I. EQUISETALES (_continued_).
(CALAMARIEAE.)
In order to minimise repetition and digression the following account of
the Calamarieae is divided into sections, under each of which a certain
part of the subject is more particularly dealt with. After a brief
sketch of the history of our knowledge of _Calamites_, and a short
description of the characteristics of the genus, the morphological
features are more fully considered. A description of the most striking
features of the better known Calamitean types is followed by a short
discussion on the question of nomenclature and classification, and
reference is made to the manner of occurrence of _Calamites_ and to
some of the possible sources of error in identification.
D. CALAMITES.
I. Historical Sketch.
In the following account of the Calamarieae the generic name
_Calamites_ is used in a somewhat comprehensive sense. As previous
writers have pointed out, it is probable that under this generic name
there may be included more than one type of plant worthy of generic
designation. Owing to the various opinions which have been held by
different authors, as to the relationship and botanical position of
plants now generally included in the Calamarieae, there has been no
little confusion in nomenclature. Facts as to the nature of the genus
_Calamites_ have occasionally to be selected from writings containing
many speculative and erroneous views, but the data at our disposal
enable us to give a fairly complete account of the morphology of this
Palaeozoic plant.
In the earliest works on fossil plants we find several figures of
_Calamites_, which are in most cases described as those of fossil
reeds or grasses. The _Herbarium diluvianum_ of Scheuchzer[580]
contains a figure of a Calamitean cast which is described as probably
a reed. Another specimen is figured by Volkmann[581] in his _Silesia
subterranea_ and compared with a piece of sugar-cane. A similar
flattened cast in the old Woodwardian collection at Cambridge is
described by Woodward[582] as “part of a broad long flat leaf,
appearing to be of some _Iris_, or rather an Aloe, but ’tis striated
without.” Schulze[583], one of the earlier German writers, figured a
Calamitean branch bearing verticils of leaves, and described the fossil
as probably the impression of an Equisetaceous plant. It has been
pointed out by another German writer that the Equisetaceous character
of _Calamites_ was recognised by laymen many years before specialists
shared this view.
One of the most interesting and important of all the older records of
_Calamites_ is that published by Suckow[584] in 1784. Suckow is usually
quoted as the author of the generic name _Calamites_; he does not
attempt any diagnosis of the plant, but merely speaks of the specimens
he is describing as “Calamiten.” The examples figured in this classic
paper are characteristic casts from the Coal-Measures of Western
Germany. Suckow describes them as ribbed stems, which were found in
an oblique position in the strata and termed by the workmen Jupiter’s
nails (“Nägel”). Previous writers had regarded the fossils as casts
of reeds, but Suckow correctly points out that the ribbed character
is hardly consistent with the view that the casts are those of reeds
or grasses. He goes on to say that the material filling up the hollow
pith of a reed would not have impressed upon it a number of ribs and
grooves such as occur on the Calamites. He considers it more probable
that the casts are those of some well-developed tree, probably a
foreign plant. _Equisetum giganteum_ L. is mentioned as a species with
which _Calamites_ may be compared, although the stem of the Palaeozoic
genus was much larger than that of the recent Horse-tail. The tree
of which the Calamites are the casts must, he adds, have possessed
a ribbed stem, and the bark must also have been marked by vertical
ribs and grooves on its _inner face_. It is clear, therefore, that
Suckow inclined to the view that _Calamites_ should be regarded as an
_internal cast_ of a woody plant. Such an interpretation of the fossils
was generally accepted by palaeobotanists only a comparatively few
years ago, and the first suggestion of this view is usually attributed
to Germar, Dawes, and other authors who wrote more than fifty years
later than Suckow.
One of the earliest notices of _Calamites_ in the present century is
by Steinhauer[585], who published a memoir in the Transactions of the
American Philosophical Society in 1818 on _Fossil reliquia of unknown
vegetables in the Carboniferous rocks_. He gives some good figures of
Calamitean casts under the generic name of _Phytolithus_, one of those
general terms often used by the older writers on fossils. Among English
authors, Martin[586] may be mentioned as figuring casts of _Calamites_,
which he describes as probably grass stems. By far the best of
the earlier figures are those by Artis[587] in his _Antediluvian
Phytology_. This writer does not discuss the botanical nature of the
specimens beyond a brief reference to the views of earlier authors.
Adolphe Brongniart[588], writing in 1822, expresses the opinion that
the Calamites are related to the genus _Equisetum_, and refers to
M. de Candolle as having first suggested this view. In a later work
Brongniart[589] includes species of _Calamites_ as figured by Suckow,
Schlotheim, Sternberg and Artis in the family _Equisetaceae_. Lindley
and Hutton[590] give several figures of Calamites in their _Fossil
flora_, but do not commit themselves to an Equisetaceous affinity.
An important advance was made in 1835 by Cotta[591], a German writer,
who gave a short account of the internal structure of some Calamite
stems, which he referred to a new genus _Calamitea_. The British Museum
collection includes some silicified fragments of the stems figured
and described by Cotta in his _Dendrolithen_. Some of the specimens
described by this author as examples of _Calamitea_ have since been
recognised as members of another family.
[Sidenote: PETZHOLDT AND UNGER.]
In 1840 Unger[592] published a note on the structure and affinities of
_Calamites_, and expressed his belief in the close relationship of the
Palaeozoic plant and recent Horse-tails.
An important contribution to our knowledge of _Calamites_ was supplied
by Petzholdt[593] in 1841. His main contention was the Equisetaceous
character of this Palaeozoic genus. The external resemblance between
Calamite casts and _Equisetum_ stems had long been recognised, but
after Cotta’s account of the internal structure it was believed that
the apparent relation between _Equisetum_ and _Calamites_ was not
confirmed by the facts of anatomy. Petzholdt based his conclusions
on certain partially preserved Permian stems from Plauenscher Grund,
near Dresden. Although his account of the fossils is not accurate his
general conclusions are correct. The specimens described by Petzholdt
differ from the common Calamite casts in having some carbonised
remnants of cortical and woody tissue. A transverse section of one of
the Plauenscher Grund fossils is shown in fig. 70. The irregular black
patches were described by Petzholdt as portions of cortical tissue,
while he regarded the spaces as marking the position of canals like the
vallecular canals in an _Equisetum_. Our more complete knowledge of the
structure of a Calamite stem enables us to correlate the patches in
which no tissue has been preserved with the broad medullary rays, which
separated the wedge-shaped groups of xylem elements; the latter being
more resistant were converted into a black coaly substance, while the
cells of the medullary rays left little or no trace in the sandstone
matrix. The thin black line, which forms the limit of the drawing in
fig. 70, external to the carbonised wood, no doubt marks the limit
of the cortex, and the appendage indicated in the lower part of the
figure may possibly be an adventitious root. It is interesting to note
that Unger[594] in 1844 expressed the opinion, which we now know to be
correct, that the coaly mass in the specimens described by Petzholdt
represented the wood, and that there was no proof of the existence of
canals in the cortex as Petzholdt believed.
[Illustration: FIG. 70. Transverse section of a Calamite stem, showing
carbonised remnants of secondary wood. From a specimen (no. 40934),
presented to the British Museum by Dr Petzholdt from Plauenscher
Grund, Dresden. ½ nat. size.]
Turning to Brongniart’s later work[595] we find an important
proposal which led to no little controversy. While retaining the
genus _Calamites_ for such specimens as possess a thin bark and a
ribbed external surface, showing occasional branch-scars at the
nodes, and having such characters as warrant their inclusion in the
Equisetaceae, he proposes a second generic name for other specimens
which had hitherto been included in _Calamites_. The fossils assigned
to his new genus _Calamodendron_ are described as having a thick woody
stem, and as differing from _Equisetum_ in their arborescent nature.
Brongniart’s genus _Calamodendron_ is made to include the plants for
which Cotta instituted the name _Calamitea_, and it is placed among
the Gymnosperms. This distinction between the Vascular Cryptogam
_Calamites_ and the supposed Gymnosperm _Calamodendron_ is based on the
presence of secondary wood in the latter type of stem. The prominence
formerly assigned to the power of secondary thickening possessed by a
plant as a taxonomic feature, is now known to have been the result of
imperfect knowledge. The occurrence of a cambium layer and the ability
of a plant to increase in girth by the activity of a definite meristem,
is a feature which some recent Vascular Cryptogams[596] share with the
higher plants; and in former ages many of the Pteridophytes possessed
this method of growth in a striking degree.
Although Brongniart’s distinction between _Calamites_ and
_Calamodendron_ has not been borne out by subsequent researches, the
latter term is still used as a convenient designation for a special
type of Calamitean structure. One of the earliest accounts of the
anatomy of _Calamodendron_ stems is by Mougeot[597], who published
figures and descriptions of two species, _Calamodendron striatum_ and
_C. bistriatum_.
Some years later Göppert[598], who was one of the greatest of the
older palaeobotanists, instituted another genus, _Arthropitys_[599],
for certain specimens of silicified stems from the Permian rocks of
Chemnitz in Saxony, which Cotta had previously placed in his genus
_Calamitea_ under the name of _Calamitea bistriata_[600]. Göppert
rightly decided that the plants so named by Cotta differed in important
histological characters from other species of _Calamitea_. The generic
name _Arthropitys_ has been widely adopted for a type of Calamitean
stem characterised by definite structural features. The great majority
of the petrified Calamite stems found in the English Coal-Measures
belong to Göppert’s _Arthropitys_.
[Sidenote: WILLIAMSON.]
The next proposal to be noticed is one by Williamson[601] in 1868;
he instituted the generic name _Calamopitys_ for a few examples
of English stems, which differed in the structure of the wood and
primary medullary rays from previously recorded types. We have thus
four names which all stand for generic types of Calamitean stems. Of
these _Calamodendron_ and _Arthropitys_ are still used as convenient
designations for stems with well-defined anatomical characters.
The genus _Calamitea_ is no longer in use, and Williamson’s name
_Calamopitys_ had previously been made use of by Unger[602] for plants
which do not belong to the Calamarieae. As it is convenient to have
some term to apply to such stems as those which Williamson made the
type of _Calamopitys_, the name _Arthrodendron_ is suggested by my
friend Dr Scott[603] as a substitute for Williamson’s genus.
The twofold division of the Calamites instituted by Brongniart has
already been alluded to, and for many years it was generally agreed
that both Pteridophytes and Gymnosperms were represented among the
Palaeozoic fossils known as Calamites. The work of Prof. Williamson
was largely instrumental in proving the unsound basis for this
artificial separation; he insisted on the inclusion of all Calamites
in the Vascular Cryptogams, irrespective of the presence or absence
of secondary wood. By degrees the adherents of Brongniart’s views
acknowledged the force of the English botanist’s contention. It is
one of the many signs of the value of Williamson’s work that there is
now almost complete accord among palaeobotanical writers as to the
affinities of Calamitean plants.
In the following account of the Calamites, the generic name
_Calamites_ is used in a wide sense as including stems possessing
different types of internal structure; when it is possible to recognise
any of these structural types the terms _Calamodendron_, _Arthropitys_
or _Arthrodendron_ are used as subgenera. The reasons for this
nomenclature are discussed in a later part of the Chapter.
╭ This term was originally applied
Genus _Calamites_, Suckow, 1714 ┤ to the common pith-casts of
│ Calamitean stems, _without reference
╰ to_ _internal structure_.
Subgenera _Calamodendron_, Brongniart, 1849 ╮ These names have
_Arthropitys_ Göppert, 1864 ├ primarily reference
_Arthrodendron_ Scott 1897 │ to _internal_
(= _Calamopitys_ Williamson, 1871) ╯ _structure_.
II. Description of the anatomy of Calamites.
_a. Stems. b. Leaves. c. Roots. d. Cones._
No fossils are better known to collectors of Coal-Measure plants than
the casts and impressions of the numerous species of _Calamites_. In
sandstone quarries of Upper Carboniferous rocks there are frequently
found cylindrical or somewhat flattened fossils, varying from one to
several inches in diameter, marked on the surface by longitudinal
ridges and grooves, and at more or less regular intervals by regular
transverse constrictions. Similar specimens are still more abundant
as flattened casts in the blocks of shale found on the rubbish heaps
of collieries. The sandstone casts are often separated from the
surrounding rock by a loose brown or black crumbling material, and the
specimens in the shale are frequently covered by a thin layer of coal.
Most of the earlier writers regarded such specimens as the impressions
of the ribbed stems of plants similar to or identical with reeds or
grasses. Suckow, and afterwards Dawes and others, expressed the opinion
that the ordinary Calamite cast represented a hardened mass of sand
or marl, which had filled up the pith of a stem either originally
fistular or rendered hollow by decay. The investigation of the internal
structure confirmed this view, and proved that the surface-features of
a Calamite stem do not represent the external markings of the original
plant, but the form of the inner face of the cylinder of wood. The ribs
represent the medullary rays of the original stem or branch, and the
intervening grooves mark the position of the strands of xylem which are
arranged in a ring round a large hollow pith[604].
With this brief preliminary account we may pass to a detailed
description of the anatomical characters of _Calamites_.
The genus _Calamites_ may be briefly defined as follows:—
Arborescent plants reaching a height of several meters, and having a
diameter of proportional size. In habit of growth the Calamites bore
a close resemblance to _Equisetum_; an underground rhizome giving off
lateral branches and erect aerial shoots bearing branches, either in
whorls from regularly recurring branch-bearing nodes, or two or three
from each node; and in some cases the stems bore occasional branches
from widely separated nodes. The leaves were disposed in whorls either
as star-shaped verticils on slender foliage shoots, or in the form
of a circle of long narrow leaves on the node of a thicker branch.
Adventitious roots were developed from the nodal regions of underground
and aerial stems. The cones had the form of long and narrow strobili
consisting of a central axis bearing whorls of sterile and fertile
appendages; the latter in the form of sporangiophores bearing groups of
sporangia. The strobili were heterosporous in some cases, isosporous
in others. The stems had a large hollow pith bridged across by a
transverse diaphragm at the nodes in the centre of the single stele;
the latter consisted of a ring of collateral bundles separated from one
another by primary medullary rays. Each group of xylem was composed of
spiral, annular, scalariform and occasionally reticulate tracheids,
the position of the protoxylem being marked by a longitudinal
carinal canal. The shoots and roots grew in thickness by means of a
regular cambium layer. The cortex consisted of parenchymatous and
sclerenchymatous cells, with scattered secretory sacs. The increase in
girth of the central cylinder was often accompanied by a considerable
development of cortical periderm. The roots differed from the shoots
in having no carinal canals, and in the possession of a solid pith and
centripetally developed primary xylem groups alternating with strands
of phloem.
The above incomplete diagnosis includes only some of the more important
structural features of the genus. Thanks to the researches begun by the
late Mr Binney of Manchester and considerably extended by Carruthers,
Williamson and later investigators, we are now in a position to give
a fairly complete account of _Calamites_. The type of stem most
frequently met with in a petrified condition in the English rocks is
that to which Göppert applied the name _Arthropitys_, and it is this
subgenus that forms the subject of the following description. Our
knowledge of Calamitean anatomy is based on the examination of numerous
fragments of petrified twigs and other portions of different specific
types of the genus. It is seldom possible to differentiate specifically
between the isolated fragments of stems and branches which are met
with in calcareous or siliceous nodules. As so frequently happens in
fossil-plant material, large specimens showing good surface features
and broken fragments with well-preserved internal structure have to be
dealt with separately.
[Sidenote: YOUNG STEM.]
_a. Stems._
A transverse section of a young twig, such as is represented in fig.
71, illustrates the chief characteristics of the _primary structure_
of a young branch of _Calamites_. The figure has been drawn from
a section originally described by Hick[605] in 1894. A very young
Calamite twig bears an exceedingly close resemblance to the stem of
a recent _Equisetum_. The axial region of the stem may be occupied
by parenchymatous cells, or the absence of cells in the centre may
indicate the beginning of the gradual formation of the hollow pith,
which is one of the characteristics of _Calamites_. The student of
petrified Palaeozoic plants must constantly be on his guard against
the possible misinterpretation of Stigmarian ‘rootlets,’ which are
frequently found in intimate association with fossil tissues. The
intrusion of these rootlets is admirably illustrated by a section
of a Calamite stem in the Williamson Collection (No. 1558) in which
the hollow pith, 2 cm. broad, contains more than a dozen Stigmarian
appendages.
[Illustration: FIG. 71. Transverse section of a young Calamite stem.
_c_, carinal canals; _mr_, primary medullary rays; _a, b, and d_,
cortex; _e_, epidermis. From a section in the Manchester Museum,
Owens College, × 60.]
In the figured specimen of a Calamite twig (fig. 71) there is a clearly
marked differentiation into a cortical region and a large stele or
central cylinder. The pith-cells are already partially disorganised,
but there still remain a few fairly large parenchymatous cells internal
to the ring of vascular bundles. The few irregular projections into the
cavity of the large pith consist of small fragments of cells, which may
be the result of fungal action. Mycelia of fungi are occasionally met
with in the tissues of older Calamite stems.
The position of the primary xylem groups is shown by the conspicuous
and regularly placed canals, _c_; these have been formed in precisely
the same manner as the corresponding spaces in an _Equisetum_ stem,
and they are spoken of in both genera as the carinal canals. Each
canal owes its origin to the disorganization and tearing apart of
the protoxylem elements and the surrounding cells. This may be
occasionally seen in examples of very young Calamites; the canals of
a young twig often contain apparently isolated rings which are coils
of elongated spiral threads. Fig. 72, _B_ represents the canal of a
twig, cut in an oblique direction, in which the remains of spiral
tracheids are distinctly seen. In the stem of fig. 71 the development
has not advanced far enough to enable us to clearly define the exact
limits of each xylem strand. The smaller elements bordering the canals
constitute the primary xylem, they are fairly distinct on the outer
margin of some of the canals seen in the section. Between the small
patches of primary xylem the outward extensions of the parenchyma of
the pith constitute the primary medullary rays, _mr_. The distinct line
encircling the canals and primary xylem has been described by Hick as
marking the position of the endodermis, but it may possibly owe its
existence to the tearing of the tissues along the line where cambial
activity is just beginning. This layer of delicate dividing cells
would constitute a natural line of weakness. External to this line
we have a zone of tissue _a_, _d_, containing here and there larger
cells with black contents, which are no doubt secretory sacs. It is
impossible to distinguish with certainty any definite phloem groups,
but in other specimens these have been recognised immediately external
to each primary xylem group; the bundles were typically collateral in
structure. Towards the periphery of the twig the preservation is much
less perfect; the outer portion of the inner cortex, _d_, consists
of rather smaller and thicker-walled cells, but this is succeeded
by an ill-defined zone containing a few scattered cells, _b_, which
have been more perfectly preserved. The twig is too young to show any
secondary tissue in the cortex; but the tangential walls in some of
the cortical cells afford evidence of meristematic activity, which
probably represents the beginning of cork-formation. The limiting line,
_e_, possibly represents the cuticularised outer walls of an epidermal
layer. The irregularly wavy character of the surface of the specimen
is probably the result of shrinking, and does not indicate original
surface features.
[Sidenote: VASCULAR SYSTEM.]
In examining sections of calcareous nodules from the coal seams one
meets with numerous fragments of small Calamitean twigs with little
or no secondary wood; in some of these there is a small number of
carinal canals, in others the canals are much more abundant. The
former probably represent the smaller ramifications of a plant, and
the latter may be regarded as the young stages of branches capable of
developing into stout woody shoots[606]. Longitudinal sections of small
branches teach us that the xylem elements next the carinal canals are
either spiral or reticulate in character, the older tracheids being
for the most part of the scalariform type, with bordered pits on the
radial walls. This and other histological characters are admirably
shown in the illustrations accompanying Williamson and Scott’s memoir
on _Calamites_. The student should treat the account of the anatomy
of _Calamites_ given in these pages as introductory to the much more
complete description by these authors. They thus describe the course of
the vascular bundles in a Calamitean branch:—
“The bundle-system of _Calamites_ bears a general resemblance to that
of _Equisetum_. A single leaf-trace enters the stem from each leaf,
and passes vertically downwards to the next node. In the simplest
cases the bundle here forks, its two branches attaching themselves to
the alternating bundles which enter the stem at this node. In other
cases both the forks attach themselves to the same bundle, so that,
in this case, there is no regular alternation. In other cases, again,
the bundle runs past one node without forking, and ultimately forms a
junction with the traces of the second node below its starting-point.
These variations may all occur in the same specimen. The xylem at the
node usually forms a continuous ring, for where the regular dichotomous
forks of the bundles are absent their place is usually taken by
anastomoses[607].”
As in _Equisetum_, the xylem at the nodes possesses certain
characteristic features which distinguish it from the internodal
strands. It has already been pointed out that the xylem of _Equisetum_
increases in breadth at the nodes (p. 251, fig. 55, 4); the same is
true of _Calamites_. In fig. 72, _C_, we have part of a radial section
of a Calamite twig in which the broad mass of short nodal tracheids is
clearly shown; this nodal wood forms a prominent projection towards
the pith. In the lower part of the section the remains of some spiral
protoxylem tracheids are seen in a carinal canal.
[Illustration: FIG. 72.
_A._ External xylem elements and cambium, _c_, with imperfect phloem,
× 100.
_B._ Carinal canal containing protoxylem, _px._ × 65.
_C._ Radial longitudinal section through nodal xylem, _px._ × 35.
_D._ Phloem elements; _s_, sieve-tubes; _p_, _p_, parenchymatous
cells.
(_A–C._ After Williamson and Scott. _D._ After Renault.)]
The tracheids of the nodal wood are often reticularly pitted, and so
differ in appearance from the ordinary scalariform elements.
It is rare to find the phloem clearly preserved, but in specimens where
it has been possible to examine this portion of the vascular bundles,
it is found to consist of elongated cambiform cells and sieve-tubes.
An unusually perfect specimen has been described by Renault[608] in
which the phloem elements are preserved in silica. Fig. 72, _D_, is
copied from one of Renault’s drawings, the sieve-tubes, _s_, _s_,
show several distinct sieve-plates on the lateral walls of the tubes,
reminding one to some extent of the sieve-tubes in a Bracken Fern.
The cells, _p_, _p_, associated with the sieve-tubes are square-ended
elongated parenchymatous elements. Another characteristic feature
illustrated by longitudinal sections is the nodal diaphragm; except in
the smallest branches the interior of each internode is hollow, and the
ring of vascular bundles is separated from the pith-cavity by a band
of parenchymatous tissue. At each node this parenchyma extends across
the central cavity in the form of a nodal diaphragm, as in the stem of
_Equisetum_.
By far the greater number of the petrified fragments of _Calamites_
afford proof of cambial activity, and possess obvious secondary
tissues. In exceptionally perfect specimens the xylem tracheids are
found to be succeeded externally by a few flattened thin-walled cells
which are in a meristematic condition (fig. 72, _A_, _c_); these
constitute the cambium zone, and it is the _secondary structure_ that
results from the activity of the meristematic cells that we have now to
consider.
[Sidenote: SECONDARY THICKENING.]
In petrified examples of branches in which the secondary thickening
has reached a fairly advanced stage, the wood is usually the outermost
tissue preserved, the more external tissues having been detached along
the line of cambium cells. It is only in a few cases that we are able
to examine all the tissues of older examples.
The specimen represented in fig. 73 illustrates very clearly the
extension of the hollow pith up to the inner surface of the vascular
ring; the disorganisation of the pith-cells which had already begun in
the twig of fig. 71 has here advanced much further. The bluntly rounded
projections represent the prominent primary xylem strands, each of
which is traversed by the characteristic carinal canal. Alternating
with the wedge-shaped groups of secondary xylem, _x_, we have the broad
principal medullary rays, _mr_, which become slightly narrower towards
the outside. The inner face of each of these wide rays has a concave
form, due to the less resistent nature of the medullary-ray cells as
compared with the stronger xylem. The regularly sinuous form of the
inner face of the vascular cylinder enables one to realise how the
Calamite-casts (figs. 82, 99, and 101) have come to have the regular
ridges and grooves on their surface. The broad ridges on the cast mark
the position of the wide medullary rays, while the grooves correspond
to the more prominent ends of the vascular strands. The tissues
external to the wood have not been preserved in the example shown in
fig. 73. Some silicified specimens described by Stur[609] from Bohemia
and now in the Museum of the Austrian Geological Survey, Vienna,
admirably illustrate the connection between the surface features of a
Calamite cast and the anatomy of the stem.
[Illustration: FIG. 73.
Transverse section of a Calamite stem.
_mr_, medullary ray. After Williamson.
_x_, _x_, xylem. (No. 1933 A.A. in the Williamson Collection.)]
[Sidenote: ARTHROPITYS.]
In the large section of a calcareous nodule diagrammatically shown in
fig. 17 II. (p. 85) the secondary wood of a slightly flattened Calamite
is the most prominent plant fragment. The pith-cavity has been almost
obliterated by the lateral compression of the woody cylinder, but the
presence of the carinal canals along the inner edge of the wood may
still be readily recognised. The appearance presented by a transverse
section of the secondary wood of a Calamite is that of regular
radial series of rather small rectangular tracheids, with occasional
secondary medullary rays consisting of narrow and radially elongated
parenchymatous cells. The principal rays[610] in the _Arthropitys_ type
of a Calamite stem are often found to gradually decrease in breadth
as they pass into the secondary wood, until in the outer portion of
the wood the primary medullary rays are practically obliterated by the
formation of interfascicular xylem.
In fig. 74, _A_, we have a portion of a single xylem group of a
thick woody stem. The stem from which the figure has been drawn was
originally described by Binney[611] as _Calamodendron commune_; we
now recognise it as a typical example of the subgenus _Arthropitys_.
The specific term _communis_ was used by Ettingshausen[612] in 1855
in a comprehensive sense to include more than twenty species of the
genus _Calamites_, but since Binney’s use of the term it has come to
be associated with a definite type of _Arthropitys_ stem, in which
the primary medullary rays decrease rapidly in breadth towards the
periphery of the wood. The wood of Binney’s stem[613] measures 2·5
cm. across, but the pith-cavity has been crushed to the limits of a
narrow band represented in the figure by the shaded portion. The strand
of cells, _s_, in the pith is a portion of a Stigmarian appendage
(“rootlet”), which penetrated into the hollow stem of the Calamite
and became petrified by the same agency to which the preservation of
the stem is due. These intruded Stigmarian appendages are of constant
occurrence in the calcareous nodules; their intimate association with
the tissues of other plants is often a serious source of error in the
identification of petrified tissues. The inner portion of one of the
xylem groups is shown in fig. 74, _A_. External to the carinal canal,
the xylem tracheids are disposed in regular series and associated with
numerous narrow secondary medullary rays. The width of the xylem wedge
increases gradually as we pass outwards, this is due to the formation
of interfascicular xylem, which in the more peripheral portion of
the stem extends across the primary medullary rays. The few primary
medullary-ray cells shown in the drawing illustrate the characteristic
tangentially elongated form and large size of the parenchymatous
elements. Williamson and Scott have pointed out that the tangentially
elongated form of the medullary-ray cells is the result of active
growth, and not merely the expression of the tangential stretching of
the stem consequent on secondary thickening.
[Illustration: FIG. 74.
_A._ Transverse section of part of a Calamite stem. [_Calamites
(Arthropitys) communis_ (Binney).]
_s_, Stigmarian appendage. _x_, xylem. From a specimen in the
Binney Collection, Cambridge, × 50.
_B._ Transverse section of a stem.
_h_, hypodermal tissue; _c_, inner cortex. From a specimen in the
Williamson Collection (no. 62). × 35.]
A glance at the complete transverse section of the stem,—of which a
small portion is shown in fig. 74 _A_,—suggests the existence of annual
rings in the wood, but this appearance of rings is merely the result
of compression. The secondary wood of a Calamite does not exhibit any
regular zones of growth comparable with the annual rings of our forest
trees.
[Illustration: FIG. 75. Longitudinal tangential section near the inner
edge of the wood of the Calamite of fig. 74.
_x_, _x_, secondary xylem and medullary rays; _m_, principal
medullary ray. From a section in the Binney Collection, × 50.]
Before passing to other examples of Calamitean stems, reference may be
made to the sections shown in figs. 75 and 76, which illustrate some
further points in the structure of Binney’s stems. In fig. 75 the xylem
tracheids are shown at _x_, and between them the secondary medullary
rays present the appearance of long and narrow parenchymatous cells;
as the section is tangential the characteristic scalariform character
of the tracheids is not shown, the ladder-like bordered pits being
confined to the radial walls of the tracheal elements. The much greater
length than breadth of the cells which form the rays associated with
the xylem tracheids, is a characteristic feature in Calamitean stems.
The breadth of the principal ray, _m_, shows that the section has
passed through the wood a short distance from the pith; in a tangential
section cut further into the wood the breadth of the principal rays
would be considerably reduced. The large medullary-ray tissue consists
of square-walled parenchymatous cells. The more highly magnified
section, in fig. 76, shows a central group of parenchyma containing
a few transversely cut tracheids, but the two kinds of elements are
not clearly differentiated in the figure; this group of cells is an
outgoing leaf-trace which is enclosed by the strongly curved tracheids
of the stem. The section is taken from the node of a stem where several
leaf-trace bundles are passing out to a whorl of leaves; the few cells
intercalated between the tracheids belong to the parenchyma of the
secondary medullary rays.
[Illustration: FIG. 76. Longitudinal tangential section of the same
Calamite as that of figs. 74 and 75, showing a leaf-trace
and curved tracheids at a node. From a section in the Binney
Collection, × 100.]
[Sidenote: ARTHROPITYS. SURFACE FEATURES.]
In the small portion of a stem represented in fig. 74 _B_, the cortical
tissues have been partially preserved; at the inner edge, next the
hollow pith, there are two xylem groups, each with a carinal canal, and
between them is part of a broad “principal” medullary ray[614]. The
cambium has not been preserved, but beyond this region we have some
of the large cells, _c_, of the inner cortex; these are followed by a
few remnants of a smaller-celled tissue, and external to this part of
the cortex there is a series of triangular groups, _h_, consisting of
small thick-walled cells alternating with spaces which were originally
occupied by more delicate parenchyma. The darker groups constitute
hypodermal strands of mechanical tissue or stereome which lent support
to the stem. The surface of a stem possessing such supporting strands
would probably assume a longitudinally wrinkled or grooved appearance
on drying; the intervening parenchyma, contracting and yielding more
readily, would tend to produce shallow grooves alternating with the
ridges above the stereome strands.
The complete section of the stem of which a small portion is shown
in fig. 74 _B_, is figured by Williamson[615] in his 12th memoir on
Coal-Measure plants. The section was obtained from Ashton-under-Lyne
in Lancashire; it illustrates very clearly a method of preservation
which is occasionally met with among petrified plants. The walls of the
various tissue elements are black in colour and somewhat ragged, and
the general appearance of the section is similar to that of a section
of a charred piece of stem. It is possible that the Calamite twig was
reduced to charcoal before petrifaction by a lightning flash or some
other cause.
It is often said that the surface of a Calamite stem was probably
marked by regular ridges and grooves similar to those of the
pith-cast, and that such external features are connected with the
arrangement of the tissues in the vascular cylinder. The indication
of grooves and ridges on the bark of fossil Calamites is no doubt the
result of the existence in the hypoderm of firm strands alternating
with strands of less resistant cells. It is very common to find
Calamite pith-casts covered with a layer of coal presenting a ribbed
surface, but this is simply due to the moulding of the coaly film on an
internal pith-cast. The broad grooves on such a specimen as that of
fig. 77 are, on the other hand, probably an indication of the existence
of hypoderm bands similar to those in fig. 74 _B_, _h_. The specimen
from which fig. 77 is drawn shows many interesting features. The figure
given by Grand’Eury, of which fig. 77 is a copy, is somewhat idealised,
but the various surfaces can be made out in the fossil. The surface
of the coaly envelope surrounding the pith-cast, _a_, is distinctly
grooved, but the depressions have nothing to do with the surface
features of the wood or the pith-cast; they are no doubt due to the
occurrence of alternating bands of thick- and thin-walled tissue in the
hypodermal region of the cortex; the peripheral strands of bast cells
would stand out as prominent ribs as the stem tissue contracted during
fossilisation. At _b_ (fig. 77) we have a view of the wood in which the
position of the principal rays is indicated by fine longitudinal lines
at regular intervals; the oval projections just below the nodal line
are probably the casts of infranodal canals (_cf._ p. 324). At _a_ the
characteristic pith-cast is seen with a small branch-scar on the node.
The scar on the middle node, _N_ 2, is probably that of a root, and a
root _R_ is still attached to the node, _N_ 3.
[Illustration: FIG. 77. Portion of a Calamite stem, showing the surface
of the bark, _c_; the wood, _b_; the surface of the pith-cast,
_a_. _N._1-_N._3. Nodes. _R._ Root. (After Grand’Eury. Partially
restored from a specimen in the École des Mines, Paris.) ¾ nat.
size.]
[Sidenote: PERIDERM IN STEMS.]
An interesting feature observed in some specimens of older Calamite
branches is the development of periderm or cork. This is illustrated on
a large scale by a unique specimen originally described by Williamson
in 1878[616]. Figs. 78 and 79 represent transverse and longitudinal
sections of this stem. This unusually large petrified stem was found
in the Coal-Measures of Oldham, in Lancashire. In the slightly reduced
drawing, fig. 78, the large and somewhat flattened pith, _p_, 4·2 cm.
in diameter, is shown towards the bottom of the figure. Surrounding
this we have 58 or 59 wedge-shaped projecting xylem groups and broad
medullary rays; the latter soon become indistinguishable as they
are traced radially through the thick mass of secondary wood, 5 cm.
wide, composed of scalariform tracheids and secondary medullary rays
(fig. 78, 3). The secondary wood presents the features characteristic
of _Calamites_ (_Arthropitys_) _communis_ (Binney). External to
the wood there is a broken-up mass, about 5·5 cm. wide composed of
regularly arranged (fig. 78, 2) and rather thick-walled cells; this
consists of periderm, a secondary tissue, which has been developed
by a cork-cambium during the increase in girth of the plant. The
more delicate cortical tissues have not been preserved, and the more
resistant portion of the bark has been broken up into small pieces of
corky tissue, among which are seen numerous Stigmarian appendages,
pieces of sporangia and other plant fragments. These associated
structures cannot of course be shown in the small-scale drawing of the
figure.
[Illustration: FIG. 78.
1. Transverse section of a thick Calamite stem.
_p_, pith; _x_, secondary wood; _c_, bark. (⅔ nat. size.)
2. Periderm cells of bark.
3. Xylem and medullary rays. (2 and 3, × 80.)
From a specimen in the Williamson Collection (no. 79).]
In the radial longitudinal section (fig. 79) we see the pith with
the projecting wood and the remains of a diaphragm at the node. The
mottled or watered appearance of the wood is due to numerous medullary
rays which sweep across the tracheids. The periderm elements, as seen
in longitudinal section, are fibrous in form.
[Illustration: FIG. 79. Longitudinal section of the specimen stem in
fig. 78. From a specimen in the Williamson Collection, British
Museum (no. 80). ⅔ nat. size.]
[Sidenote: CALLUS WOOD.]
The development of cork in a younger Calamite stem is clearly shown in
a specimen described by Williamson and Scott in their Memoir of 1894.
In a transverse section of the stem several large cells of the inner
cortex are seen to be in process of division by tangential walls, and
giving rise to radially arranged periderm tissue[617].
The section diagrammatically sketched in fig. 80 is that of a Calamite
twig in which the wood appears to have been injured, and the wound has
been almost covered over by the formation of callus wood. The young
trees in a Palaeozoic forest might easily be injured by some of the
large amphibians, which were the highest representatives of animal
life during the Carboniferous period, just as our forest trees are
often barked by deer, rabbits, and other animals. Fissures might also
be formed by the expansion of the bark under the heating influence of
the sun’s rays[618]. Such a specimen as that of fig. 80 gives an air
of living reality to the petrified fragments of the Coal period trees.
It is well known how a wound on the branch of a forest tree becomes
gradually overgrown by the activity of the cambium giving rise to a
thick callus, which gradually closes over the wounded surface in the
form of two lips of wood which finally meet over the middle of the
scar. The two lips of callus are clearly shown in the fossil branch
arching over the tear in the wood just beyond the ring of carinal
canals. The tissue external to the wood represents the imperfectly
preserved cortex. A section which was cut parallel to that of fig. 80
shows a continuous band of wood beyond the wound, and the latter has
the form of a small triangular gap; this section appears to have passed
across the wound where it was narrower and has already been closed over
by the callus. The formation of a rather different kind of callus wood
has been described by Renault[619] and by Williamson and Scott[620],
in stems where aborted or deciduous branches have been overgrown and
sealed up by cambial activity.
[Illustration: FIG. 80. Diagrammatic sketch of a transverse section of
a Calamite twig, showing callus wood. From a specimen in the
Cambridge Botanical Laboratory Collection. × _ca._ 10.]
[Illustration: FIG. 81. _Calamites._ Longitudinal section (_R_, radial;
_T_, tangential) of a small branch. _b_, position of a lateral
branch. From a specimen (no. 1937) in the Williamson Collection.
Slightly enlarged.]
Some of the features to be noticed in longitudinal sections of Calamite
stems have already been described, at least as regards younger
branches. The specimen shown in fig. 81 illustrates the general
appearance of a stem as seen in tangential and radial section. In the
lower portion, _T_, the course of the vascular bundles is shown by
the black lines which represent the xylem tracheids, bifurcating and
usually alternating at each node. Between the xylem strands are the
broad principal medullary rays. At _b_ a branch has been cut through
on its passage out from the parent stem, just above the nodal line. In
tangential sections of Calamite stems one frequently sees both branches
and leaf-trace bundles (fig. 83, _A_), passing horizontally through
the wood and enclosed by strongly curved and twisted tracheids. In the
upper part of the figure (81, _R_), the section has passed through the
centre of the stem, and the wood is seen in radial view; each node is
bridged across by a diaphragm of parenchymatous cells capable of giving
rise to a surface layer of periderm[621].
An outgoing branch, as seen in a tangential section of a stem, consists
of a parenchymatous pith surrounded by a ring of vascular bundles, in
which the characteristic carinal canals have not yet been formed, but
if the section has cut the branch further from its base, there may be
seen a circle of irregular gaps marking the position of the carinal
canals. Such gaps are often occupied by thin parenchyma, and contain
protoxylem elements. The outgoing branches, as seen in a tangential
section of a Calamite stem, are seen to be connected with the wood of
the parent stem by curved and sinuous tracheids, which give to the
stem-wood a curiously characteristic appearance[622], as if the xylem
elements had been pushed aside and contorted by the pressure of the
outgoing member. A tangential section through a Pine stem[623] in the
region of a lateral branch presents precisely the same features as in
_Calamites_. The branches are given off from the stem immediately above
a node and usually between two outgoing leaf-trace bundles.
[Sidenote: RHIZOME OF CALAMITES.]
Specimens of pith-casts occasionally present the appearance of a
curved and rapidly tapered ram’s horn, and the narrow end of such a
cast is sometimes found in contact with the node of another cast. This
juxtaposition of casts is shown unusually well in fig. 82. In some of
the published restorations of _Calamites_ the plant is represented as
having thick branches attached to the main stem by little more than
a point. Williamson[624] clearly explained this apparently unusual
and indeed physically impossible method of branching, by means of
sections of petrified stems. The branches seen in fig. 82 are of
course pith-casts, and in the living plant the pith of each branch was
surrounded by a mass of secondary wood developed from as many primary
groups of xylem as there are grooves on the surface of the cast, each
of the grooves on an internode corresponding to the projecting edge
of a xylem group. At the junction of one branch with another the pith
was much narrower and the enclosing wood thicker, so that the tapered
ends of the cast merely show the continuity by a narrow union between
the pith-cavities of different branches. Most probably the casts of
fig. 82 are those of a branched rhizome which grew underground, giving
off aerial shoots and adventitious roots. There is a fairly close
resemblance between the Calamite casts of fig. 82 and a stout branching
rhizome of a Bamboo, _e.g._ _Bambusa arundinacea_ Willd.; it is not
surprising that the earlier writers looked upon the Calamite as a
reed-like plant.
Before leaving the consideration of stem structures there is another
feature to which attention must be drawn. On the casts shown in fig. 82
there is a circle of small oval scars situated just below the nodes,
these are clearly shown at _c_, _c_, _c_. Each of the scars is in
reality a slight projection from the upper end of an internodal ridge.
As the ridges correspond to the broad inner faces of medullary rays,
the small projection at the upper end of each ridge is a cast of a
depression or canal which existed in the medullary tissue of the living
plant. There have been various suggestions as to the meaning of these
oval projections; several writers have referred to them as the points
of attachments of roots or other appendages, but Williamson proved them
to be the casts of canal-like gaps which traversed the upper ends of
principal medullary rays in a horizontal direction. In a tangential
section of a Calamite stem the summit of each primary medullary ray
often contains a group of smaller elements which are in process of
disorganisation, and in some cases these cells give place to an oval
and somewhat irregular canal. In the diagrammatic tangential section
represented in fig. 83, _A_ the upper end of each ray is perforated
by a large oval space, which has been formed as the result of the
breaking down of a horizontal band of cells. Williamson designated
these spaces _infranodal canals_. While proving that they had nothing
to do with the attachment of lateral members, he suggested that they
might be concerned with secretion; but their physiological significance
is still a matter of speculation. The casts of infranodal canals are
especially large and conspicuous in the subgenus _Arthrodendron_,
a form of Calamite characterised by certain histological features
to be referred to later. Williamson[625] originally regarded the
presence of infranodal canals as one of the distinguishing features of
_Arthrodendron_, but they occur also in the casts of the commoner type
_Arthropitys_. As a rule we have only the cast of the inner ends of the
infranodal canals preserved as slight projections like those in fig.
83, _A_; but in one exceptionally interesting pith-cast described by
Williamson, these casts of the infranodal canals have been preserved as
slender spoke-like columns radiating from the upper ends of the ridges
of the infranodal region of a pith-cast.
[Illustration: FIG. 82. Branched rhizome of _Calamites_. ½ nat. size.
_C_, _C_, nodes showing casts of infranodal canals.
From a specimen in the Manchester Museum, Owens College.]
This specimen, which was figured by Williamson[626] in two of his
papers, and by Lyell[627] in the fifth edition of his _Elementary
Geology_, is historically interesting as being one of the first
important plants obtained by Williamson early in the fifties, when he
began his researches into the structure of Carboniferous plants. A
joiner, who was employed by Williamson to make a piece of machinery
for grinding fossils, brought a number of sandstone fragments as an
offering to his employer, whom he found to be interested in stones.
The specimens “were in the main the merest rubbish, but amongst them,”
writes Williamson, “I detected a fragment which was equally elegant
and remarkable.... In later days, when the specimen so oddly and
accidentally obtained, came to be intelligently studied, its history
became clear enough, and the priceless fragment is now one of the most
precious gems in my cabinet[628].”
_Comparison of three types of structure met with in Calamitean
stems_,—Arthropitys, Arthrodendron, _and_ Calamodendron.
[Sidenote: ARTHROPITYS.]
The anatomical features which have so far been described as
characteristic of _Calamites_ represent the common type met with in the
English Coal-Measures. The same type occurs also in France, Germany and
elsewhere. It is that form of stem known as _Arthropitys_, a sub-genus
of _Calamites_.
_Arthropitys_ may be briefly diagnosed as follows,—confining our
attention to the structure of the stem: A ring of collateral bundles
surrounds a large hollow pith, each primary xylem strand terminates
internally in a more or less bluntly rounded apex traversed by a
longitudinally carinal canal. The principal medullary rays consist of
large-celled parenchyma, of which the individual elements are usually
tangentially elongated as seen in transverse section, and four or five
times longer than broad as seen in a tangential longitudinal section.
The secondary xylem consists of scalariform and reticulately pitted
tracheids; the interfascicular xylem may be formed completely across
each primary ray at an early stage in the growth of the stem[629], or
it may be developed more gradually so as to leave a tapering principal
ray of parenchyma between each primary xylem bundle. In the latter
case the principal rays present the characteristic appearance shown in
figs. 71, 74, _A_, 75 and 78, a type of stem which we may refer to as
_Calamites_ (_Arthropitys_) _communis_. In the former case the stem
presents the appearance shown in fig. 83, _D_[630]. A third variety
of _Arthropitys_ stem is one which was originally named by Göppert
_Arthropitys bistriata_; in this form the principal rays retain their
individuality as bands of parenchyma throughout the whole thickness
of the wood[631]. Such stems as those of figs. 73 and 74, _B_, may be
young examples of _Arthropitys communis_ or possibly of _A. bistriata_.
The narrow secondary medullary rays of _Arthropitys_ usually consist of
a single row of cells which are three to five times higher than broad,
as seen in tangential longitudinal section. Infranodal canals occur in
some examples of _Arthropitys_.
[Sidenote: ARTHRODENDRON.]
In the subgenus _Arthrodendron_, a type of stem first recognised
by Williamson and named by him _Calamopitys_[632], the principal
medullary rays consist of _prosenchymatous cells_ (_i.e._ elongated
pointed elements) and not parenchyma. These elongated elements are
not pitted like tracheids, and they are shorter and broader than the
xylem elements. In some examples of this subgenus the primary rays
are bridged across at an early stage by the formation of secondary
interfascicular xylem, and in others they persist as bands of ray
tissue, as in _Arthropitys_. Other characteristics of _Arthrodendron_
are the abundance of reticulated instead of scalariform tracheids in
the secondary wood, and the large size of the infranodal canals.
Fig. 83, _D_ represents part of a transverse section of
_Arthrodendron_; in this stem the rays have been occupied by
interfascicular xylem at a very early stage of the secondary growth.
The section from which fig. 83, _D_ is drawn was described by
Williamson in 1871; the complete section shows about 80 carinal canals
and primary xylem groups. The prosenchymatous form of the principal
medullary rays is seen in fig 83, _C_, and the reticulate pitting on
the radial wall of a tracheid is shown in fig. 83, _B_. Fig. 83, _A_
illustrates the large infranodal canals as seen in a tangential section
of a stem. The same section shows also the course of the vascular
bundles characteristic of _Calamites_ as of _Equisetum_, and the
position of outgoing leaf-traces is represented by unshaded areas in
the black vascular strands.
[Illustration: FIG. 83. _Calamites_ (_Arthrodendron_).
_A._ Tangential section (diagrammatic) showing the course of the
vascular strands, also leaf-traces and infranodal canals.
_B._ Radial face of a tracheid.
_C._ Prosenchymatous elements of a principal medullary ray.
_D._ Transverse section of the wood. (After Williamson.) No. 36 in
the Williamson Collection.]
The subgenus _Arthrodendron_ is very rarely met with, and our
information as to this type is far from complete[633].
The third subgenus _Calamodendron_ has not been discovered in English
rocks, and our knowledge of this type is derived from French and
German silicified specimens[634]. There is the same large hollow
pith surrounded by a ring of collateral bundles with carinal canals,
as in the two preceding subgenera. The tracheids are scalariform
and reticulate, and the secondary medullary rays consist of rows of
parenchymatous cells which are longer than broad, as in _Arthropitys_
and _Arthrodendron_.
[Illustration: FIG. 84. _Calamites_ (_Calamodendron_) _intermedium_, Ren.
Transverse section through two vascular bundles.
_a_, _a_, xylem tracheids, _b_, _b_, bands of prosenchyma, _c_,
medullary ray. (After Renault.)]
The most characteristic feature of _Calamodendron_ is the occurrence of
several rows of radially disposed thick-walled prosenchymatous elements
(fig. 84, _b_) on either flank of each wedge-shaped group of xylem.
Each principal ray is thus nearly filled up by bands of fibrous cells
on the sides of adjacent xylem groups, but the centre of each principal
ray is occupied by a narrow band of parenchyma (fig. 84, _c_). The
relative breadth of the xylem and prosenchymatous bands has been made
use of by Renault as a specific character in _Calamodendron_ stems.
Fig. 84 is copied from a drawing recently published by this French
author of a new species of _Calamodendron_, _C. intermedium_[635]. In
this case the bands of fibrous cells, _b_, are slightly broader, as
seen in a transverse section of the stem, than the bands of xylem
tracheids, _a_. The narrow band, _c_, consists of four rows of the
parenchymatous tissue of a medullary ray. At the inner end of each
group of tracheids there is a large carinal canal.
The question of the recognition of the pith-casts of stems possessing
the structure of any of the three subgenera of _Calamites_ is referred
to in a later section of this chapter.
_b._ _Leaves_
_Leaves of Calamites and Calamitean foliage-shoots, including an
account of_ (α) Calamocladus (Asterophyllites) _and_ (β)
Annularia.
Our knowledge of the structure and manner of occurrence of Calamite
leaves is very incomplete. There are numerous foliage-shoots among the
fossils of the Coal-Measures which are no doubt Calamitean, but as
they are nearly always found apart from the main branches and stems,
it is generally impossible to do more than speak of them as probably
the leaf-bearing branches of a Calamite. The familiar fossils known
as _Asterophyllites_, and in recent years often referred to the genus
_Calamocladus_, are no doubt Calamitean shoots; but they are usually
found as isolated fragments, and it is seldom that we are able to refer
them to definite forms of _Calamites_. Another common Coal-Measure
genus, _Annularia_, is also Calamitean, and at least some of the
species are no doubt leafy shoots of _Calamites_. Although it is
generally accepted that the fossils referred to as _Asterophyllites_
or _Calamocladus_ are portions of _Calamites_, and not distinct
plants, it is convenient, and indeed necessary, to retain such a term
as _Calamocladus_ as a means of recording foliage-shoots, which may
possess both a botanical and a geological value.
Some of the Calamite casts, especially those referred to the subgenus
_Calamitina_, are occasionally found with leaves attached to the nodes.
In some stems the leaves are arranged in a close verticil, and each
leaf has a narrow linear form and is traversed by a single median vein.
Figures of Calamite stems with verticils of long and narrow leaves
may be found in Lindley and Hutton[636], and in the writings of many
other authors[637]. In the specimen shown in fig. 85 the leaves are
preserved apart from the stem, but from their close association with
a Calamite cast, and from the proofs afforded by other specimens, it
is quite certain they formed part of a whorl of leaves attached to the
node of a true Calamite, and a stem having that particular type known
as _Calamitina_[638] (figs. 99, 100). It is probable that in some
Calamites, and especially in younger shoots, the leaves had the form of
narrow sheaths split up into linear segments. This question has already
been referred to in dealing with certain Palaeozoic fossils referred to
_Equisetites_[639].
[Illustration: FIG. 85. Linear leaves of a Calamite (_Calamitina_).
After Weiss, slightly reduced.]
A few years ago the late Thomas Hick[640], of Manchester, described
the structure of some leaves which he believed to be those of a
Calamite. He found them attached to a slender axis which possessed the
characteristics of a young Calamite branch. There can be little doubt
that his specimens are true Calamite leaves. The sketches of fig. 86
have been made from the sections originally described by Hick. Fig.
86, 1 shows a leaf in transverse section; on the outside there is a
well-defined epidermal layer with a limiting cuticle. Internal to
this we have radially elongated parenchymatous cells forming a loose
or spongy tissue, the cells being often separated by fairly large
spaces (fig. 86, 5), especially in the region of the blunt lateral
wings of the leaf. Some of these cells contain a single dark dot, which
in all probability is the mineralised nucleus. These pallisade-like
cells probably contained chlorophyll and constituted the assimilating
tissue of the leaf. In the centre there is a circular strand of cells
limited by a layer of larger cells with black contents, enclosing an
inner group of small-celled parenchyma and traversed by a few spiral
or scalariform tracheids constituting the single median vein. It is
hardly possible to recognise any phloem elements in the small vascular
bundle; there appear to be a few narrow tracheids surrounded by larger
parenchymatous elements (fig. 86, 2). At one point in the epidermis
of fig. 86, 1, there appears to be a stoma, but the details are not
very clearly shown (fig. 86, 4); the two cells, _s_, _s_, bordering the
small aperture are probably guard-cells.
[Illustration: FIG. 86. A leaf of _Calamites_.
1. Transverse section; _t_, vascular bundle; _x_, sheath of cells.
× 35.
2. Vascular bundle consisting of a few small tracheids, _t_.
3. A tracheid and a few parenchymatous cells, the latter with nuclei.
4. A stoma; _s_, _s_, guard-cells.
5. Pallisade cells and intercellular spaces.
From a section in the Manchester Museum, Owens College.]
The nature of the assimilating tissue, the comparatively thick band of
thin-walled cells with intercellular spaces, and the exposed position
of the stomata suggest that the plant lived in a fairly damp climate;
at least there is nothing to indicate any adaptation to a dry climate.
In the Binney collection of plants in the Woodwardian Museum,
Cambridge, there is a species of a very small shoot bearing three or
four verticils of leaves which possess the same structure as those of
fig. 86. We may probably regard such twigs as the slender terminal
branches of Calamitean shoots.
α. _Calamocladus_ (_Asterophyllites_).
The generic name _Asterophyllites_ was proposed by Brongniart[641] in
1822 for a fossil previously named by Schlotheim[642] _Casuarinites_,
and afterwards transferred to Sternberg’s genus _Annularia_. In 1828
Brongniart[643] gave the following diagnosis of the fossils which he
included under the genus _Asterophyllites_:—“Stems rarely simple,
usually branched, with opposite branches, which are always disposed in
the same plane; leaves flat, more or less linear, pointed, traversed by
a simple median vein, free to the base.” Lindley and Hutton described
examples of Brongniart’s genus as species of _Hippurites_[644], and
other authors adopted different names for specimens afterwards referred
to _Asterophyllites_.
At a later date Ettingshausen[645] and other writers expressed the
view that the fossils which Brongniart regarded as a distinct genus
were the foliage-shoots of _Calamites_, and Ettingshausen went so far
as to include them in that genus. In view of the generally expressed
opinion as to the Calamitean nature of _Asterophyllites_, Schimper[646]
proposed the convenient generic name _Calamocladus_ for “rami et
ramuli foliosi” of _Calamites_. Some recent authors have adopted this
genus, but others prefer to retain _Asterophyllites_. In a recent
important monograph by Grand’Eury[647] Calamitean foliage-shoots are
included under the two names, _Asterophyllites_ and _Calamocladus_;
the latter type of foliage-shoots he associates with the stems of
the subgenus _Calamodendron_, and the former he connects with those
Calamitean stems which belong to the subgenus _Arthropitys_.
It is an almost hopeless task to attempt to connect the various forms
of foliage-shoots with their respective stems, and to determine what
particular anatomical features characterised the plants bearing
these various forms of shoots. We may adopt Schimper’s generic name
_Calamocladus_ in the same sense as _Asterophyllites_, but as including
such other foliage-shoots as we have reason to believe belonged to
_Calamites_. Those leaf-bearing branches which conform to the type
known as _Annularia_ are however not included in _Calamocladus_, as
we cannot definitely assert that these foliage-shoots belong in all
cases to Calamitean stems. Grand’Eury’s use of _Calamocladus_ in a
more restricted sense is inadvisable as leading to confusion, seeing
that this name was originally defined in a more comprehensive manner
as including Calamitean leaf-bearing branches generally. We may define
_Calamocladus_ as follows:—
Branched or simple articulated branches bearing whorls of uni-nerved
linear leaves at the nodes; the leaves may be either free to the
base or fused basally into a cup-like sheath (_e.g._ Grand’Eury’s
_Calamocladus_). The several acicular linear leaves or segments which
are given off from the nodes spread out radially in an open manner in
all directions; they may be either almost at right angles to the axis
or inclined at different angles. Each segment is traversed by a single
vein and terminates in an acuminate apex.
As a typical example of a Calamitean foliage-shoot the species
_Calamocladus equisetiformis_ (Schloth.) may be briefly described. The
synonymy of the commoner species of fossil plants is a constant source
of confusion and difficulty; in order to illustrate the necessity of
careful comparison of specimens and published illustrations, it may be
helpful to quote a few synonyms of the species more particularly dealt
with. The exhaustive lists drawn up by Kidston in his _Catalogue of
Palaeozoic plants in the British Museum_ will be found extremely useful
by those concerned with a systematic study of the older plants.
[Illustration: FIG. 87. _Calamocladus equisetiformis_ (Schloth.).
From a specimen in the British Museum (McMurtrie Collection, no. V.
2963). _ca._ ⅓ nat. size.]
_Calamocladus equisetiformis_ (Schloth.). Fig. 87.
1809. _Phytolithus_, Martin[648].
1820. _Casuarinites equisetiformis_, Schlotheim[649].
1825. _Bornia equisetiformis_, Sternberg[650].
1828. _Asterophyllites equisetiformis_, Brongniart[651].
1836. _Hippurites longifolia_, Lindley and Hutton[652].
1855. _Calamites equisetiformis_, Ettingshausen[653].
1869. _Calamocladus equisetiformis_, Schimper[654].
1869. _Annularia calamitoides_, Schimper[654].
The above synonyms do not exhaust the list[655], but they suffice to
illustrate the necessity of a careful comparison in drawing up tables
of species, in connection with geographical distribution or for other
purposes.
_Calamocladus equisetiformis_ may be briefly defined as follows:—A
central axis possessing a hollow pith of Calamitean character, divided
externally into well-marked slightly constricted nodes and internodes;
from the nodes long narrow and free leaves are borne in whorls; from
the axils of some of the leaves lateral branches are given off inclined
at a fairly wide angle to the main axis, and bearing crowded verticils
of spreading acicular leaves.
The unusually good specimen, 38·5 cm. long, shown on a much reduced
scale in fig. 87, illustrates the characteristic habit of this form of
_Calamocladus_. It is from the Radstock coal-field of Somersetshire,
one of the best English localities for Coal-Measure plants. An
exceedingly good collection of Radstock plants has recently been
presented to the British Museum by Mr J. McMurtrie; it includes many
fine specimens of _Calamites_. A small example—probably of this
species—from Coalbrook Dale, near Dudley, in Shropshire, and now in the
British Museum, illustrates very well the appearance of a young and
partially expanded Calamitean foliage-shoot. The central axis, 6·5
cm. in length, includes about 15 internodes, and terminates in a bud
covered by several small leaves. Lateral branches are given off at a
wide angle, and small unexpanded buds occur in the axils of several of
the leaves.
As an example of the leaf-bearing branches which Grand’Eury has
recently described as _Calamocladus_, using the genus in a more
restricted sense than is adopted in the present chapter, reference may
be made to the fragment shown in fig. 68, _A_. The foliage-shoots of
this type bore verticils of linear leaves, coherent basally in the form
of a cup, at the ends of branches and not in a succession of whorls on
each branch. The association of reproductive organs, in the form of
long and narrow strobili, with _Calamocladus_ is referred to in the
sequel.
The specimens described by Grand’Eury are in the École des Mines
Museum, Paris; some of the shoots which are well preserved bear a
resemblance in habit of growth to the genus _Archaeocalamites_.
β. _Annularia._
In 1820 this generic name was applied by Sternberg[656] to some
specimens of branches bearing verticils of linear leaves. In 1828
Brongniart[657] thus defined the genus _Annularia_:—“Slender stem,
articulated, with opposite branches arising above the leaves. Leaves
verticillate, flat, frequently obtuse, traversed by a single vein,
fused basally and of unequal length.”
In the works of earlier writers we find frequent illustrations of
specimens of _Annularia_, which are compared with Asters and other
recent flowering plants. Lehmann[658] contributed a paper to the Royal
Academy of Berlin in 1756, in which he referred to certain fossil
plants as probable examples of flowers, among them being a specimen
of _Annularia_. He refers to the occurrence of fossil ferns and other
plants, and asks why we do not find flowers of the rose or tulip;
his object being “not to acquire vain glory, but to give occasion for
others to look into the matter more clearly.”
The general habit of the fossils which are now included under
_Annularia_ agrees closely with that of _Calamocladus_. There is the
same spreading form and a similar foliage in the two genera, but in
_Annularia_ the members of a whorl are always fused into a basal
sheath, and the segments are not of equal length. We may thus summarise
the characteristic features of the genus:—
Opposite branches are given off in one plane from the nodes of a
main axis; the leaves are in the form of narrow sheaths divided into
numerous and unequal linear or narrow lanceolate segments, each with
a median vein. The segments in each whorl appear to be spread out in
one plane very oblique to the axis of a branch, instead of spreading
radially in all directions; the lateral segments are usually longer
than the upper and lower members of a whorl. The vegetative branches
possess the same type of structure as _Calamites_.
A comparison of _Annularia_ and _Phyllotheca_ has already been
made in Chapter IX. (p. 282). Potonié[659] has recently given a
detailed account of Annularian leaves; he compares them with those
of _Equisetum_, and describes the occurrence on the lamina of each
leaf-segment of a broad central band or midrib, with a groove, probably
containing stomata, on either side. He shows that in well-preserved
specimens of _Annularia_, it is possible to recognise certain minute
surface-features, such as the presence of hairs and stomata, which
enable one to detect a close resemblance between the leaves of Calamite
stems and those of Annularian shoots.
It is not always easy to distinguish between _Annularia_ and
_Calamocladus_; the collar-like basal sheath in the leaves of the
former is a characteristic feature, but that cannot always be
recognised. On the other hand, the leaves of _Calamocladus_ may
sometimes be flattened out on the surface of the rock and simulate the
deeply cut sheaths of _Annularia_. It is difficult to decide how far
the manner of occurrence of Annularian leaves in one plane, which is
commonly insisted on as a generic character, is an original feature, or
how far it is the result of compression in fossilisation. Probably the
leaves of a living _Annularia_ were spread out at right angles to the
axis, as in the ‘verticils’ of such a plant as _Galium_.
Dawson[660] has described some fossils from the Devonian rocks of
Canada as species of _Asterophyllites_; the figures bear a closer
resemblance to the genus _Annularia_. The same author figures some
irregularly whorled impressions as _Protannularia_, which appear to be
identical with a fossil described by Nicholson[661] from the Skiddaw
slates (Ordovician) of Cumberland as _Buthotrephis radiata_, but the
specimens are too imperfect to admit of accurate determination.
_Annularia stellata_ (Schloth.). Fig. 88.
1820. _Casuarinites stellatus_, Schlotheim[662].
1826. _Bornia stellata_, Sternberg[663].
1828. _Annularia longifolia_, Brongniart[664].
1834. _Asterophyllites equisetiformis_, Lindley and Hutton[665].
1868. _Asterophyllites longifolius_, Binney[666].
1887. _Annularia Geinitzi_, Stur[667].
1887. _Annularia westphalica_, Stur.
This species was figured by Scheuchzer[668] in his _Herbarium
Diluvianum_, and compared by him with a species of _Galium_ (Bedstraw).
Brongniart first made use of the generic name _Annularia_ for this
common Coal-Measure species, which may be defined as follows:—
Stem reaching a diameter of about 6–8 cm., with internodes 6–12 cm.
in length, the surface either smooth or faintly ribbed. Primary
branches given off in opposite pairs from the nodes, the lateral
branches giving off smaller branches disposed in the same manner. The
smaller branches bear verticils of leaves at each node; both leaves
and ultimate branches being in one plane. The leaves are narrow,
lanceolate-spathulate in form, broadest about the middle, 1–5 cm. in
length and 1–3 mm. broad, hairy on the upper surface[669]; each leaf is
traversed by a single vein.
[Illustration: FIG. 88. Branch of _Annularia stellata_ (Schloth.). ⅘
nat. size. From a specimen in the Collection of Mr R. Kidston.
Upper Coal-Measures, Radstock.]
Each whorl contains 16–32 segments, which are connected basally into
a collar or narrow sheath; the lateral segments are usually longer
than the upper and lower. The branches are about 6–20 mm. broad, with
finely ribbed internodes 3–7 cm. long, bearing verticils of leaves; the
ultimate branches arise in pairs in the axils of the lateral segments
of the verticils.
The strobili are of the _Calamostachys_[670] type and are borne on the
main branches or possibly on the stem; they have a long and narrow form
and are attached in verticils at the nodes. Each strobilus consists of
a central axis bearing alternate whorls of linear lanceolate sterile
bracts and sporangiophores, about half as numerous as the sterile
bracts; each sporangiophore bears four ovoid sporangia.
The anatomical structure of a specimen referred to _Annularia stellata_
has been described by Renault[671]. The cortex consists of parenchyma
traversed by lacunae and limited peripherally by a denser hypodermal
tissue. In the stele Renault describes 14 xylem strands, each with a
large carinal canal. The pith was apparently large and hollow. The
same author describes an _Annularia_ strobilus in which the lower
sporangiophores bear macrosporangia, and the upper microsporangia.
[Illustration: FIG. 89. _Annularia sphenophylloides_ (Zenk.).
_A._ Strobilus (_Stachannularia calathifera_, Weiss). ⅔ nat. size.
_B._ Vegetative shoot. ⅘ nat. size.
From specimens in the Collection of Mr R. Kidston. Upper
Coal-Measures, Radstock.]
The references in the footnote should be consulted for figures of this
species of _Annularia_; it is from the examination of such specimens
as are referred to in the note that the above diagnosis has been
compiled[672].
_Annularia sphenophylloides_ (Zenk.). Fig. 89.
1833. _Galium sphenophylloides_, Zenker[673].
1865. _Annularia brevifolia_, Heer[674], Strobilus.
1876. _Calamostachys_ (_Stachannularia_) _calathifera_, Weiss[675].
Principal branches 8–12 mm. wide, with internodes 8–10 cm. in length,
giving off two opposite branches at the nodes; from the secondary
branches arise smaller branches in opposite pairs. The leaf-verticils
and branches are all in one plane. Each verticil consists of 12–18
spathulate segments, 3–10 mm. long, cuneiform at the base and broader
above, with an acuminate tip; the lateral segments are slightly longer
than the upper and lower members of a whorl.
The small and crowded leaf-whorls give to this species a characteristic
appearance, which readily distinguishes it from the larger-leaved
forms such as _Annularia stellata_. A fossil figured by Lhwyd[676]
in 1699 as _Rubeola mineralis_ is no doubt an example of _Annularia
sphenophylloides_.
Annularian branches are occasionally found with cones given off
from the axils of some of the leaf-whorls. An interesting specimen,
which is now in the Leipzig Museum, was described by Sterzel in
1882[677], showing cones attached to a vegetative shoot of _Annularia
sphenophylloides_. The long and narrow strobili—2·5 cm. long and
about 6 mm. broad—appear very large in proportion to the size of the
vegetative branches. A fertile shoot consists of a central axis bearing
whorls of bracts alternating with sporangiophores, to each of which
are attached four sporangia. The specimen in fig. 89, _A_, does not
show the details clearly; each transverse constriction represents the
attachment of a whorl of linear bracts; the whole cone appears to
consist of a series of short broad segments. The divisions in the lower
half of each segment mark the position of the sterile bracts, while
those of the upper half represent the outlines of the upper sporangia
of each whorl of sporangiophores, the lower sporangia being hidden by
the ring of linear bracts[678]. On some portions of the specimen of
fig. 89, _A_, it is possible to recognise the outlines of cells on
the coaly surface-film; these probably belong to the sporangium wall.
This type of cone is included under the genus _Calamostachys_, a name
applied to Calamitean strobili with certain morphological characters,
as described on p. 351.
_c. Roots._
In 1871 Williamson[679] described some sections of what he considered
to be a distinct variety of a Calamite stem. The chief peculiarity
which he noticed lay in the absence of carinal canals, and in the solid
pith. Some years later the same observer[680] came to the conclusion
that the specimens were probably those of a plant generically distinct
from _Calamites_; he accordingly proposed a new name _Astromyelon_.
Subsequently Cash and Hick[681] gave an account of some examples
of apparently another form of plant, to which they gave the name
_Myriophylloides Williamsonis_; and Williamson[682] suggested the term
_Helophyton_ as a more suitable generic designation. It was, however,
demonstrated by Spencer[683] that the plant described by Cash and Hick
was identical with Williamson’s _Astromyelon_. Williamson[684] then
gave an account of several specimens of this type illustrating various
stages in the growth and development of the _Astromyelon_ ‘stems,’
which he compared with the rhizome of the recent genus _Marsilia_.
In 1885 Renault[685] published an account of _Astromyelon_ in which he
brought forward good evidence in favour of regarding it as a Calamitean
root. The same author has recently given some excellent figures and
a detailed description of certain specific types of these Calamite
roots, and Williamson and Scott’s memoir on the roots of _Calamites_
has rendered our knowledge of _Astromyelon_ almost complete. Some of
the finest specimens, in which the organic connection between typical
Calamite stems and _Astromyelon_ roots is clearly demonstrated, are in
the Natural History Museum, Paris. There are several sections also from
English material which show the connection between root and stem very
clearly.