mm. The axis of the cone bears whorls of sterile leaves or bracts at

Chapter 229,008 wordsPublic domain

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_).

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.

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.

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.

The breadth of the coal on the same form of cast may vary considerably; on this account, and for various other reasons, such a character can have but little value. Our knowledge of anatomy may often help us to interpret certain features of internal casts and to appreciate apparently unimportant details. One occasionally notices that a Calamite pith-cast has large infranodal canals, and in some specimens each internodal ridge may be traversed by a narrow median line or small groove; large infranodal canal casts suggest the type of stem referred to the subgenus _Arthrodendron_, and the median line on the ridges may be due to bands of hard tissue in each principal medullary ray.

In attempting to identify pith-casts the student must keep in view the probable differences presented by the branching rhizome, the main aerial branches and the finer shoots of the same individual. The long internodal ridges of some casts may be mistaken for the parallel veins of such a leaf as _Cordaites_, a Palaeozoic Gymnosperm, if there are no nodes visible on the specimen. The fossil figured by Lindley and Hutton[755] as _Poacites_, and regarded by them as a Monocotyledon, is no doubt a portion of a Calamite with very long internodes. An interesting example of incorrect determination has recently been pointed out by Nathorst[756] in the case of certain casts from Bear Island, originally described by Heer as examples of _Calamites_; the vertical rows of leaf-trace casts on a _Knorria_ were mistaken for the ribs of a Calamite stem. The specimens in the Stockholm Museum fully bear out Nathorst’s interpretation. The undulating course of internodal ridges and grooves is not in itself a character of specific value. If a Calamite stem were bent slightly, the wood and medullary-ray tissues on the concave side might adapt themselves to the shortening of the stem by becoming more or less folded, and a cast of such a stem would show undulating ridges and grooves on one side and straight ones on the other[757].

A convenient classification of Calamite casts was proposed by Weiss in 1884, founded chiefly on the number and manner of occurrence of branch-scars—or rather branch-depressions—on the surface of pith-casts. Weiss[758] recognised the imperfection of his proposed grouping, and Zeiller[759] has also expressed reasonable doubts as to the scientific value of such group-characters. Weiss instituted three subgenera—_Calamitina_, _Eucalamites_ and _Stylocalamites_, which are made use of as convenient terms in descriptive treatment of Calamite casts. The following account of a few of the more typical casts may serve to illustrate the methods employed in the description of such specimens; the synonomy given for the different species is not intended to be complete, but it is added with a view to drawing attention to the necessity for careful comparison in systematic work.

A. _Calamitina._

This sub-genus of _Calamites_, as instituted by Weiss[760], includes Calamitean stems or branches, which are characterised by the periodic occurrence of branch-whorls usually represented by fairly large oval or circular scars just above a nodal line (figs. 99, 100 and 101). The branch-scars may form a row of contiguous discs, or a whorl may consist of a smaller number of branches which are not in contact basally. A form described by Weiss as _C. pauciramis_, Weiss[761], has only one branch in each whorl, as represented by a single large oval scar on some of the nodes of the cast. A stem of this form is by no means a typical _Calamitina_, but it serves to show the existence of forms connecting Weiss’ sub-genera _Calamitina_ and _Eucalamites_. The number of internodes and nodes between the branch whorls varies in different specimens, and is indeed not constant in the same plant. Each nodal line bears numerous elliptical scars which mark the points of attachment of leaves; each branch-whorl is situated immediately above a node, and in some forms this nodal line pursues a somewhat irregular course across the stem, following the outlines of the several branch-scars[762]. The surface of the internodes is either perfectly smooth or it is more frequently traversed by short longitudinal ridges or grooves probably representing fissures in the bark of the living stem; these are indicated by lines in fig. 99 and by elongated elliptical ridges in fig. 101. On young stems the leaves are occasionally found in place, as for example in an example figured by Weiss[763] (_C. Göpperti_), or we may have leaf-verticils still in place in much older and thicker branches[764] (cf. fig. 85, p. 330).

It occasionally happens that the bark of _Calamitina_ stems has been preserved as a detached shell[765] reminding one of the sheets of Birch bark often met with in forests, the separation being no doubt due in the fossil as in the recent trees to the manner of occurrence of the cork-cambium.

In a few cases branches have been preserved still attached to a stem or branch of higher order; examples of such specimens are figured by Lindley and Hutton[766], Stur[767], and others. Grand’Eury[768] has given an idealised drawing of a typical _Calamitina_ bearing a whorl of branches with the foliage and habit of _Asterophyllites equisetiformis_. The specimen on which this drawing is based is in the Natural History Museum, Paris; it shows Asterophyllitean branches in organic connection with a Calamitean stem, but it is not quite clear if the stem is a true _Calamitina_. A large drawing of this interesting specimen is given by Stur[769] in his monograph on _Calamites_, also a smaller sketch by Renault[770] in his _Cours de botanique fossile_. Similar branches of the _Asterophyllites_ type attached to an undoubted _Calamitina_ are figured also by Lindley and Hutton. There is, in short, good evidence that stems of this sub-genus bore branches with Asterophyllitean shoots.

The wood of stems of the Calamitina group of _Calamites_, in some instances at least, was of the _Arthropitys_ type; this has been shown to be the case in some French specimens from the Commentry coal-field[771] and in others described by Stur[772]. The pith-casts of _Calamitina_ are characterised by comparatively short internodes separated by deep nodal constrictions, as shown in fig. 100. From Permian specimens from Neu Paka in Bohemia, described by Stur[773], we learn that there were the usual Calamite diaphragms bridging across the wide pith-cavity at each node. Such a cast as that shown in fig. 100 is often referred to as _Calamites approximatus_ Brongn.; the length of the internodes and the periodic occurrence of branch-scars in the form of circular or oval depressions along a nodal line enable us to recognise the _Calamitina_ casts. Weiss[774] points out that in pith-casts of this form the branch-scars occur on the nodal constriction, and not immediately above the node as is the case on the surface of a typical _Calamitina_. This distinction is however of little or no value; the point of attachment of a branch may be above the nodal line, while on the pith-cast of the same stem the point of origin of the vascular bundles of the branch is on the nodal constriction[775].

The specimen shown in fig. 100 illustrates the appearance of a _Calamitina_ cast. There is a verticil of branch-scars on the lowest nodal constriction; on the right of the pith-cast the broad band of wood is faintly indicated by the smooth surface of the rock (_x_). Other examples demonstrating the existence of a broad woody cylinder in _Calamitina_ stems have been figured by Weiss[776] and other writers, and some good examples may be seen in the British Museum.

We have so far noticed the connection of certain forms of pith-casts (_e.g._ _Calamites approximatus_), and Asterophyllitean shoots with stems of the sub-genus _Calamitina_.

As regards the strobili our knowledge is far from satisfactory. Stur[777] figures some fertile branches bearing long and narrow strobili, either _Palaeostachya_ or _Calamostachys_, in close association with _Calamitina_ stems, and Renault and Zeiller[778] give illustrations of the association of _Calamitina_ stems with large strobili of the _Macrostachya_ form.

Before Weiss proposed the term _Calamitina_, various authors had figured this form of Calamite under a distinct generic name (_e.g._ _Hippurites_ of Lindley and Hutton[779], _Cyclocladia_[780], _Macrostachya_[781], &c.). Stems of this type have also been described by more recent writers under different names, and considerable confusion has been caused by the use of numerous generic designations for forms of _Calamitina_. Some small fragments of _Calamitina_ stems were described by Salter[782] in 1863 as portions of a new species of the Crustacean _Eurypterus_ (_E. mammatus_). In 1869 Grand’Eury proposed the generic name _Calamophyllites_[783] for stems bearing verticils of _Asterophyllites_ shoots; his description of such stems agrees with Weiss’ _Calamitina_, but as Grand’Eury’s name is used in a narrower sense as implying a connection with _Asterophyllites_, it is more convenient to adopt Weiss’ term in spite of the priority of _Calamophyllites_. In the _Fossil flora of Commentry_ we find some flattened stems of the _Calamitina_ type described under different generic names, as _Arthropitys approximatus_[784] and as _Macrostachya_[785].

The determination of distinct species of the sub-genus _Calamitina_ is rendered almost hopeless by the variation in the different branches of the same individual, and by the difficulty of connecting surface-impressions with casts of the pith-cavity.

A typical example of the _Calamitina_ type of _Calamites_ was figured by Sternberg[786] in 1821 as _Calamites varians_. This has been adopted by Weiss[787] as a comprehensive species including several different ‘forms’ of stems, which differ from Sternberg’s fossil in such points as the number of nodes between the branch-whorls and the number of branches in each whorl. The result of this system of nomenclature is the separation of portions of one specific type under different form-names. It must be clearly recognised that accurate specific diagnoses are practically impossible when we have to deal with fragments of plants, some of which are mere pith-casts, while others show the surface features. The specimen represented in fig. 99 agrees with a stem described by Ettingshausen[788] in 1855 as _Calamites Göpperti_, and as a matter of convenience a member of the _Calamitina_ group showing such characters may be referred to as _Calamites_ (_Calamitina_) _Göpperti_ (Ett.). The following list, which includes a few synonyms of this form, may suffice to illustrate the difficulties connected with accurate systematic determinations.

_Calamites_ (_Calamitina_) _Göpperti_ (Ett.). Fig. 99.

1855. _Calamites Göpperti_, Ettingshausen[789]. 1869. _Calamites_ (_Calamophyllites_) _Göpperti_, Grand’Eury[790]. 1874. _Cyclocladia major_, Feistmantel[791]. 1874. _Calamites verticillatus_, Williamson[792]. 1876. _Calamitina Göpperti_, Weiss[793]. 1884. _Calamites_ (_Calamitina_) _varians abbreviatus_, Weiss[794]. 1884. _Calamites_ (_Calamitina_) _varians inconstans_, Weiss[795]. 1887. _Calamites Sachsei_, Stur[796]. 1888. _Calamophyllites Göpperti_, Zeiller[797].

This species is characterised by the smooth bark, which may be traversed by a few irregular longitudinal fissures; most of the nodes bear a series of small leaf-scars, and at fairly regular intervals a node is immediately succeeded by a circle of contiguous branch-scars, 8–12 in a whorl. The pith-cast of this type of stem has short ribbed internodes separated by rather deep nodal constrictions; the branch-whorls being represented by a series of pits on the nodal constrictions recurring at corresponding intervals to the whorls of branch-scars on the surface of the stem. Leaves narrow and linear in form, like those on Asterophyllitean branches, are occasionally associated with this type of stem.

The fragment of a _Calamitina_ stem shown in fig. 101 is the counterpart of a specimen originally figured by Steinhauer[798] in 1818 as a species of _Phytolithus_. This may be specifically identical with _C. Göpperti_; but it is better to speak of so small a specimen as merely one of the _Calamitina_ stems, to be compared with _Calamites_ (_Calamitina_) _Göpperti_. The specimen measures 14·5 cm. in length and 7 cm. in breadth.

The form of pith-cast represented in fig. 100 is no doubt that of one of the _Calamitina_ species, but as it is seldom possible to determine the connection between such casts and the particular species of stems to which they belong, they are often referred to as _Calamites_ (_Calamitina_) _approximatus_ (Brongn.). The specimen of which fig. 100 is a photograph was originally described and figured by Mr Kidston[799] from the lower Coal-Measures of Ayrshire. Both _Calamites_ (_Calamitina_) _Göpperti_ (Ett.) and _C._ (_Calamitina_) _approximatus_ (Brongn.) are recorded from the Transition, Middle and Lower Coal-Measures[800].

B. _Stylocalamites._

In the members of this sub-genus the branch-scars are either irregular in their occurrence or absent. In some Calamites the branch-scars are very few and far between, and other species appear to have been almost without branches; pith-casts of such stems may be referred to the sub-genus _Stylocalamites_[801].

An exceedingly common Calamitean cast, _C. Suckowi_ Brongn. (fig. 82) affords a good illustration of this type of stem. In the specimen shown in fig. 82 we have a cast of a rhizome, which is rather exceptional in showing three branches in connection with one another. The appearance of the fossil suggests a rhizome, rather than an aerial shoot, bearing lateral branches; the narrowing of the branches and the rapid decrease in the length of the internodes towards the point of attachment being features associated with rhizomes rather than with aerial branches.

_Calamites_ (_Stylocalamites_) _Suckowi_, Brongn. Fig. 82.

1818. _Phytolithus sulcatus_, Steinhauer[802]. 1825. _Calamites decoratus_, Artis[803]. 1828. _Calamites Suckowi_, Brongniart[804]. 1833. _Calamites cannaeformis_, Lindley and Hutton[805].

For more complete lists of synonyms of this species reference should be made to Kidston[806], Zeiller[807], and other authors.

Casts of _Calamites Suckowi_ are characterised by flat or slightly convex internodal ridges separated by shallow depressions, the ridges are rounded at the upper end of each internode, and usually bear circular casts of infranodal canals. There are some unusually large examples of casts of this species in the British Museum from the Radstock Coal-Measures; one of these has a length of 81 cm., and a diameter of 27 cm. Specimens are not infrequently found with verticils of slender roots in close proximity to the nodes of the cast; figures of such root-bearing casts have been given by Lindley and Hutton[808], Weiss[809], and other authors.

Renault[810] has drawn attention to the thinness of the layer of wood which is often associated with large casts of _C. Suckowi_; he concludes that the stems must have possessed little or no secondary wood. In a more recent work by Grand’Eury[811] _Calamites Suckowi_ is spoken of as having had wood of the _Calamodendron_ type, but as wood of this kind has not been found in England, it is suggested that the plant may not have assumed an arborescent habit until late in the Coal-Measure period. During the Lower and Middle Coal-Measures, at which horizon it commonly occurs in England, it may have been herbaceous. This suggestion has little to commend it; the close agreement between _C. Suckowi_ from English and French localities points to a plant of the same form, and we have no satisfactory evidence as to any difference in stem-structure in the two cases.

Stur has figured a specimen of a Calamite cast, which he compares with _C. Suckowi_, surrounded by a band of silicified wood apparently of the _Arthropitys_ type. From this and other facts it would appear probable that some of the English stems with the _Arthropitys_ structure possessed casts referable to _Calamites_ (_Stylocalamites_) _Suckowi_.

We are not in a position to speak with confidence as to the strobili of _C. Suckowi_, but Stur adduces evidence in support of a connection between this species of Calamite and certain Asterophyllitean branches (_Calamocladus equisetiformis_) bearing Calamostachyan cones. He does not appear to have found the foliage-shoots and stems in organic contact, but draws this conclusion from the association of the fertile branches and stems in the same rocks[812]. This species is abundant in the Lower, Middle and Upper Coal-Measures; it has also been recorded from the Millstone Grit[813].

C. _Eucalamites._

In this sub-genus branch-scars occur on every node; the scars never form a contiguous whorl as in _Calamitina_, but there may be from 3 to 10 on each node. The scars of successive nodes often alternate in position, and thus form more or less regular vertical series as shown in fig. 102. The most obvious feature as regards the arrangement of the branch-scars is their spiral disposition on the surface of the pith-cast. The internodes are fairly uniform in length, and there is no periodic recurrence of narrower internodes as in _Calamitina_. From an examination of specimens of _Eucalamites_ in which the pith-cast is covered with a coaly layer representing the carbonised remains of the wood and cortex, it would appear that the surface of the stems was practically smooth. The coaly investment on _Eucalamites_ casts varies considerably in thickness[814]; it is very unsafe to make use of the thickness of this layer as a test of the breadth of the wood in Calamitean stems. The branch-scars as seen in a surface-view of a stem are situated a little above the nodal lines, while depressions on the pith-casts occur in the slight nodal constriction or immediately above it. Small leaf-scars have been described as occurring on the nodes between the branch-scars in specimens showing the surface features[815].

The species long known as _Calamites cruciatus_ Sternb. is usually taken as the type of the sub-genus _Eucalamites_. Weiss[816] has subdivided this species into several ‘forms,’ which he bases on the number of branch-scars on each node and on other characters; a more extended subdivision of _C. cruciatus_ has recently been made by Sterzel[817], who admits the impossibility of separating the specific forms by means of the data at our disposal, but for purposes of geological correlation he prefers to express slight differences by means of definite ‘forms’ or varieties. The more comprehensive use of the specific name _cruciatus_ as adopted by Zeiller in his _Flore de Valenciennes_[818] is, I believe, the better method to adopt. The specimen shown in fig. 102 affords a good example of a typical _Calamites cruciatus_, it was found in the Middle Coal-Measures near Barnsley, Yorkshire.

_Calamites_ (_Eucalamites_) _cruciatus_ (Sternb.). Fig. 102.

1826. _Calamites cruciatus_, Sternberg[819]. 1828. _Calamites cruciatus_, Brongniart[820]. 1831. _Calamites alternans_, Germar and Kaulfuss[821]. 1837. _Calamites approximatus_, Lindley and Hutton[822]. 1877. _Calamodendrofloyos cruciatus_, Grand’Eury[823]. 1878. _Calamodendron cruciatum_, Zeiller[824]. 1884. _Calamites_ (_Eucalamites_) _cruciatus ternarius_, Weiss[825]. 1884. „ „ „ _quaternarius_, Weiss[825]. 1884. „ „ „ _genarius_, Weiss[825]. 1884. „ „ _multiramis_, Weiss[825]. 1888. _Calamites_ (_Calamodendron_) _cruciatus_, Zeiller[826].

This species occurs in the Upper, Middle and Lower Coal-Measures[827]. The casts of the _cruciatus_ type have been found associated with wood possessing the structural features of the sub-genus _Calamodendron_[828], but our knowledge of the structure of the stem, and of the fertile branches of _C. cruciatus_ is very imperfect. A restoration of _Calamites_ (_Eucalamites_) _cruciatus_ is given by Stur[829] in his classic work on the Calamites, but he does not make quite clear the supposed connection with the stems and the fertile shoots of the Asterophyllites type[830] which he describes. Another member of the _Eucalamites_ group, which is better known as regards its foliage-shoots, is _Calamites ramosus_, a species first described by Artis[831] in 1825. Stems of this species have been found in connection with the branches and leaves of the _Annularia_[6] type, bearing _Calamostachys_[832] cones. In all probability pith-casts included in the sub-genus _Eucalamites_ belonged to stems with foliage-shoots and probably also with cones of more than one form.

[Sidenote: NOMENCLATURE.]

In the above account of a few common pith-casts it has been pointed out that there is occasionally satisfactory evidence for connecting certain casts with wood of a particular structure, and with sterile and fertile foliage-shoots of a definite type. It is, however, impossible in many cases to recognise with any certainty the leaf-bearing branches and strobili of the different casts of _Calamites_; it is equally impossible to determine what type of pith-cast or what type of foliage-shoots belongs to petrified stem-fragments in which it is possible to investigate the microscopical features. The scattered and piece-meal nature of the material on which our general knowledge of Calamitean plants is based, necessitates a system of nomenclature which is artificial and clumsy; but the apparent absurdity of attaching different names to fragments, which we believe to be portions of the same genus, is of convenience from the point of view of the geologist and the systematist. As our material increases it will be possible to further simplify the nomenclature for Calamarian plants, but it is unwise to allow our desire for a simpler terminology to lead us into proposals which are based rather on suppositions than on established fact. If it were possible to discriminate between pith-casts of stems having the different anatomical characters designated by the three sub-genera, _Arthropitys_, _Arthrodendron_ and _Calamodendron_, the genus _Calamites_ might be used in a much narrower and probably more natural sense than that which we have adopted. The tests made use of by some authors for separating pith-casts of _Calamodendron_ and _Arthropitys_ stems do not appear to be satisfactory; we want some term to apply to all Calamitean casts irrespective of the anatomical features of the stems, or of the precise nature of the foliage-branches. As used in the present chapter, _Calamites_ stands for plants differing in certain features but possessing common structural characters, which must be defined in a broad sense so as to include types which may be worthy of generic rank, but which for convenience sake are included in a comprehensive generic name. The attempts to associate certain forms of foliage with _Arthropitys_ on the one hand and with _Calamodendron_ on the other, cannot be said to be entirely satisfactory; we still lack data for a trustworthy diagnosis of distinct Calamarian genera which shall include external characters as well as histological features. If we restricted the genus _Calamites_ to stems with an _Arthropitys_ structure and an Asterophyllitean foliage, we should be driven into unavoidable error. Within certain limits it is possible to distinguish generically or even specifically between petrified branches, and we already possess material enough for fairly complete diagnoses founded on internal structure; but it is not possible to make a parallel classification for pith-casts and foliage-shoots. For this reason, and especially bearing in mind the importance of naming isolated foliage-shoots and stem-casts for geological purposes, I believe it is better to admit the artificially wide application of the name _Calamites_, and to express more accurate knowledge, where possible, by the addition of a subgeneric term. In dealing with distinctions exhibited by Calamitean stems it may be advisable to make use of specific names, but we must keep before us the probability of the pith-cast and petrified stem-fragment of the same plant receiving different specific names. If the structural type is designated by a special sub-genus, this will tend to minimise the anomaly of using more than one binominal designation for what may be the same individual.

[Sidenote: CALAMITES AND EQUISETUM.]

The following summary may serve to bring together the different generic and subgeneric terms which have been used in the foregoing account of _Calamites_.

CALAMITES.

Subgenera having reference to the method of branching as seen in casts or impressions of the stem-surface or in pith-casts.

_Calamitina_, _Eucalamites_, _Stylocalamites_.

Subgenera founded on anatomical characters in stems and branches.

_Arthropitys_, _Calamodendron_, _Arthrodendron_ (new sub-genus substituted for _Calamopitys_).

Genus proposed for roots of _Calamites_ before their real nature was recognised. The name refers to anatomical characters.

_Astromyelon._

Genera of which some species, if not all, are the leaf-bearing branches of _Calamites_.

_Calamocladus_ (including _Asterophyllites_), _Annularia_.

Generic names applied to strobili belonging to _Calamites_.

_Calamostachys_, _Palaeostachya_, _Macrostachya_, _etc._

Genus including impressions of Calamite roots.

_Pinnularia._

IV. Conclusion.

A brief sketch of the main features of _Calamites_ suffices to bring out the many points of agreement between the arborescent Calamite plants and the recent Equisetums. The slight variation in morphological character among the present-day Horse-tails, contrasts with the greater range as regards structural features among the types included in _Calamites_. The Horse-tails probably represent one of several lines of development which tend to converge in the Palaeozoic period; the Calamite itself would appear to mark the culminating point of a certain phylum of which we have one degenerate but closely allied descendant in the genus _Equisetum_. We shall, however, be in a better position to consider the general question of plant-evolution after we have made ourselves familiar with other types of Palaeozoic plants. Grand’Eury’s[833] striking descriptions of forests of Calamites in the Coal-Measures of central France, enable us to form some idea of the habit of growth of these plants with their stout branching rhizomes and erect aerial shoots.

By piecing together the evidence derived from different sources we may form some idea of the appearance of a living Calamite. A stout branching rhizome ascended obliquely or spread horizontally through sand or clay, with numerous whorls or tufts of roots penetrating into swampy soil. From the underground rhizome strong erect branches grew up as columnar stems to a height of fifty feet or more; in the lower and thicker portions the bark was fissured and somewhat rugged, but smoother nearer the summit. Looking up the stem we should see old and partially obliterated scars marking the position of a ring of lateral branches, and at a higher level tiers of branches given off at regular or gradually decreasing intervals, bearing on their upper portions graceful green branchlets with whorls of narrow linear leaves. On the younger parts of the main shoot rings of long and narrow leaves were borne at short intervals, several leaf-circles succeeding one another in the intervals between each radiating series of branches. On some of the leaf-bearing branchlets long and slender cones would be found here and there taking the place of the ordinary leafy twigs. Passing to the apical region of the stem the lateral branches given off at a less and less angle would appear more crowded, and at the actual tips there would be a crowded succession of leaf-segments forming a series of overlapping circles of narrow sheaths with thin slender teeth bending over the apex of the tree.

Thus we may feebly attempt to picture to ourselves one of the many types of Calamite trees in a Palaeozoic forest, growing in a swampy marsh or on gently sloping ground on the shores of an inland sea, into which running water carried its burden of sand and mud, and broken twigs of Calamites and other trees which contributed to the Coal Period sediments. The large proportions of a Calamite tree are strikingly illustrated by some of the broad and long pith-casts occasionally seen in Museums; in the Breslau Collection there is a cast of a stem belonging to the sub-genus _Calamitina_, which measures about 2 m. in length and 23 cm. in breadth, with 36 nodes. In the Natural History Museum, Paris, there is a cast nearly 2 metres long and more than 20 cm. wide, which is referred to the sub-genus _Calamodendron_.

E. ARCHAEOCALAMITES.

In the Upper Devonian and Culm rocks casts of a well-defined Calamitean plant are characteristic fossils; stems, leaf-bearing branches, roots and cones have been described by several authors, and the genus _Archaeocalamites_ has been instituted for their reception. Although this genus agrees in certain respects with _Calamites_, and as recent work has shown this agreement extends to internal structure, it has been the custom to regard the Lower Carboniferous and Devonian plants as genetically distinct. The surface features of the stem-casts, the form of the leaves, and apparently the cones, possess certain distinctive characters which would seem to justify the retention of a separate generic designation.

We may briefly summarise the characteristics of the genus as follows:—

Pith-casts articulated, with very slightly constricted nodes; the internodes traversed by longitudinal ribs slightly elevated or almost flat, separated by shallow grooves. The ribs and grooves are continuous from one internode to another, and do not usually show the characteristic alternation of _Calamites_[834]. Along the nodal line there are occasionally found short longitudinal depressions, probably marking the points of origin of outgoing bundles. Branches were given off from the nodes without any regular order; a pith-cast may have branch-scars on many of the nodes, or there may be no trace of branches on casts consisting of several nodes. The leaves[835] are in whorls; in some cases they occur as free, linear, lanceolate leaves, or on younger branches they are long, filiform and repeatedly forked. The structure of the wood agrees with that of some forms of _Arthropitys_. The strobili consist of an articulated axis bearing whorls of sporangiophores, and each sporangiophore has four sporangia. Our knowledge of the fertile shoots is, however, very imperfect.

Renault[836] has recently described the structure of the wood in some small silicified stems of _Archaeocalamites_ from Autun. A large hollow pith is surrounded by a cylinder of wood consisting of wedge-shaped groups of xylem tracheids associated with secondary medullary rays; at the apex of each primary xylem group there is a carinal canal. The primary medullary rays appear to have been bridged across by bands of xylem at an early stage of secondary thickening, as in the Calamite of fig. 83, _D._

Our knowledge of the cones of _Archaeocalamites_ is far from satisfactory. Renault[837] has recently described a small fertile branch bearing a succession of verticils of sporangiophores; each sporangiophore stands at right angles to the axis of the cone and bears four sporangia, as in _Calamostachys_. It is not clear how far there is better evidence than that afforded by the association of the specimen with pith-casts of stems, for referring this cone to _Archaeocalamites_, but the association of vegetative and fertile shoots certainly suggests an organic connection. The cone described by the French author agrees with _Equisetum_ in the absence of sterile bracts between the whorls of sporangiophores. It is an interesting fact that such a distinctly Equisetaceous strobilus is known to have existed in Lower Carboniferous rocks.

Stur[838] has also described _Archaeocalamites_ at considerable length; he gives several good figures of stem-casts and foliage-shoots bearing long and often forked narrow leaves. The same writer describes specimens of imperfectly preserved cones in which portions of whorls of forked filiform leaves are given off from the base of the strobilus[839]. Kidston[840] published an important memoir on the cones of _Archaeocalamites_ in 1883, in which he advanced good evidence in support of the view that certain strobili, which were originally described as Monocotyledonous inflorescences, under the generic name _Pothocites_[841], are the fertile shoots of this Calamarian genus. Kidston’s conclusions are based on the occurrence on the _Pothocites_ cones, of leaves like those of _Archaeocalamites_, on the non-alternation of the sporangiophores of successive whorls, and on the close resemblance between his specimens and those described by Stur. Good specimens of the cones, formerly known as _Pothocites_, may be seen in the Botanical Museum in the Royal Gardens, Edinburgh; as they are in the form of casts without internal structure it is difficult to form a clear conception as to their morphological features.

The fossils included under _Archaeocalamites_ have been referred by different authors to various genera, and considerable confusion has arisen in both generic and specific nomenclature. The following synonomy of the best known species, _A. scrobiculatus_ (Schloth.) illustrates the unfortunate use of several terms for the same plant.

_Archaeocalamites scrobiculatus_ (Schloth.). Fig. 103.

1720. _Lithoxylon_, Volkmann[842]. 1820. _Calamites scrobiculatus_, Schlotheim[843]. 1825. _Bornia scrobiculata_, Sternberg[844]. 1828. _Calamites radiatus_, Brongniart[845]. 1841. _Pothocites Grantoni_, Paterson[846]. 1852. _Calamites transitionis_, Göppert[847]. —— _Stigmatocanna Volkmanniana_, _ibid._ —— _Anarthrocanna tuberculata_, _ibid._ —— _Calamites variolatus_, _ibid._ —— _C. obliquus_, _ibid._ —— _C. tenuissimus_, _ibid._ —— _Asterophyllites elegans_, _ibid._ 1866. _Calamites laticulatus_, Ettingshausen[848]. —— _Equisetites Göpperti_, _ibid._ —— _Sphenophyllum furcatum_, _ibid._ 1873. _Asterophyllites spaniophyllus_, Feistmantel[849]. 1880. _Asterocalamites scrobiculatus_, Zeiller[850].

For other lists of synonyms reference may be made to Binney[851], Stur[852], Kidston[853] and other authors.

Some of the best specimens of this species are to be seen in the Museums of Breslau and Vienna, which contain the original examples described by Göppert and Stur. An examination of the original specimens, figured by Göppert under various names, enables one to refer them with confidence to the single species, _Archaeocalamites scrobiculatus_. The generic name _Archaeocalamites_, which has been employed by some authors, was suggested by Schimper[854] in 1862, as a subgenus of _Calamites_, on account of the occurrence of a deeply divided leaf-sheath, attached to the node of a pith-cast, which seemed to differ from the usual type of Calamitean leaf. The specimens described by Schimper are in the Strassburg Museum; the leaf-sheath which he figures is not very accurately represented.

The example given in fig. 103 shews very clearly the continuous course of the ribs and grooves of the pith-cast. Each rib is traversed by a narrow median groove which would seem to represent the projecting edge of some hard tissue in the middle of each principal medullary ray of the stem. The specimen was found in a Carboniferous limestone quarry, Northumberland; there is a similar cast from the same locality in the Museum of the Geological Survey.

_Affinities of_ Archaeocalamites.

This genus agrees very closely with _Calamites_ both in the anatomical structure of the stem and in the verticillate disposition of the leaves. The strobili appear to be Equisetaceous in character, and there is no satisfactory evidence of the existence of whorls of sterile bracts in the cone, such as occur in _Calamostachys_ and in other Calamitean strobili. The continuous course of the vascular bundles of the stem from one internode to the next is the most striking feature in the ordinary specimens of the genus; but it sometimes happens that the grooves on a pith-cast shew the same alternation at the node as in _Calamites_. This is the case in a specimen in the Göppert collection in the Breslau Museum, and Feistmantel[855] has called attention to such an alternation in specimens from Rothwaltersdorf. In the true _Calamites_, on the other hand, the usual nodal alternation of the vascular strands is by no means a constant character[856]. Stur[857], Rothpletz[858], and other authors have pointed out the resemblance of _Archaeocalamites_ to _Sphenophyllum_. The deeply divided leaves of some Sphenophyllums and those of _Archaeocalamites_ are very similar in form; and the course of the vascular strands in _Sphenophyllum_ may be compared with that in _Archaeocalamites_. But the striking difference in the structure of the stele forms a wide gap between the two genera. We have evidence that the Calamites and Sphenophyllums were probably descended from a common ancestral stock, and it may be that in _Archaeocalamites_, some of the _Sphenophyllum_ characters have been retained; but there is no close affinity between the two plants.

On the whole, considering the age of _Archaeocalamites_ and the few characters with which we are acquainted, it is probable that this genus is very closely related to the typical _Calamites_, and may be regarded as a type which is in the direct line of development of the more modern Calamite and the living _Equisetum_. Weiss[859] includes _Archaeocalamites_ as one of his subgenera with _Calamitina_ and others, and it is quite possible that the genus has not more claim to stand alone than other forms at present included in the comprehensive genus _Calamites_.

The student will find detailed descriptions of this genus in the works which have been referred to in the preceding pages.