CYCADOFILICES.
In this chapter are included several types represented by stems,
but which in the absence of definite information with regard to the
reproductive organs cannot be assigned to the Pteridosperms.
=MEGALOXYLEAE.=
=Megaloxylon.= Seward.
This genus is represented by a single species founded on a piece of
stem from the Lower Coal Measures of Lancashire discovered in the
Binney Collection in the Sedgwick Museum, Cambridge[466]. Nothing
is known as to the leaves, reproductive organs, or roots. The
type-specimen consists exclusively of primary and secondary xylem.
_Megaloxylon Scotti_ Seward.
The type-specimen, reproduced natural size in fig. 446, A, B, consists
of a fragment of stem which at first sight bears a close resemblance
to _Cordaites_, but the apparent pith, 1·9 cm. in diameter, is the
primary xylem of the stele and is enclosed by an incomplete cylinder
of secondary xylem 2 cm. broad. The distinction between the lighter
primary xylem and the darker secondary cylinder is especially well
marked in the longitudinal section (fig. 446, B). The central region,
shown on a larger scale in fig. 447, is occupied by groups of tracheids
varying in size and shape associated with thin-walled parenchyma: the
latter is represented by lighter patches in the section. The majority
of the tracheids are characterised by their great breadth—in some
cases ·4 mm.—and their isodiametric or even horizontally elongated
and flattened form. Some of these large metaxylem elements are seen in
longitudinal section at _m_ in fig. 448 where the shrinkage and partial
decay of the parenchymatous tissue have resulted in the separation of
transverse bands of xylem simulating the discoid pith of a _Cordaites_.
The walls of these tracheal cells are covered with multiseriate pits.
With the short and sometimes flattened xylem elements occur others of
greater length, but these are chiefly met with in the more peripheral
part of the central region where some of the tracheids are much
narrower and have the form of ordinary water-conducting elements. On
one side of the primary xylem in fig. 447 an oval and more compact
group of narrow and longer tracheids is seen at _lt¹_; this is a
leaf-trace about to enter the secondary-xylem cylinder on its outward
course. The same leaf-trace is shown at _lt_ in fig. 446, B; as it
descends the trace becomes less distinct and its elongated elements
gradually merge into the general mass of metaxylem. A portion of this
leaf-trace is seen in fig. 448, B, _lt_, close to the inner edge of
the secondary xylem, _x²_, and abutting internally on the contracted
tissue, _m_, which consists mainly of large and short tracheae with
remains of associated parenchyma. The trace includes some conjunctive
parenchyma interspersed with the tracheids: on its outer surface, that
is on the abaxial edge of the ovate xylem strand as shown at _lt¹_ in
fig. 447, there are six external protoxylem strands. In the peripheral
region of the primary xylem of the section reproduced in fig. 447
there are several more or less well-defined leaf-traces, _e.g._ _lt²_,
_lt³_; these differ from that seen at _lt¹_ in their greater tangential
breadth and in the less compact arrangement of the tracheids. As each
trace is followed downwards in the primary region of the stele it tends
to become broader, especially in a tangential direction; the spiral
protoxylem strands are more widely separated (fig. 448, A, _px_) and
the elongated and comparatively narrow tracheids as they spread out
fan-wise are reduced in length, finally passing over into the broad
and short reticulately pitted cells. Each leaf-trace can be followed
through approximately four internodes before its individuality is lost
in the general mass of metaxylem. The disposition of the peripheral
traces is such as to justify the conclusion that the phyllotaxis of the
stem is ⅖.[467]
[Illustration: Fig. 446. _Megaloxylon Scotti._ A, B. Transverse and
longitudinal sections. _x¹_, _x²_, primary and secondary xylem; _lt_,
leaf-traces. Nat. size. C. Secondary xylem.]
[Illustration: Fig. 447. _Megaloxylon Scotti._ Transverse section of
the primary portion of the stele with leaf-traces, _lt_, and the inner
edge of the secondary xylem.]
[Illustration: Fig. 448. _Megaloxylon Scotti._ A, transverse section
at the junction of the primary and secondary tissues; B, longitudinal
section showing the inner edge of the secondary xylem, _x²_, a
leaf-trace, _lt_, and the metaxylem, _m_.]
The secondary xylem (fig. 446, C) resembles that of _Lyginopteris_
and _Heterangium_ though it is less parenchymatous. The medullary
rays are numerous and vary in breadth from 1 to 5 cells, while the
tracheids, with multiseriate bordered pits on their radial walls, form
bands 1 to 8 elements in breadth. There are no regular rings of growth
but occasional arcs of narrow tracheids interfere slightly with the
otherwise uniform structure of the wood. A leaf-trace in its oblique
outward course through the wood becomes completely enclosed by a
cylinder of secondary xylem and thus appears to be concentric. Owing
to the absence of any tissue external to the secondary xylem of the
stem, statements as to the subsequent behaviour of the leaf-traces on
emerging from the stele are purely hypothetical. It is, however, not
improbable that each concentric trace lost its secondary tissue and
broke up into several collateral strands, a suggestion based on the
behaviour of the leaf-traces in _Medullosa anglica_.
The most striking characteristics of _Megaloxylon_ are: (i) the
structure of the primary xylem, particularly the unusual form of the
majority of the metaxylem tracheids, a form obviously correlated
with storage rather than with conduction of water; (ii) the gradual
spreading of the leaf-traces and their absorption as they descend into
the main mass of the xylem; (iii) the exarch structure of the primary
xylem. Confining our attention to the primary region of the stele; a
comparison is at once suggested with _Heterangium_. In _Megaloxylon_
the peculiarities are the substitution of the large storage-tracheids
for the normal xylem-elements; the greater irregularity in the groups
of metaxylem; and an exarch instead of a mesarch structure. In these
last features the primary xylem agrees with that of recent species of
the Schizaeaceous Fern _Lygodium_ and the external protoxylem is a
character shared with _Rhetinangium_. The occurrence of short tracheids
similar to those of _Megaloxylon_ in the inner portion of the stele of
the Osmundaceous Fern _Zalesskya gracilis_ (Eich.)[468] may be quoted
as an example of parallel modification but, as Scott[469] points out,
the resemblance has no phylogenetic significance. The secondary xylem
though less parenchymatous than in recent Cycads agrees more closely
with the manoxylic than with the pycnoxylic type.
_Megaloxylon_ affords an interesting example of a combination of
primary stelar anatomical features, comparable in the exarch position
of the protoxylem with the stele of _Lygodium_, and secondary wood
similar to that of _Lyginopteris_ and _Heterangium_. The large
metaxylem tracheids may be regarded as derivatives of elements which
in some ancestral type were structurally fitted for the rôle of
water-transport and made up the xylem of a _Lygodium_-like stele with
little or no secondary xylem. As the cambial activity increased and
centrifugal xylem became a prominent feature, usurping the function of
the centripetal xylem, the latter became modified and fitted for a new
service.
=RHETINANGIEAE.=
=Rhetinangium.= Gordon.
_Rhetinangium Arberi_ Gordon.
The stem on which this genus is founded was discovered by Dr
Gordon[470] in the Calciferous Sandstone series of Pettycur: a
specimen collected by Dr Kidston in Berwickshire may be specifically
identical with the Pettycur plant. We know nothing of the leaves or
reproductive organs of _Rhetinangium_. The stem, approximately 2 cm.
in diameter, was probably cylindrical; it possesses a single stele
consisting mainly of a central primary region occupied by anastomosing
groups of tracheids, 130–150μ in diameter, embedded in parenchyma
containing numerous secretory sacs and ducts. In the peripheral region
of the stele the groups of tracheids consist of narrower elements
characterised by exarch protoxylem. Each peripheral group forms the
base of a wedge of secondary xylem (fig. 450, _x²_), the primary
medullary rays being in direct connexion with some of the parenchyma of
the primary xylem. The secondary tracheids, 45–85μ in diameter, have
multiseriate bordered pits on their radial walls and the rays are broad
and deep as in _Heterangium_ and _Lyginopteris_. With the exception of
the external position of the protoxylem, the stele of _Rhetinangium_
is practically identical with that of _Heterangium_, though in
_Rhetinangium_ the primary tracheids form larger and fewer groups. The
inner cortex is composed of thin-walled cells with many secretory sacs:
there are no stereome elements. In the outer cortex (fig. 449, _cr_)
radially disposed bands of stereome form a reticulum with narrow and
very long meshes like that of Medullosan petioles.
The recently recorded occurrence of polydesmic petioles[471] in
_Heterangium_ is of special interest from the point of view of the
comparison of that genus with _Rhetinangium_ and the Medulloseae.
The very broad decurrent petiole-bases are a striking feature, the
major diameter of the pulvinus-like base of the leaf-stalk exceeding
that of the stem (fig. 449, _p_). Several xylem-strands from the
peripheral region of the primary xylem go to form a single leaf-trace:
these U-shaped strands of xylem destined for a leaf are connected
laterally by parenchyma and form an irregularly corrugated band. Fig.
450 shows a petiole-trace still enclosed on each side by the secondary
xylem cylinder _x²_.
[Illustration: Fig. 449. _Rhetinangium Arberi_ Gord. Transverse
section of stem with large leaf-base; _cr_, outer cortex of stem; _p_,
petiole. (After Gordon; × 1¾.)]
There are several protoxylem groups in a leaf-trace, one on the outer
face of each xylem-strand. In the petiole the xylem groups are more
intimately connected and the trace has the form of a flat band with
abaxial protoxylem. There is no indication that a leaf-trace undergoes
division into separate strands. The roots are described as tetrarch
with well-developed secondary xylem.
[Illustration: Fig. 450. Transverse section of leaf-trace of
_Rhetinangium Arberi_ showing the exarch structure; _x²_, secondary
xylem. (After Gordon.)]
The primary xylem of _Rhetinangium_ agrees in its exarch structure with
the Palaeozoic genera _Sutcliffia_, _Megaloxylon_ and _Stenomyelon_,
also with _Lygodium_ and some other recent Ferns: the secondary wood
is of the manoxylic type like that of _Lyginopteris_, _Heterangium_
and other genera. In the general structure of the stele _Rhetinangium_
agrees with _Sutcliffia_ and, except in the exarch structure of
the primary xylem, with the steles of _Heterangium_ and _Medullosa
anglica_[472]; but the structure and origin of the leaf-traces are
characters which mark it off from _Sutcliffia_. The sclerenchymatous
bands in the inner cortex of _Heterangium_ are unrepresented in
_Rhetinangium_, and in the latter genus the abundance of secretory sacs
and ducts is a characteristic feature, moreover in _Rhetinangium_,
the leaf-trace consists of several groups of primary xylem-elements.
Dr Gordon regards _Megaloxylon_ as the type which comes nearest to
_Rhetinangium_; but the differences in the structure of the secondary
wood and the marked contrast between the leaf-traces are too pronounced
to justify a preference for _Megaloxylon_ over _Heterangium_ in the
order of affinity. Gordon considers that the undivided leaf-trace of
_Rhetinangium_ may represent a form transitional between the simple
leaf-trace of _Lyginopteris_ and the much divided type in _Medullosa_.
The external position of the protoxylem is a character to which too
much weight may easily be attached: the difference in position between
the protoxylem of _Rhetinangium_ and _Heterangium_ is in some examples
of the latter genus hardly perceptible. Kubart[473] speaks of the
stele of his species _Heterangium Sturi_ as being almost exarch.
The inconstancy in the position of the protoxylem in the xylem of
Osmundaceous stems and in the primary bundles of _Eristophyton_ and
other Palaeozoic genera is worthy of consideration in this connexion.
[Illustration: Fig. 451. _Stenomyelon tuedianum_ Kidst. Transverse
section of stem. The black patches represent leaf-traces. (After
Kidston; × 1½.)]
=STENOMYELEAE.=
=Stenomyelon.= Kidston.
_Stenomyelon tuedianum_ Kidston. The specimens on which this monotypic
genus is founded[474] were obtained from the Lower Carboniferous
rocks (Calciferous Sandstone series) at Norham Bridge, Berwickshire,
Scotland. They consist of petrified pieces of a flattened stem, a
fragment of a rachis and portions of laminae: there is no evidence as
to the nature of the reproductive organs. The original form of the stem
is obscured by the destruction of a considerable part of the cortex and
the consequent flattening of the whole with the production of wing-like
extensions of the imperfectly preserved tissues enclosing the almost
cylindrical stele (figs. 451, 452).
The stele consists of a bluntly triangular core of primary xylem,
3–4 mm. in diameter, composed almost entirely of reticulately pitted
tracheids reaching a diameter of 160μ: a few parenchymatous cells
occur in the peripheral region and a band of parenchyma extends from
the middle of each of the three sides of the xylem to the centre of
the stele, thus dividing the primary conducting tissue into three
groups which are the expression of a phyllotaxis of ⅓. The tracheids
near the outer face of each xylem-group are narrower than the others
and have scalariform pitting. The secondary xylem first appears along
the slightly concave sides of the primary stele, eventually enclosing
the whole: it consists of tracheids with multiseriate pits on the
radial walls and numerous deep medullary rays 1–6 cells broad. No
phloem is preserved though it is probable that a narrow band was
originally present. A characteristic feature is afforded by a zone of
thick-walled cells, regarded as periderm, encircling the stele and
formed by a deep-seated phellogen. On the outer face of this band there
are projecting bosses, and similar sclerous nests are scattered in the
cortex. The outer cortex has a _Sparganum_[475] type of hypoderm, that
is long vertical strands of fibres alternating with parenchyma. The
leaf-traces are formed from the blunt angles of the primary xylem; an
angle becomes nipped off as a more or less cylindrical strand enclosed
by a zone of secondary tracheids which is very narrow on the adaxial
side (fig. 452). Protoxylem was recognised only in the leaf-traces and
not in the rest of the stele. A pair of protoxylem strands occurs on
the outer edge of a prominent angle of xylem before it becomes detached
from the stele, and these form a single strand at a lower level. As a
leaf-trace passes outwards, the exarch xylem strand becomes mesarch
and there is a single protoxylem group except at a point near the
bifurcation of a trace. In its passage through the cortex a leaf-trace
divides repeatedly, the secondary xylem on the outer face of each
strand being retained for a considerable time.
Our meagre knowledge of the nature of the leaves is based on incomplete
fragments found in association with the stem. The leaf is believed to
have been simple and characterised by a thick lamina with a hypodermal
zone of sclerous strands and several vascular bundles.
[Illustration: Fig. 452. _Stenomyelon tuedianum_. Transverse section
of stele. (After Kidston; × 7.)]
As Kidston and Gwynne-Vaughan[476] remark, _Stenomyelon_ is a very
distinct type; while resembling _Sutcliffia_ in some respects it
differs from that genus not only in the structure of the primary
stele but in the absence of the system of meristeles which form so
characteristic a feature of the latter genus.
=CYCADOXYLEAE.=
=Cycadoxylon.= Renault.
This generic term[477] is applied to a few types of Permian and Upper
Carboniferous stems possessing a vascular cylinder, which may reach a
considerable breadth, of secondary centrifugally developed xylem and
phloem enclosing a large pith containing either a narrow, peripherally
placed, and more or less continuous cylinder of inversely orientated
conducting tissue or scattered bands of centripetal xylem and phloem.
The secondary xylem is manoxylic, while the internal vascular tissue
recalls that of _Ptychoxylon_ and to a less extent the inverted arcs
that are rarely met with in _Lyginopteris_ stems. A brief diagnosis
of two species may serve to illustrate the genus: a third species is
included in _Cycadoxylon_, but it is founded on material too incomplete
to admit of satisfactory diagnosis.
_Cycadoxylon Fremyi_ Renault.
This Permian species[478] is represented by a piece of stem 2–2·5
cm. in diameter (fig. 453, B) characterised by (i) a fairly broad
parenchymatous cortex with secretory canals and several hypodermal
nests of sclerous tissue, (ii) a cylinder of secondary xylem and phloem
nearly the whole of which is centrifugal, (iii) a large pith containing
several scattered narrower bands or arcs of centripetally developed
xylem and phloem. The tracheids, with 4–6 series of hexagonal pits,
form radially disposed rows, 1–4 elements broad, separated by broad and
deep medullary rays. Renault does not mention the occurrence of any
primary xylem as distinct from the secondary centrifugal xylem, but
in a section which I examined some years ago in his laboratory there
appeared to be a group of primary tracheids. There are no anastomoses
between the main cylinder and the internal bands of inversely
orientated tissue.
_Cycadoxylon robustum_ (Seward).
This species[479] is based on a piece of stem from the Lower Coal
Measures of Lancashire first described by Williamson and identified
as an unusually large example of _Lyginopteris_. Williamson and
Scott, while recognising certain features in addition to the large
size of the stem, which must have reached 14 cm. in diameter,
expressed the view that ‘there is a presumption that it really
belonged to a _Lyginodendron_, or to some plant of the same type
of structure.’ The examination of additional material led me to
adopt the name _Lyginodendron robustum_, though I suggested
that possibly _Cycadoxylon_ might be the more appropriate genus.
Subsequently Scott[480] proposed the substitution of _Cycadoxylon_ for
_Lyginodendron_.
The type-specimen consists of secondary xylem agreeing closely in
structure with _Lyginopteris_ and recent Cycads: the pith, 2·9 cm.
in breadth, is incompletely preserved; there is a narrow band of
centripetal xylem[481] at the periphery of the perimedullary region
and close to the inner face of the main mass of wood (fig. 453, C;
the black line marks the position of the centripetal xylem). Nests of
sclerous tissue and secretory canals are scattered in the medullary
parenchyma and deeper in this region are arcs of secondary parenchyma,
possibly periderm. In places the centripetal and centrifugal xylem
are in contact and occasionally the tapered ends of the rows of
centrifugal tracheids merge into groups of primary xylem elements. The
preservation in the central region is far from complete, and although
the occurrence of primary xylem groups is probable it cannot be said to
be positively established. At the inner edge of the centrifugal xylem
and in tangential longitudinal sections a few leaf-traces are seen, but
nothing is known as to the nature of the leaf-traces in their course
beyond the stele nor have we any data with regard to the leaves or
reproductive organs.
This older species differs from _Cycadoxylon Fremyi_ in the limitation
of the centripetal xylem to the outer portion of the pith and in
the presence of sclerous nests in the medullary region, though the
latter character is probably of no great taxonomic value. _Cycadoxylon
robustum_ approaches more closely to _Lyginopteris_, and although the
differences are sufficient to justify a distinctive generic name, there
can be little doubt as to a fairly intimate relationship between this
type of _Cycadoxylon_ and _Lyginopteris_.
=Ptychoxylon.= Renault.
_Ptychoxylon_[482] _Levyi_ Renault. Like many Palaeozoic genera founded
on anatomical features, _Ptychoxylon_[483] is represented only by
stems, our knowledge of the leaves being confined to the leaf-traces
in the stem which appears to have a phyllotaxis of ⅜. The stem of
this Permian species has a diameter of 5–6 cm.: the comparatively
broad cortex contains numerous secretory canals, but in place of
hypodermal strands of stereome there is a superficial periderm. The
vascular tissue, consisting of secondary xylem and phloem, assumes
different patterns at different levels. There is an outer vascular
cylinder of centrifugally developed xylem and phloem; the xylem is
manoxylic and the tracheids have 3–5 rows of bordered pits on the
radial walls. At intervals the continuity of the main stele is broken
by the formation of leaf-gaps and before one gap is repaired a second
may be produced, thus converting the cylinder into two crescentic and
infolded bands (fig. 453, A). A striking character is the occurrence
in the large parenchymatous central region of internal vascular bands
or arcs varying in size and number at different levels and composed
of centripetally developed secondary xylem and phloem. These internal
bands differ from the outer and broader cylinder both in their inverse
orientation and their limited vertical range. The connexion between
the inner and outer vascular tissue and the alteration in plan of the
conducting tissue at different levels are illustrated by fig. 453,
1–4, simplified from some of Renault’s figures of successive sections
through a vertical distance of 4–5 cm. In section 1 the main cylinder
is continuous except for a small gap where a leaf-trace is about to be
given off: there are three internal vascular bands similar in structure
to the outer stele but inversely orientated. At a higher level (section
2) the leaf-gap is larger and in it is a double leaf-trace of two
collateral strands consisting of primary centripetal xylem and a
fan-like group of secondary xylem and phloem. The free edges of the
outer stele of section 1 have curved inwards and united with the two
lateral medullary bands, while the lower internal band of section 1
has increased in extent and forms a discontinuous arc with the upper
portions enclosed by the loops formed by the infolded ends of the outer
vascular tissue. In section 3 a second leaf-gap has been formed in
the outer stele and its invaginated ends have fused with the internal
bands. In section 4 the first leaf-gap is closed and the invaginated
bands of section 3 have broken up into an irregular circle of shorter
bands. The section reproduced in fig. 453, A, shows the main cylinder
in the form of two curved and flattened loops, each composed partly
of the centrifugally developed xylem and phloem of the main stele
and in part of the inversely orientated tissue of the inner bands.
At a lower level the two bands _b_, _b_, will become detached as the
upper leaf-gap is closed and form part of an inner cylinder like the
discontinuous ellipse formed by the two bands _c_. The section of a
branch-stele is seen at _a_.
[Illustration: Fig. 453.
A. _Ptychoxylon Levyi_; transverse section of stem; _a_, stele of
branch. (After Renault.)
B. _Cycadoxylon Fremyi_; transverse section of stem. (After Renault.)
C. _Cycadoxylon robustum_; centre of stem.
1–4, diagrams of sections of the stem of _Ptychoxylon_ at different
levels. (Simplified from Renault.)]
_Ptychoxylon_ differs considerably from _Medullosa_, which Renault
included in the _Cycadoxyleae_, in the plan of the vascular system:
there is nothing corresponding to the ‘partial pith’ or primary region
which forms the central portion of the plate- and snake-rings in
_Medullosa_. The double leaf-trace and the absence of the _Medullosa_
type of hypoderm are other distinguishing features. The paired
leaf-bundles suggest comparison with _Lyginopteris_ among other
genera and, as Scott[484] points out, the internal arcs of inversely
orientated tissue which sometimes occur in the peripheral region of
the pith of _Lyginopteris_ (fig. 405, C, _c_) behave like the internal
bands of _Ptychoxylon_ in occasionally joining the main cylinder at a
leaf-gap; but the differences outweigh the resemblances. As regards the
general arrangement of the vascular tissue in two irregular concentric
circles and their connexions with one another, but not in the structure
of the xylem and phloem, there is a similarity between this genus
and such a Fern as _Matonia_. In the varying patterns formed by the
vascular system at different levels in the stem _Ptychoxylon_ resembles
the Ferns _Polypodium quercifolium_ and _P. heracleum_[485].
=CALAMOPITYEAE.=
=Calamopitys.= Unger.
In 1856 Unger[486] described several fragmentary petrifactions from
Thuringian strata of Upper Devonian age, the majority of which he
referred to the Calamarieae and the Rhachiopterideae. In an earlier
publication[487] he gave a list of species including two families,
the Haplocalameae and the Calamoxyleae, assigned by him to the
group Calamarieae: in the Haplocalameae he placed the new genera
_Kalymma_, _Calamosyrix_, _Calamopteris_, and _Haplocalamus_. These
were subsequently examined by Graf Solms-Laubach and identified as
portions of petioles, for the most part belonging to unknown stems.
In his second family, the Calamoxyleae, Unger included the single
genus _Calamopitys_ represented by the type-species _C. Saturni_. The
type-specimens have been thoroughly investigated by Solms-Laubach[488]
who instituted the family-name Calamopityeae and recognised a close
anatomical affinity between _Calamopitys_ and _Lyginopteris_, a
conclusion which led to the incorporation of Unger’s genus in the
Pteridosperms. Further data have been supplied by Zalessky[489]
and, more recently, by Scott and Jeffrey[490] who have recognised
_Calamopitys_ in Lower Carboniferous beds in Kentucky.
_Calamopitys Saturni_ Unger.
Our knowledge of this and other species is confined to stems and
petioles. One of the largest examples of the species is a piece of
stem with a diameter of 1·5 cm.: the single stele consists of a
parenchymatous pith enclosed by secondary xylem made up of tracheids
with 4–8 rows of bordered pits and medullary rays more than one cell
broad and of considerable depth. Between the inner edge of the wood
and the pith are groups of primary xylem (fig. 454, B, _x_) which,
like those in _Lyginopteris_, constitute the leaf-traces: each has a
single internal protoxylem strand (fig. 455, B). The comparatively
wide cortex consists of parenchyma with a hypoderm of the _Sparganum_
type. Each primary xylem-strand passes out as a single leaf-trace
through the secondary xylem and on emerging divides into two as in
_Lyginopteris_: these branch in the cortex and the two are replaced by
six in the leaf-base (fig. 454, B–D). As seen in figs. 454, C, 455, A,
the boundary between the stem proper and the decurrent leaf-base is
marked by a line of stereome strands. The petioles of _Calamopitys
Saturni_ agree generally in structure with the imperfect specimens on
which Unger founded his genus _Kalymma_[491], so named in reference
to the structure of the hypodermal zone. A specimen described by
Solms-Laubach as a _Kalymma_ petiole occurs in organic connexion with
a stem of _Calamopitys_ (fig. 454, C: a detached petiole is shown in
fig. 454, D). The identification by White[492] of this attached petiole
with Unger’s _K. grandis_ has been confirmed by Scott and Jeffrey. A
fuller account of _Kalymma_ (fig. 456) is given on a later page, as the
petioles so named belong to more than one species of stem.
[Illustration: Fig. 454. _Calamopitys Saturni._ B, stem with
leaf-base; _x_, primary xylem and pith; black patches represent
leaf-traces; C, stem with two leaf-bases; D, section of petiole. (After
Solms-Laubach.)]
[Illustration: Fig. 455. _Calamopitys Saturni._ A, stem with
leaf-bases; × 5. B, stele showing leaf-trace after emerging from the
secondary xylem; × 16. (After Zalessky.)]
In _Calamopitys Saturni_ we have a plant agreeing with _Lyginopteris_
in the possession of secondary xylem of the manoxylic type and in the
structure of the common primary bundles, while it is distinguished
from _Lyginopteris_ by the greater number and by the structure of the
bundles in the axis of the leaf.
_Calamopitys annularis_ (Unger)[493].
This species, originally assigned to the genus _Stigmaria_, has
a more strongly developed primary vascular system and there is a
more decided tendency towards the formation of a continuous zone of
primary xylem on the inner edge of the secondary wood; but where
the protoxylem tracheids occur the metaxylem elements form definite
strands, like those of _C. Saturni_. It has been pointed out by
Scott and Jeffrey that there is some evidence of the occurrence of
tracheids in the parenchymatous pith of this species, an important
feature distinguishing it from _C. Saturni_ and connecting it with _C.
americana_. Information with regard to the behaviour of the leaf-traces
is far from complete, but there are indications that each trace divides
into two before emerging from the secondary xylem[494]. The leaf-traces
in the cortex are concentric as in _C. Saturni_.
_Calamopitys americana_ Scott and Jeffrey.
This Lower Carboniferous species[495] from the Waverley shales
of Kentucky is represented by portions of stems and leaf-bases
and detached petioles. The secondary wood consists of tracheids,
30–60μ in diameter, with deep and broad rays; the small pits on the
tracheids form 5–6 alternating series. Phloem and cambium are very
imperfectly preserved. The outer cortex is of the same type as in other
species. At the inner edge of the secondary xylem there is a ring of
primary xylem strands of mesarch structure composed of rather larger
tracheids, 80–120μ in diameter, separated from one another by narrow
strips of parenchyma. So far the vascular tissue agrees with that of
_C. annularis_. In the American species the axial region is not a
parenchymatous pith but a protostele, consisting of parenchyma and a
larger or smaller number of tracheal groups, the number being less in
stems with a larger central region. The peripheral strands alone are
concerned with the emission of leaf-traces, as in _Heterangium_. Each
primary xylem strand divides into two as it leaves the perimedullary
zone and passes through the secondary xylem as two bundles, each being
accompanied by an arc of secondary tracheids which, in the cortical
region, completely surrounds the primary elements. At a later stage the
single protoxylem of each trace divides into two and before entering
the leaf-base there is a further division. In some specimens leaf-bases
of the _Kalymma_ type were found attached to the stem. The occurrence
of tracheids in the axial region is a distinguishing feature and
suggests a comparison with _Heterangium_, while _C. Saturni_ agrees
more closely with _Lyginopteris_; the species _C. annularis_ would
appear, from the recent observations of Scott and Jeffrey, to occupy an
intermediate position.
_Calamopitys_, as the generic designation is here employed, is confined
to central Germany and Kentucky and occurs in Upper Devonian and
Lower Carboniferous strata. There is, however, some doubt as to the
exact geological horizon of the rocks in both countries though in
neither case is there any question of an horizon higher than Lower
Carboniferous. Certain specimens from the Lower Carboniferous of
Scotland described by Scott[496] as species of _Calamopitys_ have been
made by Zalessky the type of a new genus, _Eristophyton_, and are dealt
with under that name.
_Kalymma._ Unger.
_Kalymma grandis_ (petiole of _Calamopitys_). Under the generic name
_Kalymma_ Unger described specimens from Thuringia of Upper Devonian
age which he assigned to two species, _K. grandis_ and _K. striatum_.
Solms-Laubach has shown that _Kalymma_ is not an independent stem
as Unger believed but a petiole of _Calamopitys_, and this has been
confirmed by Scott and Jeffrey who found a leaf-base with the _Kalymma_
type of structure in connexion with a piece of _Calamopitys_ stem,
probably _C. americana_. An examination of a section (2·3 cm. broad)
of Unger’s _K. grandis_ in the collection of the Geological Survey
enables me to confirm the conclusions recently published by Scott and
Jeffrey. The best specimens of _Kalymma_, which appear to be identical
in essential features with Unger’s type-species, are from Kentucky,
some from the Genessee shales of Upper Devonian age and others from
beds (Waverley shales) believed to be Lower Carboniferous. Through
the kindness of Prof. Bower I have had an opportunity of examining
sections from the older horizon in his possession. The transverse
section reproduced in fig. 456 has a diameter of 3·8 × 2·2 cm.[497]: on
one side the radially placed plates of stereome are clearly shown, and
in the outer portion of the ground-tissue is a ring of vascular bundles
varying in size and shape but with a general tendency to a radially
elongated form. The ground-tissue consists of homogeneous parenchyma:
in one place I noticed what appeared to be a large secretory canal,
but secretory tissue, generally at least, is unrepresented. The xylem
is composed of imperfectly preserved elements, which appear to have
scalariform pits; spiral protoxylem strands, embedded in the metaxylem
as two or four groups, occur near the ends of the long axis of the
bundle and in some cases also near the centre. The phloem probably
surrounded the xylem, though it is not certain whether the arrangement
was collateral or concentric: there are no secondary-xylem tracheids,
though in some places I noticed a tendency to a radial disposition
of cells at the periphery of the vascular tissue simulating an early
stage of secondary growth. Unger’s second species _Kalymma striata_ is
characterised by an arrangement of the bundles similar to that in a
petiole described by Scott and Jeffrey as _Calamopteris Hippocrepis_
which differs from _Kalymma_ in the partial substitution of bands
of vascular tissue for separate bundles and to some extent in the
disposition of the bundles. The two types of petiole _Kalymma_ and
_Calamopteris_, as Scott and Jeffrey state, are very closely allied.
Dawson and Penhallow[498] have also described _Kalymma grandis_ from
Kentucky but they, like Unger, mistook the hypodermal stereome for an
outer zone of vascular bundles. The petioles from Germany and North
America included under the name _Kalymma grandis_, though too similar
to be referred to different species, no doubt represent petioles of
stems which are unquestionably distinct types: as in the case of
_Myeloxylon_ in its relation to the genus _Medullosa_, _Kalymma_ stands
for several closely allied forms of petioles belonging to several
species of _Calamopitys_.
[Illustration: Fig. 456. _Kalymma grandis._ (From a section in the
possession of Prof. Bower; × 4.)]
=Eristophyton.= Zalessky.
_Eristophyton fasciculare_ (Scott). The generic name
_Eristophyton_[499] was proposed by Zalessky[500] for two incomplete
stems of Lower Carboniferous age provisionally[501] referred by Scott
to _Araucarioxylon_ and subsequently to _Calamopitys_[502]. The species
_E. fasciculare_ was founded on material obtained by Dr Kidston from
the Lower Carboniferous of Dumbarton, Scotland, and on a specimen in
the Williamson collection from Northumberland. There is a small pith of
parenchyma, 2–3 mm. in diameter, with eight strands of primary xylem
of varying diameter (fig. 457) and in each a single protoxylem-group.
The primary xylem elements are considerably larger than the secondary
tracheids. These xylem-bundles are leaf-traces and their disposition
points to a phyllotaxis of ⅖. The traces attain their maximum size
when about to pass out through the secondary xylem. The tracheids are
reticulate and scalariform while some have an intermediate type of
pitting. A leaf-trace on reaching the pith gradually moves further
from the xylem-cylinder and may be separated from it by 2–6 layers
of parenchyma: as it passes down the pith the protoxylem strand
assumes an almost endarch position consequent on the reduction of the
centripetal xylem. In Scott’s words, ‘each circummedullary strand
branches at regular intervals; the one branch, that on the anodic
side [turned towards the course of the genetic spiral], becomes the
leaf-trace and passes out, while the other continues its course
up the stem as a reparatory strand, until the next leaf of the
orthostichy has to be supplied[503].’ The secondary xylem consists
almost entirely of tracheids with 3–4 rows of pits on the radial walls
and medullary rays usually one cell broad varying in depth from 1
or 2 to 16 or more cells. A characteristic feature of the secondary
xylem is the occurrence on its inner face of numerous short and broad
tracheae similar to the still larger tracheae in the primary stele
of _Megaloxylon_[504]. Nothing is known as to the behaviour of the
leaf-trace in the extrastelar region, but the fact that an outgoing
trace was found to have two protoxylems points to a subdivision similar
to that of the foliar bundles of _Calamopitys Saturni_. A well-marked
difference between _Eristophyton fasciculare_ and _Calamopitys_ and
_Lyginopteris_ is the more compact structure of the secondary wood;
it is pycnoxylic and not manoxylic. Prof. Zalessky in criticising the
use of the generic title _Calamopitys_ puts forward several arguments
in support of his institution of a new designation: (i) the primary
xylem strands of _Eristophyton_ are not confined to the periphery of
the pith as is the case in _Calamopitys Saturni_, though he speaks of
one leaf-trace in the latter species separated by several layers of
cells from the xylem-cylinder; (ii) some of the pith-cells have thick
walls and dark contents in distinction to the homogeneous parenchyma
of _Calamopitys_, a feature of little importance; (iii) the difference
in the structure of the secondary wood already alluded to, though this
loses some of its significance by the occurrence of narrower rays,
more like those of _Eristophyton_, in _C. annularis_; (iv) the more
elliptical and broader pits in the secondary tracheids in place of the
more regular hexagonal form in _Calamopitys_. While admitting a certain
degree of relationship between the two types, Zalessky asserts that
as yet we have insufficient evidence to justify their generic union.
Scott[505] maintains that Zalessky does not attach sufficient weight
to the form and mesarch structure of the primary xylem bundles as a
feature common to both genera.
[Illustration: Fig. 457. _Eristophyton fasciculare._ Transverse
section showing the relation of primary (black) to secondary xylem.
(After Scott.)]
_Eristophyton Beinertianum_ (Goeppert).
1850. _Araucarites Beinertianus_ Goeppert, Mon. Foss. Conif. p. 233,
Pls. 42, 43.
1872. _Araucarioxylon Beinertianum_ Kraus, in Schimper’s Trait. Pal.
Vol. +ii.+ p. 381.
1888. _Araucarites Beinertianus_ Goeppert and Stenzel, Abh. K. Preuss.
Akad. Wiss. p. 30, Pl. +iv.+
1902. _Calamopitys Beinertiana_ Scott, Trans. R. Soc. Edinb. Vol. 40,
p. 341, Pls. +i.+, +iv.+, +v.+
1909[506]. _Ullmannites Beinertianus_ Tuzson, Result. Wiss. Erforsch.
Balatonsees, Bd +i.+ Teil +i.+ p. 24.
1911. _Eristophyton Beinertianum_ Zalessky, Com. Geol. St Pétersb. p.
24.
The pith, 13–15 mm. in diameter, is rather larger than in _E.
fasciculare_ and is characterised by the occurrence of dark sclerotic
nests surrounded by radially disposed rows of parenchyma. The primary
xylem strands are more numerous and smaller than in _E. fasciculare_
and these increase in diameter as they approach the secondary wood. In
places the primary xylem elements form a more or less continuous band
as in _Calamopitys annularis_. The largest leaf-trace bundles at the
periphery of the pith are mesarch (fig. 458), but as each trace passes
down the pith the reduction in the centripetal xylem is carried further
than in _E. fasciculare_ until the xylem-strand becomes endarch in the
lower part of its course. The secondary tracheids have usually two
contiguous rows of pits and the medullary rays are one cell broad.
[Illustration: Fig. 458. _Eristophyton Beinertianum._ A strand of
primary xylem, showing the protoxylem, _px_, abutting on the secondary
xylem. (× 35. After Zalessky.)]
There can be no doubt as to the generic identity of the two species
referred to _Eristophyton_, but the question as to the degree of
affinity to _Calamopitys_ is more difficult to settle. There is force
in Zalessky’s contention that these two stems should not be retained in
_Calamopitys_: the recently described American species, _C. americana_
Scott and Jeffrey, gives emphasis to the view that the restriction
of _Calamopitys_ to the German (and American) types is the safer
course. While _Calamopitys_ as thus restricted is almost certainly a
Pteridosperm, the inclusion of the types referred to _Eristophyton_ in
the same category rests on a more slender basis.
=CLADOXYLEAE.=
This order was founded by Unger[507] for some imperfectly preserved
stems from Palaeozoic strata in Thuringia and in it he included the two
genera _Cladoxylon_ and _Schizoxylon_. There is some doubt as to the
precise age of the Thuringian beds; they were assigned by Richter to
the Devonian system and subsequently placed in the Culm: Solms-Laubach
in his later reference to Unger’s plants favours a Devonian
horizon[508]. Unger included the Cladoxyleae in the Lycopodiales,
and though this conclusion is not accepted the position of the order
is still uncertain. His genus _Schizoxylon_ has no claim to generic
separation from _Cladoxylon_. An inspection of the illustrations in
the memoir by Richter and Unger reveals a striking resemblance in the
main anatomical features between several types assigned to different
genera and distributed among the Cladoxyleae and Rhachiopterideae
(a term first used by Corda for petrified rachises or petioles of
ferns) and other orders. Solms-Laubach[509], to whom our more accurate
information as to Unger’s plants is chiefly due, is inclined to regard
the specimens referred by Unger to the genus _Arctopodium_ as young
stems of _Cladoxylon_, and he draws attention to a close similarity
between _Hierogramma_, another of Unger’s genera, and _Cladoxylon_.
Paul Bertrand[510] goes further in considering that the following
genera represent one generic type, namely _Syncardia_ (fig. 459, F),
_Hierogramma_, _Arctopodium_, _Cladoxylon_, and _Schizoxylon_. The same
author interprets the fossils so named by Unger as stems and does
not agree with the inclusion of any of them in the Rhachiopterideae.
Without losing sight of the fact that Bertrand’s conclusion is
not based on proof but is the expression of a view suggested by a
close agreement in general anatomical plan, I venture to adopt the
designation _Cladoxylon_ in a wide sense primarily on the ground
that Bertrand’s view is probably correct and in part for the sake of
convenience of description. As Unger’s species of _Cladoxylon_ differ
from one another in features which may fairly be regarded as of minor
importance, they are included under one specific name.
=Cladoxylon=. Unger.
_Cladoxylon mirabile_ Unger[511]. The following are regarded as
specifically identical with or closely allied to _Cladoxylon mirabile_:
_C. dubium_, _Schizoxylon taeniatum_, _Hierogramma mysticum_,
_Syncardia pusilla_, _Arctopodium insigne_ and _A. radiatum_[512].
_i. Stems._ The stems assigned to _Cladoxylon_ are characterised by a
complex system of steles, either simple or branched and occasionally
anastomosing, presenting in transverse section the form of oval or
cylindrical strands or narrow, straight or curved bands arranged on
a more or less clearly marked radial plan (fig. 459, A, B, D). In
some stems the primary vascular tissue is enclosed by secondary xylem
and phloem (fig. 460, B), while in others (Unger’s _Arctopodium_,
_Hierogramma_[513], _Syncardia_) there is no evidence of secondary
thickening. The diagrammatic drawing represented in fig. 459, F, shows
a section of a small axis, regarded by Unger and Solms as a petiole (3
mm. in diameter), containing four vascular strands composed exclusively
of primary xylem, each with one or, in the case of a double strand,
two protoxylem groups. This type may be a slender stem or branch
or possibly a petiole. The other extreme, as regards complexity of
vascular structure, is represented by such stems as those shown in
fig. 459, A, B, D. In Unger’s _Cladoxylon mirabile_ (fig. 459, A;
fig. 460, B) the stem reaches a diameter of 3 cm. and consists of
several radially disposed plates of vascular tissue with an occasional
smaller oval or cylindrical stele embedded in a ground-tissue composed
of thick-walled cells. The plates are curved like a UU or sinuous
and not infrequently anastomosing. In a section of this type figured
by Unger the vascular plates appear to form a complex anastomosing
system, but Solms[514] states that the drawing exaggerates the amount
of fusion between the strands, and an examination of a section in the
collection of the English Geological Survey cut from Unger’s specimen
enables me to confirm this statement. Each vascular plate consists of a
narrow median region composed of primary tracheids with a scalariform
type of pitting surrounded by secondary tracheids with interspersed
medullary rays one cell broad. The thickness of the secondary xylem
varies considerably in the same specimen and in places this tissue
is hardly represented, a fact of importance in view of the very
striking resemblance between _Arctopodium_ and _Cladoxylon_, the
sections referred by Unger to the former genus having steles without
any secondary xylem. The occurrence of one or two elongated spaces
(shown in black in fig. 459, A) near the distal end of each plate mark
the position of the protoxylem tracheids. Fig. 459, C, represents a
stele of a stem referred by Unger and by Solms to _Cladoxylon dubium_
which shows the typical _Cladoxylon_ structure, namely the central
primary xylem with distally placed protoxylem and the enclosing sheath
of secondary xylem. In the stem shown in fig. 459, D (_C. dubium_)
there are 12 steles, each constructed on the plan already described,
differing in their relatively broader and shorter form and in the
greater breadth of the secondary xylem from those seen in fig. 459,
A (_C. mirabile_). The black areas in fig. 459, A, show the primary
xylem, and the protoxylem is seen in fig. 459, C. A stem described by
Dawson[515] as _Asteropteris noveboracensis_ from Devonian beds is
compared by him with Unger’s _Cladoxylon mirabile_ and regarded as
possibly allied to it. The radial plates of xylem in Dawson’s plant
meet in the centre like those of _Asterochlaena_ and the leaf-traces
are of the Clepsydropsoid type.
[Illustration: Fig. 459. A, _Cladoxylon mirabile_, section of stem;
B, _Cladoxylon taeniatum_, section of stem; C, _Cladoxylon dubium_,
section of stele; D, _C. dubium_, section of stem; E, _Cladoxylon (?)
mirabile_, section of petiole; F, _Syncardia pusilla_; G, development
of leaf-trace in _Cladoxylon taeniatum_. (A–E, after Solms-Laubach; F,
after Unger; G. after P. Bertrand.)]
The type of stem for which Unger founded his genus _Schizoxylon_ is
represented in fig. 459, B; there are five small steles in the centre
and external to these eleven radially arranged plates, with oval steles
between them, in the peripheral region of the stem. Each stele consists
of primary (black in the figure) and secondary xylem and agrees with
the steles in the other stems.
From the type of stem illustrated by _Cladoxylon mirabile_ to that on
which the genera _Arctopodium_[516] and _Hierogramma_[517] were founded
is a very small step: the vascular tissue has the same characters
both as regards gross and minute anatomy, but there is no evidence of
cambial activity in the stems referred to the two latter genera, a
difference in itself hardly worthy of generic recognition.
ii. _Leaves._ Before describing a second type of stem referred to
_Cladoxylon_ it is important to consider briefly such evidence as we
have as to the vascular supply of the leaves. Nothing is known of the
reproductive organs and there is no satisfactory information with
regard to the form of the fronds. Solms-Laubach has described the only
known example of a lateral branch of a _Cladoxylon_ stem (fig. 459,
E): this has a single concentric vascular strand of plate-like form
with two blunt projections and there are four protoxylem-groups, two
in the angle of the plate and two at the base of the projections. The
structure is essentially fern-like; the xylem is wholly primary. This
type of vascular strand agrees fairly closely with that of a petiole
described by Unger as _Megalorhachis elliptica_, a section of which
is in the Museum of the Geological Survey[518]. The petiole is oval
in section and laterally winged, and the meristele is tangentially
elongated and has two blunt projections almost identical with those in
fig. 459, F. There is no evidence as to the nature of the supporting
stem, but there can be little doubt as to the close connexion with
_Megalorhachis_ and the section shown in fig. 459, F. In a note
published in 1908 P. Bertrand stated that he had identified several
of Unger’s genera as stems which bore leaf-traces having the form and
structure of _Clepsydropsis_, one of the types referred by Unger to the
Rhachiopterideae and described in the second volume of this work[519]
as a Coenopteridean petiole. Bertrand points out that in the oval or
plate-like steles of _Cladoxylon_, _Arctopodium_, _Hierogramma_, etc.,
there is a single protoxylem group near the distal end of the primary
xylem, and he adds that the leaf-traces were formed of strands cut
off from the distal portions of the vascular plates. Similarly the
hour-glass-like leaf-trace in the primary rachis of _Clepsydropsis_
gives off from each end a ring of xylem to supply a secondary rachis.
These laterally detached annular strands are, he believes, similar to
the leaf-trace cut off from the steles in a _Cladoxylon_ stem. The
conclusion is that _Cladoxylon_ is a fern stem and its leaf-trace
represents the simplest form of the _Clepsydropsis_ type, namely an
oval bundle of xylem with a central protoxylem, which is also the form
of the trace given off from the stem of _Asterochlaena_. Solms[520],
while admitting that Bertrand may be correct in uniting under one
genus _Cladoxylon_ and such types as _Syncardia_, _Hierogramma_, and
_Arctopodium_, disagrees with the view that they are _Clepsydropsis_
stems. A Clepsydropsoid leaf-trace has never been found in direct
association with any of the stems of the _Cladoxylon_ type and such
evidence as there is indicates a leaf-trace of an entirely different
form (fig. 459, E). In his more recent memoir on _Asterochlaena_
Bertrand[521] draws attention to Solms’ figures of a stele of
_Cladoxylon_ (fig. 459, C) in which the distal portion is on the point
of being separated as a small annular strand. This, Bertrand considers,
would gradually become converted into a _Clepsydropsis_ form of stele
as it passed to the petiole. Bertrand’s drawings made from a section
of _Cladoxylon taeniatum_ (Ung.) (fig. 459, G) illustrate successive
stages in the departure of a leaf-trace from one of the plate-like
steles of the stem (fig. 459, B). In fig. 459, G, 1, a piece of the
stele is detached and near its extremity is a group of thin-walled
cells with protoxylem: a later stage is seen in fig. 2, and in fig. 3
a small ring of xylem is being detached which, Bertrand assumes, would
later in its course be converted into the Clepsydropsoid strand (fig.
4), which consists of primary tissue. The weak point in Bertrand’s
contention[522] is the absence of any proof of a true Clepsydropsoid
trace in connexion with a _Cladoxylon_ stem, and there is a strong
probability that the leaf-trace of _Cladoxylon_ has the form shown in
fig. 459, E.
_Cladoxylon Kidstoni_ Solms-Laubach[523].
This species, founded on imperfectly preserved material in Dr Kidston’s
collection from Lower Carboniferous rocks in Berwickshire, is referred
to _Cladoxylon_ on evidence that cannot be regarded as convincing.
The type-specimen consists of a small piece of stem about 3 cm. in
breadth showing three complete oval steles and portions of two others
which seem to be in their original position and probably formed part
of a series of peripheral steles such as those shown in fig. 459, D.
Each stele consists mainly of secondary xylem (fig. 460, A) with some
crushed tissue, presumably phloem, on its outer face. The secondary
xylem is narrower on the inner side of each stele where a wedge-shaped
piece is partially detached. In the centre there is a narrow area
parallel to the long axis of the stele containing crushed tissue which
probably consists of parenchyma and primary xylem, but the preservation
is very imperfect. The secondary xylem has a fairly compact structure
and the rays are narrow, 1–10 cells in depth. The pits of the tracheids
are described by Solms as scalariform with occasionally two rows
of elliptical pits on the radial walls. A careful examination of
the type-specimen leads me to describe the pits as uniseriate and
transversely elongated, very like those of _Protopitys_, or biseriate
and almost circular like those of Conifers, the pits of the two rows
being alternate or sometimes opposite (fig. 460, C): in places three
rows of bordered pits are present. There is a certain degree of
resemblance between the steles of this species and those of the South
African stem _Rhexoxylon_[524], but the data are inadequate for a
satisfactory comparison.
[Illustration: Fig. 460.
A, C. A single stele and tracheids of _Cladoxylon Kidstoni_. (Kidston
Coll. 630 B, 630 C.)
B. _Cladoxylon mirabile_, part of a stele. (Museum of Practical
Geology, 15872.)]
There is a close similarity between the vascular systems of
_Cladoxylon_ and _Medullosa_, but an obvious difference is the
substitution of the oval, transversely elongated, pits on the xylem
elements for the multiseriate pitting of _Medullosa_. In _Cladoxylon
Kidstoni_ the pitting shows transitional forms between a narrow
scalariform uniseriate type and a biseriate or triseriate arrangement
similar to that in the Araucarineae and Cordaitales. In _Cladoxylon_,
as limited by Unger, the presence of secondary wood is a generic
feature, but by the inclusion of _Arctopodium_ and other forms this
character no longer holds good. The inclusion of these more fern-like
stems without secondary xylem brings _Cladoxylon_ (in the wider sense)
into closer contact with _Asterochlaena_, a comparison previously
suggested by more than one author. In _Medullosa_ the development
of secondary xylem is on a larger scale than in _Cladoxylon_, and
the vascular system of the former genus assumes a more complex form.
Moreover the _Myeloxylon_ type of petiole, which is a distinctive
feature of _Medullosa_, differs widely from any form of leaf-trace
associated with _Cladoxylon_.
=Völkelia.= Solms-Laubach.
_Völkelia refracta_ (Goeppert). The generic name _Völkelia_[525] was
proposed by Solms-Laubach[526] as a substitute for _Sphenopteris_[527]
in the case of some petrified stems or petioles associated with
fragmentary impressions of fronds from Lower Carboniferous rocks in
Silesia. Both leaf-impressions and petrifactions were included in the
genus _Sphenopteris_: Solms, while retaining Goeppert’s designation for
the leaf fragments, proposed a new generic name for the petrifactions
on the ground that there is insufficient evidence of their connexion
with the leaves. The short account of Goeppert’s petrified specimen
given by Graf Solms-Laubach in his ‘Fossil Botany[528]’ is supplemented
by a fuller description in a later paper. The fragments of highly
compound fronds are characterised by very small filiform ultimate
segments, but the specimens are too imperfect to afford a clear idea of
the habit of the leaf. The ‘stem’ bears a close superficial resemblance
to that described by Unger as _Cladoxylon dubium_ (fig. 459, C, D) and
was regarded by him as an example of that species: it contains several
radially placed steles represented by fairly well-preserved xylem,
but no phloem has been recognised. The steles vary in size and shape:
five reach almost to the centre (fig. 461, A) and smaller xylem groups
occupy a peripheral position. Each stele is excentric in structure and
consists of (i) an outer zone of secondary tracheids of horse-shoe
form in transverse section, but the apparent gap in the secondary
xylem on the outer edge of each stele is due to the crushing of the
tracheal tissue and to its smaller breadth in the distal part of each
group; this is shown in fig. 461, B, where the apparent gap is seen to
be occupied by distorted and crushed tracheids, _a_, identical with
those which form the rest of the outer zone (fig. 461, B, _b_); (ii)
a zone of tracheal tissue continuous with and originally identical
in appearance—except that the elements are rather narrower—with the
outer secondary xylem; (iii) an excentrically situated island composed
of tracheids enclosing a small central area occupied by thin-walled
parenchyma. This third region, represented by black patches in fig.
461, A, in all probability represents the primary part of each stele
to which the rest of the tissue has been added by the cambium. A
striking feature of the secondary xylem is the absence of medullary
rays: the tracheids resemble those of _Cladoxylon_ and _Protopitys_
in the transverse elongation of the pits (fig. 461, D) which form
either a single row or several irregularly distributed rows. The
primary xylem consists in the peripheral region of tracheids with very
narrow scalariform pitting which at first sight suggest close spiral
bands (fig. 461, C), while the inner tracheids are either annular or
reticulate and associated with elongated parenchyma. The imperfectly
preserved ground-tissue appears to consist of homogeneous parenchyma
with radially disposed bands of stereome in the outer cortex.
[Illustration: Fig. 461. _Völkelia refracta._ A. Transverse section
of a specimen in the Breslau Museum. B. Portion of a stele; _a_, the
crushed xylem on the outer side; _b_, the inner side. C. Longitudinal
section showing the median protoxylem. D. Tracheid from the secondary
xylem. (After Solms-Laubach.)]
Our knowledge of _Völkelia_, though far from complete, justifies its
generic separation from _Cladoxylon_ from which it differs in the
lack of medullary rays and in the structure of the primary portion
of each stele. In the form and arrangement of the pits in the
secondary tracheids _Völkelia_ differs from _Medullosa_ and resembles
_Cladoxylon_. The opinion expressed by P. Bertrand[529] that _Völkelia_
is probably the stem of one of the Zygopterideae is based on the older
accounts of the genus and not on the fuller description of 1910.
=PROTOPITYEAE.=
=Protopitys.= Goeppert.
The only species so far described is that for which Goeppert founded
the genus in 1850, substituting _Protopitys_[530] for the name
_Araucarites_, adopted in an earlier paper, on the ground that
the structure of the xylem denoted a distinct generic type. The
type-species is from the Upper Devonian rocks of Falkenberg in Silesia.
_Protopitys Buchiana_ Goeppert.
1845. _Araucarites Buchianus_ Goeppert, in Wimmer’s Flor. Schlesien
(edit. +ii.+) p. 218.
1847. _Pinites Goepperti_ Unger, Chlor. Protog. p. 31.
1847. _Dadoxylon Buchianum_ Endlicher, Syn. Conif. p. 300.
1850. _Protopitys Buchiana_ Goeppert, Foss. Conif. p. 229, Pl.
+xxxvii.+ figs. 4–7; Pl. +xxxviii.+ figs. 1, 2.
In his Monograph of Fossil Conifers Goeppert figured a large piece
of stem consisting mainly of secondary wood and described the more
important anatomical features. He recognised the narrow, transversely
elongated, pits on the radial walls of the tracheids as a feature of
special interest indicating a type of pitting transitional between that
of Ferns and Conifers. A further description was given by Kraus[531]
who included under Goeppert’s name both the Falkenberg stem and a
second specimen from Basel though the latter is Triassic in age and
a distinct plant; he suggested a comparison of _Protopitys_ with
_Sigillaria_ and _Stigmaria_ rather than with Conifers. It is, however,
to Graf Solms-Laubach[532] that we owe the most thorough account of
this species. Nothing is known of the leaves or reproductive organs.
The largest piece of stem is nearly 1 ft in diameter and consists
mainly of secondary xylem resembling that of Conifers and _Cordaites_
except in the form of the bordered pits on the radial walls of the
tracheids (fig. 462, D). The centre of the stem is occupied by a
parenchymatous pith, elliptical in transverse section, enclosed by
a band of primary xylem composed of large polygonal tracheids (fig.
462, C, _x_¹) characterised by a delicate scalariform pitting on all
their walls (fig. 462, E). The primary xylem forms a narrow layer on
the sides of the ellipse, 1–3 elements broad (fig. 462, A, B), but it
increases in breadth at the ends of the long axis where the tracheids
are intermixed with parenchyma. The primary xylem and pith-tissue at
the ends of the major axis of the central region assume different
forms at different levels, owing to the detachment of leaf-traces and
the consequent formation of foliar gaps as portions of the primary
xylem pass obliquely outwards into the secondary xylem on the way to
the distichously arranged alternate leaves. The diagram, fig. 462, A,
shows the inner part of the secondary xylem (see also fig. 462, C, x²)
which at one end, _lt_, has formed an oval group about to pass out as
a leaf-trace: at the opposite end the strand is detached and divided
into two equal branches. The two swellings of the primary xylem ellipse
shown at _a_ in figs. 462, A and B, are a characteristic feature:
these are clearly seen after the leaf-trace has become detached;
at the inner edge of each of them there appears to be a protoxylem
strand. After the formation of a foliar gap these swellings of the
xylem gradually meet and so re-establish continuity below the outgoing
leaf-trace. No protoxylem has been detected in the actual trace, which
is believed to be concentric. The formation of the leaf-gap and the
shoulders bordering it constitute interesting filicinean features,
recalling corresponding characters in solenostelic Ferns. At the
upper end of the diagram, fig. 462, B, the outgoing leaf-trace is
undergoing dichotomy while at the opposite end the trace has passed
out of view. The secondary xylem shows incomplete rings or arcs of
narrower elements, which at first sight give the impression of annual
rings: the occurrence of similar incomplete or pseudo-rings is a common
feature in _Lepidodendron_ and other Palaeozoic stems. The secondary
tracheids (54·4μ in tangential diameter, 68·5μ in radial diameter) have
usually a single series of broadly oval bordered pits on the radial
walls with here and there two rows (fig. 462, D). In one case only were
the pits of the medullary rays recognised (fig. 462, F). The rays are
uniseriate, generally 1–2 cells deep, but occasionally 3 cells in depth
and very rarely deeper. The cambium is of the normal type, and in some
specimens secondary phloem was found consisting of bands, 4–5 layers
broad, of stone-cells alternating with tubular thin-walled elements,
presumably sieve-tubes.
[Illustration: Fig. 462. _Protopitys Buchiana_. A, B. Central region
of the stem showing the pith, the primary xylem (black in fig. A) and
(A) the inner part of the secondary xylem; _a_, shoulders of xylem
at the leaf-gaps; _lt_ leaf-trace; _px_ protoxylem. C. Secondary,
_x_², and primary xylem, _x_¹. D. Radial longitudinal section of the
secondary xylem. E. Scalariform tracheid of the primary xylem. F. Pits
on the medullary-ray cells. (After Solms-Laubach.)]
As Solms-Laubach says, it is highly probable that each leaf-trace,
which forks close to its exit from the primary xylem, became further
subdivided before reaching the leaf. Morphologically, _Protopitys_ is
of special importance as a type possessing characters that indicate a
connexion with Conifers or Cordaitean genera, notably the structure
of the secondary wood, while the presence of foliar gaps is a feature
reminiscent of Ferns. The primary xylem resembles that of some of
the Palaeozoic arborescent Lycopodiales, but in _Protopitys_ the
interruptions in this tissue are due to the emission of leaf-bundles,
whereas in the discontinuous primary xylem of some Sigillarias[533]
the gaps have no connexion with leaf-traces. Moreover the distichous
leaves of _Protopitys_ and the larger, branched, leaf-traces are other
distinguishing features. The pitting of the primary xylem is like
that in the Lycopodiales and Filicales, while that of the secondary
wood shows a closer approach to the coniferous type. A comparison
may also be made with the transversely elongated pits of _Cladoxylon
Kidstoni_[534].
A piece of wood agreeing anatomically with the Silesian species of
_Protopitys_ has been found in the Yoredale rocks of England[535].
The peculiarities of the genus have been emphasised by Solms-Laubach
by the institution of a family-name Protopityeae: the genus is
essentially a generalised type exhibiting in the structure of its stem
both Filicean and Coniferous features. The bordered pits differ from
those in recent Conifers in their flatter form, but in this respect
they exhibit a closer agreement with the transversely stretched pits of
_Xenoxylon phyllocladoides_ Goth.[536], a Mesozoic species.