Chapter XXX: II. Medulloseae (1)

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.

The specimen on which the diagrammatic drawing reproduced in fig. 419 is based has been investigated by Weber and Sterzel[277] and by Solms-Laubach[278]. The figure is a slightly simplified version of that given by Weber and Sterzel; it represents the stem of _Medullosa Leuckarti_ as a transparent object, the two lower transverse sections, B and C, being seen in perspective through the longitudinal faces. The steles are shaded obliquely in the longitudinal sections, and in the three transverse sections, A, B, C, the primary xylem (partial pith) is black and the enclosing secondary vascular tissue radially shaded. The whole block is 9 cm. in length and 6 cm. broad. Only a part of the axial region is shown internal to the peripheral snake-rings and in it are the star-rings _S_, _S_, _b_, and _c_. Outside the main steles is the narrow cortex _R_ and portions of leaf-bases _I–IV_. The lowest section, C, shows part of a peripheral snake-ring with a slight swelling at _f_ on its inner side which, as seen in sections B and A, foreshadows the separation of the star-ring _S_ and the consequent break in the continuity of the snake-ring (_d_, _e_, sect. B). In section A the gap is closed: in the longitudinal section between B and A the star-ring _S_ is seen to form two branches, _a_ and _b_, the branch _a_ closing the gap between _d_ and _e_ in section B. These sections demonstrate the formation of a star-ring from the main peripheral stele and the formation of additional star-rings by branching.

Numerous vascular bundles destined for the leaves are scattered in the cortex. The course of the decurrent leaf-base _I_ is shown on the longitudinal faces, its boundary being marked by crowded stereome strands (of the _Myeloxylon Landrioti_ type); other leaf-bases are represented by _II_, _III_, and _IV_.

In habit _Medullosa Leuckarti_ differs from such a type as _M. stellata_ in its relatively shorter and stouter stem and in the shorter internodes.

+Leaves and Reproductive Organs.+

i. _Leaves._ It has already been stated that in some cases petioles occur in organic connexion with Medullosan stems, notably in _M. anglica_ and _M. Leuckarti_: in the exceptionally rich collection in the Chemnitz Museum, which forms a fitting memorial of the work of the late Prof. Sterzel, there is a stem of _M. Leuckarti_ bearing large petioles of the type known as _Myeloxylon radiatum_. The occurrence of vascular bundles in the cortex of other species of stem identical with those in the attached petioles points to a uniform type of leaf-structure so far as regards the petioles and rachises of _Medullosa_. While it is clearly unnecessary to distinguish by a special generic title the petrified portions of fronds known to belong to certain species of stems, the frequent occurrence of detached petioles necessitates some distinctive term. The name employed is _Myeloxylon_: the genus was instituted by Brongniart in 1849 for Cotta’s species _Medullosa elegans_ the petiolar nature of which was suspected by Binney in 1872.

_Myeloxylon_ (Medullosan leaf-stalks).

1832. _Medullosa elegans_ Cotta. 1865. _Stenzelia_ Goeppert. 1876.
_Myelopteris_ Renault[279]; 1877, _Aulacopteris_
Grand’Eury[280].

There is a very close agreement in general anatomical structure between the numerous specimens of _Myeloxylon_ from the Permian strata of Saxony and France and the Coal Measures and Millstone Grit of England[281]; the genus is also recorded from the Upper Carboniferous of Kansas[282]. Two well-defined types instituted by Renault are, however, readily distinguished by the form of the hypodermal stereome strands. _Myeloxylon_ may be defined as follows: Oval or cylindrical branched axes, reaching a diameter of 15 cm., bearing pinnae having the characters of _Alethopteris_, _Neuropteris_, _Odontopteris_, and some other genera that were formerly classed as Ferns. Below a single-layered epidermis, in which stomata have been recognised, occur a few layers of parenchyma: this superficial tissue, which is rarely preserved, is succeeded by a hypodermal region consisting of parenchymatous tissue and numerous vertical groups of narrow thick-walled fibres arranged as radial plates or circular, oval, or reniform strands (the _Sparganum_ type of cortex). In the hypoderm as in the ground-tissue generally secretory canals, often accompanied by stereome, are a characteristic feature. The vascular system is represented by a considerable number of collateral bundles scattered through the ground-tissue and especially abundant in the outer region: the bundles sometimes assume a more or less regular disposition in concentric circles. Each bundle consists of a small group of xylem tracheids, for the most part spiral or scalariform, though reticulately pitted elements are by no means rare, with a single protoxylem group on the outer face next the phloem (fig. 420, B, C). As a rule the xylem is wholly centripetal, but occasionally the exarch structure becomes mesarch by the occurrence of a few centrifugal tracheids. The phloem, rarely preserved (fig. 420, B), consists of narrow sieve-tubes with parenchyma, and the bundle as a whole is often partially enclosed by a sheath of fibres.

Superficially the anatomical structure is similar to that of the petioles of _Angiopteris_ or _Marattia_, and both Williamson[283] and Renault placed _Myeloxylon_ in the Ferns; but the collateral form of the vascular bundles, the position of the protoxylem, and the arrangement of the hypoderm tissues, are cycadean features.

_Myeloxylon radiatum_ (Renault).

This type is characterised by the radially elongated stereome of the hypoderm. Prof. Zeiller[284], who has given a very clear and concise description of _Myeloxylon_, is disposed to regard Cotta’s _Medullosa elegans_ as a specifically distinct form on the ground that there are two concentric zones of stereome in the hypoderm; but this feature is shown only in one of Cotta’s figures, and Weber and Sterzel[285] point out that a doubting of the hypoderm zone may be caused by accidental juxtaposition of two faulted pieces of peripheral tissue. The drawing reproduced in fig. 420, A, shows the structural plan of an unusually large petiole from the Permian of Autun: a portion of the outer tissue is seen in fig. 418, A. The vascular bundle, fig. 420, C, from a Millstone Grit specimen[286], shows the centripetal nature of the xylem and fragments of phloem in the outer half of the bundle, with imperfectly preserved fibres abutting on the xylem. The characteristic hypoderm is shown also in fig. 418, A; the double xylem strand on the left illustrates a common feature caused by the branching of vascular bundles. Several secretory canals are scattered in the ground-tissue. The pinnules of _Myeloxylon radiatum_, or at least of some specimens, have been shown by Renault to be of the _Neuropteris_ type.

_Myeloxylon Landrioti_ (Renault)[287].

In this species the distinguishing feature is the occurrence of the hypodermal stereome in the form of circular, oval, or reniform strands in place of the radial plates of _M. radiatum_. It is this form of petiole that was borne by the stems of _Medullosa anglica_ and _M. Leuckarti_. In _M. anglica_ the pinnules (fig. 420, D) are of the _Alethopteris_ type, almost certainly _A. lonchitica_. Renault and Zeiller have described French specimens of _Myeloxylon Landrioti_ bearing pinnules like those of _Alethopteris aquilina_ and _A. Grandini_.

_Myeloxylon topekense_ (Penhallow).

The occurrence of _Myeloxylon_ petioles in the New World was recorded by the late Prof. Penhallow[288] who founded this species on some imperfectly petrified specimens from Upper Carboniferous strata at Topeka, Kansas. Enough material was available to show the _Myeloxylon_ characters, but the preservation is too imperfect to admit of a complete diagnosis. The hypodermal stereome shows a tendency to form tangentially extended strands in place of the more circular or radially elongated groups in the European species.

In addition to _Alethopteris_, _Neuropteris_ (including _Cyclopteris_) and _Odontopteris_ fronds, which are known to possess rachises with the _Myeloxylon_ features, there is reason to believe that the Permian _Callipteris_ fronds and possibly some of the older _Taeniopteris_ leaves may also belong to _Medullosa_[289]. It is, however, unsafe to assume that the occurrence of _Myeloxylon_ petioles necessarily denotes the existence of _Medullosa_. The French stem _Colpoxylon aeduense_ Brongn.[290] bore leaves with the same general anatomical features as those of a typical _Myeloxylon_, and there can be little doubt that other genera of the Medulloseae also possessed fronds constructed on the same plan as those known to have been borne by _Medullosa_. An interesting illustration of an injured organ that had produced a wound-cambium is afforded by a _Myeloxylon_ petiole from the Coal Measures described by Mr Holden[291].

+Reproductive Organs.+

Reference was made in volume +ii.+ to the reasons which led to the removal of several genera of Carboniferous and Permian fronds from the Filicales to the Pteridosperms, and in Chapter +xxix.+ of the present volume certain species of _Sphenopteris_ are described as the foliage of _Lyginopteris_ and _Heterangium_. It is with such genera as _Neuropteris_, _Alethopteris_, _Linopteris_ and others that we are now more especially concerned, as they represent some of the types of leaves borne by _Medullosa_ and other members of the Medulloseae. The absence of any specimens among the large number of these common genera bearing undoubted sporangia aroused suspicion as to the correctness of the generally accepted view that these fern-like fossils were the leaves of Palaeozoic Ferns. Subsequently the suspicion based on negative evidence was confirmed by researches into the anatomical structure of the leaf-bases, petioles, and fragments of pinnae attached to and associated with stems of _Medullosa_. It is only in a few cases that actual organic connexion between reproductive organs and Medullosan leaves has been demonstrated, but from such facts as are established it is safe to make the general statement that stems of _Medullosa_—a generic term that undoubtedly includes plants which, had we a fuller knowledge of them as complete individuals, would be assigned to more than one generic type—possessed fronds simulating in habit those of certain Ferns with some of the pinnae bearing seeds often of considerable size and in all cases of complex structure, agreeing in many respects with those of existing Cycads, while other fronds, or in some cases it may be other pinnae, bore microsporangia similar in form to the sporangia of Ferns.

_a. Microsporangia._

_Neuropteris._ A. _Neuropteris heterophylla[292]._

Several examples of supposed fertile specimens of _Neuropteris_ are recorded in palaeobotanical literature, but it was not until 1887 that any satisfactory specimen was discovered. In that year Kidston[293] described a specimen of _Neuropteris heterophylla_ from the Lower Coal Measures of Scotland in which slender forked branchlets bear small bodies at their tips some of which appear to represent four-valved organs (fig. 421, D), though the imperfect state of preservation renders impossible any definite pronouncement as to their structure. To the specimen are attached a few sterile pinnules, showing that it is a portion of a frond of _N. heterophylla_ characterised by the substitution of reproductive organs for pinnules. The subsequent discovery of seeds attached to pinnae of the same species afforded strong presumptive evidence, almost amounting to proof, of the microsporangial nature of the Scotch specimen. For this specimen, although no precise diagnosis is possible, Dr P. Bertrand[294] has proposed the generic name _Neurotheca_. In 1911 the Abbé Carpentier[295] described some small ovoid bodies, 1–1·5 mm. long, from the Coal Measures of France arranged in groups of 4 to 6 and in some cases said to be borne on a slender pedicel which he found in association with _N. heterophylla_ and compared with sporangia described by Lesquereux from the Coal Measures of Arkansas as _Sorocladus stellata_[296]. These supposed microsporangia have recently been assigned by Bertrand[297] to _Sphenophyllum_.

B. _Neuropteris gigantea_, etc. _Potoniea_, Zeiller.

In 1899 Zeiller[298] instituted the name _Potoniea_ for some peculiar fertile leaves found in the Coal Measures of Heraclea consisting of a branched axis with cuneate segments, 7–10 × 6–8 mm., bearing numerous fusiform bodies, 1–1·5 mm. long, at the upper edge (fig. 421, A). These marginal bodies he regarded as sporangia and expressed the opinion that _Potoniea_ may be the fructification of some form of _Neuropteris_, _Mariopteris_ or _Alethopteris_. Carpentier[299] afterwards described similar though rather larger examples from the Pas-de-Calais coal-field as microsporophyll fragments of some Pteridosperm: he stated that similar specimens had been found by Kidston in England. In a later work Carpentier[300] described the sporangia as crowded in groups (fig. 421, B) in the substance of the thick lamina of _Potoniea_, and he connected the fertile segments with _Neuropteris gigantea_, _N. pseudogigantea_ Pot. and _Linopteris obliqua_ (Bunb.). Similar specimens are said to have been found in Holland and Silesia. Bertrand[301] also records the association of _Potoniea_ with _Neuropteris gigantea_ and _N. pseudogigantea_: he describes some specimens as belonging to _N. gigantea_ Sternb. while others, distinguished only by small differences, he attributes to _N. pseudogigantea_. Kidston[302] has recently drawn attention to the inconstancy of the characters mentioned by Potonié as distinguishing features of _N. pseudogigantea_, and he shows good cause for referring the examples so named to _N. gigantea_. The fertile lamina is almost orbicular in surface-view and attached to a slightly excentric pedicel; the microsporangia are borne on the lower surface and probably in groups as described by Carpentier.

_Neuropteris Carpentieri_ Kidston. Kidston[303] has recently described some fertile leaflets under this name from the Westphalian series of South Staffordshire which he identifies with French examples referred by Carpentier[304] to _Potoniea adiantiformis_ Zeill. The fertile pinnules are thick and sub-cyclopteroid in form; the upper surface bears densely packed, narrow and long, microsporangia, 4 × 0·5 mm., containing more or less spherical microspores 45–60μ in diameter; the ventral face of the lamina on the removal of the spores shows several strong veins. With these are associated sterile pinnules of the Neuropteroid type, and examples are described intermediate between the sterile and fertile leaflets. Kidston believes the specimens to be microsporophylls of some species of _Neuropteris_, but as the material does not suffice for identification with any known species a new name is proposed. It is pointed out that in some states of preservation the leaflets resemble Goeppert’s Permian species _Dictyothalamus Schrollianus_[305].

_Alethopteris. Linopteris._

No specimens of _Alethopteris_ fronds have so far been described that afford any information as to the nature of the microsporangia, and we have no means of knowing whether they were borne on naked pedicels as in _Neuropteris heterophylla_, or on modified pinnules as in _N. gigantea_.

Zeiller in 1888[306] described some fertile pinnules of _Dictyopteris Schützii_ Roem. from Commentry bearing two rows of long sporangia: he subsequently transferred this species to the genus _Linopteris_[307] and expressed the opinion that the sporangia occur singly and not in groups as he originally believed. Zeiller compares the fertile pinnules with the type _Crossotheca_. Bertrand[308], as the result of examining similar specimens, has suggested that the fringe of pendulous bodies regarded by Zeiller as sporangia may be tooth-like lobes of modified pinnules which served to protect microsporangia borne on the lower surface of the lamina. The nature of the impressions is not clear, though there is little doubt that they are microsporophylls. The fertile pinnae of _Linopteris obliqua_ (Bunb.) described by Carpentier[309] and Bertrand[310] closely resemble the microspore-bearing organs which have been referred to _Neuropteris gigantea_; they have the characters of _Potoniea_ and consist of oval laminae similar to the sterile pinnules but about half their size: the lamina was attached excentrically to a slender stalk (fig. 421, C, E) and traversed by numerous occasionally anastomosing veins. No actual microsporangia have been discovered in organic connexion with the lamina.

_b. Megasporangia._

_Neuropteris._

Kidston’s discovery of undoubted seeds attached to pinnae of _Neuropteris_[311] marked an important step in our more exact knowledge of the morphology of Medullosan sporophylls. Specimens from the ironstone balls (Coal Measures) of Coseley near Dudley showed seeds attached to portions of pinnae bearing pinnules of _Neuropteris heterophylla_. The seeds are approximately 3 cm. long and from 1·10 to 1·40 cm. broad; oblong and gradually tapering from the middle to a slightly curved and obtuse apical snout (fig. 422). The outer surface shows numerous longitudinal ribs which no doubt represent hypodermal fibres. The preservation of the seeds, which appear to be circular in section, is not such as to render possible a description of structural features. In their asymmetrical form the seeds agree with the genus _Platyspermum_ as recently defined by Arber, but Kidston’s specimens are in all probability radiospermic. Kidston compares the _Neuropteris_ seeds with _Rhabdocarpus tunicatus_ as figured from the Commentry coalfield[312] and with specimens from Gard named by Grand’Eury _R. subtunicatus_[313].

Additional proof of the occurrence of seeds on _Neuropteris_ fronds is furnished by examples from the Coal Measures of Holland described by Kidston and Jongmans[314]: these seeds are of the same general type as those from Coseley but nearly twice as large, and they were borne at the tips of a dichotomously branched pedicel of _Neuropteris obliqua_. Grand’Eury in 1904[315] recorded the association of radiospermic seeds with _Neuropteris_ fronds though no case of actual attachment was found. It is, however, noteworthy that he speaks of the frequent association with _Neuropteris_ of seeds characterised by six or rarely twelve longitudinal keels, a feature recalling the sclerotesta of _Trigonocarpus_ and allied seeds. Until petrified specimens are available it is impossible to refer the seeds of _Neuropteris_ to a generic type founded on structural features: the seeds described by Kidston are, as he says, very similar in external characters to species assigned to _Rhabdocarpus_, and there can be little doubt as to the generic identity of the _Neuropteris_ seeds and some of the impressions referred to _Rhabdocarpus_ which are characterised by a similarity in form, an apical snout that gives an asymmetrical appearance to the specimens and the presence of numerous longitudinal striations[316]. It is, however, by no means certain that these seeds possessed the morphological features of _Rhabdocarpus_ as described by Brongniart in petrified examples from St Étienne[317].

The seeds of _Neuropteris_ may, as Kidston suggests, agree anatomically more closely with _Pachytesta_[318], a type that Grand’Eury associates with _Alethopteris_ fronds. With a view to avoid the danger of incorrectly identifying petrified specimens and impressions that cannot be proved to belong to the same generic type, I have suggested the restriction of the name _Rhabdocarpus_[319] to seeds that do not furnish evidence as to internal structure, and the employment of the designation _Rhabdospermum_ for seeds that conform to those described by Brongniart as _Rhabdocarpus_. The seeds of _Neuropteris_ may be of the _Rhabdospermum_ type or they may agree anatomically more closely with _Trigonocarpus_ or _Pachytesta_; they are members of either the Trigonocarpales or the Cardiocarpales, probably the former group. Dr P. Bertrand[320] assigns to _Neuropteris gigantea_ some seeds of the _Hexapterospermum_ (= _Hexagonocarpus_[321]) type which occur in association with cupule-like organs. These supposed cupules resemble the _Potoniea_ leaflets with microsporangia also referred to the same species of _Neuropteris_; they are characterised by a laciniate edge and may be compared with the Indian fossil described by Zeiller as _Ottokaria bengalensis_[322] (fig. 433). Dr Arber[323] and Dr P. Bertrand[324] have independently proposed the generic name _Neurospermum_ for the seeds of _Neuropteris heterophylla_ and _N. obliqua_ in preference to _Rhabdocarpus_: the former author speaks of the seeds of _N. heterophylla_ as _Neurospermum Kidstoni_. The generic term _Neuropterocarpus_ used by Grand’Eury in 1904[325], though not defined by him, has priority and avoids the adoption of a new designation for seeds attached to _Neuropteris_ fronds. In his definition of _Neurospermum_ Arber makes no reference to the obliquity of the apical snout that is clearly shown in fig. 422. There is no evidence that _Neuropterocarpus_ possessed a cupular investment comparable with that of _Lagenostoma_. Additional instances of the association of seeds with _Neuropteris_ fronds are recorded by Renier from the Belgian Coal Measures, also by Bertrand and Chodat from France: Renier found seeds associated with _N. Schlehani_ Stur and with the same species of frond Bertrand found impressions of oval ribbed seeds. Prof. Chodat[326] has figured some fragments of _Neuropteris_ pinnules referred to _N. auriculata_ Brongn. from the Stephanian of France which show small seed-like bodies apparently in organic connexion with the lamina; but the specimens are too imperfect to afford any satisfactory evidence as to the nature of the reproductive organs.

Lotsy[327] has expressed the opinion that the bodies attached to _Neuropteris_ pinnae described by Kidston and other authors as seeds may possibly be vegetative buds, but if this were the case one would expect to find some evidence of the bud-nature in some at least of the specimens that have already been found.

It would seem that the microsporophylls of _Neuropteris_ were constructed on different plans, some being of the type described by Zeiller and other observers as _Potoniea_, while others bore sporangia on pinnae without any accompanying laminae; but our knowledge of the latter form represented by Kidston’s specimen of _Neuropteris heterophylla_ (fig. 421, D) is very incomplete. On the other hand the seeds appear to have been characterised by features suggesting a close affinity to _Trigonocarpus_ and pointing to membership of the same family.

_Alethopteris._

Although no specimens have been discovered showing actual connexion between fronds and seeds, it is practically certain that _Alethopteris_ leaves, or at least some species of the genus, bore seeds of the _Trigonocarpus_ type. The association of _Trigonocarpus Parkinsoni_ with _Alethopteris lonchitica_ is too frequent to be fortuitous and there is further evidence afforded by certain anatomical resemblances. In France other species of _Alethopteris_, _e.g._ _Alethopteris Serlii_ and _A. Grandini_, occur in association with _Pachytesta_[328], a large seed similar to _Trigonocarpus_, and _Trigonocarpus_ is found in the Pas-de-Calais coal-field with _Alethopteris Serlii_.

=TRIGONOCARPUS.= Brongniart.