CHAPTER XXX.
II. MEDULLOSEAE.
The term Medulloseae was first employed by Goeppert and Stenzel[236] for a family of Palaeozoic plants that appears to have reached its maximum development in the Permian period: the oldest representatives so far discovered are of Upper Carboniferous age. Our knowledge of the family is chiefly derived from a study of the anatomical characters of stems, and it is therefore on this basis that any grouping of genera or species should be attempted. Although there is little information with regard to the reproductive organs of Medullosa, the type-genus, it is certain that the Medulloseae are Pteridosperms differing from members of that group included in the Lyginopterideae in the presence of more than one stele in the stem, in the habit of the fronds, and in the structure of the rachis, as also in the structure of the seeds, though these organs bear a fairly close resemblance to the seeds of Lyginopteris and Heterangium. The fronds of the Lyginopterideae are of the Sphenopteris type while in the case of such species of Medullosa as afford evidence of connexion between stems and leaves the latter have the characters of Neuropteris, Alethopteris, Odontopteris, Linopteris, and other form-genera usually included in the Neuropterideae. Dr Lotsy[237] speaks of Lyginopteris and Heterangium as members of the Sphenopteridophylla and assigns species of Medullosa either to the Neuropteridophylla or to the Pecopteridophylla, the latter subdivision including species with fronds of the Alethopteris type. There is, however, little doubt that other forms of leaves, such as Odontopteris and possibly Taeniopteris, were borne on Medullosan stems. It is undesirable except in the absence of more trustworthy criteria to make use of so protean a feature as leaf-form as a basis of classification. The name Neuropterideae has been frequently employed for Pteridosperms other than the Lyginopterideae on the ground that the foliage of Medullosa is represented by species assigned to form-genera included in the Neuropterideae. It is, however, preferable to restrict the family-name Neuropterideae to fronds and to speak of the second family of Pteridosperms as the Medulloseae, including the genera Medullosa, Sutcliffia, and Rhexoxylon.
MEDULLOSA.
Some species of Medullosa probably resembled in habit Angiopteris evecta and the larger Marattias; they had short and relatively thick stems clothed with the large decurrent bases of long compound fronds superficially like those of some recent Ferns and the leaves of the Cycad Bowenia. It is probable that, as Zeiller[238] has pointed out, the fronds of Medullosa and of other Pteridosperms had a greater tendency than those of true Ferns to a dichotomy of the rachis. In other types the stems reached a considerable length and leaves and branches were separated by several feet of bare stem. The large size of the leaf-stalks in proportion to the diameter of the stem as shown by such species as Medullosa anglica and M. Leuckarti (fig. 416) suggests either a short and thick main axis or, in the case of long stems bearing scattered leaves, a plant that supported itself partially at least by a habit of growth comparable with that of tropical Aroids or other lianes. While Medullosa anglica with its contiguous leaf-bases affords an example of the first type, the occurrence of stems of a Permian species, M. stellata, 3½ metres long without branches or leaf-scars, suggests the habit of a liane; similarly a specimen of Medullosa Leuckarti in the Chemnitz Museum bearing a few spreading petioles but little narrower than the stem and given off at a wide angle would seem to favour the view that some species were ill adapted to be mechanically self-supporting plants. The longest piece of stem that has come under my notice is a specimen of M. stellata in the Chemnitz Museum reaching a length of nearly 8 metres: some species attained a diameter of about 50 centimetres.
Medullosa is always polystelic: the plan of the vascular system varies considerably as regards both the number and form of the steles, but there is a uniform type of structure within the limits of each stele that recalls the single stele of Heterangium. The steles consist of a central region composed of primary xylem, originally surrounded by phloem, which in its mesarch or exarch structure agrees with the vascular tissue of some species of Gleichenia or Lygodium. To this central region a cambium added secondary xylem and phloem either in the form of a cylinder of uniform breadth, or more frequently the centrifugally developed xylem exceeded in amount the secondary conducting tissue added to the inner side of the primary region. Apart from anatomical details a Medullosan stem with its several steles, each with secondary tissue, embedded in parenchymatous ground-tissue resembles the stems of some Dicotyledonous climbers such as Thinouia scandens, species of Serjania and Paullinia[239].
Anatomically the main features of the stelar system of Medullosa, neglecting the secondary xylem and phloem, are in closer agreement with the stems of Ferns than with those of any other plants. It has been shown that the genus Heterangium bears a close resemblance to Gleichenia in the structure of the primary stele (fig. 418, C): one of the oldest types of Medullosa, M. anglica, may be described as a Heterangium with three steles and may be compared with a dictyostelic Fern in which the irregular vascular framework is made up of three main strands. In certain types of Medullosa (fig. 416) the ground-plan of the vascular system recalls that of a solenostelic Fern, while in others the greater complexity suggests comparison with such Ferns as Matonia, Angiopteris, Psaronius, or Cyathea; ‘it is as though Nature were at the Carboniferous moment in the midst of a series of amazing engineering experiments, most of which were either buried deep in Palaeozoic oblivion, or permitted to survive only as vestigial relics and atavistic ghosts’[240]. Though many Medullosae resemble Ferns there is an important difference between the two groups in the origin of the various plans of Medullosan stelar systems: in Ferns the leaf is the determining factor in the evolution of stelar arrangement, while in Medullosa the occasional interruption of a solenostele or the development of an apparently complex dictyostele are features independent of the leaf and leaf-traces. In the structure of the secondary xylem and phloem and in root-structure Medullosa agrees with recent Cycads. The genus is in short a generalised type with filicinean and cycadean affinities. In the possession of seeds borne on modified pinnae of compound fronds, Medullosa resembles both Cycas and the Lyginopterideae. The seeds exhibit a fairly close agreement with those of Lyginopteris, Heterangium and recent Cycads, but they appear to have advanced further towards the cycadean type than is the case with the closely related seeds of the Lyginopterideae. The microsporophylls are very imperfectly known but they were undoubtedly much less advanced and more fern-like than the megasporophylls.
The genus Medullosa is recorded from the Permian strata of Saxony, France, and Bohemia[241]; also from the Coal Measures of England, and the discovery of petrified petioles of Myeloxylon, the type borne on Medullosan stems in European species, may be taken as evidence of the existence of the genus in North America during the Carboniferous period[242].
The name Medullosa was applied by Cotta[243] to three types, Medullosa elegans, M. stellata, and M. porosa, from the Rothliegende of the Chemnitz district. The first of these was recognised by Brongniart[244] as a distinct genus for which he proposed the designation Myeloxylon and this was afterwards identified by Renault, Williamson, and other palaeobotanists as a petiole and not a stem. Further reference is made to Myeloxylon on a later page. Cotta spoke of Medullosa as the most puzzling of the genera dealt with in his ‘Dendrolithen,’ and in spite of the many additions to our knowledge the position of this Palaeozoic genus is still a fertile source of speculation. The generic designation Medullosa is applied to stems, with or without petioles; petioles or rachises of fronds that frequently occur apart from stems are referred to the genus Myeloxylon. The leaves of Medullosa include several well-known species of Carboniferous and Permian genera such as Alethopteris, Neuropteris and others that have in recent years been transferred from the Filicales to the Pteridosperms. In a few instances seeds have been found in organic connexion with Medullosan foliage, and there can be no reasonable doubt that Trigonocarpus, some forms of Rhabdocarpus, Pachytesta, and other seeds represent the integumented megasporangia of Medullosa or some closely allied genus.
Before attempting to summarise the salient features of Medullosa a description of a few selected types will serve to place us in a better position to consider the genus as a whole. The British species are placed first on the ground that they are both geologically the oldest though, historically, the most recently described, representatives of the genus; and in the organisation of the stem they are simpler than the continental species. Their resemblance to Heterangium serves to some extent to bridge the gap between the majority of species of Medullosa and the simpler types of Pteridosperms represented by Heterangium and Lyginopteris.
Medullosa anglica Scott[245].
Prior to the discovery of this species the genus Medullosa had not been recorded from Britain. A section in the Williamson collection recognised by Scott as that of a Medullosa had been identified by Williamson as a large Heterangium stem. An undescribed specimen was found by Arber[246] in the Binney collection at Cambridge which afforded some additional information as to the structure of the roots.
The specimens on which Scott’s thorough description is based were obtained by Messrs Wilde and Lomax from the Lower Coal Measures of Lancashire. The stem of this oldest species has the habit of a tree-fern and is almost completely invested by the stout decurrent bases of the petioles of large spirally disposed compound fronds with a phyllotaxis of ⅖, the leaves of the same orthostichy being separated from one another by a vertical distance of approximately 10 cm.
A transverse section of a slightly flattened stem is shown in fig. 416, A, the bases of three petioles give to it an angular form. Its dimensions are approximately 10 × 4 cm. The ground-tissue of two of the petioles is continuous with that of the stem, while that of the third leaf-stalk is cut through near its separation from the stem and its adaxial face is already defined by a hypodermal band of stereome, d. The surface of the stem is characterised by fine longitudinal ribs caused by the slightly projecting stereome in the outer cortex, and from the narrow furrows between the leaf-bases adventitious roots emerge in vertical series. The position of an interfoliar furrow is shown by a small arrow in fig. 416, A. There are three steles, 2–3 cm. × 6–10 mm. in diameter: each agrees very closely in structure with the single stele of Heterangium. Medullosa anglica may be described as a polystelic Heterangium and as having the same relation to Heterangium as regards the stelar system as Primula auricula bears to the monostelic Primula. The central core of the stele (the black patches in the diagram, fig. 416, A) consists of an anastomosing system of tracheal groups embedded in an irregular parenchymatous reticulum. The large primary tracheids reach a diameter of 150μ and have multiseriate pitting: at the periphery of the primary xylem there is a more definite grouping of tracheids as in Heterangium, and the slightly internal (mesarch) protoxylem elements are associated with scalariform and densely spiral tracheids (fig. 416, B, C) narrower than the more internal reticulate elements. The secondary xylem is manoxylic as in Cycads, tracheids in 2–4 radial series alternating with medullary rays 1–3 cells broad and usually of considerable depth (fig. 416, B). The principal rays are continuous with the parenchymatous matrix of the central core. Thick-walled tubular elements, no doubt of the nature of sieve-tubes, form a conspicuous feature in the phloem.
The three steles occasionally divide and fuse with one another. The tissue between the steles is crushed and disorganised and in the living plant was probably small in amount. In the imperfectly preserved inner cortical region there is a sinuous band of secondary parenchyma (periderm; fig. 416, A, c) developed from a deep-seated phellogen; in older stems this formed the superficial tissue after the fall of the leaves. There is no definite boundary between the cortex of the stem and the petiole-bases except when the hypoderm cuts across the cortex preparatory to the separation of a leaf-stalk. The stem-cortex and the ground-tissue of the petioles consist of parenchyma with numerous secretory canals, not sacs only as in Heterangium, and are abundantly supplied with scattered vascular bundles of collateral and exarch structure.
The leaf-traces are furnished by the peripheral tracheal groups at the free surface of the primary portion of each stele: each trace is at first concentric and consists of primary xylem with one or more protoxylem strands near the outer surface and is completely or partially enclosed by secondary xylem and phloem. In the course of its passage to the leaf a leaf-trace loses its secondary tissues, which were added by the cambium during the traverse of the zone of secondary wood, and divides into small collateral bundles consisting mainly of spiral and scalariform tracheids. The collateral bundles accompanied by some narrow fibres are of the Myeloxylon type (fig. 420), the xylem being wholly centripetal. In the behaviour of the leaf-traces and in the vascular system of the petioles Medullosa differs from Heterangium and Lyginopteris. Each leaf-base is supplied by sets of vascular strands which pass into it from the stem at different levels; a large leaf-base reaching 4 cm. in diameter receives as many as 70–80 bundles. The hypoderm is like that first described in the French species Myeloxylon Landriotii[247] and often spoken of as the Sparganum type of hypoderm. The branching of the rachises points to a compound frond, and the occurrence of numerous linear pinnules with revolute margins (fig. 420, D) in association with the stem suggests that the ultimate segments were of the Alethopteris form. This inference receives confirmation from the occurrence of petrified specimens of undoubted Alethopteris rachises with the structure of Myeloxylon. It is practically certain that the leaves borne on the stems of Medullosa anglica are those long known as Alethopteris lonchitica (Vol. ii. A, p. 553, fig. 364).
An interesting feature in the stems is the occurrence of cortical vascular strands (fig. 416, A, a, b), reaching a diameter of 7 mm., containing scattered tracheids in a parenchymatous core surrounded by secondary xylem and phloem. These cauline bundles are almost identical both in structure and distribution with the accessory steles in the stem of a recent Cycas, and the agreement is emphasised by the presence of short square-ended tracheids in the primary xylem.
The roots branch freely and may attain a diameter of more than 1 cm.: they are generally triarch and the triangular primary xylem is enclosed by secondary xylem except opposite the protoxylem. The cortex is like that of Lyginopteris roots and a conspicuous double layer of superficial tissue is another feature common to both (cf. fig. 410). The exceptionally well preserved specimens described by Arber[248] show very clearly the thick zone of periderm which forms the covering of older roots, and in some of the sieve-tubes groups of dark brown patches show the form and arrangement of the sieve-plates.
Reproductive organs. We have as yet no precise information in regard to the reproductive organs of Medullosa anglica, but there can be little or no doubt that the fronds bore seeds that have long been known under the generic name of Trigonocarpus. Many years ago Mr Hemingway noticed the almost constant association of the fronds of Alethopteris lonchitica with Trigonocarpus, and Dr Kidston’s discovery[249] of seed-bearing Neuropteris pinnae considerably strengthened the evidence derived from mere association. The structure of Trigonocarpus is described later (p. 117) in a section devoted to reproductive organs attributed to Medullosa. Nothing is known as to the microspore-bearing organs.
While in the structure of each of the steles Medullosa anglica agrees very closely with Heterangium, it differs from that genus in the presence of three steles and in the structure of the petioles which are much less fern-like than the simpler petioles of Heterangium and Lyginopteris. From the continental species the British species is distinguished by its simpler stelar system, though there is a close correspondence as regards individual steles.
Medullosa pusilla Scott.
This species, briefly referred to by Scott in 1909[250] and fully described in a recent paper[251], is founded on material from the Lower Coal Measures of Colne, Lancashire. It agrees in essential features with Medullosa anglica, but differs in the following particulars: the linear dimensions of the stem are about one quarter those of a typical stem of the older species; the leaf-traces possess little or no secondary xylem and the relatively large decurrent leaf-bases have a narrower and simpler hypoderm. The stem has a tri-stelar vascular system enclosed in a ring of internal periderm, and each stele (3 mm. in diameter) consists of a roughly triangular strand of reticulate tracheids and a small amount of scattered parenchyma. The protoxylem is either exarch or, as in M. anglica, mesarch, the exact position being difficult to determine in the available material. The secondary xylem closely resembles that of M. anglica.
Scott suggests the possibility that Alethopteris decurrens may be the foliage of Medullosa pusilla. It is possible that there is no specific difference between M. pusilla and M. anglica, but on the present evidence the employment of a distinctive name is desirable.
Medullosa centrofilis de Fraine.
This species was founded by Miss de Fraine[252] on a petrified stem from the Lower Coal Measures of Lancashire. The maximum diameter of the flattened stem including four decurrent leaf-bases is 5 cm. The vascular system consists of an outer group of four steles, reduced to three by fusion in the upper part of the specimen, enclosing a central smaller stele or star-ring (fig. 417). It is the presence of the star-ring that distinguishes this type from the other two British species and forms a connecting link with certain continental Medullosae. The peripheral steles agree with the steles of M. anglica but, as in M. pusilla, there is some doubt as to the exarch or mesarch position of the protoxylem. In the structure of the xylem the central stele conforms to the rest of the vascular system and a strand of protoxylem is preserved that is almost certainly exarch. There is evidence that the peripheral steles occasionally anastomose, but the central stele follows an independent course at least in the piece of stem examined. Leaf-traces are furnished by the primary xylem of the outer steles, and they appear to be without secondary tracheids as in M. pusilla. A zone of secondary cortex encloses the vascular system as in the other British stems: it is pointed out by Miss de Fraine[253] that this tissue, usually described as a deep-seated periderm, must have differed from cork in that there is no sign of drying up or decay in the tissues external to it. The leaf-bases are of the usual Myeloxylon type. In size this species is intermediate between Medullosa anglica and M. pusilla.
Medullosa stellata Cotta.
Cotta[254] described Medullosa stellata as a stem characterised by the occurrence of several many-rayed stellate columns (‘vielstrahlige Sternsäule’) in a pith enclosed by a double cylinder of secondary xylem. The so-called pith is the central ground-tissue of the stem and the double ‘striated ring’ of Cotta is a cylindrical stele identical in structure with each of the steles of Medullosa anglica but having a tubular form instead of forming a relatively broad and short band (cf. fig. 416, D and A). Goeppert[255] in his Permian Flora gave a detailed account of the species, some of his sections being cut from Cotta’s material, and by the employment of varietal epithets emphasised the range of variation within the limits of the type. Goeppert and Stenzel[256] and, several years later, Weber and Sterzel[257] adopted the same plan as a convenient method of drawing attention to differences in anatomical characters. As Schenk[258] pointed out, there is a considerable risk in the case of small pieces of stems of attaching excessive importance to structural variations, and it is by no means improbable, as he said, that differences which are the expression of states of preservation or stages in development have been incorrectly regarded as distinguishing marks of individual plants. It is, however, convenient to recognise some of the more striking deviations from the type-species by speaking of the different forms as varieties though, as Weber and Sterzel fully admit, such varieties and even some of the species must be looked upon as provisional. Weber and Sterzel give expression to the provisional nature of their grouping by classifying the species with their varieties into form-cycles. Under the form-cycle Medullosa stellata five more or less well defined forms are recognised, the type-species being Medullosa stellata var. typica[259].
Medullosa stellata var. typica.
Part of a transverse section of a cylindrical stem is represented diagrammatically in fig. 416, D. Very little of the cortex is preserved: a parenchymatous axial region with scattered secretory canals contains four oval or cylindrical vascular steles, the stellate columns of Cotta or star-rings of later authors. These are of the same nature as the small central stele in the English Medullosa centrofilis. The central region of the stem in this specimen is completely surrounded by a narrow cylinder of inversely orientated secondary xylem and phloem (fig. 416, D), the phloem being on the inner side of the xylem. Beyond the xylem is a parenchymatous band containing scattered groups of primary xylem tracheids with spiral, scalariform, and reticulate pitting, and this zone, which is usually designated the ‘partial pith,’ is succeeded by a second and broader, normally orientated, cylinder of secondary xylem and phloem. In this section the two concentric cylinders separated by the partial pith form a solenostele like that of several recent Ferns except in the presence of secondary tissue. The term ‘partial pith’ applied to the tissue between the two cylinders of secondary tissue is misleading: this tissue (fig. 416, D, p) is the primary xylem of the stele and is homologous with the primary portion of the stele of Heterangium and of the steles of M. anglica. In many sections the continuity of the tubular stele is broken. In a section in the British Museum cut from one of Cotta’s specimens[260], 6 × 3·5 cm. in diameter to the outer edge of the vascular tissue, the cylindrical stele is interrupted at two places. An example of the interrupted type of stele is shown in fig. 416, F, and in fig. 416, H: the latter belongs to a distinct species. The complete type of cylindrical stele is exceptional and occurs occasionally at different levels in the stem. An important point is that the frequent breaks in the cylinder are not connected with the exit of leaf-traces and do not, therefore, correspond to the foliar gaps in the solenostele or dictyostele of a Fern.
The secondary xylem is of the cycadean type (fig. 418, B, D) like that of Heterangium and Lyginopteris and several other stems. Each of the star-rings in the axial region consists of a parenchymatous core with scattered primary tracheids enclosed by secondary vascular tissue (fig. 418, B). The star-ring shown in fig. 418, B, from a Chemnitz stem illustrates the characteristic cycadean character of the secondary xylem with broad medullary rays: some of the innermost elements are in contact with the primary tracheids. The phloem is rendered conspicuous by the black contents in some of the elements. Both the star-rings and the larger peripheral steles are constructed on the same plan and agree with the steles of M. anglica. The star-rings occasionally branch and anastomose with one another and with the encircling stele. The star-ring in fig. 416, D at a is about to give off a small strand.
Leaf-traces are furnished by the primary xylem at the edge of the ‘partial pith’ of the outer stele: as a leaf-trace passes outwards through the outer cylinder of secondary xylem the cambium invests it with secondary xylem and phloem, but as it passes through the cortex of the stem it becomes reduced to its primary elements, and by successive branching gives rise to small collateral bundles which enter the petioles. The piece of stem shown in fig. 416, G, illustrates the exit of leaf-traces from the stele and their subsequent division into several small bundles, v, which are scattered in the cortex with strands of sclerenchyma. In a specimen identified with Medullosa stellata, Schenk[261] found part of a leaf-base attached to the stem: its vascular system was of the Myeloxylon type, the bundles being identical with those in the cortex of the stem seen in fig. 416, G.
In some stems of M. stellata the outer, centrifugally developed, portion of the main stele is very much broader than in the example represented in fig. 416, D. The diagrammatic sketch reproduced in fig. 416, F, represents a section of a Chemnitz specimen in the British Museum[262] in which the axial region containing several star-rings is almost enclosed by an inner zone of secondary xylem, and beyond the narrow primary xylem (black in the sketch) the rest of the block consists exclusively of secondary xylem 5·5 cm. broad. This example illustrates a common tendency in Medullosa towards a large excess of centrifugal over centripetal secondary vascular tissue. A similar specimen of Medullosa stellata is figured by Mougeot[263] from the Vosges showing a considerable development of centrifugal xylem comparable with that in the British Museum stem. Weber and Sterzel[264] describe stems of Medullosa stellata showing slight periodic swellings which it is suggested, though there is no evidence in support of the opinion, may be connected with reproductive organs.
Medullosa stellata var. corticata[265]. The specimen referred to this variety, represented in fig. 416, G, has already been quoted as affording data with regard to the origin and behaviour of the leaf-traces. In this type of stem the outer portion of the main stele is narrower than in M. stellata var. typica and the stele never forms a complete tube. The star-rings in the centre of the stem are more numerous than in the type-species of the genus. In the axial region of some stems included in the form-cycle to which M. stellata belongs there may be flatter and tangentially elongated vascular strands in addition to the cylindrical star-rings; these are termed plate-rings.
In Medullosa stellata var. lignosa[266] the outer xylem reaches a breadth of 4 cm. and the star-rings are reduced to one. The form M. stellata var. gigantea[267] (fig. 416, K) is of special interest as an example of a stem reaching a diameter of nearly 50 cm. and having as many as 43 large and small star-rings in the axial region. A large tubular stele like that of the type-species (fig. 416, D) surrounds the central region, but in this form the cylindrical stele a is succeeded by concentric cylinders of normally orientated xylem and phloem (fig. 416, K, bb) produced by successive cambiums either cortical or pericyclic in origin. This type of stem presents a striking resemblance to stems of Cycas and Macrozamia except in the possession of a double cylindrical stele consisting of both centripetal and centrifugal secondary xylem and phloem separated by a zone of primary xylem (partial pith).
Medullosa gigas Renault.
This species was founded on a piece of stem from the Permian of Autun[268], consisting almost entirely of secondary xylem, which Brongniart had previously placed in his genus Palaeoxylon[269]. The secondary xylem reaches a diameter of 45–50 cm. and in the portion of the central region preserved there are a few vascular strands like the star-rings of other species. The considerable development of secondary xylem indicates a form of stem similar to some forms of M. stellata (e.g. fig. 416, F), but as the available data are insufficient for accurate determination Renault’s specific name is retained. Renault describes the internal xylem cylinder (i.e. the centripetal xylem) as very slightly developed or as hardly visible, a feature in which the French specimen shows a nearer approach to the structure of a recent Cycad.
Medullosa porosa Cotta.
The second of Cotta’s species[270], which has been fully investigated by Weber and Sterzel, is constructed on the same plan as that of M. stellata, but the stem is distinguished by the greater number of star-rings and, more especially, by the presence of an outer system of vascular strands in the axial region (fig. 416, M): these form a frequently interrupted cylinder of anastomosing strands characterised by the feeble development of secondary xylem and phloem or by the absence of this tissue on the outer face of the strands. The component parts of this outer series occasionally fuse with the internal star-rings.
Medullosa Solmsi Schenk[271] var. typica Web. and Ster.[272]
This type has a large axial region containing several very small star-rings enclosed by two concentric zones of separate plate-rings (fig. 416, E) each consisting of a complete flattened cylinder of secondary xylem and phloem enclosing primary xylem. As the complete cylindrical stele of the stem of Medullosa stellata shown in fig. 416, D, was compared with the solenostele of a Fern, so in this stem (fig. 416, E) the vascular cylinder may be compared at least superficially with a dictyostele. From the inner circle of plate-rings strands are given off in the form of star-rings and these pass through the gaps in the outer system, eventually breaking up in the cortex into numerous collateral bundles. In another form of this species, var. lignosa (fig. 416, L), the axial region is enclosed by a circle of plate-rings like those in the type-form, but these are succeeded by a circle of very asymmetrically developed and large steles with the outer xylem and phloem much broader than the inner. Moreover in this form additional cylinders of normally orientated vascular tissue are added as in M. stellata var. gigantea and in some recent Cycads. It is noteworthy that the secondary wood of Medullosa Solmsi is rather more compact than in other species, a feature in which it to some extent agrees with the South African genus Rhexoxylon.
Medullosa Leuckarti Goeppert and Stenzel.
In this species[273], also from the Permian of Saxony, the central region including some star-rings is surrounded by sinuous flattened concentric steles (snake-rings) agreeing anatomically with the steles of other species and characterised by the comparatively small breadth of the secondary xylem and phloem (fig. 416, H). Leaf-traces are given off, as in M. anglica and other species, from the outer edge of the primary xylem. In some forms there is a single set of snake-rings; in others there is a double series. Fig. 418, D, shows part of the secondary xylem of a stele of this species from Chemnitz: the tracheids are in some places continuous with the primary xylem, and on the outer edge of the secondary wood is a cylinder of phloem. A section of Medullosa Leuckarti figured by Goeppert and Stenzel[274] shows some radial rows of very thick-walled elements in the secondary phloem which they describe as bast sclerenchyma, but Solms-Laubach[275] believes them to be sieve-tubes. Precisely similar elements are figured by Scott[276] in M. anglica and as this author suggests the thick walls are probably not an original feature. The structure of the primary xylem is more clearly seen in fig. 418, C, and the relation between primary and secondary xylem is shown in fig. 416, I, where the position of the protoxylem may be either exarch or mesarch. The protoxylem is only occasionally recognisable but some of the peripheral primary tracheal groups are undoubtedly mesarch. External to the stele, a part of which is reproduced in fig. 418, D, are strands of stereome elements and beyond them a band of radially elongated cells that may be ‘periderm’: still farther out there are some imperfectly preserved vascular bundles that are leaf-traces. This species is important as affording a complete demonstration of the organic connexion between the stem and petioles of the Myeloxylon Landrioti type which indicate that the fronds were probably Alethopteroid.