CORDAITALES.
A. =POROXYLEAE=.
=Poroxylon=. Renault.
In 1879 Renault[537] briefly summarised the anatomical features of
some silicified vegetative shoots from the Permian of Autun for which
he instituted a new family, the Poroxyleae. The more complete account
contains a description of two species, _Poroxylon Boysseti_ and
_P. Duchartrei_: the latter was afterwards recognised as a stem of
_Heterangium_. Renault considered this new genus to be closely allied
to _Sigillaria_ and _Sigillariopsis_ and pointed out its resemblance
to _Cordaites_. Additional species have since been described but as
yet the genus has not been found outside France in Permo-Carboniferous
strata of Autun and the St Étienne district. The results of a more
detailed investigation of the anatomy of the genus were published by
Bertrand and Renault in 1882 and since then[538] Bertrand, Renault,
and Scott have added to our knowledge of this interesting type. In
several respects _Poroxylon_ stems present a striking resemblance to
_Lyginopteris_, but the recent discovery of the genus _Mesoxylon_ has
given greater significance to the characters in which _Poroxylon_
agrees with representatives of the Cordaitales. Our knowledge of
the genus, though exceptionally full with regard to the anatomy of
vegetative shoots, does not include any precise information as to the
reproductive organs.
The slender cylindrical stems, not exceeding 2–3 cm. in diameter in
specimens so far recorded, bore large broadly linear leaves similar
in form and venation to those of some species of _Cordaites_ which
were attached singly to slightly swollen nodes separated from one
another by internodes several centimetres long. The base of the
rather fleshy lamina passes imperceptibly from the narrow lower
portion into a tangentially expanded petiole which forms a decurrent
ridge on the stem. Axillary buds frequently occur. Little is known
of the leaf-impressions, but if Grand’Eury[539] is correct in his
identification of certain specimens from French Stephanian beds as the
leaves of _Poroxylon_, the lamina reached a length of 1 met. and a
breadth of 15–20 cm. In habit the stems probably resembled some of the
larger-leaved Bamboos. The only evidence bearing on the nature of the
reproductive organs is furnished by Grand’Eury who believes that some
_Rhabdocarpus_ seeds and bractless inflorescences associated with the
leaves assigned to _Poroxylon_ belong to that genus.
The single cylindrical stele has a relatively large solid pith, the
perimedullary region being characterised by the occurrence of a row
of primary crescentic strands of centripetal xylem of exarch type,
though not improbably in some cases slightly mesarch, varying in
size and shape and forming single or paired bundles. These strands
represent the xylem of collateral leaf-traces similar to those of
_Lyginopteris_ but differing in the absence of well-defined centrifugal
elements: the curved form of some of the xylem strands gives them an
appearance similar to that of the leaf-traces of _Lyginopteris_. The
leaf-traces, except in the lower part of their course through the pith,
are double and pass through several internodes before the centripetal
tracheids die out. The secondary xylem (fig. 463) is manoxylic and very
similar to that of _Lyginopteris_ though rather less parenchymatous.
The secondary phloem and cambium are often very well preserved. No
endodermis and no distinct pericycle has been recognised. The cortex
is parenchymatous and, like the pith and to some extent the phloem,
contains numerous secretory sacs; in the outer cortex the presence of
hypodermal strands is a prominent feature. At an early stage in the
growth of the stem a deep-seated phellogen forms secondary tissue both
externally and internally and decortication ensues.
[Illustration: Fig. 463. _Poroxylon Edwardsii_. A. Transverse section
of stem. B. The central region of another stem of the same species. (A,
× 9; University College Collection; B, from a photograph supplied by
Prof. Bertrand of a specimen in the Renault Collection, Paris.)]
The bundle of each leaf-trace is accompanied by an arc of secondary
centrifugal xylem as it passes through the secondary wood and this is
retained in the leaf except in the finer veins. After entering the
petiole the leaf-trace branches and an arc of bundles is produced, the
concave side facing the upper surface of the thick lamina (fig. 464,
A). Further reference is made to the structure of the leaves in the
description of _Poroxylon stephanense_. The specimens of roots so far
described are characterised by a diarch plate of primary xylem and two
masses of secondary vascular tissue separated by two medullary rays
opposite the protoxylems. Bertrand mentions the occurrence of roots of
_P. stephanense_ with more than two protoxylem strands. The phellogen
was produced in the pericycle as in the roots of recent Gymnosperms. It
is suggested by Lignier[540] that some silicified rootlets from Grand’
Croix (Loire) described by him as _Radiculites reticulatus_ and at
first compared with roots of _Sequoia_ may belong to some Cordaitalean
plant, possibly _Poroxylon_.
_Poroxylon Edwardsii_ Renault.
This species[541] affords a good illustration of the generic
characters already summarised. The strap-like leaves are fleshy
and the occasionally forked, parallel or slightly divergent, veins
are embedded in a homogeneous mesophyll with hypodermal strands of
mechanical tissue. The pith consists of parenchyma in vertical series
with scattered secretory sacs and differs from that of _Cordaites_
and _Mesoxylon_ in the absence of transverse discs. There are 13
primary-xylem strands close to the inner edge of the secondary wood:
the centripetal tracheids are scalariform or have multiseriate pitting
like that in the secondary xylem. The structure of the leaf-traces is
clearly shown in fig. 464: the double trace seen in fig. 464, C, has
two protoxylem-strands accompanied by some parenchyma, and these are
almost enveloped by the metaxylem tracheids which abut on the secondary
wood. At this stage in its course, that is just before bending
outwards, the centripetal xylem reaches its maximum development and the
trace forms a prominent and broad twin-strand in striking contrast to
the two narrower and tangentially extended strands shown in fig. 464,
E, D. Each of these strands with a single protoxylem-group would at a
higher level assume the broader and more compact form and contain two
protoxylems as in fig. 464, C. The tracheids of the secondary xylem
have 4–7 alternate rows of contiguous alternate pits on the radial
walls: the medullary rays are 2–3 cells broad and may be 60 cells deep.
According to Renault[542] several small oblique pits occur on the
radial walls of the ray cells. The secondary phloem, separated by a
normal cambium from the xylem, forms a broad band of sieve-tubes with
lateral sieve-plates like those in _Medullosa anglica_ alternating
with tangential rows of parenchyma. The cortex is relatively narrow
and in older stems is chiefly occupied by secondary tissue formed from
deep-seated phellogens.
[Illustration: Fig. 464. _Poroxylon_. A, B, _Poroxylon Boysseti_.
A, transverse section of leaf in the region of lateral expansion of
the petiole; B, portion of vascular tissue of A. C–E, _Poroxylon
Edwardsii_. C, leaf-trace showing recent separation of the two
protoxylems; D, leaf-trace at a lower level with more widely separated
protoxylems, _px_; E, leaf-trace intermediate between C and D. (From
photographs supplied by Prof. Bertrand.)]
_Poroxylon Boysseti_ Renault.
The stems of this species agree closely with those of _P. Edwardsii_,
the chief difference being in the structure of the secondary phloem
which does not show the regular concentric alternation of sieve-tubes
and parenchyma.
_Poroxylon stephanense_ Bertrand and Renault.
This the oldest species, from Stephanian beds at Grand’ Croix,
differs in no essential features from the other representatives
of the genus. It is from a study of the leaves of this type that
Bertrand and Renault have obtained most of the facts with regard to
the anatomy of _Poroxylon_ foliage. In the median region of the fleshy
leaf the bundles are characterised by a comparatively large amount
of centripetal xylem accompanied by a considerable development of
secondary centrifugal tracheids: the bundles are connected laterally
by both centripetal and centrifugal xylem and thus at certain levels
in the lamina the vascular tissue has the form of a continuous plate
(fig. 464, A, B). The veins become independent on branching and near
the edge of the lamina they consist only of primary elements. Secretory
sacs of elongated form are scattered in the homogeneous mesophyll, and
thick stereome-strands underlie the epidermis. The epidermal cells are
rectangular and rows of stomata occur on both surfaces.
B. =CORDAITEAE=.
=Cordaites=. Unger.
A preliminary statement with regard to nomenclature may serve to
remove possible misconceptions in connexion with the application
of the generic name _Cordaites_. It has been the general practice
to apply this name to certain forms of linear leaves which are
particularly abundant in Carboniferous and Permian strata in Europe
and North America, and in recent years a few palaeobotanists have
substituted _Cordaites_ for _Noeggerathiopsis_ as the more suitable
designation for Permo-Carboniferous specimens abundant in the rocks of
Gondwana Land. It has been customary to assign to _Cordaites_ certain
reproductive shoots, seeds, and stems described under the generic
names _Cordaianthus_, _Cordaicarpus_, _Cordaicladus_, _Cordaioxylon_,
etc. Stems agreeing anatomically in their main features with those of
recent Araucarineae have long been attributed to _Cordaites_, but a
few years ago a new type of stem was discovered which, though almost
identical with that of _Cordaites_, is distinguished by the character
of the primary xylem. For this new type the name _Mesoxylon_[543]
was proposed. Nothing is known as to the reproductive organs borne
on _Mesoxylon_ stems, but the leaves are externally at least
indistinguishable from those referred to _Cordaites_. It is therefore
obvious that when we apply the name _Cordaites_ to leaves or other
plant-organs, under that designation are undoubtedly included specimens
belonging both to _Mesoxylon_ stems and to stems with the characters
of _Cordaites_ (_Cordaioxylon_). Further research may enable us to
subdivide _Cordaites_ into more precisely defined types distinguished
by well-marked morphological characters, but at present the only course
would seem to be to restrict the term _Mesoxylon_ to petrified stems
exhibiting the features of that genus and to retain _Cordaites_ as a
comprehensive designation in accordance with the general account of the
genus given in the following pages. This widely distributed and mainly
Palaeozoic genus is especially well represented in the coalfields of
France where in some localities it contributed largely to the formation
of seams of coal[544], and it is chiefly from the researches of French
Palaeobotanists that our knowledge of its morphology is derived.
_Cordaites_ has shared the fate of most other abundant fossil plants
in the distribution of its _disjuncta membra_ among several genera
and classes, but on the whole the information that is now available
enables us to reconstruct the complete plant with a greater degree of
confidence than is usually attainable.
_Cordaites_ may be described as a forest-tree closely resembling
in habit and probably in size the recent Conifer _Agathis_, more
especially such species as _A. macrophyllus_, _A. vitiensis_ and others
with leaves considerably longer than those of the Kauri Pine (_A.
australis_)[545]. The main stem reached a considerable height before
giving off scattered branches bearing spirally disposed, sessile, and
often crowded leaves[546] like the foliage of _Agathis_. The absence of
any evidence of a two-ranked arrangement of leaves on lateral branches
suggests a general tendency towards a vertical rather than a horizontal
direction of growth. The sessile and closely set leaves for the most
part of leathery texture vary considerably in length and breadth in
different types (figs. 466–472): in some the broadly linear lamina with
its parallel veins and perfectly constructed I-shaped girders (fig.
465) reached a length of nearly 100 cm., in shape like the blade of
a straight broad-sword or the leaves of a _Yucca_, torn by the wind
into strips; in other forms the lamina is shorter and more obovate,
while in some the leafy shoots must have looked like slender stems of
the smaller-leaved Bamboos. There is no proof that young vegetative
branches with their spirally rolled leaves[547] were protected by
bud-scales, but some oval triangular scales (fig. 468, C), occasionally
found in association with larger foliage-leaves, may have served that
purpose. The branches from which leaves had recently fallen at the
time of fossilisation are characterised by transversely elongated
oval scars, occasionally showing a slightly curved row of pits like
the marks of leaf-traces on the scars of a Horse Chestnut, sometimes
terminating a feebly projecting decurrent leaf-cushion (fig. 466, C).
The leaves persisted for a comparatively long period as in _Araucaria
imbricata_, and on older leafless branches the scars are transversely
stretched; the leaf-cushion loses its individuality and eventually the
development of secondary cortical tissue causes the exfoliation of the
superficial bark.
In the form and structure of the fertile shoots _Cordaites_ parts
company with _Agathis_; the trees bore no cones in the ordinary sense,
but unisexual inflorescences—whether on one plant or on different
individuals is uncertain—were produced in the axils or from a
supra-axillary position as compound spikes or compact racemes. Both the
longer female shoots and the shorter and more compact male branches are
constructed on a similar plan. The ovulate inflorescence may exceed 30
cm. in length (fig. 479); a stout axis bears two-ranked linear bracts
subtending short lateral bud-like shoots with one or several sessile
or stalked ovules (fig. 480) between the sterile scales. The seeds
are platyspermic and agree much more closely with those of Cycads
and _Gingko_ than with the seeds of Conifers. The male inflorescence
is on a smaller scale, in habit not unlike the elongated male shoot
of _Cephalotaxus pedunculata_ and some other Conifers; each bract
subtends a small oval bud composed of imbricate scales and highly
modified microsporophylls borne singly or in clusters (figs. 481,
F; 482). A microsporophyll consists of a comparatively long pedicel
bearing at its apex a few long microsporangia. The term microsporophyll
implies a morphological interpretation which is not accepted by all
palaeobotanists, some of whom prefer to regard the microsporangia as
stamens or microsporophylls reduced to their simplest terms and sessile
on an elongated flower-stalk.
The stem agrees very closely in its more important features with that
of an _Araucaria_ or an _Agathis_: the primary xylem forms the inner
surface of the thick cylinder of secondary wood, merging gradually into
it as in recent Conifers; there are no separate bundles of primary
centripetal xylem. The medullary rays are narrow: in other words
the secondary xylem is of the pycnoxylic type. The pitting of the
tracheids is Araucarian and, as in _Agathis_, the leaf-traces arise
as twin-bundles. The pith is larger than in the Araucarineae and more
homogeneous in structure; it shares with the pith of _Juglans_ and some
other recent plants an almost constant tendency to assume a discoid
structure. Anatomically the leaves agree more closely in the structure
of the vascular bundles with Cycads than with Conifers though there are
points of contact with both of these classes. The roots branch freely
and their horizontally extended arms (figs. 468, A; 478) suggest growth
in swampy ground; anatomically they conform to the recent Gymnospermous
type and there is good evidence that in some cases fungal mycelia lived
symbiotically in the cortex of coralline rootlets.
Sternberg[548] figured some leaves of _Cordaites_ from Carboniferous
rocks in Bohemia under the generic name _Flabellaria_ in the belief
that they belonged to a Palm. Brongniart substituted a new name
_Pycnophyllum_[549] on the ground that Corda had disproved the
supposed relationship with Monocotyledons. The name _Cordaites_ was
instituted by Unger[550], his definition being based on leaf-form as
well as on stem-anatomy. It has recently been proposed to revive the
forgotten designation _Pycnophyllum_[551], but the reasons given are
hardly likely to induce botanists to discard the familiar generic
name which perpetuates the memory of Corda. As already pointed out,
the name _Cordaites_, even though employed in what has always been
regarded a legitimate sense, is no doubt often given to specimens
of some other allied member of the Cordaitales which can only be
recognised as such in the case of more completely preserved material.
The naming of wood of the Cordaitean type, but which may equally well
belong to another genus, raises a difficult question: if there is
satisfactory evidence from collateral sources that the wood is that
of a _Cordaites_ Grand’Eury’s name _Cordaixylon_[552] or Schenk’s
form _Cordaioxylon_[553] may be used, though there seems to be no
adequate reason against the use of the name _Cordaites_. If there is no
confirmatory evidence available and it is impossible to say whether the
wood is that of a Conifer or a _Cordaites_, or some other plant with
the same type of secondary xylem, Endlicher’s term _Dadoxylon_[554] is
most conveniently employed. The confusion liable to follow from the
use of the two generic names _Dadoxylon_ and _Araucarioxylon_ for
wood of the same type differing only in geological age is an argument
in favour of extending _Dadoxylon_ to all specimens having certain
anatomical characters, which cannot be certainly assigned either
to the Araucarineae or the Cordaitales, irrespective of geological
age. The term _Cordaicladus_ sometimes applied to branches is hardly
necessary, but the subgeneric names _Eu-Cordaites_, _Dory-Cordaites_,
and _Poa-Cordaites_, instituted by Grand’Eury for different forms of
leaf, are frequently employed and serve a useful purpose as descriptive
terms though the characters which they connote are of small importance
and by no means always well defined or constant. For inflorescences it
is customary to adopt the name _Cordaianthus_ suggested by Grand’Eury
as a substitute for _Antholithus_ and some other terms. The same author
uses _Rhizo-Cordaites_ for roots.
The nomenclature of seeds is more difficult: in a few instances seeds
occur in organic connexion with Cordaitean shoots, but there is no
doubt that many platyspermic Palaeozoic seeds preserved as detached
fossils belong to _Cordaites_ or some other member of the group.
The difficulty is that in the present state of knowledge we cannot
definitely determine in many cases whether a seed is Cordaitean or
whether it belonged to a genus of Pteridosperms. For this reason the
account of several seeds that were probably borne on _Cordaites_ or
some allied genus is given in a later chapter devoted to Gymnospermous
seeds. There is no doubt that under the generic names _Cardiocarpus_,
_Cordaicarpus_, and _Samaropsis_ are included true Cordaitean seeds,
though it would be incorrect to say that all the seeds so named belong
to members of the Cordaitales.
_Cordaites_ reached its maximum development in the Carboniferous and
Permian periods; the genus or some closely allied types persisted into
the Triassic and Rhaetic periods, and there is reason to believe that
the group was represented in some post-Rhaetic floras. The genus is
one of many remarkable examples of the high degree of specialisation
attained by Palaeozoic plants. The complex mechanisms represented by
_Cordaites_ and similar types give force to the conviction that we
cannot hope to penetrate below the higher branches of the genealogical
tree which had its roots in a period of the earth’s history
inaccessible to botanical investigation. The plants of the present
age are to a large extent the result of evolutionary tendencies more
correctly described as the result of degeneration or simplification
than as the latest phase in a series composed of a succession of
types gradually growing in complexity. _Cordaites_ is essentially a
generalised type, a composite product of an age characterised by an
activity in the elaboration of the complex from the simple. Botanical
records furnished by the geological series available for investigation
furnish evidence of the sorting of characters among gradually
diverging races and of changes in plant-organisation tending towards
simplification and increased efficiency.
_Cordaites_, using the generic designation in a wide sense, occurs
in Carboniferous and Permian strata in Europe, North America, and
China; it is recorded from several localities in Russia and Siberia
for the most part from Permian rocks, from Permo-Carboniferous (Lower
Gondwana) beds in India, Australia, South Africa, and South America.
Wood agreeing generally in the structure of its secondary tracheids
with that of _Cordaites_ is represented in Devonian rocks, and
there can be no doubt as to the existence of Cordaitalean plants in
pre-Carboniferous floras. It is represented in the Rhaetic flora of
Tonkin and has recently been discovered in strata probably of Rhaetic
age in Mexico.
+Leaves.+
It is important to recognise the fact that leaves included under
the generic name _Cordaites_ were in many cases not borne on stems
or branches with the anatomical characters of _Cordaites_. Scott in
his account of the genus _Mesoxylon_ says, ‘I feel no doubt that
most of the British specimens of Cordaitean leaves really belong to
_Mesoxylon_, which is a much commoner type of stem in the Coal Measure
petrifications than that of _Cordaites_ itself[555].’ Some of the
Cordaitean leaves were probably attached to stems of the _Poroxylon_
type[556] and it is not improbable that, as investigations are
extended, additional genera of vegetative shoots will be discovered
provided with leaves similar at least in external characters to those
which it is customary to refer to _Cordaites_. In the present state of
our knowledge we cannot make use of anatomical characters as criteria
by which to distribute the foliage of the _Cordaites_ form among the
genera _Cordaites_, _Mesoxylon_, and _Poroxylon_, using these names as
designations of certain types of anatomical structure. The specimen
reproduced in fig. 465 is in all probability a piece of a leaf of
_Cordaites principalis_, but on anatomical grounds Miss Benson[557]
has made it the type of a new species, _C. Felicis_, and more recently
Scott[558] has brought forward evidence supporting the view that it
is a leaf of _Mesoxylon_. As, therefore, neither impressions nor
petrifications of Cordaitean leaves can in the great majority of cases
be referred with confidence to their respective genera of stems,
pending fuller information the only course would seem to be to use
the name _Cordaites_ in a comprehensive sense indicating in special
cases where evidence is available the more precise systematic position
of the specimen. The classification of Cordaitean leaves proposed by
Grand’Eury[559] is based partly on the form of the lamina and in part
on the equality or inequality of the ‘veins.’ The actual veins, which
are embedded in the fairly thick mesophyll, do not directly affect the
superficial ribbing on the carbonised impression of the leaves and, as
seen in fig. 465, the most prominent hypodermal strands of supporting
tissue which would appear as the main veins or primary ribs on an
impression do not correspond in position with the vascular bundles.
Although in some cases the largest stereome-strands coincide with the
veins, forming the upper and lower parts of I-shaped girders the centre
of which is occupied by the veins, this is by no means always the case.
Grand’Eury has drawn attention to the difference between the upper and
lower surface of some carbonised leaves: in _C. crassifolius_ (fig.
468, D)[560] there are five to seven finer ribs between each pair of
primary ribs on one face while the other shows ridges and grooves with
a rib corresponding to each. Attention is called on a later page to the
variable character of the ribbing even on different parts of the same
lamina. The lower surface of the leaf, seen in section in fig. 465,
would show a number of approximately equal ribs, or possibly primary
ribs (midway between the veins) separated by two interstitial ribs,
while on the upper face there would be three rather smaller secondary
ribs. In a section of a leaf called by Renault _C. crassus_[561], a
specific name used also by Lesquereux[562] for an impression of a
leaf originally described by Goeppert as _Noeggerathia crassa_, there
are deep stereome-strands between the veins next the lower epidermis
alternating with single smaller strands, while on the upper surface
the hypodermal strands occur only immediately above the veins. In a
section figured by Felix[563] from North Germany as _C. robustus_,
the hypodermal stereome forms continuous bands; on the upper face the
bands are uniform in thickness but next the lower epidermis they form a
series of ribs.
Grand’Eury’s subgeneric terms _Cordaites_, _Dory-Cordaites_, and
_Poa-Cordaites_ have therefore very little value as regards differences
in the ribbing of leaf-impressions: the large size of leaves included
in _Dory-Cordaites_ and the more acute apex of the lamina as compared
with the obtuse apex of smaller leaves of _Cordaites_ are features of
limited application and of minor importance as diagnostic characters.
The name _Poa-Cordaites_ is, however, usefully employed for the
narrower linear leaves with an obtuse apex.
The structure of a _Cordaites_ leaf is clearly shown in fig. 465; the
lamina is approximately 1 mm. thick and there are about 30 veins in a
breadth of 2 cm. Strong I-shaped girders with the webbing composed of
thick-walled cells divide the mesophyll into rectangular compartments:
the intervening hypodermal strands differ in number and size on the two
faces. The epidermis is not preserved: specimens of other leaves show
that the stomata[564] occur in rows on the lower surface. The mesophyll
shows no differentiation into palisade and spongy parenchyma, and in
this respect the leaf agrees with many other forms; but in some leaves
the palisade-tissue is well developed, as in _C. lingulatus_ Ren.[565]
The central region of the lamina consisted of lacunar tissue, portions
of which are preserved, with a more compact sheath of parenchyma
enclosing each vein. In some leaves there is a narrower sheath of
thick-walled cells more sharply contrasted with the mesophyll. The
vascular bundles agree in structure with those in the rachis of a
Cycadean frond more closely than with the veins of an Araucarian
or other Coniferous leaf. The xylem consists mainly of centripetal
elements which form a deltoid strand with the protoxylem at the apex,
and in close association with this is a larger or smaller amount of
narrower centrifugal tracheids: in the section shown in fig. 465 the
centrifugal xylem may extend all round the centripetal tracheids,
but it usually forms an irregular arch with its base attached to the
sides of the larger tracheal strand, _cp_, separated, except at the
base of the arch, by a small amount of conjunctive parenchyma from the
centripetal xylem. The phloem is not preserved and is represented only
by a few patches, _ph_, below the centrifugal tracheids. Dr Benson[566]
in her account of this type of leaf gives additional details and
compares the anatomical features with those in other species. The
dual nature of the xylem like that characteristic of recent Cycads
has usually been regarded as a definite feature of Cordaites leaves;
Dr Stopes, on the other hand, interprets the narrower tracheids
(occupying a position similar to those in fig. 465) in some sections
of a leaf identified with _C. principalis_ from Grand’ Croix, as an
inner sheath of transfusion elements (‘primitive transfusion tissue’)
possibly derived from the centripetal xylem with which it is clearly
connected at the sides precisely as in fig. 465; but in the Grand’
Croix leaf the phloem is enclosed within the sheath of narrower
tracheids and not external to it as it is in the section shown in fig.
465 and in a section of _C. lingulatus_ figured by Dr Stopes. It is,
however, difficult to recognise any fundamental difference between the
‘inner transfusion tissue’ and centripetal xylem. The cells of the
outer sheath in Dr Stopes’s specimens of _C. principalis_ have bordered
pits on their walls and this character is mentioned also by Renault in
other specimens.
[Illustration: Fig. 465. _Cordaites_ leaf (_Cordaites Felicis_
Bens.), probably borne on a _Mesoxylon_ stem. _cp_, centripetal xylem;
_ph_, phloem; _px_, protoxylem. (Kidston Coll. No. 2194.)]
Prof. Lignier[567] has described the structure of fragments of adult
leaves from the Stephanian of Grand’ Croix (Loire) which he refers
to _Cordaites lingulatus_, and the same author gives an interesting
account of the anatomical features of a bud of the same species. The
bud, which resembles in general appearance that of _Dolerophyllum_
(fig. 430, p. 133) is 3 cm. long, oval in transverse section—as the
result of compression—and consists of four convolute leaves and a piece
of a fifth. The outer leaves have 75 to 80 veins: the inner laminae are
sinistral in their curvature while the three outer leaves are dextral.
In the second, the first in which the tissues are recognisable, the
small desmogen-strands afford some evidence that the phloem preceded
the xylem in the order of differentiation as is often the case in
recent plants. The first tracheids occur almost in the centre of
the desmogen-strand and to these are added the other tracheids of
the centripetal xylem, the oldest elements being spiral, the next
scalariform and the later tracheids reticulate. The centrifugal
xylem is formed at a later stage, and at about the same time are
differentiated the elements called by Dr Stopes the inner sheath and
by Lignier the ‘bois diaphragmatique.’ Lignier also describes the
development and structural features of the other tissues of the young
leaves and compares the anatomical features of the French leaves with
those of _Cordaites Felicis_ described by Prof. Benson.
The main features of _Cordaites_ leaves are (i) the presence of two
kinds of xylem in the veins, the larger centripetal tracheids, or chief
water-conducting elements, and the narrower tracheids, in some cases
attached to the sides of the centripetal xylem, in others forming free
groups, usually between the protoxylem and the phloem, but sometimes
enclosing the phloem; (ii) the frequent presence of a well-defined
sheath of cells round each vein composed of comparatively thick-walled
elements comparable with the transfusion-tracheids in Conifers; (iii)
the presence of lacunar tissue in the centre of the mesophyll and in
some cases of transversely extended tracheids similar to those in some
Podocarp leaves; (iv) a well-developed system of stereome-strands
and I-shaped girders. The structural features on the whole suggest a
xerophilous type, and the frequent absence or feeble development of
palisade tissue points to diffused rather than to brilliant sunlight.
The considerable range in size and form among Cordaitean leaves as well
as the obvious dependence on conditions of preservation or growth of
such a relatively unimportant feature as the presence or absence of
the so-called false or interstitial veins—the variability of which has
been demonstrated in several instances—renders specific determination
exceedingly difficult. The following species are briefly described
rather with a view to illustrate the nature of the characters employed
by authors than as implying the existence of so many well-defined types.
_Cordaites principalis_ (Germar).
This species was founded[568] on a large specimen from the Coal
Measures of Wettin showing a fan-like cluster of longitudinally torn
and partially overlapping leaves spread out in the position that
would be assumed on the compression of a shoot with a close spiral
phyllotaxis. This form of _Cordaites_ is the most abundant in the
British Coal Measures. The broadly linear lamina is characterised by an
obtuse apex (fig. 466, A), a tendency to split into strips, close-set
parallel ribs, the stouter ribs or veins separated from one another
by 2–3 or it may be as many as 5 finer ribs or interstitial ‘veins.’
A statement by Weiss that in Germar’s type-specimen the longitudinal
ribbing of the lamina is very imperfectly preserved confirms the
scepticism that is justly felt as to the validity of this character as
a satisfactory specific criterion.
[Illustration: Fig. 466. A. _Cordaites principalis_, part of leaf.
_B. Artisia_ _transversa_, pith-cast. C. _Cordaites principalis_,
branch with leaf-scars. (A, ⅖ nat. size; B, C, nat. size; Kidston
Coll.)]
The incomplete example shown in fig. 466, A, is 19·5 cm. long and has
a maximum breadth of 3·5 cm., but the complete leaf was much larger
and tapered gradually to the comparatively broad and slightly concave
or amplexicaul base. The narrow elliptical proximal end of a specimen
figured by Kidston[569] from the Middle Coal Measures of Yorkshire is
2·8 cm. broad indicating that the tangentially expanded leaf-scars on
a branch recently deprived of its foliage must have been a conspicuous
feature. In his synonymy of this species Kidston[570] includes _Knorria
taxina_, a species founded by Lindley and Hutton[571] on a piece of
stem from the Coal Measures of Newcastle. The type-specimen, as Mr
Howse[572] states, is much larger than the published drawing and
closely resembles in the decurrent leaf-bases with broad apices the
piece of stem represented in fig. 466, C, which Kidston identifies as
_C. principalis_. Geinitz[573] refers to this species the seeds named
_Cordaicarpus Cordai_ (Gein.), but there is no evidence of connexion.
Kidston[574] points out that this seed is rare in Britain: he believes
that _Cordaianthus Pitcairniae_ (Lind. and Hutt.) is probably the
inflorescence of _C. principalis_.
_Cordaites principalis_ occurs in both Carboniferous and Permian
strata. The leaves described by Lesquereux[575] from Pennsylvania as
_C. Mansfieldi_ agree closely with _C. principalis_. Another similar or
possibly identical form is represented by _C. Ottonis_ Gein.[576]
_Cordaites borassifolius_ (Sternberg).
The leaves of this species, originally referred to _Flabellaria_[577],
resemble those of _C. principalis_ but differ in the ovate-lanceolate
and less obtuse apex and in the presence of only one or rarely two
finer striations between the stronger ribs. Corda’s drawing[578]
affords a good illustration of the crowded spiral disposition of the
foliage comparable with that on an _Agathis_ shoot. The lamina is
usually 4–8 cm. broad but in exceptional cases may reach a breadth
of 12 cm. The species occurs in the Coal Measures, especially in the
Westphalian series and in Permian rocks. Feistmantel[579] unites with
this type _Cordaianthus Pitcairniae_ (fig. 480, A), but as in other
cases there may be a confusion between _C. borassifolius_ and _C.
principalis_. Leaves described by Lesquereux as _C. communis_[580]
are, as White says, not distinguished by any well-marked characters
from this species. White[581] figures some good examples of _C.
borassifolius_ from Missouri, reaching in one case a length of 40 cm.,
showing on the lamina the fructifications of a fungus, _Hysterites
cordaitis_[582] Grand’Eury. The leaves described from Canada and the
United States as _C. Robbii_[583] Daws. are closely allied to if not
identical with Sternberg’s type. Among other species differing in
no definite character from _C. borassifolius_ is _C. lancifolius_
described by Schmalhausen[584] from the Permian of Russia.
_Cordaites lingulatus_ Grand’Eury.
The leaves of this species[585] are characterised by the obovate
lamina and bluntly rounded or almost truncate apex; it affords a
good illustration of the uncertainty of the ribbing as a diagnostic
character. The lamina of a well-preserved specimen from the Blanzy
coalfield described by Zeiller[586] reaches a length of 35 cm. and a
breadth of 10–11 cm. decreasing to 4 cm. at the base (fig. 467). In the
lower part of the lamina Zeiller describes the ribs as unequal in
prominence, the stronger ones being separated by 1–3 finer ribs, while
in the middle and upper portions the ribs appear to be of equal size.
Some of the finer ribs are due to folding of the lamina and are not
represented, as are the ribs due to the presence of stereome-strands,
by dark streaks in the detached cuticle.
[Illustration: Fig. 467. _Cordaites lingulatus._ (After Zeiller; ⅚
nat. size.)]
Reference has already been made to the anatomical features of leaves of
this species described by Lignier[587] and other authors.
_Cordaites grandifolius_ Lesquereux.
The leaves so named by Lesquereux[588], from the Coal Measures of
Pennsylvania, are distinguished by the elongate cuneate lamina, which
reaches a length of 38 cm. with a narrow base and a slightly rounded
truncate distal end, 16 cm. broad, characterised by a few broad and
shallow crenulations. By contrast with some American specimens in Dr
Kidston’s collection Lesquereux’s figures convey an imperfect idea of
the size of the leaf. A large leaf from the Coal Measures of Belgium
described by Cambier and Renier as a new species of _Psygmophyllum_,
_P. Delvali_[589], is perhaps identical with the American type; the
lamina of sub-triangular form has approximately the same dimensions;
the veins are numerous and repeatedly forked. The leaf is much longer
than any known _Psygmophyllum_ and the veins are much more numerous
than in _P. majus_ Arb.[590], the largest representative of that genus.
Palaeobotanists who have seen the type-specimen inform me that they
have no doubt as to the Cordaitean nature of the Belgian specimens,
which may be designated _Cordaites Delvali_. It is, however, not
impossible that _Psygmophyllum_ and _Cordaites_ are allied genera: our
knowledge of the former is limited to unimportant characters.
_Cordaites_ (_Dory-Cordaites_) _palmaeformis_ (Goeppert).
This Permian type, originally described by Goeppert[591] as
_Noeggerathia palmaeformis_, is characterised by numerous slender
veins: according to Weiss[592] there may be as many as 3–5 in 1 mm.
The leaf is broadly lanceolate; it tapers gradually to an acute
apex reaching a length of 80 cm. and a breadth of 10 cm. In habit
the young foliage-shoots[593] resemble those of _C. principalis_ and
_C. borassifolius_. Grand’Eury records the frequent association of
_Samaropsis_ seeds with this species; it occurs in Upper Carboniferous
and in Permian strata and is recorded from a few British localities.
_Cordaites_ (_Poa-Cordaites_) _microstachys_ Goldenberg.
Weiss[594] first figured this species from drawings supplied by
Goldenberg at whose suggestion the name _C. microstachys_ was adopted.
The type-specimen consists of a slender axis bearing numerous narrow
linear leaves and a few imperfect fertile axillary shoots. A specimen
is figured by Kidston[595] from the Upper Coal Measures of Radstock: it
is a rare type in Britain. The species is readily distinguished from
_C. principalis_ and similar forms by the narrow lamina which varies
considerably in length, rarely as long as 30 cm. and not exceeding 1
cm. in breadth. The apex is obtuse and the ribs are either equal in
strength or 1–2 finer striae may alternate with the stronger ribs. The
base of the lamina is 3–4 mm. wide and the leaf-scars have a slightly
arched upper margin and an almost straight lower edge[596]. The foliage
of this species, generally regarded as identical with _C. linearis_
Grand’Eury, bears a close resemblance to that of the Mesozoic genus
_Phoenicopsis_ from which it is distinguished by the occurrence of the
leaves in bunches.
_C. gracilis_ Lesq.[597] is a similar type. The shoot on which
Lesquereux founded his genus _Desmiophyllum_[598] may perhaps be an
example of _Poa-Cordaites_. _Poa-Cordaites tenuifolius_ Schmal.[599]
from the Permian of Russia may be identical with _C. microstachys_.
As examples of other forms of leaf referred to _Cordaites_, though as
in other cases without any proof of connexion with branches having the
anatomical features of the genus, reference may be made to _Cordaites
circularis_ Grand’Eury[600] from Gard (fig. 468, B) and a smaller leaf
from the same locality compared with _C. Lacoei_ (fig. 468, C) Lesq.
_Cordaites circularis_ is characterised by the almost orbicular lamina
traversed by slightly spreading veins; it recalls some of the larger
_Cyclopteris_ pinnules of Pteridosperm fronds and is indistinguishable
from some leaves assigned to the genus _Dolerophyllum_[601].
[Illustration: Fig. 468. A, _Cordaites_ root-system
(_Rhizo-Cordaites_); B, _Cordaites circularis_ leaf; C, _Cordaites_
sp., cf. _Cordaites Lacoei_; D, _Cordaites crassifolius_, upper and
lower surface of leaf. (After Grand’Eury.)]
The species _C. Lacoei_ was founded by Lesquereux[602] on some detached
specimens 3–12 cm. long and 1·5–5 cm. broad; it is by no means certain
that a specimen referred by Grand’Eury[603] with some hesitation to
this species is Cordaitean.
The generic name _Scuto-Cordaites_ was proposed by Renault[604] for
a specimen from Commentry consisting of a flattened branch bearing
a few imperfectly preserved leaves. The surface of the branch shows
semicircular leaf-scars on decurrent, spirally disposed leaf-cushions
and bears a certain resemblance to a slender stem of a Clathrarian
_Sigillaria_. The leaves of the type-specimen of _Scuto-Cordaites
Grand’Euryi_ appear to be broadly linear, 13 cm. long, the breadth
gradually increasing from the base: a short distance from the proximal
end the lamina is broken up into narrow segments; the veins are ·5 mm.
apart with finer striations between them.
Some specimens from Pennsylvania made by Dawson[605] the type of
a new sub-genus and named _Dictyo-Cordaites Lecoi_ agree in shape
and arrangement with some species of _Cordaites_, but differ in an
occasional anastomosis of the veins as in _Psygmophyllum flabellatum_.
It is, however, impossible to determine the true nature of the fossils
from the published figures.
+Cordaitean leaves from India, the Southern Hemisphere, and Siberia.
_Noeggerathiopsis_, Feistmantel; _Rhiptozamites_, Schmalhausen;
_Euryphyllum_, Feistmantel.+
{_Cordaites aequalis_ Goeppert.
{_Cordaites_ (_Noeggerathiopsis_) _Hislopi_ (Bunbury).
In 1845 Goeppert[606] instituted the species _Noeggerathia aequalis_
(fig. 469) and _N. distans_ for incomplete broadly linear and
obovate leaves, from Siberian Permian strata, having a contracted
base and equal parallel veins. The specimens so named are no doubt
specifically identical. Goeppert’s species _N. aequalis_ has recently
been carefully investigated by Zalessky[607] who agrees with
Kosmovsky[608] in identifying it with _Noeggerathiopsis Hislopi_
(Bunb.) and _Rhiptozamites Goepperti_ Schmal. Schmalhausen[609] had
previously pointed out the probable identity of his species with
_Noeggerathia palmaeformis_ Goepp. (= _Cordaites_). The question of
specific identity of these leaves from different localities and of
other hardly distinguishable forms is of secondary importance; the main
point is that they are all examples of Cordaitean leaves, _Cordaites_
or some allied genus, and point to the existence of this group of
Gymnosperms during Permo-Carboniferous times in Siberia, China, India,
Australia, South Africa, and S. America, also in the Rhaetic floras
of Tonkin[610] and Mexico[611]. The fragments from Devonian strata at
Iguana Creek, Australia, named by McCoy[612] _Cordaites australis_ are
probably pieces of the rachis of some large frond.
[Illustration: Fig. 469. A–C, E, _Cordaites aequalis_; D, _Cordaites
Clerci_. (After Zalessky; ⅚ nat. size.)]
Wieland[613] recently discovered Cordaitean leaves exhibiting a wide
range in size and shape in the Mixteca flora of Mexico in the lower
members of a series which extends from the ‘upper borders of the
Rhaetic’ through the Liassic to the lower beds of the Inferior Oolite.
These leaves are referred to _Noeggerathiopsis Hislopi_, and it is
clear from an examination of photographs received from Dr Wieland,
one of which is reproduced in fig. 470, that the Mexican _Cordaites_
cannot be specifically distinguished from Bunbury’s type as represented
by specimens described from India, South Africa, Siberia, Tonkin, and
elsewhere.
The occurrence of _Noeggerathiopsis_ is also recorded by Newberry from
the Rhaetic series of Honduras[614].
_Noeggerathiopsis._ This genus was founded by Feistmantel[615] for
some leaves from Lower Gondwana rocks in India originally described
by Bunbury[616] as _Noeggerathia (Cyclopteris?) Hislopi_ (figs.
470–472) and regarded by him as probably Cycadean. Several authors
have added to our knowledge of this widely spread southern type and
in many localities the leaves occur in association with platyspermic
seeds of the _Samaropsis_ or _Cordaicarpus_ type, pieces of stems with
Cordaitean leaf-scars, and petrified wood agreeing in the structure
of the secondary xylem with that of European species of _Cordaites_.
In some Permo-Carboniferous sandstones at Vereeniging, South Africa,
stumps and spreading roots (fig. 478) resembling those described from
France by Grand’Eury (cf. fig. 468, A) have also been discovered. A
remarkable occurrence of roots and prostrate stems of some forest-tree
was recorded some years ago in the bed of the Vaal river near
Vereeniging where the surface of a seam of coal was exposed over an
area of more than two acres[617]. Large branched roots (fig. 478)
spreading over the coal for a distance of several feet and thick stems
40–50 ft in length with very few branches and but little decrease in
diameter afford a striking picture of a forest-floor. The frequent
occurrence of _Cordaites (Noeggerathiopsis) Hislopi_ in the associated
strata suggests a reference of the stems and roots to that species.
Moreover the structure of the secondary xylem of some petrified pieces
of stem sent to me by Mr Leslie from Vereeniging agrees closely with
that of a European Cordaitean stem.
[Illustration: Fig. 470. _Cordaites (Noeggerathiopsis) Hislopi_. From
Mexico. (After Wieland; ⅚ nat. size.)]
[Illustration: Fig. 471. _Cordaites (Noeggerathiopsis) Hislopi_. From
Vereeniging. (A, _ca_, ⅕ nat. size; B, _ca_, ⅙ nat. size.)]
[Illustration: Fig. 472. _Cordaites (Noeggerathiopsis) Hislopi_. From
India. (Calcutta Museum; ⅔ nat. size.)]
The leaves of _Cordaites (Noeggerathiopsis) Hislopi_ vary considerably
in size, in some cases reaching a length of 80 cm. (fig. 471); the
lamina tapers gradually from a short distance behind the obtuse apex
to a relatively narrow base: in venation and form the leaves are
very similar to those of _C. principalis_ and other European and
North American species. The specimen from India represented in fig.
472 shows several spathulate leaves attached in a close spiral to
a branch. As White[618] and Zalessky have shown, the stronger ribs
are separated by less prominent striations indicating the presence
of two sizes of hypodermal strands. The obvious resemblance between
_Noeggerathiopsis Hislopi_ and species of _Cordaites_ has long been
recognised and many authors have included Feistmantel’s genus in the
Cordaitales[619]. Prof. Zeiller[620] preferred to retain the name
_Noeggerathiopsis_ as a precautionary measure, chiefly on the ground
that the stomata appeared to be less definitely arranged in rows
and more scattered than in the European leaves of _Cordaites_, and
because of the absence of interstitial veins. We have as yet little
information as to the arrangement of the stomata, but in view of the
irregularity in stomatal grouping in recent leaves this feature is,
perhaps, of minor importance. The presence of interstitial ‘veins’ has
now been established in Indian[621] and South American[622] leaves. In
a paper published in 1908[623] the name _Cordaites_ was substituted for
_Noeggerathiopsis_ and Zalessky’s recent work supports this step. The
description by Zalessky of the ribbing in Goeppert’s species _Cordaites
aequalis_ from Siberia shows how uncertain and variable a character the
venation is even in different parts of the same leaf.
_Cordaites Clerci_ Zalessky.
This species (fig. 469, D) was instituted for some small lanceolate
or spathulate leaves from the Petschora basin (Adzva River)[624]
reaching a length of 6 cm. and a breadth of 1 cm. It is separated from
_Cordaites aequalis_ on the ground that the veins are more numerous, as
many as 44 in a breadth of 1 cm.
A recent investigation by Miss Holden[625] of the carbonised
cuticles of some Indian specimens, sent to Cambridge by the Director
of the Indian Geological Survey, and a comparison of them with
preparations made from European _Cordaites_ leaves, have revealed
certain distinguishing features which support Zeiller’s view that
the Gondwana-Land leaves, though similar superficially to those of
_Cordaites_, are probably distinct. It is, however, impossible in many
cases to obtain any information with regard to epidermal characters,
and though it would seem probable that had we a fuller knowledge of
the Indian and southern hemisphere plants represented for the most
part by leaf-impressions well-defined distinguishing features would
be recognised, the comprehensive name _Cordaites_ may conveniently
be retained on the ground that in the absence of well-preserved
cuticles no satisfactory distinguishing features are exhibited by the
impressions of _Noeggerathiopsis_.
=Phylladoderma.= Zalessky.
_Phylladoderma Arberi_ Zalessky.
Zalessky[626] founded this genus on some Permian leaves from the
Petschora basin (Adzva River) which closely resemble those of
_Cordaites_ but are characterised by a coarser venation. The lanceolate
lamina reaches a length of 18 cm. and a breadth of 4·2 cm.; the veins
are 2 mm. apart and occasionally forked near the base of the leaf. The
epidermal cells have straight walls and stomata are abundant on the
lower surface. As Zalessky says, the systematic position of the leaves
is uncertain though they are probably Cordaitean. The coarseness of the
venation is a feature of minor importance and hardly worthy of generic
recognition.
_Rhiptozamites_ Schmalhausen.
This genus was instituted by Schmalhausen[627] for leaves from beds
in the Kusnezk basin regarded by him as Jurassic. These strata are
now recognised as Permian[628] and homotaxial with those from which
Schmalhausen[629] subsequently recorded the same species. The leaves,
though smaller than many of the Indian and South African specimens
of _Cordaites_ (_Noeggerathiopsis_) _Hislopi_, may belong to that
species. Zeiller and others definitely assigned the Russian leaves to
_Cordaites_.
_Euryphyllum._ The Indian leaves for which Feistmantel[630] proposed
this name are, as several writers have pointed out, in all probability
referable to _Cordaites_.
The general conclusion to be drawn from this imperfect summary of
an extensive literature is that the employment of the generic names
_Noeggerathiopsis_, _Rhiptozamites_, _Euryphyllum_, and others has
tended to exaggerate the difference between the European and Southern
botanical provinces during the Permo-Carboniferous period.
_Scale-leaves, seeds, and stems._
The occurrence of small scale-like leaves of the type represented in
fig. 468, C, in association with _Cordaites_ (_Noeggerathiopsis_)
_Hislopi_[631] in India, Brazil, Siberia, and elsewhere may mean that
these organs are scales of large foliar buds. The occurrence of several
forms of platyspermic seeds, in some cases apparently identical with
European forms and sometimes distinct types, in close association with
_Cordaites_ (_Noeggerathiopsis_) _Hislopi_ has already been mentioned.
Examples of such seeds are described in Chapter +xxxv.+ under the genus
_Samaropsis_.
There are very few satisfactory examples of Cordaitean branches from
the southern hemisphere. Schmalhausen[632] figures good specimens from
Siberian rocks from which his _Rhiptozamites_ leaves were obtained.
Branches with spirally disposed leaf-scars figured by Zeiller[633] from
the Rhaetic of Tonkin closely resemble _Cordaicladus_. Feistmantel’s
drawing of a fossil from the Karharbari series, compared by him
with a Fern rhizome[634], may be a Cordaitean branch, and the same
author describes a stem[635] from New South Wales as _Caulopteris
Adamsi_ which bears a close resemblance to a branch of _Cordaites_.
Similarly a leafy shoot described from India by Zeiller as _Araucarites
Oldhami_[636] may be compared with branches of the _Poa-Cordaites_ type.
[Illustration: Fig. 473. _Cordaites_ (or _Mesoxylon_?) stem showing
the discoid pith partially enclosed by wood. (½ nat. size.) M. S.]
+Stems.+ i. _Pith-casts._
=Artisia.= Sternberg.
A character to which authors tend to attach excessive importance
as a diagnostic feature is the almost invariable tendency of the
parenchymatous pith of _Cordaites_ to break up on contraction into
transverse diaphragms, thus producing what is known as a discoid pith.
In the stem shown in fig. 473 the pith is represented by a more or
less cylindrical cast characterised by fairly regular transverse ribs
and narrow grooves; in the upper part of the fossil the peripheral
tissue of the pith is preserved in the form of narrow plates projecting
from the inner face of the wood. As Renault[637] pointed out, this
type of pith is the expression of certain conditions of growth and is
not a satisfactory distinguishing feature of any particular genus or
family. The same tendency to form a discoid pith is characteristic
of _Mesoxylon_, and it occurs also in some other Palaeozoic genera.
Corda long ago figured a stem attributed by him to _Lomatofloyos_ with
a typical discoid pith, and a similar pith is recorded in a stem of
_Dicranophyllum_[638]. Among recent plants _Juglans regia_ affords
perhaps the most familiar instance of an identical form of pith: the
same type occurs in the white Jasmine, in _Ceropegia peltata_, and
some other flowering plants. An interesting case is that of the tree
Groundsel, _Senecio praecox_ D.C.[639], of Mexico: in this plant,
which grows in arid districts, the pith serves as a water-store and as
the water is drawn off the thick turgescent discs contract and form
thin transverse diaphragms separated by wide spaces, as is also the
case on drying in some succulent _Euphorbia_ stems. It may be that in
_Cordaites_ the medullary region also served as a water-reservoir and
the depth of the medullary discs would vary according to the state of
their contents.
The earlier writers regarded the pith-casts as stems with scars of
amplexicaul leaves: Artis[640] described specimens from the English
Coal Measures as _Sternbergia_, one of which he stated to be 6 ft
long; a few years later Sternberg[641] proposed the name _Artisia_ and
this has been generally used on the ground that _Sternbergia_ is the
name of a recent flowering plant. A specimen of _Artisia transversa_
(Art.) from the Coal Measures of Yorkshire is shown in fig. 466, B, and
similar specimens varying considerably in diameter up to about 10 cm.
are abundant in European and American Coal Measures. The prominence and
depth of the transverse ridges, the presence or absence of anastomoses
between adjacent discs are, as Zeiller[642] says, of very doubtful
value as specific characters. Dawson in 1846[643] spoke of _Artisia_ as
probably the pith of a tree, a view suggested to him by Mr Dawes. In
1851 Williamson[644] published a description of some specimens in which
a pith-cast, _Artisia approximata_ Lind. and Hutt., was enclosed by
wood showing very clearly Cordaitean characters. Further demonstration
of the true nature of _Artisia_ was supplied by Grand’Eury from St
Étienne material. If the generic name _Artisia_ is applied to all
pith-casts showing the transverse ridges and grooves like those seen
in fig. 466, B, it must be remembered that it is not safe to assume a
connexion with _Cordaites_ or _Mesoxylon_. A Liassic species described
by Lignier[645] from France as _Artisia alternans_ is quoted by
authors as evidence of the persistence of _Cordaites_ into the Jurassic
period; but in view of the fact that the discoid type of pith is not
by any means confined to _Cordaites_ or even to the Cordaitales the
occurrence of _Artisia_ is in itself of no great botanical significance.
It is also true that a discoid pith is not an invariable attribute
of stems closely allied to the genus _Cordaites_; but if these
reservations are made the use of the generic term _Artisia_ serves a
useful purpose.
ii. _Petrified stems._
=Dadoxylon.= Endlicher.
Palaeobotanical literature contains numerous descriptions of Palaeozoic
petrified wood occasionally enclosing an _Artisia_ pith-cast described
under such names as _Dadoxylon_, _Cordaioxylon_, _Araucarioxylon_,
etc., and regarded as portions of Cordaitean stems. It is, however,
certain that much of this material belonged to stems other than those
of _Cordaites_. Recent research has demonstrated the insufficiency of
the secondary xylem alone, however well preserved, as a safe guide to
generic position: stems identical in the structure of the secondary
xylem differ in that of the primary portion of the stele, and it is on
the characters of the latter tissues that several genera have recently
been founded. _Mesoxylon_ affords a striking example of the importance
of the primary xylem as a distinctive feature. As Gothan[646] points
out, the species of _Calamopitys_ recently made the type of a new
genus _Eristophyton_[647] would, in the absence of the primary xylem,
probably be regarded as Cordaitean. It is important to recognise the
limitations imposed by the imperfection of the material; we cannot
in most cases determine whether a specimen should be referred to
_Cordaites_ or _Mesoxylon_, and while it may be described as probably
Cordaitean in affinity there remains the possibility that some of the
Palaeozoic plants with secondary wood like that of _Cordaites_, if
their reproductive organs were known, would not be included in the
Cordaitales. Goeppert’s species _Araucarites Tchihatcheffi_, which
Renault[648] quotes as _Cordaites_, has recently been assigned to a
new genus _Mesopitys_[649] because of certain distinctive features of
the primary xylem. Additional examples might be quoted pointing to the
tendency of recent and more thorough investigation to establish the
fact that the occurrence of Permo-Carboniferous wood of the Araucarian
type does not necessarily denote the existence of _Cordaites_. The
question of nomenclature is necessarily raised in this connexion.