Chapter XXX: II. Medulloseae (3)

This form, described by Miss de Fraine as _Sutcliffia insignis_[412], is represented by a stem rather smaller than the type-specimen described by Scott, though it is probably an older example of the same species. It is distinguished by a greater development of secondary xylem and phloem both on the main stele and the meristeles; it differs also in the absence of the greater part of the cortex and leaf-bases which have been cut off as the result of the formation of a deep-seated periderm (fig. 439, D, C). The meristeles are smaller and fewer than in the larger form and are distinguished by some other peculiarities. At _m′_ in fig. 439, D, a meristele is seen attached to the main stele. In _Sutcliffia insignis forma α_ the main stele is enclosed by an irregular network of subsidiary steles or meristeles and these form the points of departure of the leaf-traces, but the meristeles are not completely used up in the process of conversion into leaf-traces. In _Sutcliffia insignis forma β_ the meristeles agree in structure with the main stele except in the smaller amount of secondary tissue: they do not, however, form a network as in _forma α_ but occur as strands parallel to the central stele, ‘giving off leaf-traces and ultimately dividing up into smaller strands, often unequal in size, the primary wood of the meristeles being entirely used up in the production of radially symmetrical or unilateral bundles.’ In this respect, as Miss de Fraine points out, _forma β_ agrees more closely than _forma α_ with _Medullosa anglica_ and supports Scott’s view that _Sutcliffia_ is a primitive type of Medullosan stem. The leaves were given off at fairly long intervals as in some species of _Medullosa_: the leaf-traces are exarch and similar to those of the type-species. The most striking feature of the new stem is the presence of a vascular network (fig. 439, D, _e_) which encloses both the main steles and the meristeles; it consists of extrafascicular strands composed of normally orientated bands of secondary xylem and phloem often assuming a fan-like arrangement and occasionally almost concentric or inversely orientated. These strands are always accompanied by short, usually reticulate, tracheids on the inner margin of the xylem: similar isodiametric tracheids also occur in the pericyclic region. The extrafascicular strands are believed to be secondary structures phylogenetically independent of the meristeles and main stele, comparable with the successive cylinders or arcs of secondary xylem and phloem in some recent Cycads and in some species of _Medullosa_[413].

The stele is exarch and roughly triangular; except in the broader zone of secondary tissue it agrees with the protostele of _forma α_. The secondary xylem (fig. 439, C) possesses numerous medullary rays 3–4 cells broad and of considerable depth: the secondary phloem is characterised by the presence of thick-walled elements, presumably sieve-tubes, like those of _Medullosa Leuckarti_ and _M. anglica_.

The ground-tissue is rich in secretory tissue and the stem-surface, from which the leaf-bases have been detached, is limited by a wide zone of secondary tissue produced by a phellogen.

_Sutcliffia Williamsoni_ (Seward).

1876. _Myelopteris_ (_pars_) Williamson, Phil. Trans. R. Soc. Vol. 166, Pl. +ii.+ figs. 7, 8; Pl. +iv.+ fig. 17. 1893. _Rachiopteris Williamsoni_ Seward, Ann. Bot. Vol. +vii.+ p. 1. 1894. _Rachiopteris Williamsoni_ Seward, _ibid._ Vol. +viii.+ p. 207, Pl. +xiii.+ 1906. _Sutcliffia Williamsoni_ Scott, Trans. Linn. Soc. Vol. +vii.+ pt +iv.+ p. 62.

In an account of _Myelopteris_ (= _Myeloxylon_) published in 1876 Williamson included some sections of petioles from the Lower Coal Measures which I afterwards with his concurrence transferred to the genus _Rachiopteris_ as _R. Williamsoni_. In _Rachiopteris Williamsoni_ the vascular bundles are concentric and not collateral, and are further distinguished from those of _Myeloxylon_ by the association of parenchyma with the tracheids. In the arrangement of the bundles and in the structure of the ground-tissue the petioles of _Rachiopteris Williamsoni_ agree with those of _Myeloxylon_. An outstanding feature of the vascular strands of the former is the occurrence at fairly regular intervals in the peripheral part of the phloem of comparatively large tubes described by me as secretory canals on the ground that evidence was furnished of their development by the schizogenous separation of cells to form a central canal. The canals were compared with the large sieve-tubes of the Marattiaceae, but with the qualification that ‘their mature form and their manner of development are strongly suggestive of small secretory canals.’ Nothing was known as to the stem which bore these petioles until Scott’s discovery of _Sutcliffia_ with vascular bundles in the cortex and leaf-bases of the same type as those of _Rachiopteris Williamsoni_. As Scott says, there are a few distinguishing features which suggest that _R. Williamsoni_ is not specifically identical with the petioles of _Sutcliffia insignis_, though the agreement is such as to justify the substitution of _Sutcliffia_ for _Rachiopteris_. The tubular elements in the phloem which I considered to be secretory canals are regarded by Scott and Miss de Fraine as sieve-tubes.

Miss de Fraine gives an excellent summary of our knowledge of the genus and discusses, in the light of the additional facts furnished by the second stem, the position of _Sutcliffia_ in a phylogenetic series. The new form strengthens the comparison instituted by Scott between _Sutcliffia_ and _Medullosa_ and gives support to a close connexion between the Medulloseae and the Cycadaceae. Scott lays stress on the fact that the Medulloseae, except _Sutcliffia_, are polystelic, while the Cycads, at least the adult stems, are monostelic. The views of Worsdell and other botanists who believe that the Medulloseae and the Cycads are intimately related are discussed on another page: in reference to these views Miss de Fraine adds a caveat as to the danger of attaching excessive importance to evidence based on seedling anatomy when we are concerned with broad phylogenetic questions. The stele of a cycadean stem may be derived from a protostelic type such as that of _Sutcliffia_ by the gradual disappearance of the internal tracheids: in _Sutcliffia_, as in some Cycads and species of _Medullosa_, extrafascicular strands and cylinders are a characteristic feature and these may well have arisen independently of the central stele in response to physiological requirements. From such a type as _Sutcliffia_ evolution may have proceeded along two lines; in one direction new types were produced in which increasing complexity, as represented by a multiplication of steles, was an outstanding feature. These forms, illustrated by _Medullosa anglica_ and other more elaborate species, proved inefficient and were unproductive. Along another line the protostelic condition was maintained though in some cases extrafascicular strands or cylinders and cortical steles were superadded: it was this line that led to the recent Cycads.

_General considerations suggested by the anatomical features
of_ Medullosa.

A comparison of the stems described under the generic name _Medullosa_ reveals a considerable range in the grosser anatomical features superadded to certain fundamental characters denoting a common origin[414]. If additional data were available giving us a fuller knowledge of individual plants differences between species would be more clearly defined and would provide adequate grounds for the institution of new genera for some of the types now included in the comprehensive genus _Medullosa_. A proposal by Dr Lotsy[415] to adopt the names _Pecopteromedullosa_ and _Neuropteromedullosa_ rests primarily on the relatively unimportant difference between the fronds associated with certain Medullosan stems and, as Scott[416] points out, this two-fold division if applied to such a species as _Medullosa Leuckarti_ would result in its separation from species which anatomically are clearly of the same generic type. In _Medullosa anglica_, one of the oldest and simplest types, there are three steles of equal importance, and each of them is practically identical with the single stele of _Heterangium_. Each stele—as indeed all Medullosan steles—consists of a strand of primary xylem enclosed by secondary xylem and phloem, and it is obvious that the development of a constantly increasing cylinder of secondary conducting tissue about three centres would lead to serious mechanical difficulty: a stem constructed on the plan of _Medullosa anglica_ or the smaller _M. pusilla_ could not increase the thickness of its secondary vascular tissue beyond a certain point without detriment to its efficiency. In some types this difficulty is partially overcome by the production of complete concentric cylinders of centrifugally developed conducting tissue external to an inner system of concentric steles agreeing individually with those of _Heterangium_ (fig. 415, B). _Medullosa anglica_, regarded from the point of view of the architectural efficiency of its vascular system, affords a much less promising _point d’appui_ for further evolution than some of the forms described under _Medullosa stellata_ in which the mechanical impasse is avoided by the adoption of the cycadean plan as represented by such genera as _Cycas_ and _Macrozamia_. The English species _Medullosa centrofilis_ (fig. 417) affords the first example of a characteristic Medullosan feature, namely the presence of a small concentric stele in the central region of the stem: this so-called star-ring differs not only in its smaller dimensions but in its more cylindrical form from the larger peripheral steles. In the later Permian species, _e.g._ _Medullosa porosa_ and _M. Solmsi_, the single star-ring of the older _M. centrofilis_ is replaced by a large number of precisely similar conducting strands. These star-rings are structurally comparable with the cortical steles of _Cycas_ and, in position, with the medullary system of bundles in a _Macrozamia_; they are essentially cauline and take no part in the emission of leaf-traces. _Medullosa Leuckarti_ (fig. 416, H) resembles in its vascular plan _M. centrofilis_, but in this larger stem there are several star-rings and the enlarged peripheral steles are more or less sinuous. In _Medullosa Solmsi_ (fig. 416, E) the star-rings are still more numerous and the main vascular system consists of a double series of concentric steles, each agreeing with the larger peripheral steles of _M. Leuckarti_.

Some of the forms included in _Medullosa stellata_ appear to be very different from _M. anglica_ and _M. Leuckarti_ (_cf._ fig. 416, F, A, H), but their similarity is apparent if we imagine _Medullosa anglica_ with only one main stele (with the addition of star-rings) which is stretched tangentially until it becomes a long and narrow plate-ring and is then rolled into a hollow cylinder like that in fig. 416, D. A modification of the tubular type of stele is seen in _Medullosa stellata_ var. _corticata_ (fig. 416, G) in which the vascular cylinder is broken up into two or more curved plate-rings, a change superficially similar to that by which a dictyostele is produced from a solenostele, but in _Medullosa_ it is not the overlapping of leaf-gaps that is the cause of the change. A striking feature in the stem of _Medullosa stellata_ represented in fig. 416, D and F (especially F), is the inequality in breadth of the centrifugal and centripetal xylem: this inequality is an expression of the difficulty caused by the presence of an internal as well as an external addition of secondary conducting tissue. A limit is set to the production of secondary centripetal xylem by the space available for extension, whereas there is unlimited room for increase in the case of the centrifugal tissue. This tendency to a greater development of xylem and phloem on the outer side of the primary portion of the steles is illustrated also in _M. Solmsi_ var. _lignosa_ (fig. 416, L) where the outer of the two series of peripheral steles has a much larger proportion of centrifugal xylem. In _M. anglica_ the secondary xylem on the inner side of the steles is not infrequently broader than the corresponding tissue on the abaxial side[417]; but this is exceptional in the genus. A further development of centrifugal conducting tissue without any corresponding development of centripetal tissue is effected in certain cases (fig. 416, K, L) by the addition of concentric cylinders of centrifugal xylem and phloem beyond the original concentric steles. The type illustrated by _Medullosa Solmsi_ var. _lignosa_ and _M. stellata_ var. _gigantea_ (fig. 416, L, K) at once suggests comparison with stems of _Cycas_, _Macrozamia_, and _Encephalartos_, the chief difference being the presence in _Medullosa_ of an inner series of concentric steles and a central ground-tissue containing star-rings, though the latter may be regarded as corresponding to the medullary system of bundles in _Macrozamia_. In _Macrozamia_ the central region of the stem is considered to be the pith of a monostelic stem, whereas in _Medullosa_ the stem is polystelic. In recent cycadean stems it is not uncommon to find patches of inversely orientated xylem and phloem internal to one or more of the cylinders of centrifugal vascular tissue. These abnormal developments are considered by Worsdell[418] to be relics of the inner portions of concentric steles possessed by the Medullosan ancestors of recent Cycads. This interpretation affords a means of bringing into closer relationship the polystelic Medulloseae and the monostelic Cycadaceae, the apparent simplicity of the latter being the result of the progressive loss of centripetal xylem and phloem, the normal cycadean cylinder being therefore regarded as a one-sided remnant of a concentric Medullosan stele. In other words, the Cycads are descended from polystelic ancestors. As further evidence in support of this view Worsdell points to the occurrence of concentric steles in the cortex of _Cycas_ and their occasional presence in the pith of other genera. Matte[419] has shown that in the seedling stem of _Encephalartos Barteri_ (fig. 396, K) there are three concentric steles each similar to a normal Medullosan stele: at a higher level in the axis the steles become ‘unrolled’ and assume the form of one-sided cylinders of centrifugal xylem and phloem.

In the peduncles of some recent Cycads, _e.g._ _Stangeria_, there is a tendency towards a somewhat irregular orientation of the collateral bundles that constitute the vascular cylinder, and tracheids occasionally occur internal to the protoxylem of the individual bundles[420]. Worsdell regards these features as evidence of a Medullosan ancestry. If the sinuous plate-rings of a stem of _Medullosa Leuckarti_ (fig. 416, H) were broken up into separate portions and wholly or in part deprived of the centripetal xylem, the result would be an arrangement of bundles comparable with that in a _Stangeria_ peduncle[421]. The scattered centripetal tracheids discovered by Scott in _Stangeria_ and other cycadean peduncles are interpreted by Scott and by Worsdell as relics of some ancestral centripetal xylem, but with this important difference in the point of view; Scott believes that they represent the almost completely aborted centripetal xylem of a single stele like that of _Lyginopteris_, while Worsdell sees in them fragmentary vestiges of the central primary xylem of two or more Medullosan steles. An abnormal seedling of _Araucaria Bidwillii_ described by Shaw[422] exhibits features analogous to those in some cycadean seedlings: within the normal stele a cambium forms an inversely orientated vascular cylinder which at a lower level becomes continuous with the outer centrifugal tissue, the whole vascular system being eventually represented by two concentric steles. The polystelic stage is a development of a monostelic condition, and the inner or inverted portion of each of the two concentric steles is derived from an inversely orientated cylinder in the central region of the root. This abnormal root does not, however, supply an argument in favour of the derivation of a monostelic type of stem from one that was polystelic, but it shows a close relation between the two plans in one organ. The seedling is not altogether normal in form apart from structure and it is not improbable that the anatomical abnormality is connected with some pathological cause.

It has been suggested[423] that Worsdell attaches too much phylogenetic significance to the irregularities in the disposition and form of the vascular bundles in the peduncle of _Stangeria_, and the criticism that insufficient allowance is made for the possible reaction on structure of the special physiological requirements of reproductive shoots is well founded. Granting an overestimate of the arguments drawn from the occasional occurrence of concentric vascular strands, a considerable body of evidence remains in favour of Worsdell’s main contention.

Mrs Thoday[424] has drawn attention to certain features exhibited by the inflorescence-axes of _Welwitschia_, particularly the occurrence of concentric and inversely orientated bundles, similar to those characteristic of the seedling of _Cycas siamensis_[425] described by Matte, and to anatomical characters occasionally present in adult cycadean stems and normally represented in _Medullosa_. She is of opinion that the occurrence in _Welwitschia_ of certain Medullosan features has a phylogenetic significance. The differences between the Gnetales, Medulloseae, and Cycadales are considerable, and it would seem unlikely that the anatomical resemblances described by Mrs Thoday are of great value as criteria of close relationship. The comparison of _Lagenostoma_ with gnetalean seeds is alluded to elsewhere. To the statement that the presence of concentric and inversely orientated steles in _Welwitschia_ are reminiscent of the polystelic Medulloseae, Mrs Thoday adds the qualifying remark that the occurrence of four concentric groups of vascular tissue in the hypocotyl of _Welwitschia_ is not sufficient to justify the conclusion that the ancestral type was polystelic. This reservation accords with the contention of Scott and other botanists, that the occasional occurrence in cycadean seedlings and adult stems of anatomical features suggestive of polystely does not in itself furnish an adequate reason for doubting that the apparent monostely of Cycads is phylogenetically what it seems to be, namely, an indication of monostelic ancestry. This brings us to the question of a possible monostelic ancestor. It may be that the Upper Carboniferous genus _Sutcliffia_ affords a clue to the problem of the origin of the polystelic type illustrated in various forms by _Medullosa_. The protostele of _Sutcliffia_ bears a close resemblance to each of the three steles of _Medullosa anglica_; the fact that _Sutcliffia_ is exarch and that _Medullosa anglica_ has mesarch xylem is of secondary importance, particularly as exarchy is represented within the genus _Medullosa_. The extrafascicular strands of xylem and phloem and the accessory strands are points in which _Sutcliffia_ and _Medullosa anglica_ agree and, as Miss de Fraine[426] adds, the meristeles of _Sutcliffia_ may be homologous with the leaf-trace strands of _Medullosa_. Scott[427] gave expression to the characters shared by these two types by describing _Sutcliffia_ as the most primitive of the Medulloseae. It is suggested that the protostelic axis of _Sutcliffia_ may be regarded as the starting-point of the monostelic Cycads, the central mass of tracheal tissue being replaced by a parenchymatous pith, while the extrafascicular and accessory strands arose independently of the central stele in response to increased physiological demands consequent on the increase in size of the stem. From the same starting-point evolution may have progressed along another line through such a type as _Medullosa anglica_ leading to the more complex Permian species of _Medullosa_. Chodat’s view[428] that the Medulloseae are Protocycadaceae, if we include _Sutcliffia_ as well as _Medullosa_ in the Medulloseae, is probably correct. There is clear evidence of a close bond of union between recent Cycads and the Medulloseae, and _Sutcliffia_ offers a possible means of deriving complex polystelic types from a monostelic ancestor.

Worsdell’s opinion[429] that the stele of _Lyginopteris_ affords evidence of derivation from a polystelic ancestor and is not homologous with the true monostele of _Heterangium_ is opposed to the undoubted signs of intimate connexion exhibited by these genera. The Lyginopterideae are, as Scott[430] says, a less advanced group than the Medulloseae and, it may be added, they are more remote from the modern representatives of the Cycadales. The Lyginopterideae and the Medulloseae are probably offshoots of a common stock, but the Medulloseae occupy a position farther removed from the filicinean ancestry than _Heterangium_ and _Lyginopteris_[431].

The relative meagreness of our knowledge of the reproductive organs of the Medulloseae gives precedence to anatomical data in phylogenetic considerations, but the evidence furnished by _Trigonocarpus_ and other seeds that may fairly be assigned to Medullosan plants is in harmony with the conclusions based on vegetative characters with regard to a close affinity between the Medulloseae and Cycads.

The comparative examination of recent Cycads naturally suggested by any attempt to compare the group as a whole with Palaeozoic types leads to some apparently contradictory results. The habit of the megasporophyll of _Cycas_ is usually quoted as a primitive attribute: the close resemblance in plan and in manner of occurrence on the stem between megasporophylls and foliage leaves recalls both Ferns and Medullosan fronds. On the other hand the production of eight body-cells in the pollen-tube of _Microcycas_[432] in place of the usual single cell may also be regarded as a primitive character. It is perhaps possible, as Miss Dorety[433] says, that the polyspermy may be a case of recurrence and not a direct inheritance. _Microcycas_ differs from _Cycas_ in having only one vascular cylinder, and if the presence of several concentric cylinders in _Cycas_ be interpreted as an indication of a closer connexion with a Medullosan ancestry, the _Microcycas_ type would represent a more advanced stage in evolution. Attempts to arrange plants according to a natural sequence are frequently frustrated by instances of unequal progress in the development of vegetative and reproductive organs; one or other set of members lags behind; some characters point to the retention of primitive traits while others indicate a marked progressive tendency. It is noteworthy that the Mesozoic Bennettitales are characterised by a greater simplicity of stem-structure than is the rule in recent Cycads, and both in their vegetative features and in the structure of the seeds they are further removed from the Medullosan type.

II. A. =STELOXYLEAE.=

=Steloxylon.= Solms-Laubach.

_Steloxylon Ludwigii_ (Goeppert and Stenzel). The genus is founded on a piece of stem from Siberia, possibly of Permian age though not improbably older, which was originally described as _Medullosa Ludwigii_[434]. It is characterised by numerous cylindrical and band-like vascular strands forming an irregular anastomosing system (fig. 441, B) and by crowded spiral leaf-scars on the exposed face. The appearance presented by the transverse section figured by these authors, while suggesting comparison with _Medullosa_, reveals a distinctive character, namely the absence of a definite peripheral system of vascular rings such as forms a striking feature of the continental Medulloseae. A more complete description was afterwards published by Schenk[435] who recognised more fully the peculiar features and hinted at the possibility that the species might more appropriately be regarded as a member of a distinct group. Solms-Laubach[436] went a step further and instituted the generic name _Steloxylon_, and in a later publication gave a fuller account of the anatomical characters. The complete stem must have reached a diameter of approximately 13 cm. The homogeneous ground-tissue forms a matrix enclosing an anastomosing vascular system of cylindrical or oval steles (fig. 441, C). Each strand consists of a band of secondary xylem tracheids with one or several rows of circular or oval bordered pits on the radial walls and narrow medullary rays usually 1–2 cells broad and 1–4 cells deep, though occasionally deeper. No phloem is preserved. The tissue in the centre of each stele is very imperfectly preserved, but it is clear that the secondary xylem enclosed a central region (‘partial pith’) like that in the steles of a _Medullosa_, doubtless consisting of primary xylem and conjunctive parenchyma.

The stem is covered with leaf-bases of oval or circular section and between them are small organs, probably multicellular hairs (fig. 441, A). A leaf-base consists of an outer zone of strengthening tissue and a parenchymatous ground-tissue traversed by two or more small vascular strands which assume various forms. These petiolar strands are simply portions of the main vascular system which bend outwards at the periphery of the anastomosing network. The more noteworthy features in which _Steloxylon_ differs from _Medullosa_, particularly such species as _M. stellata_ and _M. Leuckarti_, are (i) the crowded and comparatively small leaf-bases in place of the massive decurrent petioles of _Medullosa_; (ii) the supply of the leaves by compact branches of the stelar network instead of the bundles detached as leaf-traces from a stem-stele of _Medullosa_ (the origin of a leaf-trace in _Steloxylon_ is shown in fig. 441, C); (iii) the absence of a peripheral system of vascular plate-rings and the irregular distribution of cylindrical and plate-steles in the ground-tissue. Nothing is known of the reproductive organs or leaves beyond the structure of the attached leaf-bases. The opinion expressed by P. Bertrand[437] that the fossil described by Stenzel as _Asterochlaena_ (_Clepsydropsis_) _kirgisica_ is the petiole of _Steloxylon_ was abandoned after the additional facts published by Solms-Laubach.

As regards the affinities of _Steloxylon_: the structure of the steles agrees closely with that of the star- and plate-rings of a _Medullosa_, while the pitting of the tracheids is more like that in _Medullosa_ than _Cladoxylon_. In the tendency to a more radial than tangential disposition of the band-like steles _Steloxylon_ recalls _Cladoxylon_ rather than _Medullosa_, but in _Cladoxylon_ the vascular system does not form an irregular network as in _Steloxylon_. The information as to the structure of the primary xylem is very meagre, but it points to a closer connexion with _Medullosa_ than with _Cladoxylon_. On the whole _Steloxylon_ may perhaps be defined as a genus allied to the Medulloseae in the anatomical features of the stem more closely than to other genera, but sufficiently distinct to be excluded from the Medulloseae as at present understood[438].