Fossil plants, Vol. 3

In this chapter are included several types represented by stems, but which in the absence of definite information with regard to the reproductive organs cannot be assigned to the Pteridosperms.

MEGALOXYLEAE.

Megaloxylon. Seward.

This genus is represented by a single species founded on a piece of stem from the Lower Coal Measures of Lancashire discovered in the Binney Collection in the Sedgwick Museum, Cambridge[466]. Nothing is known as to the leaves, reproductive organs, or roots. The type-specimen consists exclusively of primary and secondary xylem.

Megaloxylon Scotti Seward.

The type-specimen, reproduced natural size in fig. 446, A, B, consists of a fragment of stem which at first sight bears a close resemblance to Cordaites, but the apparent pith, 1·9 cm. in diameter, is the primary xylem of the stele and is enclosed by an incomplete cylinder of secondary xylem 2 cm. broad. The distinction between the lighter primary xylem and the darker secondary cylinder is especially well marked in the longitudinal section (fig. 446, B). The central region, shown on a larger scale in fig. 447, is occupied by groups of tracheids varying in size and shape associated with thin-walled parenchyma: the latter is represented by lighter patches in the section. The majority of the tracheids are characterised by their great breadth—in some cases ·4 mm.—and their isodiametric or even horizontally elongated and flattened form. Some of these large metaxylem elements are seen in longitudinal section at m in fig. 448 where the shrinkage and partial decay of the parenchymatous tissue have resulted in the separation of transverse bands of xylem simulating the discoid pith of a Cordaites. The walls of these tracheal cells are covered with multiseriate pits. With the short and sometimes flattened xylem elements occur others of greater length, but these are chiefly met with in the more peripheral part of the central region where some of the tracheids are much narrower and have the form of ordinary water-conducting elements. On one side of the primary xylem in fig. 447 an oval and more compact group of narrow and longer tracheids is seen at lt¹; this is a leaf-trace about to enter the secondary-xylem cylinder on its outward course. The same leaf-trace is shown at lt in fig. 446, B; as it descends the trace becomes less distinct and its elongated elements gradually merge into the general mass of metaxylem. A portion of this leaf-trace is seen in fig. 448, B, lt, close to the inner edge of the secondary xylem, x², and abutting internally on the contracted tissue, m, which consists mainly of large and short tracheae with remains of associated parenchyma. The trace includes some conjunctive parenchyma interspersed with the tracheids: on its outer surface, that is on the abaxial edge of the ovate xylem strand as shown at lt¹ in fig. 447, there are six external protoxylem strands. In the peripheral region of the primary xylem of the section reproduced in fig. 447 there are several more or less well-defined leaf-traces, e.g. lt², lt³; these differ from that seen at lt¹ in their greater tangential breadth and in the less compact arrangement of the tracheids. As each trace is followed downwards in the primary region of the stele it tends to become broader, especially in a tangential direction; the spiral protoxylem strands are more widely separated (fig. 448, A, px) and the elongated and comparatively narrow tracheids as they spread out fan-wise are reduced in length, finally passing over into the broad and short reticulately pitted cells. Each leaf-trace can be followed through approximately four internodes before its individuality is lost in the general mass of metaxylem. The disposition of the peripheral traces is such as to justify the conclusion that the phyllotaxis of the stem is ⅖.[467]

The secondary xylem (fig. 446, C) resembles that of Lyginopteris and Heterangium though it is less parenchymatous. The medullary rays are numerous and vary in breadth from 1 to 5 cells, while the tracheids, with multiseriate bordered pits on their radial walls, form bands 1 to 8 elements in breadth. There are no regular rings of growth but occasional arcs of narrow tracheids interfere slightly with the otherwise uniform structure of the wood. A leaf-trace in its oblique outward course through the wood becomes completely enclosed by a cylinder of secondary xylem and thus appears to be concentric. Owing to the absence of any tissue external to the secondary xylem of the stem, statements as to the subsequent behaviour of the leaf-traces on emerging from the stele are purely hypothetical. It is, however, not improbable that each concentric trace lost its secondary tissue and broke up into several collateral strands, a suggestion based on the behaviour of the leaf-traces in Medullosa anglica.

The most striking characteristics of Megaloxylon are: (i) the structure of the primary xylem, particularly the unusual form of the majority of the metaxylem tracheids, a form obviously correlated with storage rather than with conduction of water; (ii) the gradual spreading of the leaf-traces and their absorption as they descend into the main mass of the xylem; (iii) the exarch structure of the primary xylem. Confining our attention to the primary region of the stele; a comparison is at once suggested with Heterangium. In Megaloxylon the peculiarities are the substitution of the large storage-tracheids for the normal xylem-elements; the greater irregularity in the groups of metaxylem; and an exarch instead of a mesarch structure. In these last features the primary xylem agrees with that of recent species of the Schizaeaceous Fern Lygodium and the external protoxylem is a character shared with Rhetinangium. The occurrence of short tracheids similar to those of Megaloxylon in the inner portion of the stele of the Osmundaceous Fern Zalesskya gracilis (Eich.)[468] may be quoted as an example of parallel modification but, as Scott[469] points out, the resemblance has no phylogenetic significance. The secondary xylem though less parenchymatous than in recent Cycads agrees more closely with the manoxylic than with the pycnoxylic type.

Megaloxylon affords an interesting example of a combination of primary stelar anatomical features, comparable in the exarch position of the protoxylem with the stele of Lygodium, and secondary wood similar to that of Lyginopteris and Heterangium. The large metaxylem tracheids may be regarded as derivatives of elements which in some ancestral type were structurally fitted for the rôle of water-transport and made up the xylem of a Lygodium-like stele with little or no secondary xylem. As the cambial activity increased and centrifugal xylem became a prominent feature, usurping the function of the centripetal xylem, the latter became modified and fitted for a new service.

RHETINANGIEAE.

Rhetinangium. Gordon.

Rhetinangium Arberi Gordon.

The stem on which this genus is founded was discovered by Dr Gordon[470] in the Calciferous Sandstone series of Pettycur: a specimen collected by Dr Kidston in Berwickshire may be specifically identical with the Pettycur plant. We know nothing of the leaves or reproductive organs of Rhetinangium. The stem, approximately 2 cm. in diameter, was probably cylindrical; it possesses a single stele consisting mainly of a central primary region occupied by anastomosing groups of tracheids, 130–150μ in diameter, embedded in parenchyma containing numerous secretory sacs and ducts. In the peripheral region of the stele the groups of tracheids consist of narrower elements characterised by exarch protoxylem. Each peripheral group forms the base of a wedge of secondary xylem (fig. 450, x²), the primary medullary rays being in direct connexion with some of the parenchyma of the primary xylem. The secondary tracheids, 45–85μ in diameter, have multiseriate bordered pits on their radial walls and the rays are broad and deep as in Heterangium and Lyginopteris. With the exception of the external position of the protoxylem, the stele of Rhetinangium is practically identical with that of Heterangium, though in Rhetinangium the primary tracheids form larger and fewer groups. The inner cortex is composed of thin-walled cells with many secretory sacs: there are no stereome elements. In the outer cortex (fig. 449, cr) radially disposed bands of stereome form a reticulum with narrow and very long meshes like that of Medullosan petioles.

The recently recorded occurrence of polydesmic petioles[471] in Heterangium is of special interest from the point of view of the comparison of that genus with Rhetinangium and the Medulloseae.

The very broad decurrent petiole-bases are a striking feature, the major diameter of the pulvinus-like base of the leaf-stalk exceeding that of the stem (fig. 449, p). Several xylem-strands from the peripheral region of the primary xylem go to form a single leaf-trace: these [** symbol U]-shaped strands of xylem destined for a leaf are connected laterally by parenchyma and form an irregularly corrugated band. Fig. 450 shows a petiole-trace still enclosed on each side by the secondary xylem cylinder x².

There are several protoxylem groups in a leaf-trace, one on the outer face of each xylem-strand. In the petiole the xylem groups are more intimately connected and the trace has the form of a flat band with abaxial protoxylem. There is no indication that a leaf-trace undergoes division into separate strands. The roots are described as tetrarch with well-developed secondary xylem.

The primary xylem of Rhetinangium agrees in its exarch structure with the Palaeozoic genera Sutcliffia, Megaloxylon and Stenomyelon, also with Lygodium and some other recent Ferns: the secondary wood is of the manoxylic type like that of Lyginopteris, Heterangium and other genera. In the general structure of the stele Rhetinangium agrees with Sutcliffia and, except in the exarch structure of the primary xylem, with the steles of Heterangium and Medullosa anglica[472]; but the structure and origin of the leaf-traces are characters which mark it off from Sutcliffia. The sclerenchymatous bands in the inner cortex of Heterangium are unrepresented in Rhetinangium, and in the latter genus the abundance of secretory sacs and ducts is a characteristic feature, moreover in Rhetinangium, the leaf-trace consists of several groups of primary xylem-elements. Dr Gordon regards Megaloxylon as the type which comes nearest to Rhetinangium; but the differences in the structure of the secondary wood and the marked contrast between the leaf-traces are too pronounced to justify a preference for Megaloxylon over Heterangium in the order of affinity. Gordon considers that the undivided leaf-trace of Rhetinangium may represent a form transitional between the simple leaf-trace of Lyginopteris and the much divided type in Medullosa. The external position of the protoxylem is a character to which too much weight may easily be attached: the difference in position between the protoxylem of Rhetinangium and Heterangium is in some examples of the latter genus hardly perceptible. Kubart[473] speaks of the stele of his species Heterangium Sturi as being almost exarch. The inconstancy in the position of the protoxylem in the xylem of Osmundaceous stems and in the primary bundles of Eristophyton and other Palaeozoic genera is worthy of consideration in this connexion.

STENOMYELEAE.

Stenomyelon. Kidston.

Stenomyelon tuedianum Kidston. The specimens on which this monotypic genus is founded[474] were obtained from the Lower Carboniferous rocks (Calciferous Sandstone series) at Norham Bridge, Berwickshire, Scotland. They consist of petrified pieces of a flattened stem, a fragment of a rachis and portions of laminae: there is no evidence as to the nature of the reproductive organs. The original form of the stem is obscured by the destruction of a considerable part of the cortex and the consequent flattening of the whole with the production of wing-like extensions of the imperfectly preserved tissues enclosing the almost cylindrical stele (figs. 451, 452).

The stele consists of a bluntly triangular core of primary xylem, 3–4 mm. in diameter, composed almost entirely of reticulately pitted tracheids reaching a diameter of 160μ: a few parenchymatous cells occur in the peripheral region and a band of parenchyma extends from the middle of each of the three sides of the xylem to the centre of the stele, thus dividing the primary conducting tissue into three groups which are the expression of a phyllotaxis of ⅓. The tracheids near the outer face of each xylem-group are narrower than the others and have scalariform pitting. The secondary xylem first appears along the slightly concave sides of the primary stele, eventually enclosing the whole: it consists of tracheids with multiseriate pits on the radial walls and numerous deep medullary rays 1–6 cells broad. No phloem is preserved though it is probable that a narrow band was originally present. A characteristic feature is afforded by a zone of thick-walled cells, regarded as periderm, encircling the stele and formed by a deep-seated phellogen. On the outer face of this band there are projecting bosses, and similar sclerous nests are scattered in the cortex. The outer cortex has a Sparganum[475] type of hypoderm, that is long vertical strands of fibres alternating with parenchyma. The leaf-traces are formed from the blunt angles of the primary xylem; an angle becomes nipped off as a more or less cylindrical strand enclosed by a zone of secondary tracheids which is very narrow on the adaxial side (fig. 452). Protoxylem was recognised only in the leaf-traces and not in the rest of the stele. A pair of protoxylem strands occurs on the outer edge of a prominent angle of xylem before it becomes detached from the stele, and these form a single strand at a lower level. As a leaf-trace passes outwards, the exarch xylem strand becomes mesarch and there is a single protoxylem group except at a point near the bifurcation of a trace. In its passage through the cortex a leaf-trace divides repeatedly, the secondary xylem on the outer face of each strand being retained for a considerable time.

Our meagre knowledge of the nature of the leaves is based on incomplete fragments found in association with the stem. The leaf is believed to have been simple and characterised by a thick lamina with a hypodermal zone of sclerous strands and several vascular bundles.

As Kidston and Gwynne-Vaughan[476] remark, Stenomyelon is a very distinct type; while resembling Sutcliffia in some respects it differs from that genus not only in the structure of the primary stele but in the absence of the system of meristeles which form so characteristic a feature of the latter genus.

CYCADOXYLEAE.

Cycadoxylon. Renault.

This generic term[477] is applied to a few types of Permian and Upper Carboniferous stems possessing a vascular cylinder, which may reach a considerable breadth, of secondary centrifugally developed xylem and phloem enclosing a large pith containing either a narrow, peripherally placed, and more or less continuous cylinder of inversely orientated conducting tissue or scattered bands of centripetal xylem and phloem. The secondary xylem is manoxylic, while the internal vascular tissue recalls that of Ptychoxylon and to a less extent the inverted arcs that are rarely met with in Lyginopteris stems. A brief diagnosis of two species may serve to illustrate the genus: a third species is included in Cycadoxylon, but it is founded on material too incomplete to admit of satisfactory diagnosis.

Cycadoxylon Fremyi Renault.

This Permian species[478] is represented by a piece of stem 2–2·5 cm. in diameter (fig. 453, B) characterised by (i) a fairly broad parenchymatous cortex with secretory canals and several hypodermal nests of sclerous tissue, (ii) a cylinder of secondary xylem and phloem nearly the whole of which is centrifugal, (iii) a large pith containing several scattered narrower bands or arcs of centripetally developed xylem and phloem. The tracheids, with 4–6 series of hexagonal pits, form radially disposed rows, 1–4 elements broad, separated by broad and deep medullary rays. Renault does not mention the occurrence of any primary xylem as distinct from the secondary centrifugal xylem, but in a section which I examined some years ago in his laboratory there appeared to be a group of primary tracheids. There are no anastomoses between the main cylinder and the internal bands of inversely orientated tissue.

Cycadoxylon robustum (Seward).

This species[479] is based on a piece of stem from the Lower Coal Measures of Lancashire first described by Williamson and identified as an unusually large example of Lyginopteris. Williamson and Scott, while recognising certain features in addition to the large size of the stem, which must have reached 14 cm. in diameter, expressed the view that ‘there is a presumption that it really belonged to a Lyginodendron, or to some plant of the same type of structure.’ The examination of additional material led me to adopt the name Lyginodendron robustum, though I suggested that possibly Cycadoxylon might be the more appropriate genus. Subsequently Scott[480] proposed the substitution of Cycadoxylon for Lyginodendron.

The type-specimen consists of secondary xylem agreeing closely in structure with Lyginopteris and recent Cycads: the pith, 2·9 cm. in breadth, is incompletely preserved; there is a narrow band of centripetal xylem[481] at the periphery of the perimedullary region and close to the inner face of the main mass of wood (fig. 453, C; the black line marks the position of the centripetal xylem). Nests of sclerous tissue and secretory canals are scattered in the medullary parenchyma and deeper in this region are arcs of secondary parenchyma, possibly periderm. In places the centripetal and centrifugal xylem are in contact and occasionally the tapered ends of the rows of centrifugal tracheids merge into groups of primary xylem elements. The preservation in the central region is far from complete, and although the occurrence of primary xylem groups is probable it cannot be said to be positively established. At the inner edge of the centrifugal xylem and in tangential longitudinal sections a few leaf-traces are seen, but nothing is known as to the nature of the leaf-traces in their course beyond the stele nor have we any data with regard to the leaves or reproductive organs.

This older species differs from Cycadoxylon Fremyi in the limitation of the centripetal xylem to the outer portion of the pith and in the presence of sclerous nests in the medullary region, though the latter character is probably of no great taxonomic value. Cycadoxylon robustum approaches more closely to Lyginopteris, and although the differences are sufficient to justify a distinctive generic name, there can be little doubt as to a fairly intimate relationship between this type of Cycadoxylon and Lyginopteris.

Ptychoxylon. Renault.

Ptychoxylon[482] Levyi Renault. Like many Palaeozoic genera founded on anatomical features, Ptychoxylon[483] is represented only by stems, our knowledge of the leaves being confined to the leaf-traces in the stem which appears to have a phyllotaxis of ⅜. The stem of this Permian species has a diameter of 5–6 cm.: the comparatively broad cortex contains numerous secretory canals, but in place of hypodermal strands of stereome there is a superficial periderm. The vascular tissue, consisting of secondary xylem and phloem, assumes different patterns at different levels. There is an outer vascular cylinder of centrifugally developed xylem and phloem; the xylem is manoxylic and the tracheids have 3–5 rows of bordered pits on the radial walls. At intervals the continuity of the main stele is broken by the formation of leaf-gaps and before one gap is repaired a second may be produced, thus converting the cylinder into two crescentic and infolded bands (fig. 453, A). A striking character is the occurrence in the large parenchymatous central region of internal vascular bands or arcs varying in size and number at different levels and composed of centripetally developed secondary xylem and phloem. These internal bands differ from the outer and broader cylinder both in their inverse orientation and their limited vertical range. The connexion between the inner and outer vascular tissue and the alteration in plan of the conducting tissue at different levels are illustrated by fig. 453, 1–4, simplified from some of Renault’s figures of successive sections through a vertical distance of 4–5 cm. In section 1 the main cylinder is continuous except for a small gap where a leaf-trace is about to be given off: there are three internal vascular bands similar in structure to the outer stele but inversely orientated. At a higher level (section 2) the leaf-gap is larger and in it is a double leaf-trace of two collateral strands consisting of primary centripetal xylem and a fan-like group of secondary xylem and phloem. The free edges of the outer stele of section 1 have curved inwards and united with the two lateral medullary bands, while the lower internal band of section 1 has increased in extent and forms a discontinuous arc with the upper portions enclosed by the loops formed by the infolded ends of the outer vascular tissue. In section 3 a second leaf-gap has been formed in the outer stele and its invaginated ends have fused with the internal bands. In section 4 the first leaf-gap is closed and the invaginated bands of section 3 have broken up into an irregular circle of shorter bands. The section reproduced in fig. 453, A, shows the main cylinder in the form of two curved and flattened loops, each composed partly of the centrifugally developed xylem and phloem of the main stele and in part of the inversely orientated tissue of the inner bands. At a lower level the two bands b, b, will become detached as the upper leaf-gap is closed and form part of an inner cylinder like the discontinuous ellipse formed by the two bands c. The section of a branch-stele is seen at a.

Ptychoxylon differs considerably from Medullosa, which Renault included in the Cycadoxyleae, in the plan of the vascular system: there is nothing corresponding to the ‘partial pith’ or primary region which forms the central portion of the plate- and snake-rings in Medullosa. The double leaf-trace and the absence of the Medullosa type of hypoderm are other distinguishing features. The paired leaf-bundles suggest comparison with Lyginopteris among other genera and, as Scott[484] points out, the internal arcs of inversely orientated tissue which sometimes occur in the peripheral region of the pith of Lyginopteris (fig. 405, C, c) behave like the internal bands of Ptychoxylon in occasionally joining the main cylinder at a leaf-gap; but the differences outweigh the resemblances. As regards the general arrangement of the vascular tissue in two irregular concentric circles and their connexions with one another, but not in the structure of the xylem and phloem, there is a similarity between this genus and such a Fern as Matonia. In the varying patterns formed by the vascular system at different levels in the stem Ptychoxylon resembles the Ferns Polypodium quercifolium and P. heracleum[485].

CALAMOPITYEAE.

Calamopitys. Unger.

In 1856 Unger[486] described several fragmentary petrifactions from Thuringian strata of Upper Devonian age, the majority of which he referred to the Calamarieae and the Rhachiopterideae. In an earlier publication[487] he gave a list of species including two families, the Haplocalameae and the Calamoxyleae, assigned by him to the group Calamarieae: in the Haplocalameae he placed the new genera Kalymma, Calamosyrix, Calamopteris, and Haplocalamus. These were subsequently examined by Graf Solms-Laubach and identified as portions of petioles, for the most part belonging to unknown stems. In his second family, the Calamoxyleae, Unger included the single genus Calamopitys represented by the type-species C. Saturni. The type-specimens have been thoroughly investigated by Solms-Laubach[488] who instituted the family-name Calamopityeae and recognised a close anatomical affinity between Calamopitys and Lyginopteris, a conclusion which led to the incorporation of Unger’s genus in the Pteridosperms. Further data have been supplied by Zalessky[489] and, more recently, by Scott and Jeffrey[490] who have recognised Calamopitys in Lower Carboniferous beds in Kentucky.

Calamopitys Saturni Unger.

Our knowledge of this and other species is confined to stems and petioles. One of the largest examples of the species is a piece of stem with a diameter of 1·5 cm.: the single stele consists of a parenchymatous pith enclosed by secondary xylem made up of tracheids with 4–8 rows of bordered pits and medullary rays more than one cell broad and of considerable depth. Between the inner edge of the wood and the pith are groups of primary xylem (fig. 454, B, x) which, like those in Lyginopteris, constitute the leaf-traces: each has a single internal protoxylem strand (fig. 455, B). The comparatively wide cortex consists of parenchyma with a hypoderm of the Sparganum type. Each primary xylem-strand passes out as a single leaf-trace through the secondary xylem and on emerging divides into two as in Lyginopteris: these branch in the cortex and the two are replaced by six in the leaf-base (fig. 454, B–D). As seen in figs. 454, C, 455, A, the boundary between the stem proper and the decurrent leaf-base is marked by a line of stereome strands. The petioles of Calamopitys Saturni agree generally in structure with the imperfect specimens on which Unger founded his genus Kalymma[491], so named in reference to the structure of the hypodermal zone. A specimen described by Solms-Laubach as a Kalymma petiole occurs in organic connexion with a stem of Calamopitys (fig. 454, C: a detached petiole is shown in fig. 454, D). The identification by White[492] of this attached petiole with Unger’s K. grandis has been confirmed by Scott and Jeffrey. A fuller account of Kalymma (fig. 456) is given on a later page, as the petioles so named belong to more than one species of stem.

In Calamopitys Saturni we have a plant agreeing with Lyginopteris in the possession of secondary xylem of the manoxylic type and in the structure of the common primary bundles, while it is distinguished from Lyginopteris by the greater number and by the structure of the bundles in the axis of the leaf.

Calamopitys annularis (Unger)[493].

This species, originally assigned to the genus Stigmaria, has a more strongly developed primary vascular system and there is a more decided tendency towards the formation of a continuous zone of primary xylem on the inner edge of the secondary wood; but where the protoxylem tracheids occur the metaxylem elements form definite strands, like those of C. Saturni. It has been pointed out by Scott and Jeffrey that there is some evidence of the occurrence of tracheids in the parenchymatous pith of this species, an important feature distinguishing it from C. Saturni and connecting it with C. americana. Information with regard to the behaviour of the leaf-traces is far from complete, but there are indications that each trace divides into two before emerging from the secondary xylem[494]. The leaf-traces in the cortex are concentric as in C. Saturni.

Calamopitys americana Scott and Jeffrey.

This Lower Carboniferous species[495] from the Waverley shales of Kentucky is represented by portions of stems and leaf-bases and detached petioles. The secondary wood consists of tracheids, 30–60μ in diameter, with deep and broad rays; the small pits on the tracheids form 5–6 alternating series. Phloem and cambium are very imperfectly preserved. The outer cortex is of the same type as in other species. At the inner edge of the secondary xylem there is a ring of primary xylem strands of mesarch structure composed of rather larger tracheids, 80–120μ in diameter, separated from one another by narrow strips of parenchyma. So far the vascular tissue agrees with that of C. annularis. In the American species the axial region is not a parenchymatous pith but a protostele, consisting of parenchyma and a larger or smaller number of tracheal groups, the number being less in stems with a larger central region. The peripheral strands alone are concerned with the emission of leaf-traces, as in Heterangium. Each primary xylem strand divides into two as it leaves the perimedullary zone and passes through the secondary xylem as two bundles, each being accompanied by an arc of secondary tracheids which, in the cortical region, completely surrounds the primary elements. At a later stage the single protoxylem of each trace divides into two and before entering the leaf-base there is a further division. In some specimens leaf-bases of the Kalymma type were found attached to the stem. The occurrence of tracheids in the axial region is a distinguishing feature and suggests a comparison with Heterangium, while C. Saturni agrees more closely with Lyginopteris; the species C. annularis would appear, from the recent observations of Scott and Jeffrey, to occupy an intermediate position.

Calamopitys, as the generic designation is here employed, is confined to central Germany and Kentucky and occurs in Upper Devonian and Lower Carboniferous strata. There is, however, some doubt as to the exact geological horizon of the rocks in both countries though in neither case is there any question of an horizon higher than Lower Carboniferous. Certain specimens from the Lower Carboniferous of Scotland described by Scott[496] as species of Calamopitys have been made by Zalessky the type of a new genus, Eristophyton, and are dealt with under that name.

Kalymma. Unger.

Kalymma grandis (petiole of Calamopitys). Under the generic name Kalymma Unger described specimens from Thuringia of Upper Devonian age which he assigned to two species, K. grandis and K. striatum. Solms-Laubach has shown that Kalymma is not an independent stem as Unger believed but a petiole of Calamopitys, and this has been confirmed by Scott and Jeffrey who found a leaf-base with the Kalymma type of structure in connexion with a piece of Calamopitys stem, probably C. americana. An examination of a section (2·3 cm. broad) of Unger’s K. grandis in the collection of the Geological Survey enables me to confirm the conclusions recently published by Scott and Jeffrey. The best specimens of Kalymma, which appear to be identical in essential features with Unger’s type-species, are from Kentucky, some from the Genessee shales of Upper Devonian age and others from beds (Waverley shales) believed to be Lower Carboniferous. Through the kindness of Prof. Bower I have had an opportunity of examining sections from the older horizon in his possession. The transverse section reproduced in fig. 456 has a diameter of 3·8 × 2·2 cm.[497]: on one side the radially placed plates of stereome are clearly shown, and in the outer portion of the ground-tissue is a ring of vascular bundles varying in size and shape but with a general tendency to a radially elongated form. The ground-tissue consists of homogeneous parenchyma: in one place I noticed what appeared to be a large secretory canal, but secretory tissue, generally at least, is unrepresented. The xylem is composed of imperfectly preserved elements, which appear to have scalariform pits; spiral protoxylem strands, embedded in the metaxylem as two or four groups, occur near the ends of the long axis of the bundle and in some cases also near the centre. The phloem probably surrounded the xylem, though it is not certain whether the arrangement was collateral or concentric: there are no secondary-xylem tracheids, though in some places I noticed a tendency to a radial disposition of cells at the periphery of the vascular tissue simulating an early stage of secondary growth. Unger’s second species Kalymma striata is characterised by an arrangement of the bundles similar to that in a petiole described by Scott and Jeffrey as Calamopteris Hippocrepis which differs from Kalymma in the partial substitution of bands of vascular tissue for separate bundles and to some extent in the disposition of the bundles. The two types of petiole Kalymma and Calamopteris, as Scott and Jeffrey state, are very closely allied. Dawson and Penhallow[498] have also described Kalymma grandis from Kentucky but they, like Unger, mistook the hypodermal stereome for an outer zone of vascular bundles. The petioles from Germany and North America included under the name Kalymma grandis, though too similar to be referred to different species, no doubt represent petioles of stems which are unquestionably distinct types: as in the case of Myeloxylon in its relation to the genus Medullosa, Kalymma stands for several closely allied forms of petioles belonging to several species of Calamopitys.