Fossil plants, Vol. 3

Eristophyton. Zalessky.

Eristophyton fasciculare (Scott). The generic name Eristophyton[499] was proposed by Zalessky[500] for two incomplete stems of Lower Carboniferous age provisionally[501] referred by Scott to Araucarioxylon and subsequently to Calamopitys[502]. The species E. fasciculare was founded on material obtained by Dr Kidston from the Lower Carboniferous of Dumbarton, Scotland, and on a specimen in the Williamson collection from Northumberland. There is a small pith of parenchyma, 2–3 mm. in diameter, with eight strands of primary xylem of varying diameter (fig. 457) and in each a single protoxylem-group. The primary xylem elements are considerably larger than the secondary tracheids. These xylem-bundles are leaf-traces and their disposition points to a phyllotaxis of ⅖. The traces attain their maximum size when about to pass out through the secondary xylem. The tracheids are reticulate and scalariform while some have an intermediate type of pitting. A leaf-trace on reaching the pith gradually moves further from the xylem-cylinder and may be separated from it by 2–6 layers of parenchyma: as it passes down the pith the protoxylem strand assumes an almost endarch position consequent on the reduction of the centripetal xylem. In Scott’s words, ‘each circummedullary strand branches at regular intervals; the one branch, that on the anodic side [turned towards the course of the genetic spiral], becomes the leaf-trace and passes out, while the other continues its course up the stem as a reparatory strand, until the next leaf of the orthostichy has to be supplied[503].’ The secondary xylem consists almost entirely of tracheids with 3–4 rows of pits on the radial walls and medullary rays usually one cell broad varying in depth from 1 or 2 to 16 or more cells. A characteristic feature of the secondary xylem is the occurrence on its inner face of numerous short and broad tracheae similar to the still larger tracheae in the primary stele of Megaloxylon[504]. Nothing is known as to the behaviour of the leaf-trace in the extrastelar region, but the fact that an outgoing trace was found to have two protoxylems points to a subdivision similar to that of the foliar bundles of Calamopitys Saturni. A well-marked difference between Eristophyton fasciculare and Calamopitys and Lyginopteris is the more compact structure of the secondary wood; it is pycnoxylic and not manoxylic. Prof. Zalessky in criticising the use of the generic title Calamopitys puts forward several arguments in support of his institution of a new designation: (i) the primary xylem strands of Eristophyton are not confined to the periphery of the pith as is the case in Calamopitys Saturni, though he speaks of one leaf-trace in the latter species separated by several layers of cells from the xylem-cylinder; (ii) some of the pith-cells have thick walls and dark contents in distinction to the homogeneous parenchyma of Calamopitys, a feature of little importance; (iii) the difference in the structure of the secondary wood already alluded to, though this loses some of its significance by the occurrence of narrower rays, more like those of Eristophyton, in C. annularis; (iv) the more elliptical and broader pits in the secondary tracheids in place of the more regular hexagonal form in Calamopitys. While admitting a certain degree of relationship between the two types, Zalessky asserts that as yet we have insufficient evidence to justify their generic union. Scott[505] maintains that Zalessky does not attach sufficient weight to the form and mesarch structure of the primary xylem bundles as a feature common to both genera.

The pith, 13–15 mm. in diameter, is rather larger than in E. fasciculare and is characterised by the occurrence of dark sclerotic nests surrounded by radially disposed rows of parenchyma. The primary xylem strands are more numerous and smaller than in E. fasciculare and these increase in diameter as they approach the secondary wood. In places the primary xylem elements form a more or less continuous band as in Calamopitys annularis. The largest leaf-trace bundles at the periphery of the pith are mesarch (fig. 458), but as each trace passes down the pith the reduction in the centripetal xylem is carried further than in E. fasciculare until the xylem-strand becomes endarch in the lower part of its course. The secondary tracheids have usually two contiguous rows of pits and the medullary rays are one cell broad.

There can be no doubt as to the generic identity of the two species referred to Eristophyton, but the question as to the degree of affinity to Calamopitys is more difficult to settle. There is force in Zalessky’s contention that these two stems should not be retained in Calamopitys: the recently described American species, C. americana Scott and Jeffrey, gives emphasis to the view that the restriction of Calamopitys to the German (and American) types is the safer course. While Calamopitys as thus restricted is almost certainly a Pteridosperm, the inclusion of the types referred to Eristophyton in the same category rests on a more slender basis.

CLADOXYLEAE.

This order was founded by Unger[507] for some imperfectly preserved stems from Palaeozoic strata in Thuringia and in it he included the two genera Cladoxylon and Schizoxylon. There is some doubt as to the precise age of the Thuringian beds; they were assigned by Richter to the Devonian system and subsequently placed in the Culm: Solms-Laubach in his later reference to Unger’s plants favours a Devonian horizon[508]. Unger included the Cladoxyleae in the Lycopodiales, and though this conclusion is not accepted the position of the order is still uncertain. His genus Schizoxylon has no claim to generic separation from Cladoxylon. An inspection of the illustrations in the memoir by Richter and Unger reveals a striking resemblance in the main anatomical features between several types assigned to different genera and distributed among the Cladoxyleae and Rhachiopterideae (a term first used by Corda for petrified rachises or petioles of ferns) and other orders. Solms-Laubach[509], to whom our more accurate information as to Unger’s plants is chiefly due, is inclined to regard the specimens referred by Unger to the genus Arctopodium as young stems of Cladoxylon, and he draws attention to a close similarity between Hierogramma, another of Unger’s genera, and Cladoxylon. Paul Bertrand[510] goes further in considering that the following genera represent one generic type, namely Syncardia (fig. 459, F), Hierogramma, Arctopodium, Cladoxylon, and Schizoxylon. The same author interprets the fossils so named by Unger as stems and does not agree with the inclusion of any of them in the Rhachiopterideae. Without losing sight of the fact that Bertrand’s conclusion is not based on proof but is the expression of a view suggested by a close agreement in general anatomical plan, I venture to adopt the designation Cladoxylon in a wide sense primarily on the ground that Bertrand’s view is probably correct and in part for the sake of convenience of description. As Unger’s species of Cladoxylon differ from one another in features which may fairly be regarded as of minor importance, they are included under one specific name.

Cladoxylon. Unger.

Cladoxylon mirabile Unger[511]. The following are regarded as specifically identical with or closely allied to Cladoxylon mirabile: C. dubium, Schizoxylon taeniatum, Hierogramma mysticum, Syncardia pusilla, Arctopodium insigne and A. radiatum[512].

i. Stems. The stems assigned to Cladoxylon are characterised by a complex system of steles, either simple or branched and occasionally anastomosing, presenting in transverse section the form of oval or cylindrical strands or narrow, straight or curved bands arranged on a more or less clearly marked radial plan (fig. 459, A, B, D). In some stems the primary vascular tissue is enclosed by secondary xylem and phloem (fig. 460, B), while in others (Unger’s Arctopodium, Hierogramma[513], Syncardia) there is no evidence of secondary thickening. The diagrammatic drawing represented in fig. 459, F, shows a section of a small axis, regarded by Unger and Solms as a petiole (3 mm. in diameter), containing four vascular strands composed exclusively of primary xylem, each with one or, in the case of a double strand, two protoxylem groups. This type may be a slender stem or branch or possibly a petiole. The other extreme, as regards complexity of vascular structure, is represented by such stems as those shown in fig. 459, A, B, D. In Unger’s Cladoxylon mirabile (fig. 459, A; fig. 460, B) the stem reaches a diameter of 3 cm. and consists of several radially disposed plates of vascular tissue with an occasional smaller oval or cylindrical stele embedded in a ground-tissue composed of thick-walled cells. The plates are curved like a U or sinuous and not infrequently anastomosing. In a section of this type figured by Unger the vascular plates appear to form a complex anastomosing system, but Solms[514] states that the drawing exaggerates the amount of fusion between the strands, and an examination of a section in the collection of the English Geological Survey cut from Unger’s specimen enables me to confirm this statement. Each vascular plate consists of a narrow median region composed of primary tracheids with a scalariform type of pitting surrounded by secondary tracheids with interspersed medullary rays one cell broad. The thickness of the secondary xylem varies considerably in the same specimen and in places this tissue is hardly represented, a fact of importance in view of the very striking resemblance between Arctopodium and Cladoxylon, the sections referred by Unger to the former genus having steles without any secondary xylem. The occurrence of one or two elongated spaces (shown in black in fig. 459, A) near the distal end of each plate mark the position of the protoxylem tracheids. Fig. 459, C, represents a stele of a stem referred by Unger and by Solms to Cladoxylon dubium which shows the typical Cladoxylon structure, namely the central primary xylem with distally placed protoxylem and the enclosing sheath of secondary xylem. In the stem shown in fig. 459, D (C. dubium) there are 12 steles, each constructed on the plan already described, differing in their relatively broader and shorter form and in the greater breadth of the secondary xylem from those seen in fig. 459, A (C. mirabile). The black areas in fig. 459, A, show the primary xylem, and the protoxylem is seen in fig. 459, C. A stem described by Dawson[515] as Asteropteris noveboracensis from Devonian beds is compared by him with Unger’s Cladoxylon mirabile and regarded as possibly allied to it. The radial plates of xylem in Dawson’s plant meet in the centre like those of Asterochlaena and the leaf-traces are of the Clepsydropsoid type.

The type of stem for which Unger founded his genus Schizoxylon is represented in fig. 459, B; there are five small steles in the centre and external to these eleven radially arranged plates, with oval steles between them, in the peripheral region of the stem. Each stele consists of primary (black in the figure) and secondary xylem and agrees with the steles in the other stems.

From the type of stem illustrated by Cladoxylon mirabile to that on which the genera Arctopodium[516] and Hierogramma[517] were founded is a very small step: the vascular tissue has the same characters both as regards gross and minute anatomy, but there is no evidence of cambial activity in the stems referred to the two latter genera, a difference in itself hardly worthy of generic recognition.

ii. Leaves. Before describing a second type of stem referred to Cladoxylon it is important to consider briefly such evidence as we have as to the vascular supply of the leaves. Nothing is known of the reproductive organs and there is no satisfactory information with regard to the form of the fronds. Solms-Laubach has described the only known example of a lateral branch of a Cladoxylon stem (fig. 459, E): this has a single concentric vascular strand of plate-like form with two blunt projections and there are four protoxylem-groups, two in the angle of the plate and two at the base of the projections. The structure is essentially fern-like; the xylem is wholly primary. This type of vascular strand agrees fairly closely with that of a petiole described by Unger as Megalorhachis elliptica, a section of which is in the Museum of the Geological Survey[518]. The petiole is oval in section and laterally winged, and the meristele is tangentially elongated and has two blunt projections almost identical with those in fig. 459, F. There is no evidence as to the nature of the supporting stem, but there can be little doubt as to the close connexion with Megalorhachis and the section shown in fig. 459, F. In a note published in 1908 P. Bertrand stated that he had identified several of Unger’s genera as stems which bore leaf-traces having the form and structure of Clepsydropsis, one of the types referred by Unger to the Rhachiopterideae and described in the second volume of this work[519] as a Coenopteridean petiole. Bertrand points out that in the oval or plate-like steles of Cladoxylon, Arctopodium, Hierogramma, etc., there is a single protoxylem group near the distal end of the primary xylem, and he adds that the leaf-traces were formed of strands cut off from the distal portions of the vascular plates. Similarly the hour-glass-like leaf-trace in the primary rachis of Clepsydropsis gives off from each end a ring of xylem to supply a secondary rachis. These laterally detached annular strands are, he believes, similar to the leaf-trace cut off from the steles in a Cladoxylon stem. The conclusion is that Cladoxylon is a fern stem and its leaf-trace represents the simplest form of the Clepsydropsis type, namely an oval bundle of xylem with a central protoxylem, which is also the form of the trace given off from the stem of Asterochlaena. Solms[520], while admitting that Bertrand may be correct in uniting under one genus Cladoxylon and such types as Syncardia, Hierogramma, and Arctopodium, disagrees with the view that they are Clepsydropsis stems. A Clepsydropsoid leaf-trace has never been found in direct association with any of the stems of the Cladoxylon type and such evidence as there is indicates a leaf-trace of an entirely different form (fig. 459, E). In his more recent memoir on Asterochlaena Bertrand[521] draws attention to Solms’ figures of a stele of Cladoxylon (fig. 459, C) in which the distal portion is on the point of being separated as a small annular strand. This, Bertrand considers, would gradually become converted into a Clepsydropsis form of stele as it passed to the petiole. Bertrand’s drawings made from a section of Cladoxylon taeniatum (Ung.) (fig. 459, G) illustrate successive stages in the departure of a leaf-trace from one of the plate-like steles of the stem (fig. 459, B). In fig. 459, G, 1, a piece of the stele is detached and near its extremity is a group of thin-walled cells with protoxylem: a later stage is seen in fig. 2, and in fig. 3 a small ring of xylem is being detached which, Bertrand assumes, would later in its course be converted into the Clepsydropsoid strand (fig. 4), which consists of primary tissue. The weak point in Bertrand’s contention[522] is the absence of any proof of a true Clepsydropsoid trace in connexion with a Cladoxylon stem, and there is a strong probability that the leaf-trace of Cladoxylon has the form shown in fig. 459, E.

Cladoxylon Kidstoni Solms-Laubach[523].

This species, founded on imperfectly preserved material in Dr Kidston’s collection from Lower Carboniferous rocks in Berwickshire, is referred to Cladoxylon on evidence that cannot be regarded as convincing. The type-specimen consists of a small piece of stem about 3 cm. in breadth showing three complete oval steles and portions of two others which seem to be in their original position and probably formed part of a series of peripheral steles such as those shown in fig. 459, D. Each stele consists mainly of secondary xylem (fig. 460, A) with some crushed tissue, presumably phloem, on its outer face. The secondary xylem is narrower on the inner side of each stele where a wedge-shaped piece is partially detached. In the centre there is a narrow area parallel to the long axis of the stele containing crushed tissue which probably consists of parenchyma and primary xylem, but the preservation is very imperfect. The secondary xylem has a fairly compact structure and the rays are narrow, 1–10 cells in depth. The pits of the tracheids are described by Solms as scalariform with occasionally two rows of elliptical pits on the radial walls. A careful examination of the type-specimen leads me to describe the pits as uniseriate and transversely elongated, very like those of Protopitys, or biseriate and almost circular like those of Conifers, the pits of the two rows being alternate or sometimes opposite (fig. 460, C): in places three rows of bordered pits are present. There is a certain degree of resemblance between the steles of this species and those of the South African stem Rhexoxylon[524], but the data are inadequate for a satisfactory comparison.

There is a close similarity between the vascular systems of Cladoxylon and Medullosa, but an obvious difference is the substitution of the oval, transversely elongated, pits on the xylem elements for the multiseriate pitting of Medullosa. In Cladoxylon Kidstoni the pitting shows transitional forms between a narrow scalariform uniseriate type and a biseriate or triseriate arrangement similar to that in the Araucarineae and Cordaitales. In Cladoxylon, as limited by Unger, the presence of secondary wood is a generic feature, but by the inclusion of Arctopodium and other forms this character no longer holds good. The inclusion of these more fern-like stems without secondary xylem brings Cladoxylon (in the wider sense) into closer contact with Asterochlaena, a comparison previously suggested by more than one author. In Medullosa the development of secondary xylem is on a larger scale than in Cladoxylon, and the vascular system of the former genus assumes a more complex form. Moreover the Myeloxylon type of petiole, which is a distinctive feature of Medullosa, differs widely from any form of leaf-trace associated with Cladoxylon.

Völkelia. Solms-Laubach.

Völkelia refracta (Goeppert). The generic name Völkelia[525] was proposed by Solms-Laubach[526] as a substitute for Sphenopteris[527] in the case of some petrified stems or petioles associated with fragmentary impressions of fronds from Lower Carboniferous rocks in Silesia. Both leaf-impressions and petrifactions were included in the genus Sphenopteris: Solms, while retaining Goeppert’s designation for the leaf fragments, proposed a new generic name for the petrifactions on the ground that there is insufficient evidence of their connexion with the leaves. The short account of Goeppert’s petrified specimen given by Graf Solms-Laubach in his ‘Fossil Botany[528]’ is supplemented by a fuller description in a later paper. The fragments of highly compound fronds are characterised by very small filiform ultimate segments, but the specimens are too imperfect to afford a clear idea of the habit of the leaf. The ‘stem’ bears a close superficial resemblance to that described by Unger as Cladoxylon dubium (fig. 459, C, D) and was regarded by him as an example of that species: it contains several radially placed steles represented by fairly well-preserved xylem, but no phloem has been recognised. The steles vary in size and shape: five reach almost to the centre (fig. 461, A) and smaller xylem groups occupy a peripheral position. Each stele is excentric in structure and consists of (i) an outer zone of secondary tracheids of horse-shoe form in transverse section, but the apparent gap in the secondary xylem on the outer edge of each stele is due to the crushing of the tracheal tissue and to its smaller breadth in the distal part of each group; this is shown in fig. 461, B, where the apparent gap is seen to be occupied by distorted and crushed tracheids, a, identical with those which form the rest of the outer zone (fig. 461, B, b); (ii) a zone of tracheal tissue continuous with and originally identical in appearance—except that the elements are rather narrower—with the outer secondary xylem; (iii) an excentrically situated island composed of tracheids enclosing a small central area occupied by thin-walled parenchyma. This third region, represented by black patches in fig. 461, A, in all probability represents the primary part of each stele to which the rest of the tissue has been added by the cambium. A striking feature of the secondary xylem is the absence of medullary rays: the tracheids resemble those of Cladoxylon and Protopitys in the transverse elongation of the pits (fig. 461, D) which form either a single row or several irregularly distributed rows. The primary xylem consists in the peripheral region of tracheids with very narrow scalariform pitting which at first sight suggest close spiral bands (fig. 461, C), while the inner tracheids are either annular or reticulate and associated with elongated parenchyma. The imperfectly preserved ground-tissue appears to consist of homogeneous parenchyma with radially disposed bands of stereome in the outer cortex.

Our knowledge of Völkelia, though far from complete, justifies its generic separation from Cladoxylon from which it differs in the lack of medullary rays and in the structure of the primary portion of each stele. In the form and arrangement of the pits in the secondary tracheids Völkelia differs from Medullosa and resembles Cladoxylon. The opinion expressed by P. Bertrand[529] that Völkelia is probably the stem of one of the Zygopterideae is based on the older accounts of the genus and not on the fuller description of 1910.

PROTOPITYEAE.

Protopitys. Goeppert.

The only species so far described is that for which Goeppert founded the genus in 1850, substituting Protopitys[530] for the name Araucarites, adopted in an earlier paper, on the ground that the structure of the xylem denoted a distinct generic type. The type-species is from the Upper Devonian rocks of Falkenberg in Silesia.

In his Monograph of Fossil Conifers Goeppert figured a large piece of stem consisting mainly of secondary wood and described the more important anatomical features. He recognised the narrow, transversely elongated, pits on the radial walls of the tracheids as a feature of special interest indicating a type of pitting transitional between that of Ferns and Conifers. A further description was given by Kraus[531] who included under Goeppert’s name both the Falkenberg stem and a second specimen from Basel though the latter is Triassic in age and a distinct plant; he suggested a comparison of Protopitys with Sigillaria and Stigmaria rather than with Conifers. It is, however, to Graf Solms-Laubach[532] that we owe the most thorough account of this species. Nothing is known of the leaves or reproductive organs. The largest piece of stem is nearly 1 ft in diameter and consists mainly of secondary xylem resembling that of Conifers and Cordaites except in the form of the bordered pits on the radial walls of the tracheids (fig. 462, D). The centre of the stem is occupied by a parenchymatous pith, elliptical in transverse section, enclosed by a band of primary xylem composed of large polygonal tracheids (fig. 462, C, x¹) characterised by a delicate scalariform pitting on all their walls (fig. 462, E). The primary xylem forms a narrow layer on the sides of the ellipse, 1–3 elements broad (fig. 462, A, B), but it increases in breadth at the ends of the long axis where the tracheids are intermixed with parenchyma. The primary xylem and pith-tissue at the ends of the major axis of the central region assume different forms at different levels, owing to the detachment of leaf-traces and the consequent formation of foliar gaps as portions of the primary xylem pass obliquely outwards into the secondary xylem on the way to the distichously arranged alternate leaves. The diagram, fig. 462, A, shows the inner part of the secondary xylem (see also fig. 462, C, x²) which at one end, lt, has formed an oval group about to pass out as a leaf-trace: at the opposite end the strand is detached and divided into two equal branches. The two swellings of the primary xylem ellipse shown at a in figs. 462, A and B, are a characteristic feature: these are clearly seen after the leaf-trace has become detached; at the inner edge of each of them there appears to be a protoxylem strand. After the formation of a foliar gap these swellings of the xylem gradually meet and so re-establish continuity below the outgoing leaf-trace. No protoxylem has been detected in the actual trace, which is believed to be concentric. The formation of the leaf-gap and the shoulders bordering it constitute interesting filicinean features, recalling corresponding characters in solenostelic Ferns. At the upper end of the diagram, fig. 462, B, the outgoing leaf-trace is undergoing dichotomy while at the opposite end the trace has passed out of view. The secondary xylem shows incomplete rings or arcs of narrower elements, which at first sight give the impression of annual rings: the occurrence of similar incomplete or pseudo-rings is a common feature in Lepidodendron and other Palaeozoic stems. The secondary tracheids (54·4μ in tangential diameter, 68·5μ in radial diameter) have usually a single series of broadly oval bordered pits on the radial walls with here and there two rows (fig. 462, D). In one case only were the pits of the medullary rays recognised (fig. 462, F). The rays are uniseriate, generally 1–2 cells deep, but occasionally 3 cells in depth and very rarely deeper. The cambium is of the normal type, and in some specimens secondary phloem was found consisting of bands, 4–5 layers broad, of stone-cells alternating with tubular thin-walled elements, presumably sieve-tubes.

As Solms-Laubach says, it is highly probable that each leaf-trace, which forks close to its exit from the primary xylem, became further subdivided before reaching the leaf. Morphologically, Protopitys is of special importance as a type possessing characters that indicate a connexion with Conifers or Cordaitean genera, notably the structure of the secondary wood, while the presence of foliar gaps is a feature reminiscent of Ferns. The primary xylem resembles that of some of the Palaeozoic arborescent Lycopodiales, but in Protopitys the interruptions in this tissue are due to the emission of leaf-bundles, whereas in the discontinuous primary xylem of some Sigillarias[533] the gaps have no connexion with leaf-traces. Moreover the distichous leaves of Protopitys and the larger, branched, leaf-traces are other distinguishing features. The pitting of the primary xylem is like that in the Lycopodiales and Filicales, while that of the secondary wood shows a closer approach to the coniferous type. A comparison may also be made with the transversely elongated pits of Cladoxylon Kidstoni[534].

A piece of wood agreeing anatomically with the Silesian species of Protopitys has been found in the Yoredale rocks of England[535].

The peculiarities of the genus have been emphasised by Solms-Laubach by the institution of a family-name Protopityeae: the genus is essentially a generalised type exhibiting in the structure of its stem both Filicean and Coniferous features. The bordered pits differ from those in recent Conifers in their flatter form, but in this respect they exhibit a closer agreement with the transversely stretched pits of Xenoxylon phyllocladoides Goth.[536], a Mesozoic species.