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.
[Illustration: Fig. 416. _Medullosa_ stems. A-C, _Medullosa anglica_;
A, transverse section; _a_, accessory vascular strand; _b_, accessory
strand enclosed by periderm; _c_, band of periderm encircling steles;
_d_, sclerenchyma between leaf and stem. B and C, longitudinal
sections. (After Scott.) D, _Medullosa stellata_; _a_, star-rings;
_p_, ‘partial pith.’ (After Weber and Sterzel.) E, _Medullosa Solmsi_.
(After Weber and Sterzel.) F, Medullosa stellata, from a specimen in
the British Museum (No. 13767). G, _Medullosa stellata_ var. _cortica_;
_v_, leaf-bundles. (After Weber and Sterzel.) H, I, _Medullosa
Leuckarti_. (After Weber and Sterzel and Solms-Laubach.) K, _Medullosa
stellata_ var. _gigantea_; _a_, concentric stele; _b_, _b_, later
cylinders of centrifugal tissue. (Adapted from Weber and Sterzel.) L,
_Medullosa Solmsi_ var. _lignosa_. M, _Medullosa porosa_. (L, M, after
Weber and Sterzel.)]
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_.
[Illustration: Fig. 417. _Medullosa centrofilis._ Transverse section
showing the deep-seated ‘periderm’ (broken line) surrounding four
steles. (After de Fraine.)]
_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.
[Illustration: Fig. 418. A, _Myeloxylon radiatum_, part of petiole.
B, _Medullosa stellata_; star-ring; × 14. C, D, _Medullosa Leuckarti_;
C, primary xylem; D, part of primary stelar tissues and secondary xylem
and phloem; × 6. (A–D, Kidston Coll., 1014, 1307, 1238.)]
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.
[Illustration: Fig. 419. _Medullosa Leuckarti._ Diagrammatic sketch
of stem showing longitudinal and transverse sections: for explanation
see text. (After Weber and Sterzel.)]
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).
[Illustration: Fig. 420. A, B, _Myeloxylon radiatum_; A, petiole ⅔
nat. size; B, vascular bundle. C, _Myeloxylon_ sp. vascular bundle;
_px_, protoxylem. D, _Medullosa anglica_; section of pinnule. (A, B,
after Zeiller; D, after Scott.)]
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_.
[Illustration: Fig. 421.
A. _Potoniea adiantiformis_ Zeill. (very slightly enlarged).
B. Microsporangia of _Potoniea_ (enlarged).
C. _Linopteris obliqua_; side-view of microsporophyll.
D. _Neuropteris heterophylla_, pinnules and microsporangia (?).
E. _Linopteris obliqua_, surface-view of microsporophyll.
(A, C, E, after P. Bertrand; B, after Carpentier; D, after Kidston.)]
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].
[Illustration: Fig. 422. _Neuropterocarpus Kidstoni_ (Arber). (After
Kidston; × 3.)]
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.
The generic name _Trigonocarpum_ was instituted by Brongniart in
1828[329] for ovoid longitudinally ribbed ‘fruits’ from Upper
Carboniferous strata, the type-species being named _T. Parkinsoni_. The
generic name is often altered to _Trigonocarpon_: Williamson[330], who
adopted this form, states that Brongniart substituted _Trigonocarpon_
for _Trigonocarpum_ in his _Tableau_[331], but in that work the
original termination is used, the form _Trigonocarpon_, probably the
result of a slip, appearing only in the index. In his later work on
seeds Brongniart adopted the name _Trigonocarpus_, and in recent years
this has been widely employed. Among other species named by Brongniart
are two previously referred by Sternberg to _Palmacites_. Several
examples of Brongniart’s genus were described by Lindley and Hutton,
and in their description of _T. Noeggerathii_ the statement is made
that a fractured specimen demonstrated that ‘the fossil in its ordinary
state is an interior part divested of fleshy covering’[332]; this
suspicion of the true nature of the nut-like fossils was afterwards
proved correct by the investigations of Hooker and Binney[333] and by
the later work of Williamson. The specimens on which the genus was
founded are casts of seed-cavities and it is in this state that the
seeds are usually preserved, often in large numbers, in the sandstones
of the Coal Measures, as in the block shown in fig. 423 from the famous
quarry at Peel near Bolton, Lancashire. Another type of preservation is
represented by the seeds figured by Lindley and Hutton as _Carpolithes
alata_[334], but the generic identity of the two states was not
recognised until the discovery of petrified material afforded the clue.
Figs. 424, 1, 425 illustrate the appearance of _Trigonocarpus_ when
preserved as a carbonised impression showing a thick fleshy envelope
enclosing an oval kernel with a hard wall prolonged upwards as a longer
or shorter micropyle. Casts of the seed-cavity are represented in figs.
423; 424, 2, 3. The surface of these casts occasionally shows one or
more short cylindrical projections which are probably extensions of the
sand or mud into holes formed in the testa by boring insects. The view
that _Trigonocarpus_ seeds are ‘obviously Palm fruits’ was not accepted
by Hooker and Binney who inclined to regard them as the seeds of
Conifers and compared them especially with the similar nuts of _Ginkgo_
seeds (_cf._ fig. 631, C). It was Mr Wild[335] who was first struck
by the association of _Trigonocarpus_ and the petioles of _Medullosa_
(_Myeloxylon_) and by some resemblances in structure between the testa
and the hypoderm of the petioles; though, as Scott and Maslen[336]
point out, the agreement is not so close as Wild believed, his view of
a possible connexion between the reproductive and vegetative organs
has been confirmed. Williamson extended our knowledge of the genus by
his account of _Trigonocarpus olivaeformis_ Lind. and Hutt., a form
that is specifically identical with _T. Parkinsoni_ Brongn. This author
also drew attention to the close resemblance between Brongniart’s three
genera _Trigonocarpus_, _Hexapterospermum_, _Tripterospermum_ and
expressed doubts as to the possibility of founding specific differences
on casts of the _Trigonocarpus_ type without the evidence of anatomy.
Our knowledge of the structure of _Trigonocarpus_ has in recent years
been considerably extended by the researches of Oliver, Scott and
Maslen, and Salisbury.
[Illustration: Fig. 423. Casts of the seed-cavity of _Trigonocarpus
Parkinsoni_.
(Manchester Museum. ½ nat. size.)]
_Trigonocarpus Parkinsoni_[337] Brongniart.
The seeds of this species like all examples of the genus are
radiospermic, that is radially symmetrical in contrast to the flattened
or platyspermic seeds. The complete seed is elongate oval in form when
preserved as an impression (fig. 425, A) and reaches a length of 4–5
cm.: the casts of the seed-cavity are ovoid and provided with three
prominent ridges (fig. 424, 2, 3). The testa forms a thick covering
differentiated into three regions, an outer flesh or sarcotesta,
a sclerous shell or sclerotesta, and an inner flesh. Transverse
sections show that the sclerotesta has three sharp longitudinal keels
with corresponding furrows on the inner face, and between each pair
of main ribs are 2–3 less prominent ridges, usually 12 in all (fig.
426). The sarcotesta consists of thin-walled parenchyma passing
externally into a more lacunar tissue with a palisade-like hypoderm:
the sclerotesta consists of thick cells which interlace and form an
efficient protective shell. Both the sarcotesta and sclerotesta are
continued into the apical region as the wall of the long micropyle,
the sarcotesta being prolonged beyond the sclerotesta at the apex of
the integument[338]. The micropyle is triangular in section and may
exceed in length the whole seed (figs. 425; 426, A). Its form as seen
in transverse section (fig. 426, B) suggests the presence of wings:
this appearance may be deceptive and due to pressure or, more probably,
it represents an original feature. The seed-body, that is the portion
enclosed by the integument, consists of the nucellus, represented by a
few crushed layers of cells, bounded by a well-defined epidermis; the
nucellus is separated from the integument from the base of the seed
upwards, an important feature in which this and some other Palaeozoic
seeds differ from _Lagenostoma_ and the seeds of recent Cycads which
are characterised by an integument adnate to the nucellus up to
the level of the shoulder; the seeds of the Conifer _Phyllocladus_
afford an example of separation of integument and nucellus as in
_Trigonocarpus_. The innermost layer of the nucellus consists mainly
of tracheal tissue investing the large megaspore (fig. 426, A, C,
_m_) which is preserved as a contracted membrane detached from the
nucellus after the death of the seed. At the summit of the nucellus is
a relatively small pollen-chamber (fig. 426, A, _Pc_) like a broad and
low cupola bearing a terminal beak which extended some distance into
the micropylar tube. No microspores have been found in this species,
but Oliver[339] records the occurrence of multicellular microspores in
_Trigonocarpus pusillus_. The pedicel of the seed had a central strand
of sclerous tissue penetrated by a concentric vascular bundle which
gives off six strands to supply the sarcotesta (fig. 426, C, _v_) and
then passes into the nucellus where it forms a tracheal sheath (fig.
426, A, _nt_) surrounding the lower part of the megaspore and at a
higher level breaks up into anastomosing strands of tracheids which
reach up to the plane of insertion of the pollen-chamber.
[Illustration: Fig. 424. _Trigonocarpus._ 1. Section of imperfect
seed showing a micropyle; _b_, sclerotesta; _c_, cast of seed-cavity;
2, 3, apical and basal view of seed-cast. (Approximately nat. size.) M.
S.]
[Illustration: Fig. 425. _Trigonocarpus Parkinsoni._
A. Impression showing the nucule and sarcotesta. (= _Carpolithes
alata_ Lind. and Hutt.)
B. Specimen with long micropyle. (A, nat. size; from the Lower Coal
Measures of Kilmarnock; B, nat. size; Middle C. M. of Yorkshire.
Kidston Coll., 1579 and 1062.)]
[Illustration: Fig. 426. _Trigonocarpus Parkinsoni_. A, longitudinal
section; _Sa_, _Sc_, _if_, sarcotesta, sclerotesta, inner flesh; _mi_,
micropyle; _Pc_, pollen-chamber; _v_, vascular bundles; _m_, megaspore
and prothallus; _t_, tracheal disc; _nt_, tracheids in the nucellus.
B, transverse section of the upper region showing the sarcotesta,
sclerotesta (black) and the micropyle. C, transverse section of _T.
Parkinsoni_. D, transverse section of _Trigonocarpus shorensis_. (A, C,
after Scott; B, after Wild; D, after Salisbury.)]
A second species described by Scott and Maslen as _Trigonocarpus
Oliveri_ has been further investigated by Salisbury who finds
that it is an 8-angled seed which cannot be retained in the genus
_Trigonocarpus_: its systematic position ‘must for the present remain
uncertain[340].’ Dr Arber has recently described a new species of
_Trigonocarpus_, _T. Moyseyi_[341], from the Nottingham Coal-field
(Middle Coal Measures), similar to _T. Parkinsoni_ but much broader
in proportion to its length: this species is founded on an impression
without structure.
The species _Trigonocarpus Dawesi_ Lind. and Hutt.[342], from the
Middle Coal Measures of Lancashire, was founded on casts differing
in their large dimensions from those of _T. Parkinsoni_: specimens
referred to this species were described by Fiedler[343] from Saxony
in 1857 and Lesquereux[344] figures similar casts from the Upper
Carboniferous and Permian rocks of North America.
_Trigonocarpus shorensis_ Salisbury.
This species, founded on specimens from the Lower Coal Measures of
Shore, Lancashire[345], may exceed 4 cm. in length and has a breadth
of 2·5 cm. In general plan it agrees with _T. Parkinsoni_ but there
are certain well-marked differences: the micropyle is much shorter;
the thick sarcotesta, attaining a breadth of 6 mm. at the base of the
micropyle, is characterised by the presence of six peripherally placed
vascular bundles (fig. 426, D, _v_) in contrast to the deeply embedded
bundles of _T. Parkinsoni_. Below the epidermis of the sarcotesta
is a hypoderm formed of radially disposed plates of sclerous tissue
similar to that of _Myeloxylon_ and different from the palisade-like
hypoderm of the type-species. Within the sarcotesta is a hard shell,
the sclerotesta, characterised by three prominent ribs extending from
base to apex and three shorter ribs which reach from the chalaza to
about a third of the length of the seed. The fact that the sarcotesta
and sclerotesta pass gradually into one another is a point in favour of
the view that the integument is a single structure. There appears to be
good evidence of the restriction of an inner flesh to the micropylar
region, whereas this tissue in _T. Parkinsoni_ was probably continuous
over the whole inner face of the sclerotesta. The sarcotesta is
lacunar in its outer part as in some other types of Palaeozoic seeds,
a feature probably connected with floating efficiency. _Trigonocarpus
shorensis_ occurs in association with _Myeloxylon_ petioles, and there
is a resemblance between the seed and the vegetative organs in the
structure of the hypoderm as also in the structure of the secretory
sacs which are particularly numerous in this species. Salisbury draws
attention to the close resemblance between the form of _T. shorensis_
and the seeds found in organic connexion with pinnae of _Neuropteris
obliqua_[346].
The species _T. corrugatus_ described by Renault[347] bears a close
resemblance to _T. shorensis_.
+Other Genera founded in part on Reproductive Organs which may belong
to the Medulloseae.+
=Codonotheca=, =Schützia=, =Whittleseya=, =Dolerophyllum=, =Ottokaria=,
=Strobilites=.
=CODONOTHECA.= Sellards.