embedded in sandstone and shale. (After Grand’Eury.) Much reduced.]
Casts of the hollow pith of Calamite rhizomes or aerial branches are
occasionally found in which slender appendages are given off either
singly or in tufts from the nodal regions. Many examples of such
casts have been figured by Lindley and Hutton[686], Binney[687],
Grand’Eury[688], Weiss[689], Stur, and other writers[690]. The large
stem-cast of fig. 90 illustrates the manner of occurrence of long
branched roots on the nodes of a Calamite growing in sandy or clay
soil. The lower and more darkly shaded portion of the specimen is
covered by a layer of coal representing the carbonised wood and cortex,
which has been moulded on to the sandstone pith-cast. In fig. 77 (p.
316) a fairly thick root is seen, in organic connection with one of the
nodes, _N_ 3, and on _N_ 2 there is a scar of another root.
There are certain external characters by which one may often recognise
a Calamitean root. There is no division into nodes and internodes as
in stems, and as the pith of the root was usually solid the parallel
ribs and grooves of stem-casts are not present. In smaller flattened
roots there may sometimes be seen a central or excentric black line
representing the stele, and the surface of the root presents a curious
wrinkled or shagreen texture, probably due to the shrinkage of the
loose lacunar cortex. The occasional excentric position of the stele is
no doubt due to the displacement of the vascular cylinder as a result
of the rapid decay of the cortical tissues. In the Bergakademie of
Berlin there are some unusually good examples of Calamite casts bearing
well-preserved root-impressions; these include the original specimens
figured by Weiss[691].
No doubt some of the roots figured by various writers under the names
_Pinnularia_[692] and _Hydatica_[693] belong to _Calamites_, but it is
often impossible to identify detached specimens with any certainty.
The section figured diagrammatically in fig. 91 _A_ shows the
characteristic single series of large lacunae, _l_, in the middle
cortical region. In the centre there is a wide solid pith surrounded by
a ring of vascular tissue, _x_. The appearance of the middle cortex is
very like that of the stem of a water-plant such as _Myriophyllum_, the
Water Milfoil; it shows that the Calamite roots grew either in water or
swampy ground. In fig. 91 _B_, the root characters are clearly seen;
the centre of the stele is occupied by large parenchymatous cells which
are rather longer than broad in longitudinal view; at the periphery
there are four protoxylem groups _px_, alternating with four groups
of phloem, _ph_, the latter being situated a little further from the
centre of the stele. The structure is therefore that of a typical
tetrarch root. In the example represented in the figure secondary
thickening has begun, and the cambial cells internal to each phloem
group have given rise to a few radially disposed tracheids, _x²_.
Beyond the phloem there are two layers of parenchyma representing,
as regards position, a pericycle and an endodermis. In the ordinary
pericycle and endodermis of the roots of most plants the cells of
the two layers are on alternate radii, but in the Calamite root, as
in _Equisetum_ roots, the cells of these layers are placed on the
same radii, as seen in the neighbourhood of _x²_ in the figure. This
correspondence of the radial walls of the endodermal and pericyclic
cells points to the development of both layers from one mother-layer,
and suggests the ‘double endodermis’ or phloeoterma of _Equisetum_
(p. 254). The cells in the outer of these two layers have slight
thickenings on the radial walls recalling the usual character of
endodermal cells. The phloeoterma is succeeded by a few layers of
parenchyma, constituting the inner cortex, and beyond this we have
the large lacunae separated from one another by slender trabeculae
of cells. The outer cortex is limited by a well-defined layer of
thick-walled cells, which may be spoken of as the _epidermoidal[694]
layer_. Roots possessing this superficial layer of thicker cells have
no doubt lost the original surface-layer which produced the absorptive
root-hairs.
[Illustration: FIG. 91. _A._ Diagrammatic sketch of a transverse
section of a young root of _Calamites_. _x_, xylem; _l_, lacuna.
After Hick.
_B._ Central cylinder (stele) of root, _px_, protoxylem; _ph_,
phloem; _x²_, secondary xylem; _l_, phloeoterma. × 75. After
Williamson and Scott.]
The xylem elements have the form of spiral, reticulate and scalariform
tracheids.
In roots or rootlets smaller than that shown in fig. 91 _B_, the
primary xylem may extend to the centre of the stele, and form a
continuous axial strand; in such examples the structure may be
diarch, triarch or tetrarch. The origin of the cambium agrees
with that in recent roots, the cells immediately external to the
protoxylem tracheids become meristematic, as also those internal to
the phloem. Another root-character is seen in the endogenous origin
of lateral members. Good examples of branching roots are figured by
Williamson[695] and by Williamson and Scott[696].
Older roots[697] are usually found in a decorticated condition. A
transverse section of root in which secondary thickening has been
active for some time presents on a superficial view a close resemblance
to a stem of _Calamites_, but a careful comparison at once reveals
important points of difference. The specimen diagrammatically sketched
in fig. 92 illustrates very clearly the origin of a root from the
node of a Calamite stem. The section has passed through a stem in a
tangential direction, showing the characteristic arrangement of the
vascular bundles _x_, and principal medullary rays _m_. The small
leaf-traces, _t_, _t_, afford another feature characteristic of a
Calamite stem. The portion of stem to the right of the figure has been
slightly displaced, and between this piece and the root _R_, one of
the ubiquitous Stigmarian appendages, _s_, has inserted itself. At _R_
a fairly thick and decorticated root is seen in oblique transverse
section; at the upper end the root tracheids are seen in direct
continuity with the xylem of the stem. In the centre of the root
is the large solid pith surrounded by twelve bluntly pointed xylem
groups, composed in the main of radially disposed scalariform elements
with narrow secondary medullary rays like those in a stem. Between
each xylem group there is a broad medullary ray, which tapers rapidly
towards the outside, and is soon obliterated by the formation of
interfascicular secondary xylem. At _R′_ a portion of another root is
seen in transverse section, and _R″_ the inner part of a single xylem
group is shown more clearly. The solid pith and the absence of carinal
canals are the two most obvious distinguishing features of the roots.
[Illustration: FIG. 92. Tangential section through a node of
_Calamites_, showing a root in organic connection with the stem.
_R_, _R′_. Root (_Astromyelon_) in transverse and oblique section,
_x_, xylem; _m_, primary medullary ray; _t_, leaf-trace; _s_,
Stigmarian appendage.
_R″_, the inner portion of one of the xylem wedges of _R′_ more
highly magnified. Sketched from a section in the Cambridge
Botanical Laboratory Collection.]
As Renault points out, roots of _Calamites_ have been figured by some
writers[698] as examples of stems, but it is usually comparatively
easy to distinguish between roots and stems. On examining the xylem
groups more closely, one notices that the apex of each is occupied by
a triangular group of centripetally-developed primary tracheids, the
narrow spiral protoxylem elements occupying the outwardly directed
apex. The protoxylem apex is usually followed externally by a ray of
one or two radially disposed series of parenchymatous cells. This ray
is not distinguished in fig. 92 _R″_ from the rows of xylem tracheids.
Each xylem group is thus formed partly of centripetal xylem and in part
of secondary centrifugal xylem; the latter is associated with secondary
medullary rays, as in stems, and contains a broader ray (_fascicular
ray_ of Williamson and Scott[699]) immediately opposite each
protoxylem strand. In the roots of recent plants (_e.g._ _Cucurbita_,
_Phaseolus_, &c.) a broad medullary ray is often found opposite the
protoxylem, and such an arrangement is a perfectly normal structure in
roots[700].
Renault has recently described several species of Calamite roots which
he designates by specific names, some of them belonging to stems with
the _Arthropitys_ structure, and others to _Calamodendron_. Some of
the roots figured by the French author have an axial strand of xylem
with 7–15 projecting angles of protoxylem[701]. These he considers
true roots, but the larger specimens with a wide pith he prefers to
regard as stolons. In the latter he mentions the union of the primary
centripetal with the secondary centrifugal wood as a distinguishing
feature. It has been shown, however, that each group of secondary xylem
includes a median ray of parenchyma, and that the whole structure is
essentially that of a root, and not that of a modified stem or stolon.
The organs described by Renault as true roots are probably rootlets,
and as Williamson and Scott have demonstrated, there is every gradation
between the smaller specimens with a solid xylem axis and those with a
large central pith.
It is interesting to note that Renault’s figures of _Calamodendron_
roots show the closest resemblance to those of the subgenus
_Arthropitys_.
_d. Cones._
The occurrence of fossil plants in the form of isolated fragments is a
constant source of difficulty, and is well illustrated by the numerous
examples of strobili which cannot be connected with their parent
stems. We are, however, usually able to recognise Calamitean cones
if the impressions or petrified specimens are fairly well preserved,
but it is seldom possible to correlate particular types of cones with
the corresponding species of foliage-shoots or stems. Palaeobotanical
literature contains numerous illustrations and descriptions of
long and narrow strobili designated by different generic terms such
as _Volkmannia_, _Brukmannia_, _Calamostachys_, _Macrostachya_ and
others; many of these have since been recognised as the cones of
_Calamites_, while some species of _Volkmannia_ have been identified
with _Sphenophyllum_ stems. Before further considering the general
question of Calamite cones, a few examples may be described in detail
as types of fructification which are known to have been borne by
_Calamites_. The examples selected are species of the two provisional
genera _Calamostachys_ and _Palaeostachya_.
The usual form of a Calamite cone is illustrated in fig. 93, which
represents a fertile shoot bearing a few narrow linear leaves of the
_Calamocladus_ type; in the axils of some of these are borne the long
strobili.
[Illustration: FIG. 93. _Calamostachys_ sp. A fertile Calamitean shoot.
From a specimen in the Geological Survey Museum, Jermyn Street,
London. From the Upper Coal-Measures of Monmouthshire (No. 5539).]
_Calamostachys Binneyana_ (Carr.). Figs. 94 and 95.
In 1867 Carruthers[702] gave an account of the structural features
of the species of cones named by him _Volkmannia Ludwigi_ and
_V. Binneyi_, the generic term having been originally used by
Sternberg[703] for some impressions of Carboniferous strobili.
Brongniart[704] in 1849 referred to the various forms of _Volkmannia_
as cones of Asterophyllitean branches, and the latter he regarded
as the foliage-shoots of a Calamite stem. In 1868 Binney[705]
published a description, with several illustrations, of the cones
named by Carruthers _Volkmannia Binneyi_, and referred to them
as the fructification of that type of Calamite stem spoken of
in a previous section of this chapter (p. 311) as _Calamites_
(_Arthropitys_) _communis_ (Binney). This cone is now usually spoken
of as _Calamostachys Binneyana_; the specific name _Binneyana_
being suggested by Schimper[706] in 1869 as more euphonious than
that proposed by Carruthers. In recent years our knowledge of both
_C. Binneyana_ and _C. Ludwigi_ has been considerably extended. We
shall confine our attention in the following account to the former
species[707]. Some excellent figures of the latter species may be found
in Weiss’ Memoir[708] on Calamarieae.
One of the largest examples of _Calamostachys Binneyana_ so far
recorded has a length of 3–4 cm. and a maximum diameter of about 7·5
mm. The axis of the cone bears whorls of sterile leaves or bracts at
equal distances; the linear bracts of each whorl are coherent basally
as a disc or plate of tissue attached at right angles to the central
axis of the cone. The periphery of each of these discs divides up into
twelve linear segments, which curve upwards in a direction more or less
parallel to the strobilus axis, and at right angles to the coherent
portion of each whorl. The manner of occurrence of the whorls is
shown in fig. 94, which has been sketched from a large section in the
Williamson collection. The segments of the successive sterile verticils
alternate with one another, so that in the surface-view of a cone the
long and narrow free bracts appear spirally disposed. Midway between
these alternating sterile verticils there is a series of fertile
appendages, also given off in regular whorls. Each fertile whorl
consists of about half as many members as the segments of a sterile
whorl, and the members of the several fertile whorls are superposed and
not alternate. Each member has the form of a stalk or sporangiophore
given off at right angles from the cone axis; this is expanded distally
into a peltate disc bearing four sporangia attached to its inner face.
In fig. 94 we can only see the basal portions of the sporangiophores,
which are shown in the upper part of the sketch as pointed projections,
_Sp_, from the cone axis. Each sporangiophore is traversed by a
vascular strand which sends off a branch to the base of a sporangium
(fig. 95, A, _t_).
[Illustration: FIG. 94. _Calamostachys Binneyana_ (Carr.) in
longitudinal (radial and tangential) section.
_Sp_, sporangiophores; _S_, sporangia.
(From specimen no. 1022 in the Williamson Collection, British
Museum.)]
The axis of the cone is occupied by a single stele, usually triangular
in section; the stele consists of a solid pith of elongated cells
surrounded by six vascular bundles, two at each corner. A somewhat
irregular gap marks the position of the protoxylem of each strand,
and portions of spiral or annular tracheids may occasionally be
seen in the cavity. These cavities, which may be spoken of as the
carinal canals, disappear at the nodes, where there is a mass of
short reticulately pitted tracheids, as in a Calamite stem. Vascular
bundles pass upwards in an oblique direction from the central stele
to supply the bracts, each of which is traversed by a single strand
of tracheids. The coherent portion, or disc, of each sterile whorl
consists of sclerenchymatous elements towards the upper surface, and
of parenchyma below. The pedicel of the sporangiophores consists of
fairly thick-walled cells traversed by a single vascular strand,
and the peltate distal portions are made up of parenchymatous cells
arranged in a palisade-like form at right angles to the free surface
of the sporangiophores. The vascular strand of the pedicel forks into
two halves just below the peltate head, and these branches again
bifurcate to send a branch to each sporangium. The four sporangia of
each sporangiophore are attached by a narrow band of tissue to the
shield-shaped distal expansion (fig. 95, A).
In a tangential section of a cone, such as the lower portion of fig. 94
and in fig. 95, B, the sporangiophores present the appearance of narrow
stalks (fig. 95, B, _a_) in the middle of a cluster of sporangia,
and the latter appear more or less square in outline. The wall of a
sporangium is made of a single layer of cells (fig. 95, B) which
present a characteristic appearance in surface-view (fig. 95, C), the
thin walls being crossed at right angles by small vertical plates. In
the tangential section of the coherent sterile whorls (fig. 95, B,
_b_ and _b_) the vascular strands are occasionally seen in transverse
section (fig. 95, B, _t_), as they pass outwards to the several free
bracts.
[Illustration: FIG. 95. _Calamostachys Binneyana_ (Carr.).
_A._ A sporangiophore and one sporangium. _t_, vascular bundle. × 45.
_B._ Tangential section showing portions of two sterile discs, _b_,
_b_; a sporangiophore, _a_, with its four sporangia, in two of
which are seen the spores; _t_, vascular bundle. × 35.
_C._ Surface-view of cells of a sporangium wall. × 130.
_D._ Spores and remains of mother-cells. × 130.
(After Williamson and Scott.)]
The spores in _Calamostachys Binneyana_ are all of the same size,
and no macrospores have ever been seen. In well preserved specimens
tetrads of spores may be seen, still enclosed by the wall of the
spore-mother-cell (fig. 95, A and D); and the torn remnants of
the mother-cell sometimes simulate in appearance the elaters of
an _Equisetum_ spore. In surface-view a spore often shows clearly
the three-rayed marking, which is a characteristic feature of
daughter-cells formed in a tetrad from a mother-cell. The spores of
a tetrad are in some cases of unequal size, some having developed
more vigorously than others. This unequal growth and nourishment of
spores is clearly shown in fig. 96, which represents a sporangium
of a heterosporous Calamitean strobilus, _C. Casheana_. Williamson
and Scott[709] have described striking examples of spores in
different stages of abortion, and these authors draw attention to
the importance of the phenomenon from the point of view of the
origin of a heterosporous form of cone. The abortion of some of the
members of a spore-tetrad and the consequent increased nutrition of
the more favoured daughter-cells, might well be the starting-point
of a process, which would ultimately lead to the production of well
defined macrospores and microspores. The young microsporangia and
macrosporangia of recent Vascular Cryptogams such as _Selaginella_,
_Salvinia_ and other heterosporous genera are identical in
appearance[710]; it is not until the spore-producing tissue begins to
differentiate into groups of spores, that the sporangia assume the
form of macrosporangia and microsporangia. During the evolution of
the various known types of pteridophytic plants heterospory gradually
succeeded isospory, and this no doubt occurred several times and in
different phyla of the plant kingdom. In the mature sporangia of some
of the Calamitean strobili we have in the inequality of the spores in
one sporangium an indication of the steps by which heterospory arose;
and in the immature sporangia of some recent genera we are carried back
to a stage still nearer the starting-point of the substitution of the
heterosporous for the isosporous condition.
_Calamostachys Casheana_ Will. Fig. 96.
To Williamson[711] again is largely due the information we possess as
to the structure of this type of Calamitean strobilus. Its special
interest lies in the occurrence of macrospores and microspores in the
same cone.
The strobilus axis agrees in structure with that of _C. Binneyana_,
but in _C. Casheana_ a band of secondary xylem forms the peripheral
portion of the triangular stele. Were any further proof needed of the
now well-established fact that secondary growth in thickness is by no
means unknown as an attribute of Vascular Cryptogams, the co-existence
in the same cone of a cambium layer producing secondary wood and
bark, and cryptogamic macrospores and microspores, affords conclusive
evidence[712]. The dogma accepted by many writers for a considerable
number of years that the power of secondary thickening is evidence
against a cryptogamic affinity, has been responsible for no little
confusion in palaeobotanical nomenclature.
On the axis of _Calamostachys Casheana_ there are borne alternate
whorls of fertile and sterile appendages similar to those in the
homosporous _C. Binneyana_, but they are inclined more obliquely to
the axis of the cone. Macrospores and microspores have been found in
sporangia borne on the same sporangiophore.
[Illustration: FIG. 96. _Calamostachys Casheana_ Will.
A sporangium with macrospores and abortive spores. × 65.
(After Williamson and Scott.)]
The spore-tetrads in the macrosporangia occasionally include aborted
sister-cells like those noticed in _C. Binneyana_; this phenomenon
is well illustrated by the unequally nourished spores in the
sporangium of fig. 96, but no such starved spores have been found
in the microsporangia. In this cone, then, heterospory has become
firmly established, but the occurrence of undersized spores in a
macrospore-tetrad leads us back to the probable lines of development of
heterospory, which are seen in _C. Binneyana_ at their starting-point.
In the two species of strobili which have been described,
_Calamostachys Binneyana_ and _C. Casheana_, the sporangiophores or
sporophylls are given off at right angles to the axis, and midway
between the sterile whorls. These are two of the most important
distinguishing features of the Calamitean cones included under the
generic term _Calamostachys_. In another form of cone, which also
belongs to Calamitean stems, the sporangiophores arise in the axil
of the sterile leaves, and are inclined obliquely to the axis of the
cone. To this type the generic name _Palaeostachya_ has been applied
by the late Prof. Weiss[713] of Berlin. The portion of a cone shown in
fig. 97 shows the arrangement of the sterile and fertile appendages
characteristic of _Palaeostachya_.
[Illustration: FIG. 97. _Palaeostachya pedunculata_ Will. Part of a
cone, × 3. (After Weiss.)]
It is practically impossible to distinguish between cones of the
_Calamostachys_ and _Palaeostachya_ type in the case of imperfectly
preserved impressions; indeed we cannot assume that all long and narrow
cones with spirally disposed verticillate bracts are Calamitean. We
must have the additional evidence of internal structure or of the
direct association of the cones with Calamitean foliage.
_Palaeostachya vera_ sp. nov. Fig. 98.
In 1869 Williamson[714] described a fragment of a strobilus which
showed certain anatomical features indicative of a close relationship
or even identity with _Calamites_. Some years later[715] a much more
perfect example was obtained from the Coal-Measures of Lancashire,
and the additional evidence which it afforded definitely confirmed
the earlier views of Williamson. The cone was more fully described by
Williamson in 1888, as “the true fruit of _Calamites_.” It is clearly a
form of Weiss’ genus _Palaeostachya_; Williamson and Scott[716] refer
to it in their Memoir as _Calamites pedunculatus_. It is preferable,
however, to retain the generic designation _Palaeostachya_ for cones
of this type. As the name _P. pedunculata_ has previously been adopted
by Weiss[717] for a cone figured by Williamson[718] in 1874, and
afterwards referred to by that author in writing as _P. pedunculata_,
it is proposed to substitute the specific name _vera_; this specific
name being chosen with a view to put on record the fact that it was
this type of cone that Williamson first proved to be the _true_
fructification of the Calamite.
The axis of _P. vera_ is practically identical in structure with a
Calamitean twig. There is a hollow pith in the centre of the stele
surrounded by a ring of 16–20 collateral bundles, each of which is
accompanied by a carinal canal as in a vegetative shoot. As the
pedicel of the strobilus passes into the cone proper it undergoes some
modification in structure, but retains the characteristic features of
a Calamite. The diagrammatic longitudinal section of fig. 98, which is
copied from a drawing by Williamson[719], shows the broadening of the
vascular strands at the nodes, and here and there a carinal canal is
seen internal to the wood.
The axis of the cone bears whorls of bracts at right angles to the
central column. Each whorl consists of about 30–40 segments coherent
basally into a disc of prosenchymatous and parenchymatous tissue.
The free linear bracts curve sharply upwards from the periphery of
the disc, approximately parallel to the axis of the cone. From each
of these sterile whorls there are given off 16–20 long and slender
obliquely-inclined sporangiophores, _sp_, which arise from the upper
surface of the disc close to the axis. Each sporangiophore no doubt
bore four sporangia, _S_, containing spores of one size,—about ·075
mm. in diameter. The specimens of _Palaeostachya vera_ so far obtained
do not show the actual manner of attachment of the sporangia, but more
complete examples of other species of _Palaeostachya_[720] enable us to
assume with certainty that the sporangiophores terminated in a distal
peltate expansion bearing four sporangia on its inner face.
[Illustration: FIG. 98. Diagrammatic longitudinal section of
_Palaeostachya vera_, sp. nov. _S_, _S_, _S_, sporangia; _x_,
xylem; _sp_, sporangiophore. (After Williamson.)]
A transverse section of the axis of the cone in the region of the
sterile and fertile appendages shows the vascular bundles arranged
in pairs. In a section through the peduncle of the cone, below the
lowest whorl of bracts, the bundles of the stele are situated at equal
distances apart. The cortical tissue of the peduncle is traversed by
a ring of large canals[721] similar to the vallecular canals of an
Equisetum stem.
Isospory is not a constant characteristic of _Palaeostachya_; some
forms have been found with macrospores and microspores[722].
_Other Calamitean cones, and examples illustrating the
connection between Cones and Vegetative Shoots._
It would be out of place in an introduction to Palaeobotany to attempt
an exhaustive account of the various cones which were probably borne
by Calamitean plants, but there are a few general points to which the
attention of the student should be directed. The examples dealt with
in the foregoing description illustrate the fact, that plants included
under the comprehensive genus _Calamites_ bore cones possessing
distinct morphological features. There are, however, other types of
strobili which have been found in organic connection with _Calamites_;
and some of these must be taken into account in dealing with
Calamarian plants. The genera _Volkmannia_, _Brukmannia_, _Huttonia_,
_Macrostachya_, in addition to _Calamostachys_ and _Palaeostachya_ and
others, have been applied by different writers to Calamitean cones. As
Solms-Laubach[723] has suggested, it is wiser to discard _Volkmannia_
and _Brukmannia_, as they have been made to do duty for cones of widely
different forms. It is better to adhere to the provisional generic
names used by Weiss, as they enable us to conveniently systematise the
various Calamarian strobili.
The following classification may be given of the better known cones,
some of which we are able to describe in considerable detail, while
others are still very imperfectly known. We have good evidence that
all these strobili were borne by vegetative shoots of the type of
_Calamites_, _Calamocladus_ or _Annularia_.
1. _Calamostachys_[724] (including _Paracalamostachys_ and
_Stachannularia_).
Cones long and narrow, consisting of a central axis bearing alternate
whorls of sterile and fertile appendages, the latter having the form of
sporangiophores attached at right angles to the axis midway between the
sterile verticils, and bearing four sporangia on the inner face of a
peltate distal expansion.
_Calamostachys Binneyana_ Schimp., _C. Ludwigi_ Carr., _C. Casheana_
Will., may be referred to as examples of this type of cone; also
some of the strobili described by different authors as species of
_Volkmannia_[725], _Brukmannia_[726], &c.
Although one cannot make out the detailed structure of a Calamite cone
in the absence of internal structure, it is often possible to recognise
the essential features in specimens preserved in ironstone nodules,
such as those from Coalbrook Dale in Shropshire, or by carefully
examining the carbonised impressions on shale under a simple microscope.
Weiss applies the term _Paracalamostachys_[727] to cones of the
_Calamostachys_ form, but in which the manner of attachment cannot be
made out. Such a cone as that of fig. 93 should probably be referred to
this sub-type of _Calamostachys_ in the absence of definite evidence as
to the position of the sporangia.
Another term _Stachannularia_, originally used by Weiss as a
genus[728], was afterwards[729] applied to cones of the same general
type as _Calamostachys_, in which the sporangiophores have the form of
thorn-like structures bearing on their upper side a lamellar expansion.
There is however some doubt as to the correct interpretation of the
features associated with cones included in _Stachannularia_; for an
account of such forms reference must be made to the writings of Weiss,
Renault[730], Solms-Laubach[731] and others[732].
_Calamostachys_ cones have been found in organic union with branches
bearing leaves of the _Annularia_ type, also with _Calamocladus_
foliage, and the branches bearing such cones have been found in
actual connection with Calamitean stems. The association of cones and
vegetative stems and branches is shown in tabular form on p. 363.
2. _Palaeostachya[733]._
In this genus the general habit agrees with that of _Calamostachys_,
and in imperfectly preserved specimens it may be impossible to
discriminate between _Calamostachys_ and _Palaeostachya_. The latter
form is characterised by the attachment of the sporangiophores in the
axil of the sterile bracts, or immediately above them, as shown in
figs. 97 and 98.
EXAMPLES. _Palaeostachya vera_ sp. nov., _P. pedunculata_ Will. afford
examples of this form of strobilus. The genus _Palaeostachya_ includes
several species previously described under the genus _Volkmannia_[734].
Strobili of this generic type are known in organic association with
Annularian branches, as well as with _Calamocladus_ and _Calamites_.
3. _Macrostachya._
This generic name was originally applied by Schimper[735] to certain
forms of Calamitean stems, of the type afterwards referred to the
sub-genus _Calamitina_ by Weiss, bearing long and thick cones. The name
is, however, more appropriately restricted to strobili, which differ
from the two preceding genera in their greater length (14–16 cm.) and
in the more crowded and imbricating whorls of bracts. The internodes of
the cones are very short, and each whorl of bracts consists of about
20 coherent members separated at the periphery of the disc into short
pointed teeth. The internal structure of _Macrostachya_ has not been
satisfactorily determined. An account by Renault[736] of a petrified
specimen does not present a very clear idea as to the structural
features of this form of Calamitean strobilus.
THE ASSOCIATION OF CALAMITEAN VEGETATIVE SHOOTS AND CONES.
Strobilus | Foliage-shoot | Stem
------------------------+----------------------+-------------------------
_Calamostachys_ | _Annularia ramosa_ | _Calamites ramosus_
(_Stachannularia_) | Weiss | Artis
_ramosa_ Weiss[737] | |
| |
_C._ (_Stachannularia_) | _A. sphenophylloides_| Stem bearing verticils of
_calathifera_ | Zenk. | long and narrow
Weiss[738] | | leaves[739]. Probably a
| | young _Calamites_
| |
_C._ (_Stachannularia_) | _A. stellata_ |
_tuberculata_ (Stern.)| (Schloth.)[740] | _Calamites_ sp.[741]
|(_A. longifolia_ |
| Brongn.) |
| |
_C. Solmsi_[742] Weiss | _Calamocladus_ sp. | _Calamites_ (_Calamitina_)
| | sp.
| |
_C. longifolia_ | _Calamocladus_ sp. |
(Stern.)[743] | |
| |
_Palaeostachya_ | _Calamocladus_ |
_pedunculata_ | |
Will.[744] | |
| |
_P. arborescens_ | | _Calamites_
(Stern.)[745] | | (_Stylocalamites_)
| | _arborescens_ (Stern.)
| |
_Macrostachya_[746] | _Calamocladus_ | _Calamites_ (_Calamitina_)
| _equisetiformis_ | sp.
| (Schloth.) |
[Sidenote: HUTTONIA.]
The generic name _Huttonia_, suggested by Sternberg[747] in 1837,
is applied to cones which closely resemble _Macrostachya_ in habit,
but differ—so far as our scanty knowledge enables us to judge—in the
arrangement of the members. The student must refer to Weiss[748],
Solms-Laubach[749] and other writers[750] for a further account of
these types, and of another rare and little-known form of cone, called
by Weiss Cingularia[751].
Macrostachyan cones have been found attached to stems of _Calamites_
which are included in the sub-genus _Calamitina_ (p. 367). The larger
size of _Macrostachya_ as a distinguishing feature is not always a
safe test; some cones which belong to _Palaeostachya_ [_e.g._ _P.
arborescens_ Sternb.] and _Calamostachys_ (_e.g._ _C. Solmsi_) are much
thicker and larger than the majority of species of these two genera.
It would appear from the examples selected to illustrate the
connection between strobili and vegetative shoots, that the
_Annularia_ type of branch usually bears cones which conform to the
genus _Calamostachys_ (_Stachannularia_); while the Asterophyllitean
branches—_Calamocladus_—are associated with _Palaeostachya_ and
_Macrostachya_. But this rule is not constant, and we are not in
a position to speak of cones of a particular type as necessarily
characteristic of definite types of Calamitean shoots.
• • • • •
Although it is admitted by the great majority of Palaeobotanists that
the Calamites were all true Vascular Cryptogams, the older view that
some members of the Calamarieae are gymnospermous has not been given up
by Renault[752]. This observer has recently described some seeds which
he believes were borne by Calamitean stems; he admits, however, that
no undoubted female cones of _Calamodendron_ have so far been found.
In view of the unsatisfactory evidence on which Renault’s opinion is
based, we need not further discuss the questions which he raises.
[The following specimens in the Williamson Cabinet in the British
Museum, may be found useful in illustration of the structure of
_Calamites_.
_Stems._ (i. _Arthropitys._) _Young twigs and small branches_ 1, 2,
6, 10, 14, 19, 116*, 1002, 1007, 1020.
_Older stems_ (_transverse sections_) 15–17, 62, 77–87, 115 _a_,
117*, 118*, 120, 122*–124*, 1933 A, 1934, 1941.
(_Tangential sections_) 20, 24, 26, 37, 38, 49, 90, 91, 130, 138,
1937, 1943.
(_Radial sections_) 20, 20 A, 21, 22, 48, 65–68, 83–91, 137*,
138*, 1937.
(ii. _Arthrodendron)_ 36, 37, 38, 52, 54.
_Roots._ 1335, 1347, 1350, 1356.
_Strobili._ i. _Calamostachys Binneyana._ 991, 996, 997, 1000, 1003,
1005, 1007, 1008, 1011, 1013, 1016, 1017, 1022, 1023, 1037 A,
1043.
ii. _C. Casheana._ 1024, 1025, 1587, 1588.
iii. _Palaeostachya vera._ 110, 1564, 1567, 1569, 1579, 1583.]
III. Pith-casts of Calamites.
A. _Calamitina._ B. _Stylocalamites._ C. _Eucalamites._
Palaeobotanical literature contains a large number of species of
_Calamites_ founded on pith-casts alone. Many of these so-called
species are of little or no value botanically, but while we may admit
the futility of attempting to recognise specific types in the same
sense as in the determination of recent plants, it is necessary to
pay attention to such characters as are likely to prove of value for
descriptive and comparative purposes. From the nature of the specimens
it is clear that many of the differences may be such as are likely to
be met with in different branches of the same species, while in others
the pith-casts of distinct species or genera may be almost identical.
The most striking differences observable in Calamite casts are in the
character of the internodes, the infranodal canals, the number and
disposition of branch-scars, and other surface features. Occasionally
it is possible to recognise certain anatomical characters in the coaly
layer which often surrounds a shale- or sandstone-cast, and the surface
of a well preserved cast may give a clue to the nature of the wood in
the faint outlines of cells which can sometimes be detected on the cast
itself[753]. The breadth of the carbonaceous envelope on a cast has
been frequently relied on by some writers as an important character. It
has been suggested[754] that we may arrive at the original thickness
of the wood of a stem by measuring the coaly layer and multiplying
the breadth by 27; the explanation being that a zone of wood 27 mm. in
thickness is reduced in the process of carbonisation to a layer 1 mm.
thick.
The breadth of the coal on the same form of cast may vary considerably;
on this account, and for various other reasons, such a character can
have but little value. Our knowledge of anatomy may often help us
to interpret certain features of internal casts and to appreciate
apparently unimportant details. One occasionally notices that a
Calamite pith-cast has large infranodal canals, and in some specimens
each internodal ridge may be traversed by a narrow median line or small
groove; large infranodal canal casts suggest the type of stem referred
to the subgenus _Arthrodendron_, and the median line on the ridges may
be due to bands of hard tissue in each principal medullary ray.
In attempting to identify pith-casts the student must keep in view
the probable differences presented by the branching rhizome, the main
aerial branches and the finer shoots of the same individual. The long
internodal ridges of some casts may be mistaken for the parallel veins
of such a leaf as _Cordaites_, a Palaeozoic Gymnosperm, if there are
no nodes visible on the specimen. The fossil figured by Lindley and
Hutton[755] as _Poacites_, and regarded by them as a Monocotyledon,
is no doubt a portion of a Calamite with very long internodes. An
interesting example of incorrect determination has recently been
pointed out by Nathorst[756] in the case of certain casts from Bear
Island, originally described by Heer as examples of _Calamites_; the
vertical rows of leaf-trace casts on a _Knorria_ were mistaken for the
ribs of a Calamite stem. The specimens in the Stockholm Museum fully
bear out Nathorst’s interpretation. The undulating course of internodal
ridges and grooves is not in itself a character of specific value. If
a Calamite stem were bent slightly, the wood and medullary-ray tissues
on the concave side might adapt themselves to the shortening of the
stem by becoming more or less folded, and a cast of such a stem would
show undulating ridges and grooves on one side and straight ones on the
other[757].
A convenient classification of Calamite casts was proposed by Weiss
in 1884, founded chiefly on the number and manner of occurrence of
branch-scars—or rather branch-depressions—on the surface of pith-casts.
Weiss[758] recognised the imperfection of his proposed grouping,
and Zeiller[759] has also expressed reasonable doubts as to the
scientific value of such group-characters. Weiss instituted three
subgenera—_Calamitina_, _Eucalamites_ and _Stylocalamites_, which are
made use of as convenient terms in descriptive treatment of Calamite
casts. The following account of a few of the more typical casts may
serve to illustrate the methods employed in the description of such
specimens; the synonomy given for the different species is not intended
to be complete, but it is added with a view to drawing attention to the
necessity for careful comparison in systematic work.
A. _Calamitina._
[Illustration: FIG. 99. _Calamites_ (_Calamitina_) _Göpperti_ (Ett.).
_b_, branch scars.
From a specimen in the Manchester Museum, Owens College. ¼ nat.
size.]
This sub-genus of _Calamites_, as instituted by Weiss[760], includes
Calamitean stems or branches, which are characterised by the periodic
occurrence of branch-whorls usually represented by fairly large oval
or circular scars just above a nodal line (figs. 99, 100 and 101).
The branch-scars may form a row of contiguous discs, or a whorl may
consist of a smaller number of branches which are not in contact
basally. A form described by Weiss as _C. pauciramis_, Weiss[761],
has only one branch in each whorl, as represented by a single large
oval scar on some of the nodes of the cast. A stem of this form is by
no means a typical _Calamitina_, but it serves to show the existence
of forms connecting Weiss’ sub-genera _Calamitina_ and _Eucalamites_.
The number of internodes and nodes between the branch whorls varies
in different specimens, and is indeed not constant in the same
plant. Each nodal line bears numerous elliptical scars which mark
the points of attachment of leaves; each branch-whorl is situated
immediately above a node, and in some forms this nodal line pursues
a somewhat irregular course across the stem, following the outlines
of the several branch-scars[762]. The surface of the internodes is
either perfectly smooth or it is more frequently traversed by short
longitudinal ridges or grooves probably representing fissures in the
bark of the living stem; these are indicated by lines in fig. 99 and
by elongated elliptical ridges in fig. 101. On young stems the leaves
are occasionally found in place, as for example in an example figured
by Weiss[763] (_C. Göpperti_), or we may have leaf-verticils still in
place in much older and thicker branches[764] (cf. fig. 85, p. 330).
It occasionally happens that the bark of _Calamitina_ stems has been
preserved as a detached shell[765] reminding one of the sheets of Birch
bark often met with in forests, the separation being no doubt due in
the fossil as in the recent trees to the manner of occurrence of the
cork-cambium.
In a few cases branches have been preserved still attached to a stem
or branch of higher order; examples of such specimens are figured
by Lindley and Hutton[766], Stur[767], and others. Grand’Eury[768]
has given an idealised drawing of a typical _Calamitina_ bearing a
whorl of branches with the foliage and habit of _Asterophyllites
equisetiformis_. The specimen on which this drawing is based is in the
Natural History Museum, Paris; it shows Asterophyllitean branches in
organic connection with a Calamitean stem, but it is not quite clear if
the stem is a true _Calamitina_. A large drawing of this interesting
specimen is given by Stur[769] in his monograph on _Calamites_, also
a smaller sketch by Renault[770] in his _Cours de botanique fossile_.
Similar branches of the _Asterophyllites_ type attached to an undoubted
_Calamitina_ are figured also by Lindley and Hutton. There is, in
short, good evidence that stems of this sub-genus bore branches with
Asterophyllitean shoots.
The wood of stems of the Calamitina group of _Calamites_, in some
instances at least, was of the _Arthropitys_ type; this has been
shown to be the case in some French specimens from the Commentry
coal-field[771] and in others described by Stur[772]. The pith-casts
of _Calamitina_ are characterised by comparatively short internodes
separated by deep nodal constrictions, as shown in fig. 100. From
Permian specimens from Neu Paka in Bohemia, described by Stur[773], we
learn that there were the usual Calamite diaphragms bridging across
the wide pith-cavity at each node. Such a cast as that shown in fig.
100 is often referred to as _Calamites approximatus_ Brongn.; the
length of the internodes and the periodic occurrence of branch-scars
in the form of circular or oval depressions along a nodal line
enable us to recognise the _Calamitina_ casts. Weiss[774] points out
that in pith-casts of this form the branch-scars occur on the nodal
constriction, and not immediately above the node as is the case on
the surface of a typical _Calamitina_. This distinction is however
of little or no value; the point of attachment of a branch may be
above the nodal line, while on the pith-cast of the same stem the
point of origin of the vascular bundles of the branch is on the nodal
constriction[775].
The specimen shown in fig. 100 illustrates the appearance of a
_Calamitina_ cast. There is a verticil of branch-scars on the lowest
nodal constriction; on the right of the pith-cast the broad band of
wood is faintly indicated by the smooth surface of the rock (_x_).
Other examples demonstrating the existence of a broad woody cylinder in
_Calamitina_ stems have been figured by Weiss[776] and other writers,
and some good examples may be seen in the British Museum.
[Illustration: FIG. 100. _Calamites_ (_Calamitina_) _approximatus_
Brongn. Lower Coal-Measures of Ayrshire.
_x_, impression of the wood.
(From a specimen in the collection of Mr R. Kidston.)]
We have so far noticed the connection of certain forms of pith-casts
(_e.g._ _Calamites approximatus_), and Asterophyllitean shoots with
stems of the sub-genus _Calamitina_.
As regards the strobili our knowledge is far from satisfactory.
Stur[777] figures some fertile branches bearing long and narrow
strobili, either _Palaeostachya_ or _Calamostachys_, in close
association with _Calamitina_ stems, and Renault and Zeiller[778] give
illustrations of the association of _Calamitina_ stems with large
strobili of the _Macrostachya_ form.
Before Weiss proposed the term _Calamitina_, various authors had
figured this form of Calamite under a distinct generic name (_e.g._
_Hippurites_ of Lindley and Hutton[779], _Cyclocladia_[780],
_Macrostachya_[781], &c.). Stems of this type have also been described
by more recent writers under different names, and considerable
confusion has been caused by the use of numerous generic designations
for forms of _Calamitina_. Some small fragments of _Calamitina_ stems
were described by Salter[782] in 1863 as portions of a new species
of the Crustacean _Eurypterus_ (_E. mammatus_). In 1869 Grand’Eury
proposed the generic name _Calamophyllites_[783] for stems bearing
verticils of _Asterophyllites_ shoots; his description of such stems
agrees with Weiss’ _Calamitina_, but as Grand’Eury’s name is used in
a narrower sense as implying a connection with _Asterophyllites_,
it is more convenient to adopt Weiss’ term in spite of the priority
of _Calamophyllites_. In the _Fossil flora of Commentry_ we find
some flattened stems of the _Calamitina_ type described under
different generic names, as _Arthropitys approximatus_[784] and as
_Macrostachya_[785].
The determination of distinct species of the sub-genus _Calamitina_
is rendered almost hopeless by the variation in the different
branches of the same individual, and by the difficulty of connecting
surface-impressions with casts of the pith-cavity.
A typical example of the _Calamitina_ type of _Calamites_ was figured
by Sternberg[786] in 1821 as _Calamites varians_. This has been
adopted by Weiss[787] as a comprehensive species including several
different ‘forms’ of stems, which differ from Sternberg’s fossil in
such points as the number of nodes between the branch-whorls and
the number of branches in each whorl. The result of this system of
nomenclature is the separation of portions of one specific type under
different form-names. It must be clearly recognised that accurate
specific diagnoses are practically impossible when we have to deal with
fragments of plants, some of which are mere pith-casts, while others
show the surface features. The specimen represented in fig. 99 agrees
with a stem described by Ettingshausen[788] in 1855 as _Calamites
Göpperti_, and as a matter of convenience a member of the _Calamitina_
group showing such characters may be referred to as _Calamites_
(_Calamitina_) _Göpperti_ (Ett.). The following list, which includes a
few synonyms of this form, may suffice to illustrate the difficulties
connected with accurate systematic determinations.
_Calamites_ (_Calamitina_) _Göpperti_ (Ett.). Fig. 99.
1855. _Calamites Göpperti_, Ettingshausen[789].
1869. _Calamites_ (_Calamophyllites_) _Göpperti_, Grand’Eury[790].
1874. _Cyclocladia major_, Feistmantel[791].
1874. _Calamites verticillatus_, Williamson[792].
1876. _Calamitina Göpperti_, Weiss[793].
1884. _Calamites_ (_Calamitina_) _varians abbreviatus_, Weiss[794].
1884. _Calamites_ (_Calamitina_) _varians inconstans_, Weiss[795].
1887. _Calamites Sachsei_, Stur[796].
1888. _Calamophyllites Göpperti_, Zeiller[797].
This species is characterised by the smooth bark, which may be
traversed by a few irregular longitudinal fissures; most of the nodes
bear a series of small leaf-scars, and at fairly regular intervals a
node is immediately succeeded by a circle of contiguous branch-scars,
8–12 in a whorl. The pith-cast of this type of stem has short ribbed
internodes separated by rather deep nodal constrictions; the
branch-whorls being represented by a series of pits on the nodal
constrictions recurring at corresponding intervals to the whorls of
branch-scars on the surface of the stem. Leaves narrow and linear
in form, like those on Asterophyllitean branches, are occasionally
associated with this type of stem.
[Illustration: FIG. 101. _Calamites_ (_Calamitina_) sp. From a specimen
in the British Museum. (After Carruthers.) Slightly reduced.]
The fragment of a _Calamitina_ stem shown in fig. 101 is the
counterpart of a specimen originally figured by Steinhauer[798] in 1818
as a species of _Phytolithus_. This may be specifically identical with
_C. Göpperti_; but it is better to speak of so small a specimen as
merely one of the _Calamitina_ stems, to be compared with _Calamites_
(_Calamitina_) _Göpperti_. The specimen measures 14·5 cm. in length and
7 cm. in breadth.
The form of pith-cast represented in fig. 100 is no doubt that of
one of the _Calamitina_ species, but as it is seldom possible to
determine the connection between such casts and the particular
species of stems to which they belong, they are often referred to as
_Calamites_ (_Calamitina_) _approximatus_ (Brongn.). The specimen of
which fig. 100 is a photograph was originally described and figured
by Mr Kidston[799] from the lower Coal-Measures of Ayrshire. Both
_Calamites_ (_Calamitina_) _Göpperti_ (Ett.) and _C._ (_Calamitina_)
_approximatus_ (Brongn.) are recorded from the Transition, Middle and
Lower Coal-Measures[800].
B. _Stylocalamites._
In the members of this sub-genus the branch-scars are either irregular
in their occurrence or absent. In some Calamites the branch-scars are
very few and far between, and other species appear to have been almost
without branches; pith-casts of such stems may be referred to the
sub-genus _Stylocalamites_[801].
An exceedingly common Calamitean cast, _C. Suckowi_ Brongn. (fig. 82)
affords a good illustration of this type of stem. In the specimen shown
in fig. 82 we have a cast of a rhizome, which is rather exceptional in
showing three branches in connection with one another. The appearance
of the fossil suggests a rhizome, rather than an aerial shoot, bearing
lateral branches; the narrowing of the branches and the rapid decrease
in the length of the internodes towards the point of attachment being
features associated with rhizomes rather than with aerial branches.
_Calamites_ (_Stylocalamites_) _Suckowi_, Brongn. Fig. 82.
1818. _Phytolithus sulcatus_, Steinhauer[802].
1825. _Calamites decoratus_, Artis[803].
1828. _Calamites Suckowi_, Brongniart[804].
1833. _Calamites cannaeformis_, Lindley and Hutton[805].
For more complete lists of synonyms of this species reference should
be made to Kidston[806], Zeiller[807], and other authors.
Casts of _Calamites Suckowi_ are characterised by flat or slightly
convex internodal ridges separated by shallow depressions, the ridges
are rounded at the upper end of each internode, and usually bear
circular casts of infranodal canals. There are some unusually large
examples of casts of this species in the British Museum from the
Radstock Coal-Measures; one of these has a length of 81 cm., and a
diameter of 27 cm. Specimens are not infrequently found with verticils
of slender roots in close proximity to the nodes of the cast; figures
of such root-bearing casts have been given by Lindley and Hutton[808],
Weiss[809], and other authors.
Renault[810] has drawn attention to the thinness of the layer of
wood which is often associated with large casts of _C. Suckowi_; he
concludes that the stems must have possessed little or no secondary
wood. In a more recent work by Grand’Eury[811] _Calamites Suckowi_
is spoken of as having had wood of the _Calamodendron_ type, but as
wood of this kind has not been found in England, it is suggested that
the plant may not have assumed an arborescent habit until late in
the Coal-Measure period. During the Lower and Middle Coal-Measures,
at which horizon it commonly occurs in England, it may have been
herbaceous. This suggestion has little to commend it; the close
agreement between _C. Suckowi_ from English and French localities
points to a plant of the same form, and we have no satisfactory
evidence as to any difference in stem-structure in the two cases.
Stur has figured a specimen of a Calamite cast, which he compares with
_C. Suckowi_, surrounded by a band of silicified wood apparently of the
_Arthropitys_ type. From this and other facts it would appear probable
that some of the English stems with the _Arthropitys_ structure
possessed casts referable to _Calamites_ (_Stylocalamites_) _Suckowi_.
We are not in a position to speak with confidence as to the strobili
of _C. Suckowi_, but Stur adduces evidence in support of a connection
between this species of Calamite and certain Asterophyllitean branches
(_Calamocladus equisetiformis_) bearing Calamostachyan cones. He does
not appear to have found the foliage-shoots and stems in organic
contact, but draws this conclusion from the association of the fertile
branches and stems in the same rocks[812]. This species is abundant in
the Lower, Middle and Upper Coal-Measures; it has also been recorded
from the Millstone Grit[813].
C. _Eucalamites._
In this sub-genus branch-scars occur on every node; the scars never
form a contiguous whorl as in _Calamitina_, but there may be from 3
to 10 on each node. The scars of successive nodes often alternate in
position, and thus form more or less regular vertical series as shown
in fig. 102. The most obvious feature as regards the arrangement of
the branch-scars is their spiral disposition on the surface of the
pith-cast. The internodes are fairly uniform in length, and there is
no periodic recurrence of narrower internodes as in _Calamitina_. From
an examination of specimens of _Eucalamites_ in which the pith-cast
is covered with a coaly layer representing the carbonised remains of
the wood and cortex, it would appear that the surface of the stems
was practically smooth. The coaly investment on _Eucalamites_ casts
varies considerably in thickness[814]; it is very unsafe to make use
of the thickness of this layer as a test of the breadth of the wood in
Calamitean stems. The branch-scars as seen in a surface-view of a stem
are situated a little above the nodal lines, while depressions on the
pith-casts occur in the slight nodal constriction or immediately above
it. Small leaf-scars have been described as occurring on the nodes
between the branch-scars in specimens showing the surface features[815].
The species long known as _Calamites cruciatus_ Sternb. is usually
taken as the type of the sub-genus _Eucalamites_. Weiss[816] has
subdivided this species into several ‘forms,’ which he bases on the
number of branch-scars on each node and on other characters; a more
extended subdivision of _C. cruciatus_ has recently been made by
Sterzel[817], who admits the impossibility of separating the specific
forms by means of the data at our disposal, but for purposes of
geological correlation he prefers to express slight differences by
means of definite ‘forms’ or varieties. The more comprehensive use
of the specific name _cruciatus_ as adopted by Zeiller in his _Flore
de Valenciennes_[818] is, I believe, the better method to adopt.
The specimen shown in fig. 102 affords a good example of a typical
_Calamites cruciatus_, it was found in the Middle Coal-Measures near
Barnsley, Yorkshire.
[Illustration: FIG. 102. _Calamites_ (_Eucalamites_) _cruciatus_,
Sternb. From a specimen in the Barnsley Museum, Yorkshire. ½ nat.
size.]
_Calamites_ (_Eucalamites_) _cruciatus_ (Sternb.). Fig. 102.
1826. _Calamites cruciatus_, Sternberg[819].
1828. _Calamites cruciatus_, Brongniart[820].
1831. _Calamites alternans_, Germar and Kaulfuss[821].
1837. _Calamites approximatus_, Lindley and Hutton[822].
1877. _Calamodendrofloyos cruciatus_, Grand’Eury[823].
1878. _Calamodendron cruciatum_, Zeiller[824].
1884. _Calamites_ (_Eucalamites_) _cruciatus ternarius_, Weiss[825].
1884. „ „ „ _quaternarius_, Weiss[825].
1884. „ „ „ _genarius_, Weiss[825].
1884. „ „ _multiramis_, Weiss[825].
1888. _Calamites_ (_Calamodendron_) _cruciatus_, Zeiller[826].
This species occurs in the Upper, Middle and Lower Coal-Measures[827].
The casts of the _cruciatus_ type have been found associated
with wood possessing the structural features of the sub-genus
_Calamodendron_[828], but our knowledge of the structure of the stem,
and of the fertile branches of _C. cruciatus_ is very imperfect. A
restoration of _Calamites_ (_Eucalamites_) _cruciatus_ is given by
Stur[829] in his classic work on the Calamites, but he does not make
quite clear the supposed connection with the stems and the fertile
shoots of the Asterophyllites type[830] which he describes. Another
member of the _Eucalamites_ group, which is better known as regards its
foliage-shoots, is _Calamites ramosus_, a species first described by
Artis[831] in 1825. Stems of this species have been found in connection
with the branches and leaves of the _Annularia_[6] type, bearing
_Calamostachys_[832] cones. In all probability pith-casts included in
the sub-genus _Eucalamites_ belonged to stems with foliage-shoots and
probably also with cones of more than one form.
[Sidenote: NOMENCLATURE.]
In the above account of a few common pith-casts it has been pointed
out that there is occasionally satisfactory evidence for connecting
certain casts with wood of a particular structure, and with sterile and
fertile foliage-shoots of a definite type. It is, however, impossible
in many cases to recognise with any certainty the leaf-bearing
branches and strobili of the different casts of _Calamites_; it is
equally impossible to determine what type of pith-cast or what type
of foliage-shoots belongs to petrified stem-fragments in which it is
possible to investigate the microscopical features. The scattered
and piece-meal nature of the material on which our general knowledge
of Calamitean plants is based, necessitates a system of nomenclature
which is artificial and clumsy; but the apparent absurdity of attaching
different names to fragments, which we believe to be portions of the
same genus, is of convenience from the point of view of the geologist
and the systematist. As our material increases it will be possible to
further simplify the nomenclature for Calamarian plants, but it is
unwise to allow our desire for a simpler terminology to lead us into
proposals which are based rather on suppositions than on established
fact. If it were possible to discriminate between pith-casts of stems
having the different anatomical characters designated by the three
sub-genera, _Arthropitys_, _Arthrodendron_ and _Calamodendron_, the
genus _Calamites_ might be used in a much narrower and probably more
natural sense than that which we have adopted. The tests made use
of by some authors for separating pith-casts of _Calamodendron_
and _Arthropitys_ stems do not appear to be satisfactory; we want
some term to apply to all Calamitean casts irrespective of the
anatomical features of the stems, or of the precise nature of the
foliage-branches. As used in the present chapter, _Calamites_ stands
for plants differing in certain features but possessing common
structural characters, which must be defined in a broad sense so as
to include types which may be worthy of generic rank, but which for
convenience sake are included in a comprehensive generic name. The
attempts to associate certain forms of foliage with _Arthropitys_ on
the one hand and with _Calamodendron_ on the other, cannot be said
to be entirely satisfactory; we still lack data for a trustworthy
diagnosis of distinct Calamarian genera which shall include external
characters as well as histological features. If we restricted the
genus _Calamites_ to stems with an _Arthropitys_ structure and an
Asterophyllitean foliage, we should be driven into unavoidable error.
Within certain limits it is possible to distinguish generically or
even specifically between petrified branches, and we already possess
material enough for fairly complete diagnoses founded on internal
structure; but it is not possible to make a parallel classification
for pith-casts and foliage-shoots. For this reason, and especially
bearing in mind the importance of naming isolated foliage-shoots and
stem-casts for geological purposes, I believe it is better to admit
the artificially wide application of the name _Calamites_, and to
express more accurate knowledge, where possible, by the addition of a
subgeneric term. In dealing with distinctions exhibited by Calamitean
stems it may be advisable to make use of specific names, but we
must keep before us the probability of the pith-cast and petrified
stem-fragment of the same plant receiving different specific names.
If the structural type is designated by a special sub-genus, this
will tend to minimise the anomaly of using more than one binominal
designation for what may be the same individual.
[Sidenote: CALAMITES AND EQUISETUM.]
The following summary may serve to bring together the different generic
and subgeneric terms which have been used in the foregoing account of
_Calamites_.
CALAMITES.
Subgenera having reference to the method of branching as seen in casts
or impressions of the stem-surface or in pith-casts.
_Calamitina_,
_Eucalamites_,
_Stylocalamites_.
Subgenera founded on anatomical characters in stems and branches.
_Arthropitys_,
_Calamodendron_,
_Arthrodendron_ (new sub-genus substituted for _Calamopitys_).
Genus proposed for roots of _Calamites_ before their real nature was
recognised. The name refers to anatomical characters.
_Astromyelon._
Genera of which some species, if not all, are the leaf-bearing branches
of _Calamites_.
_Calamocladus_ (including _Asterophyllites_),
_Annularia_.
Generic names applied to strobili belonging to _Calamites_.
_Calamostachys_,
_Palaeostachya_,
_Macrostachya_,
_etc._
Genus including impressions of Calamite roots.
_Pinnularia._
IV. Conclusion.
A brief sketch of the main features of _Calamites_ suffices to bring
out the many points of agreement between the arborescent Calamite
plants and the recent Equisetums. The slight variation in morphological
character among the present-day Horse-tails, contrasts with the
greater range as regards structural features among the types included
in _Calamites_. The Horse-tails probably represent one of several
lines of development which tend to converge in the Palaeozoic period;
the Calamite itself would appear to mark the culminating point of a
certain phylum of which we have one degenerate but closely allied
descendant in the genus _Equisetum_. We shall, however, be in a better
position to consider the general question of plant-evolution after we
have made ourselves familiar with other types of Palaeozoic plants.
Grand’Eury’s[833] striking descriptions of forests of Calamites in the
Coal-Measures of central France, enable us to form some idea of the
habit of growth of these plants with their stout branching rhizomes and
erect aerial shoots.
By piecing together the evidence derived from different sources we
may form some idea of the appearance of a living Calamite. A stout
branching rhizome ascended obliquely or spread horizontally through
sand or clay, with numerous whorls or tufts of roots penetrating into
swampy soil. From the underground rhizome strong erect branches grew
up as columnar stems to a height of fifty feet or more; in the lower
and thicker portions the bark was fissured and somewhat rugged, but
smoother nearer the summit. Looking up the stem we should see old and
partially obliterated scars marking the position of a ring of lateral
branches, and at a higher level tiers of branches given off at regular
or gradually decreasing intervals, bearing on their upper portions
graceful green branchlets with whorls of narrow linear leaves. On the
younger parts of the main shoot rings of long and narrow leaves were
borne at short intervals, several leaf-circles succeeding one another
in the intervals between each radiating series of branches. On some
of the leaf-bearing branchlets long and slender cones would be found
here and there taking the place of the ordinary leafy twigs. Passing to
the apical region of the stem the lateral branches given off at a less
and less angle would appear more crowded, and at the actual tips there
would be a crowded succession of leaf-segments forming a series of
overlapping circles of narrow sheaths with thin slender teeth bending
over the apex of the tree.
Thus we may feebly attempt to picture to ourselves one of the many
types of Calamite trees in a Palaeozoic forest, growing in a swampy
marsh or on gently sloping ground on the shores of an inland sea, into
which running water carried its burden of sand and mud, and broken
twigs of Calamites and other trees which contributed to the Coal Period
sediments. The large proportions of a Calamite tree are strikingly
illustrated by some of the broad and long pith-casts occasionally
seen in Museums; in the Breslau Collection there is a cast of a stem
belonging to the sub-genus _Calamitina_, which measures about 2 m. in
length and 23 cm. in breadth, with 36 nodes. In the Natural History
Museum, Paris, there is a cast nearly 2 metres long and more than 20
cm. wide, which is referred to the sub-genus _Calamodendron_.
E. ARCHAEOCALAMITES.
In the Upper Devonian and Culm rocks casts of a well-defined Calamitean
plant are characteristic fossils; stems, leaf-bearing branches, roots
and cones have been described by several authors, and the genus
_Archaeocalamites_ has been instituted for their reception. Although
this genus agrees in certain respects with _Calamites_, and as recent
work has shown this agreement extends to internal structure, it has
been the custom to regard the Lower Carboniferous and Devonian plants
as genetically distinct. The surface features of the stem-casts,
the form of the leaves, and apparently the cones, possess certain
distinctive characters which would seem to justify the retention of a
separate generic designation.
We may briefly summarise the characteristics of the genus as follows:—
Pith-casts articulated, with very slightly constricted nodes; the
internodes traversed by longitudinal ribs slightly elevated or
almost flat, separated by shallow grooves. The ribs and grooves are
continuous from one internode to another, and do not usually show
the characteristic alternation of _Calamites_[834]. Along the nodal
line there are occasionally found short longitudinal depressions,
probably marking the points of origin of outgoing bundles. Branches
were given off from the nodes without any regular order; a pith-cast
may have branch-scars on many of the nodes, or there may be no trace
of branches on casts consisting of several nodes. The leaves[835]
are in whorls; in some cases they occur as free, linear, lanceolate
leaves, or on younger branches they are long, filiform and repeatedly
forked. The structure of the wood agrees with that of some forms of
_Arthropitys_. The strobili consist of an articulated axis bearing
whorls of sporangiophores, and each sporangiophore has four sporangia.
Our knowledge of the fertile shoots is, however, very imperfect.
Renault[836] has recently described the structure of the wood in some
small silicified stems of _Archaeocalamites_ from Autun. A large hollow
pith is surrounded by a cylinder of wood consisting of wedge-shaped
groups of xylem tracheids associated with secondary medullary rays;
at the apex of each primary xylem group there is a carinal canal. The
primary medullary rays appear to have been bridged across by bands of
xylem at an early stage of secondary thickening, as in the Calamite of
fig. 83, _D._
[Illustration: FIG. 103. _Archaeocalamites scrobiculatus_ (Schloth.).
From a specimen in the Woodwardian Museum, Cambridge. From the
Carboniferous limestone of Northumberland. ½ nat. size.]
Our knowledge of the cones of _Archaeocalamites_ is far from
satisfactory. Renault[837] has recently described a small fertile
branch bearing a succession of verticils of sporangiophores; each
sporangiophore stands at right angles to the axis of the cone and
bears four sporangia, as in _Calamostachys_. It is not clear how far
there is better evidence than that afforded by the association of
the specimen with pith-casts of stems, for referring this cone to
_Archaeocalamites_, but the association of vegetative and fertile
shoots certainly suggests an organic connection. The cone described by
the French author agrees with _Equisetum_ in the absence of sterile
bracts between the whorls of sporangiophores. It is an interesting fact
that such a distinctly Equisetaceous strobilus is known to have existed
in Lower Carboniferous rocks.
Stur[838] has also described _Archaeocalamites_ at considerable
length; he gives several good figures of stem-casts and foliage-shoots
bearing long and often forked narrow leaves. The same writer describes
specimens of imperfectly preserved cones in which portions of whorls
of forked filiform leaves are given off from the base of the
strobilus[839]. Kidston[840] published an important memoir on the cones
of _Archaeocalamites_ in 1883, in which he advanced good evidence
in support of the view that certain strobili, which were originally
described as Monocotyledonous inflorescences, under the generic
name _Pothocites_[841], are the fertile shoots of this Calamarian
genus. Kidston’s conclusions are based on the occurrence on the
_Pothocites_ cones, of leaves like those of _Archaeocalamites_, on the
non-alternation of the sporangiophores of successive whorls, and on the
close resemblance between his specimens and those described by Stur.
Good specimens of the cones, formerly known as _Pothocites_, may be
seen in the Botanical Museum in the Royal Gardens, Edinburgh; as they
are in the form of casts without internal structure it is difficult to
form a clear conception as to their morphological features.
The fossils included under _Archaeocalamites_ have been referred
by different authors to various genera, and considerable confusion
has arisen in both generic and specific nomenclature. The following
synonomy of the best known species, _A. scrobiculatus_ (Schloth.)
illustrates the unfortunate use of several terms for the same plant.
_Archaeocalamites scrobiculatus_ (Schloth.). Fig. 103.
1720. _Lithoxylon_, Volkmann[842].
1820. _Calamites scrobiculatus_, Schlotheim[843].
1825. _Bornia scrobiculata_, Sternberg[844].
1828. _Calamites radiatus_, Brongniart[845].
1841. _Pothocites Grantoni_, Paterson[846].
1852. _Calamites transitionis_, Göppert[847].
—— _Stigmatocanna Volkmanniana_, _ibid._
—— _Anarthrocanna tuberculata_, _ibid._
—— _Calamites variolatus_, _ibid._
—— _C. obliquus_, _ibid._
—— _C. tenuissimus_, _ibid._
—— _Asterophyllites elegans_, _ibid._
1866. _Calamites laticulatus_, Ettingshausen[848].
—— _Equisetites Göpperti_, _ibid._
—— _Sphenophyllum furcatum_, _ibid._
1873. _Asterophyllites spaniophyllus_, Feistmantel[849].
1880. _Asterocalamites scrobiculatus_, Zeiller[850].
For other lists of synonyms reference may be made to Binney[851],
Stur[852], Kidston[853] and other authors.
Some of the best specimens of this species are to be seen in the
Museums of Breslau and Vienna, which contain the original examples
described by Göppert and Stur. An examination of the original
specimens, figured by Göppert under various names, enables one to
refer them with confidence to the single species, _Archaeocalamites
scrobiculatus_. The generic name _Archaeocalamites_, which has been
employed by some authors, was suggested by Schimper[854] in 1862, as
a subgenus of _Calamites_, on account of the occurrence of a deeply
divided leaf-sheath, attached to the node of a pith-cast, which seemed
to differ from the usual type of Calamitean leaf. The specimens
described by Schimper are in the Strassburg Museum; the leaf-sheath
which he figures is not very accurately represented.
The example given in fig. 103 shews very clearly the continuous course
of the ribs and grooves of the pith-cast. Each rib is traversed by a
narrow median groove which would seem to represent the projecting edge
of some hard tissue in the middle of each principal medullary ray of
the stem. The specimen was found in a Carboniferous limestone quarry,
Northumberland; there is a similar cast from the same locality in the
Museum of the Geological Survey.
_Affinities of_ Archaeocalamites.
This genus agrees very closely with _Calamites_ both in the anatomical
structure of the stem and in the verticillate disposition of the
leaves. The strobili appear to be Equisetaceous in character, and there
is no satisfactory evidence of the existence of whorls of sterile
bracts in the cone, such as occur in _Calamostachys_ and in other
Calamitean strobili. The continuous course of the vascular bundles of
the stem from one internode to the next is the most striking feature
in the ordinary specimens of the genus; but it sometimes happens that
the grooves on a pith-cast shew the same alternation at the node as in
_Calamites_. This is the case in a specimen in the Göppert collection
in the Breslau Museum, and Feistmantel[855] has called attention to
such an alternation in specimens from Rothwaltersdorf. In the true
_Calamites_, on the other hand, the usual nodal alternation of the
vascular strands is by no means a constant character[856]. Stur[857],
Rothpletz[858], and other authors have pointed out the resemblance of
_Archaeocalamites_ to _Sphenophyllum_. The deeply divided leaves of
some Sphenophyllums and those of _Archaeocalamites_ are very similar
in form; and the course of the vascular strands in _Sphenophyllum_
may be compared with that in _Archaeocalamites_. But the striking
difference in the structure of the stele forms a wide gap between the
two genera. We have evidence that the Calamites and Sphenophyllums were
probably descended from a common ancestral stock, and it may be that in
_Archaeocalamites_, some of the _Sphenophyllum_ characters have been
retained; but there is no close affinity between the two plants.
On the whole, considering the age of _Archaeocalamites_ and the few
characters with which we are acquainted, it is probable that this
genus is very closely related to the typical _Calamites_, and may be
regarded as a type which is in the direct line of development of the
more modern Calamite and the living _Equisetum_. Weiss[859] includes
_Archaeocalamites_ as one of his subgenera with _Calamitina_ and
others, and it is quite possible that the genus has not more claim to
stand alone than other forms at present included in the comprehensive
genus _Calamites_.
The student will find detailed descriptions of this genus in the works
which have been referred to in the preceding pages.