_ have so far been found. In view of the unsatisfactory evidence on which Renault’s opinion is based, we need not further discuss the questions which he raises.

[The following specimens in the Williamson Cabinet in the British
Museum, may be found useful in illustration of the structure of
_Calamites_.

_Stems._ (i. _Arthropitys._) _Young twigs and small branches_ 1, 2,
6, 10, 14, 19, 116*, 1002, 1007, 1020.

_Older stems_ (_transverse sections_) 15–17, 62, 77–87, 115 _a_,
117*, 118*, 120, 122*–124*, 1933 A, 1934, 1941.

(_Tangential sections_) 20, 24, 26, 37, 38, 49, 90, 91, 130, 138,
1937, 1943.

(_Radial sections_) 20, 20 A, 21, 22, 48, 65–68, 83–91, 137*,
138*, 1937.

(ii. _Arthrodendron)_ 36, 37, 38, 52, 54.

_Roots._ 1335, 1347, 1350, 1356.

_Strobili._ i. _Calamostachys Binneyana._ 991, 996, 997, 1000, 1003,
1005, 1007, 1008, 1011, 1013, 1016, 1017, 1022, 1023, 1037 A,
1043.

ii. _C. Casheana._ 1024, 1025, 1587, 1588.

iii. _Palaeostachya vera._ 110, 1564, 1567, 1569, 1579, 1583.]

III. Pith-casts of Calamites.

A. _Calamitina._ B. _Stylocalamites._ C. _Eucalamites._

Palaeobotanical literature contains a large number of species of _Calamites_ founded on pith-casts alone. Many of these so-called species are of little or no value botanically, but while we may admit the futility of attempting to recognise specific types in the same sense as in the determination of recent plants, it is necessary to pay attention to such characters as are likely to prove of value for descriptive and comparative purposes. From the nature of the specimens it is clear that many of the differences may be such as are likely to be met with in different branches of the same species, while in others the pith-casts of distinct species or genera may be almost identical.

The most striking differences observable in Calamite casts are in the character of the internodes, the infranodal canals, the number and disposition of branch-scars, and other surface features. Occasionally it is possible to recognise certain anatomical characters in the coaly layer which often surrounds a shale- or sandstone-cast, and the surface of a well preserved cast may give a clue to the nature of the wood in the faint outlines of cells which can sometimes be detected on the cast itself[753]. The breadth of the carbonaceous envelope on a cast has been frequently relied on by some writers as an important character. It has been suggested[754] that we may arrive at the original thickness of the wood of a stem by measuring the coaly layer and multiplying the breadth by 27; the explanation being that a zone of wood 27 mm. in thickness is reduced in the process of carbonisation to a layer 1 mm. thick.

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Fossil plants, Vol. 1Chapter X: I. EQUISETALES (continued) (2)

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