at the base, which shows the stumps of four branches recalling the
dichotomously branched arms of _Stigmaria_ and _Pleuromeia_. If these
are in reality the remains of Stigmaria-like horizontal branches the
species affords an interesting example of a Lepidodendron axis with
a subterranean rhizome of the type which has been found in several
Sigillarian stems. In the upper end of the axis the stele consists of a
solid strand of xylem which is not sufficiently well preserved to show
the position of the protoxylem groups. A transverse section taken near
the base reveals a type of stele differing from that at the upper end
in being composed of radially disposed tracheids and in its resemblance
to the stele of _Stigmaria_.
4. _Lepidodendron fuliginosum_, Williamson. Figs. 162–172, 179, E.
1871. _Lepidodendron Harcourtii_, Binney, Palæont. Soc., p. 48, Pl.
VII. fig. 6.
1872. _Halonia regularis_, Binney, Palæont. Soc., p. 89, Pl. XV.
1881. _Lepidodendron Harcourtii_, Williamson, Phil. Trans. Roy.
Soc., Vol. 172, p. 288, Pls. XLIX–LII.
1887. _Lepidodendron fuliginosum_, Williamson, Proc. Roy. Soc.,
Vol. XLII. p. 6.
1891. _Lepidodendron Williamsoni_, Solms-Laubach, Fossil Botany, p.
226.
1893. _Lepidophloios fuliginosus_, Kidston, Trans. Roy. Soc.
Edinburgh, Vol. XXXVIII. p. 548.
The name _Lepidodendron fuliginosum_ was proposed by Williamson in
1887 for petrified stems previously included by him in Witham’s
species _L. Harcourtii_, but subsequently recognised as a distinct
type characterised by “the greater uniformity in the composition
of the entire cortex” and by other features some of which do not
constitute distinctive characters. The species agrees with _L.
Harcourtii_ and with _L. Veltheimianum_ in having a medullated stele;
it is distinguished not only by the more frequent preservation of
the middle cortex, a fact due to a difference in minute structure,
but chiefly by the peculiar structure of the secondary tissue
added to the stele; this is in part composed of radial series of
parenchymatous cells and of a varying amount of tracheal tissue
the elements of which are narrower than in other species and are
characterised also by their sinuous vertical course. As is pointed
out in the sequel, the anatomical features of _L. fuliginosum_, as at
present understood, are not confined to one type of _Lepidodendron_
stem. Specimens have been described with leaf-cushions of the form
characteristic of _L. aculeatum_, _L. obovatum_ and _Lepidophloios_
combined with the anatomical features of Williamson’s species: it is
possible that the two species _L. obovatum_ and _L. aculeatum_ are
not really distinct[323], but it is certain that shoots with both the
_Lepidodendron_ and _Lepidophloios_ cushions may have the same type of
anatomical structure.
A more detailed knowledge of the structural features of Lepidodendron
shoots may enable us to define anatomical species with more exactness
than is possible at present. There can, however, be little doubt that
well-marked anatomical features may be associated with more than one
specific form of shoot as defined by the form of the leaf-cushions.
Solms-Laubach proposed the name _Lepidodendron Williamsoni_ for the
anatomical type _L. fuliginosum_ of Williamson, but the latter name has
been generally adopted.
In the following account special attention is directed to the nature
and origin of the secondary stelar tissue and to the secretory
zone, as difference of opinion exists as to the interpretation of
these features. Among the best examples of shoots of _Lepidodendron
fuliginosum_ without secondary tissue or in which it is feebly
developed are those originally described by Binney. The stele includes
a large parenchymatous pith, the cells of which frequently show signs
of recent division, a feature observed also in the pith of the large
stem of _L. Wünschianum_, represented in figs. 181, 182. The primary
xylem cylinder has an irregularly crenulate outer edge like that of _L.
Wünschianum_ and _L. Harcourtii_ and the protoxylem elements occupy an
exarch position. Isodiametric reticulately-pitted elements are met with
both on the inner and outer edge of the xylem.
[Illustration: FIG. 162. _Lepidodendron fuliginosum._ Part of the
stele in transverse section. (Binney Collection, Sedgwick Museum,
Cambridge.)]
[Illustration: FIG. 163. _Lepidodendron fuliginosum._ Longitudinal
section. (Binney Collection, Cambridge.)]
Figs. 162 and 163 illustrate the structure of the outer portion of the
xylem and adjacent tissues in a section of a shoot 3·8 cm. × 2·5 cm. in
diameter, which is in the act of branching, as shown by the occurrence
of two steles of equal size. A figure of the complete section will
be found in Binney’s memoir[324], and additional illustrations were
published in 1899[325].
[Illustration: FIG. 164. _Lepidodendron fuliginosum_. Leaf-trace.
(Binney Collection, Cambridge.)]
The primary xylem (figs. 162, 163, _x_) is succeeded by 2–3 rows of
polygonal cells with dark contents and associated with isodiametric
tracheae: these pass into clearer parenchymatous tissue, _a_,
characterised by the arrangement of the cells in vertical series,
to which the term meristematic zone has been applied. The secretory
zone, _s_, abutting on the meristematic zone, consists of more or less
disorganised parenchymatous cells and broader and more elongated
spaces; it is interrupted here and there by an outgoing leaf-trace,
as at _lt_ 1 and _lt_ 2 in fig. 162. The secretory zone is succeeded
by a homogeneous inner cortex like that described in _L. vasculare_;
part of this region is seen at the upper edge of fig. 162. The broad
middle cortex, which is separated from the inner cortex by a sharply
defined boundary, is composed of rather small lacunar parenchymatous
tissue consisting of sinuous tubular elements interspersed among
isodiametric cells of various sizes (fig. 166, _p_). In the middle
cortical region the leaf-traces pursue an almost horizontal course; one
is shown in fig. 164, in oblique longitudinal section, in a reversed
position; the xylem, _x_, should be on the inner side of the secretory
tissue, _s_. The clear space between the two parts of the vascular
bundle was originally occupied by a few layers of parenchymatous
cells, as seen in the transverse sections, figs. 165 and 166. In some
specimens the leaf-traces pass through the middle cortex in a much
more vertical course, as shown by the section represented in fig. 165.
This section illustrates the structure of a typical leaf-trace with
unusual clearness; it shows the tangentially elongated group of xylem,
the strand of tissue which occupies the position of phloem, _s_ (to
which the term secretory zone is applied), the compact parenchyma
between the two parts of the bundle, and surrounding the whole a
narrow sheath sharply contrasted by the smaller and more uniform size
of the cells from the middle cortex, a few cells of which are seen in
the photograph. The middle cortex shows a well-defined junction with
the more compact outer cortical region, which consists of primary
parenchyma passing externally into a zone of phelloderm composed of
thick-walled and more elongated cells. A noticeable feature in many
Lepidodendron shoots is the occurrence of a circle of strands of
secretory cells often surrounding fairly large ducts just internal to
the edge of phelloderm: similar strands form irregularly concentric
circles, as was pointed out in the case of _L. vasculare_, in the
phelloderm itself.
[Illustration: FIG. 165. _Lepidodendron fuliginosum._ Leaf-trace: _x_,
xylem; _s_, secretory zone. (Binney Collection, Cambridge.)]
[Illustration: FIG. 166. _Lepidodendron fuliginosum._ Leaf-trace: _p_,
parichnos. (Binney Collection, Cambridge.)]
Fig. 166 shows a leaf-trace in the outer cortex accompanied by its
crescent-shaped parichnos, _p_, derived from the middle cortex and
by means of which the outer cortex and the lamina of the leaves are
connected with the inner region of the shoot. This lacunar middle
cortex and parichnos doubtless constitute an aerating tissue-system
which after leaf-fall is exposed directly to the air at the ends of the
parichnos arms on the leaf-scars.
Some of the sections in the Binney Collection (Sedgwick Museum,
Cambridge) show early stages in the production of secondary xylem: in
the section represented in fig. 167 the secretory zone is succeeded
on its inner face by a zone of radially elongated cells, _m_, which
are clearly in a meristematic condition. The same section shows also
the more radially extended form of the xylem of a leaf-trace with its
internal protoxylem, _px_, in contrast to the tangentially elongated
form which is assumed during its passage through the cortex (cf. figs.
165, 166).
[Illustration: FIG. 167. _Lepidodendron fuliginosum._ (Binney
Collection, Cambridge.)]
Some sections of _Lepidodendron fuliginosum_ in the Manchester
University Collection are of special interest from the point of view of
the method of secondary thickening. In the section reproduced in fig.
168, B, the meristematic zone is seen to consist in part of radially
elongated elements, _m_, with parallel cross-walls evidently of recent
origin. The same tissue is shown also in fig. 168, C, _a_, D, _a_,
and in fig. 169, A, _a_ This band of meristem, which we may speak of
as the cambium, occurs in the outer region of the meristematic zone
immediately internal to the secretory zone, _sc_.
[Illustration: FIG. 168.
A. _Lepidodendron vasculare._ (Botany School, Cambridge.)
B. _Lepidodendron fuliginosum._ (From a specimen from Shore,
Lancashire, in the Cambridge Botany School Collection).
C. _L. fuliginosum._ (“Biseriate Halonia” of Weiss No. 257,
Manchester University Museum.)
D. _L. fuliginosum._ (Manchester Univ. Museum.)]
The result of the activity of this cambium band is the production of
secondary parenchyma and tracheal tissue. In fig. 179, E, drawn from
a portion of the section represented in fig. 168, B, a projecting arm
of primary xylem is seen at _x_; this is followed by 2–3 layers of
parenchymatous cells, some of which have dark contents, and beyond
this is seen a group of secondary elements, _tr_, cut across somewhat
obliquely, which are evidently products of the cambial cells on the
inner margin of the secretory zone, _sc_. The longitudinal section
(fig. 169, D) shows the cambial cells, a, next the secretory zone,
_sc_, passing internally into crushed and imperfectly preserved
elongated elements which are presumably miniature tracheae, and these
are succeeded by older and more completely lignified xylem elements,
_x_. In larger shoots the amount of secondary tissue is considerably
greater; it may consist almost entirely of short-celled parenchyma
(fig. 168, C, from _x_ to _sc_), or it may include a large proportion
of radially disposed and vertically elongated tracheae (fig. 168,
D, _x_², and fig. 170, A, _x_²), or it may consist of parenchyma
containing scattered groups of tracheae (fig. 169, A, _x_²)[326].
[Illustration: FIG. 169. _Lepidodendron fuliginosum._
A, B. (Manchester University Collection. No. Q. 645 A.)
B, C. (Manchester. No. 257.)
D. (Manchester. No. 6.)]
Fig. 169, A, is a diagrammatic sketch of the tissues—1 mm. wide—between
the primary xylem, _x_, and the inner cortex. The primary xylem is
succeeded by short parenchymatous cells followed by a zone of radially
elongated elements passing occasionally into rows of narrow scalariform
tracheae, some of which, owing to their sinuous longitudinal course
(fig. 171, C), are seen in oblique section, as at C, fig. 169, A. At
its outer edge this secondary tissue, _x_², consisting of parenchyma
and tracheae, passes into the cambial band (fig. 169, B, _a_).
[Illustration: FIG. 170. _Lepidodendron fuliginosum._ (From sections in
the Manchester Museum.)]
The radial longitudinal section represented in fig. 168, C, is
taken from the fossil described by Weiss as a biseriate _Halonia_;
it agrees sufficiently closely in structure with others referred
to _Lepidodendron fuliginosum_ to be classed as an example of this
anatomical type. A complete transverse section of the stem measures 9
× 6·3 cm.; the breadth of the tissues between the edge of the primary
xylem and the outer edge of the secretory zone is 2·5 mm. The middle
cortical region, characterised by the sooty appearance, which led
Williamson to choose the specific name _fuliginosum_, is traversed
by the leaf-traces and is sharply differentiated from both the inner
and outer cortex. The longitudinal section (fig. 168, C) shows the
outer edge of the primary xylem, _x_, abutting on a band of dark and
small-celled parenchyma which passes into the broad zone of secondary
tissue, _m_, the inner region of which consists of fairly thick-walled
elements in radial series passing externally into the thin-walled cells
of the cambial region, _a_, on the inner edge of the secretory zone,
_sc_. This section shows also the interruption of the secretory zone
by an outgoing leaf-trace, _lt_, the lower part of which, _sc_, is
continued downwards into the secretory zone. The exit of a leaf-trace
produces a gap in the secretory zone of the stem, but not in the xylem.
If we applied the term phloem to the secretory zone—a course adopted by
Prof. F. E. Weiss and some other authors, but which I do not propose
to follow—we should speak of a phloem foliar-gap as a characteristic
feature of a Lepidodendron shoot. This applies to other species of the
genus as well as to _L. fuliginosum_.
[Illustration: FIG. 171. _Lepidodendron fuliginosum._ (From sections in
the Manchester Museum.)]
Fig. 171, A, shows more clearly the broad zone of secondary parenchyma
with the thinner-walled cambial region, _a_; the latter is represented
on a larger scale in fig. 171, B. The section shown in fig. 168,
D, and in fig. 170, A, affords an example of a stem in which the
secondary tissue consists largely of narrow scalariform tracheae,
_x_²; the primary stele has a diameter of 1 cm.; the secondary xylem,
_x_², forms a fairly broad zone of parenchyma and tracheal elements
through which leaf-traces pass vertically, a fact of some interest in
comparison with the horizontal course which they pursue through the
medullary rays in the normal secondary wood of _L. vasculare_ and _L.
Wünschianum_. The secondary tracheae pass gradually into thin-walled
cambial cells (_a_, fig. 168, D; 170, A) with parallel tangential
walls. Fig. 171, C, shows the sinuous course of the secondary
tracheae as seen in longitudinal section, and a few small groups of
parenchymatous cells, _mr_, which may be of the nature of medullary
rays, enclosed between the winding scalariform tracheae.
[Illustration: FIG. 172. _Lepidodendron fuliginosum._ From a section
(4 × 3·4 cm.) in the Williamson Collection, British Museum (No.
379), figured by Williamson, _Phil. Trans. R. Soc._ =1881=, Pl. 52.]
The secretory zone of _Lepidodendron fuliginosum_ agrees essentially
with that of other species; it usually presents the appearance shown
in fig. 168, B, _sc_; fig. 169, B and C; fig. 170, B (longitudinal
section); fig. 171, D, _sc_. The comparatively large clear spaces which
characterise this tissue, as seen in fig. 168, B, appear to owe their
origin to groups of small cells which gradually break down and give
rise to spaces containing remnants of the disorganised elements, as in
fig. 171, D, and fig. 169, B, _b_. The secretory tissue seen in fig.
170, B, consists of large and small parenchymatous cells without any of
the broad sacs or spaces such as are shown in fig. 169, C.
Fig. 172 represents a diagrammatic sketch of a transverse section (4
× 3·4 cm. in diameter) of a young shoot from the Lower Coal-Measures
of Lancashire figured by Williamson[327] in 1881 as _Lepidodendron
Harcourtii_. It shows the features characteristic of _L. fuliginosum_
and is of importance as affording an example of a shoot giving
off a branch from the stele to supply a lateral axis of the type
characteristic of _Halonia_. The exit of the branch-stele forms a
gap in the main stele; a ramular gap as distinguished from a foliar
gap. The outgoing vascular strand is at first crescentic, but becomes
gradually converted into a solid stele. The primary xylem of the main
stele (black in the figure) consists of a ring six tracheae in breadth;
this is succeeded by a few layers of dark parenchymatous cells and a
band of radially elongated elements, _a_, which abuts on the secretory
zone. The middle lacunar cortex, _c_², with _Stigmaria_ rootlets,
_s_, is fairly well preserved. In the outer cortex occur several
leaf-traces, _lt_, accompanied by spaces originally occupied by the
parichnos strand, _p_. A band of secondary cortex, consisting chiefly
of phelloderm, is seen at _pd_. The prominent leaf-cushions, some of
which show the parichnos, _p_, appear to be of the Lepidophloios type.
• • • • •
It remains to consider the external characters of Lepidodendroid shoots
possessing the anatomical features represented by the comprehensive
species _Lepidodendron fuliginosum_.
Certain sections exhibiting this type of structure were described by
Binney in 1872 as _Halonia regularis_[328] on evidence supplied by Mr
Dawes, who stated that they were cut from a specimen bearing Halonia
tubercles. The section represented in fig. 172 is no doubt from an
Halonia axis. In 1890 Cash and Lomax[329] stated that they had in
their possession a stem of the _L. fuliginosum_ type with the external
features of _Lepidophloios_; this identification has been confirmed by
Kidston[330] and Weiss[331]. It is, however, equally clear that certain
species with the elongated leaf-cushions of _Lepidodendron_ must be
included among examples of shoots with the anatomical characters of _L.
fuliginosum_.
[Illustration: FIG. 173. _Lepidodendron obovatum._ (From a specimen
lent by Dr D. H. Scott.)]
Dr Scott[332] published in 1906 a short account of the structure
of a specimen from the Lower Coal-Measures of Lancashire, the
external features of which were identified by Kidston with those of
_Lepidodendron obovatum_ Sternb. Dr Scott generously allowed me to have
drawings made from his specimen; these are reproduced in fig. 173. The
form of the leaf-cushion is by no means perfect; there is a well-marked
median ridge, and the small circular scar near the upper end of some
of the cushions may represent the ligular cavity. At the base of the
leaf-cushions a cortical meristem has produced a zone of secondary
cortex; at _c_ a second meristem is seen in the outer cortex: the dark
dots in the cortex mark the positions of leaf-trace bundles. The inner
cortex, _d_, is a more compact tissue surrounding the imperfectly
preserved secretory zone. From the medullated stele a lateral branch,
_b_, is being given off; its crescentic form becoming changed to
circular as it passes nearer to the surface.
[Illustration: FIG. 174. _Lepidodendron aculeatum._
(Cambridge Botany School.)]
[Illustration: FIG. 175. _Lepidodendron aculeatum._
(Cambridge Botany School.)]
A type of _Lepidodendron_, _L. Hickii_, founded on anatomical
characters by Mr Watson[333], is believed by him to possess
leaf-cushions like those of _L. obovatum_; if this is so, it is
interesting, as he points out, to find two distinct anatomical types
associated with one species. Watson thinks it probable that the
“species” _L. obovatum_ includes at least two widely different species.
This merely emphasizes the importance of correlating structure and
external characters as far as available data permit.
[Illustration: FIG. 176. _Lepidodendron aculeatum._
(Cambridge Botany School.)]
The specimen, of which part of the surface is shown in fig. 174, is
in all probability _L. aculeatum_ Sternb. This was described by me
in detail in _The Annals of Botany_ (1906) as another example of the
co-existence of the _Lepidodendron fuliginosum_ type of anatomy with
a true _Lepidodendron_. The locality of the specimen is not known.
The leaf-cushions are 1·5 cm. long with tapered upper and lower ends;
a ligular cavity may be recognised on some parts of the fossil,
also faint indications of leaf-trace scars. The tubercles (fig.
174, A–C, _t_) probably represent leaf-traces which the shrinkage
of the superficial tissues has rendered visible in the lower part
of their course. The circular scar, _s_ (fig. B), on the partially
decorticated surface is apparently a wound. The stele is sufficiently
well preserved to justify its reference to _L. fuliginosum_. The
irregularly crenulated edge of the primary xylem, _x_ (fig. 175),
is succeeded by a broad band of parenchyma (the meristematic zone),
_m_, and beyond this are remnants of the secretory zone, _s_. The
structure of the leaf-traces corresponds with that of other specimens
of the type, but the much steeper course of these vascular strands,
_lt_, _lt′_ (fig. 176), is a feature in which this example differs
from most of those referred to _L. fuliginosum_. Such evidence as is
available would seem to point to the absence of trustworthy criteria
enabling us to separate, on anatomical grounds, _Lepidophloios_ and
_Lepidodendron_[334].
[Illustration: FIG. 177. _Stigmaria radiculosa_ (Hick). (From sections
in the Manchester University Collection.)]
_Stigmaria radiculosa_ (Hick).
We have no proof of the nature of the subterranean organs of
_Lepidodendron fuliginosum_, though it is not improbable that the
specimens described below may be correctly assigned by Weiss to that
species. Prof. Weiss[335] has made an interesting contribution to
our knowledge of a type first described by Hick[336] under the name
_Tylophora radiculosa_, a designation which he afterwards altered
to _Xenophyton radiculosum_[337] and for which we may now substitute
_Stigmaria radiculosa_ (Hick). Prof. Williamson expressed the opinion
that _Xenophyton_ exhibited considerable affinity with _Stigmaria
ficoides_ and Weiss’s further study of the species leads him to regard
Hick’s plant as probably the Stigmarian organ of _Lepidodendron
fuliginosum_. The diagrammatic transverse section represented in fig.
177, A (4·5 cm. in diameter), shows an outer cortex of parenchyma,
_c_³, consisting in part of radial rows of secondary tissue and of a
band of compact parenchyma bounded by the wavy line _a_; at _sc_ is
a series of secretory strands exactly like those in a corresponding
position in _Lepidodendron fuliginosum_ and other species of the genus.
The greater part of the organ is occupied by a lacunar and hyphal
middle cortex identical in structure with that shown in fig. 178, B,
drawn from a rootlet. At _d_, fig. 177, A, the middle cortex has been
invaded by a narrow tongue of outer cortical tissue. The stele is
characterised by a large pith filled with parenchyma; in _Stigmaria
ficoides_[338] the general absence of pith-tissue has led to the
inference that the stele was hollow. The xylem is represented by a ring
of bundles separated by broad medullary rays; each bundle contains a
few small, apparently primary, elements on its inner edge but is mainly
composed of radial rows of secondary tracheae _x_², fig. 177, B. On
the outer face of the secondary xylem occur a few smaller and thinner
walled cells, _c_, having the appearance of meristematic tissue; from
these additional tracheae were added to the xylem. This meristematic
zone occurs, as in the stems of _Lepidodendron_, immediately internal
to the secretory tissue, sc; at _c_¹, fig. 177, B, is seen the inner
cortical tissue.
[Illustration: FIG. 178. _Rootlet of Stigmaria._ (From a section in the
Manchester Collection.)]
In surface-view a specimen figured by Hick[339] shows a number of
circular scars agreeing in shape and arrangement with the rootlet scars
of _Stigmaria ficoides_. At _b_ in fig. 177, A, the basal portion of
a rootlet is shown in organic connexion with the outer cortex. The
rootlet-bundles are given off from the stele as in other examples of
_Stigmaria_; each bundle consists of a triangular strand of xylem with
an endarch protoxylem at the narrow end accompanied by a portion of
the secretory tissue as in the leaf-traces. As in _Stigmaria ficoides_
the rootlets are attached to the outer cortex above a cushion of small
cells. It is interesting to find that rootlet-bundles, as seen in
tangential section of the main axis, are associated with a parichnos
strand, but this is on the xylem side of the vascular strand, whereas
in the case of leaf-traces the parichnos is on the other side of the
bundle.
Fig. 178, A, represents a transverse section of a rootlet (6 mm.
in diameter) associated with _Stigmaria radiculosa_ and probably
belonging to this species. The xylem strand _x_ is composed of a group
of tracheae with a single protoxylem strand, _px_, at the pointed end
and with small metaxylem elements at the broad end next the space
originally occupied by the so-called phloem. A parenchymatous sheath,
_c′_, surrounds the bundle, and beyond this is the broad middle cortex,
a small portion of which is shown on a larger scale in fig. 178, B; as
Weiss points out, some of the outermost cells of the lacunar cortex
(_m_) are clearly in a state of meristematic activity.
The preservation of the middle cortex and the small quantity of
secondary xylem are characters which this _Stigmaria_ shares with
_Lepidodendron fuliginosum_, and although decisive evidence is still to
seek, we may express the opinion that Weiss’s surmise of a connexion
between _Stigmaria radiculosa_ and _Lepidodendron fuliginosum_ is
probably correct.
5. _Lepidodendron Harcourtii._ Fig. 179, A–D.
In 1831 Mr Witham[340] published an anatomical description of a
fragment of a _Lepidodendron_ which he named _Lepidodendron Harcourtii_
after Mr C. G. V. Vernon Harcourt from whom the specimen was originally
obtained. The fossil was found in rocks belonging to the Calciferous
series in Northumberland. Witham reproduced the account of this
species in his classic work on _Fossil Vegetables_[341], and Lindley
and Hutton[342], who examined Mr Harcourt’s material, published a
description of it in their _Fossil Flora_ in which they expressed
the view that _Lepidodendron_ is intermediate between Conifers and
Lycopods. Adolphe Brongniart[343] included in his memoir on _Sigillaria
elegans_ an account of Witham’s species based on material presented to
the Paris Museum by Mr Hutton and Robert Brown. Dr Kidston[344] has
shown that the actual transverse section figured by Witham is now in
the York Museum; a piece of stem in the same Museum, which is not the
specimen from which Witham’s section was cut, supplied the transverse
section figured by Brongniart. The figures given by Lindley and Hutton
do not appear to have been made from the York specimens. In 1887
Williamson[345] published a note in which he pointed out that some of
the specimens described by him as _L. Harcourtii_ should be transferred
to a distinct species, which he named _L. fuliginosum_. Subsequently
in 1893 he gave a fuller account of Witham’s species; it has, however,
been shown by Dr Kidston and by Mr Watson[346] that certain specimens
identified by Williamson as _L. Harcourtii_ differ sufficiently from
that type to be placed in another species, for which Watson proposes
the name _L. Hickii_.
A paper on _L. Harcourtii_ published by Bertrand[347] in 1891 extends
our knowledge of this type in regard to several anatomical details.
It was recognised by Williamson that the absence of secondary wood
in shoots possessing the anatomical characters of _L. Harcourtii_
is a feature to which no great importance should be attached. It is
possible that the large stems from the Isle of Arran described by
Williamson[348] as _Lepidodendron Wünschianum_, in which the secondary
wood is well developed, may be specifically identical with the smaller
specimens from Northumberland and elsewhere which are recognised as
examples of Witham’s type.
The diagrammatic sketch shown in fig. 179, A, was made from a section
figured by Williamson in 1893[349]; it has a diameter of 9 × 8·5 cm.
The stele is of the medullated type like that of _L. Wünschianum_,
and the outer edge of the primary xylem is characterised by sharp and
prominent projecting ridges similar to those of _L. fuliginosum_ but
rather more prominent. Parenchymatous cells succeed the xylem, as in
other species, but in this case there is no indication of meristematic
activity; beyond this region occur occasional patches of a partially
destroyed secretory zone. Remains of a lacunar tissue are seen in the
middle cortical region; also numerous leaf-traces, _lt_, consisting
of a tangentially elongated xylem strand accompanied by a strand of
secretory zone tissue enclosed in a sheath of delicate parenchyma. In
the inner part of the outer cortex, _c_³, the leaf-traces lie in a
space originally occupied by the parichnos; in the outer portion of
the same region a band of secondary cortex, _pd_, has been formed;
immediately internal to this occur numerous patches of secretory
tissue, represented by small dots in the drawing close to _pd_; one is
shown on a larger scale in fig. B.
The position of the phellogen is seen at _a_; external to this are
radial rows of rather large cells with dark contents.
[Illustration: FIG. 179.
A–D. _Lepidodendron Harcourtii_, Witham.
E. _Lepidodendron fuliginosum_, Shore, Lancashire.
A, B. From a specimen in the Williamson Collection, British Museum
(No. 380), from Airdrie, Scotland.
C, D. From sections in the Collection of Dr Kidston, from Shore,
Lancashire.]
Fig. 179, C, _x_, shows the characteristic form of the primary xylem
edge, beyond which are seen oval or circular leaf-traces with a mesarch
protoxylem, _lt_, _px_. It is possible that this specimen may not
be specifically identical with Witham’s species, but it represents
a very similar if not identical type; it may on the other hand be
referable to _L. fuliginosum_. The importance of the specimen, apart
from its precise specific position, is that it serves to illustrate the
general appearance of the xylem surface met with in both species, _L.
Harcourtii_ and _L. fuliginosum_. A tangential longitudinal section,
taken through the line _ab_ in fig. C, is represented in fig. 179, D.
The xylem of the leaf-traces _lt_, consisting chiefly of scalariform
tracheae, alternates with patches of crushed and delicate parenchyma
which immediately abut on the primary xylem; at _p_, _p_, the section
passes through some of the projecting arms of the xylem cylinder; at
_m_ is seen a patch of meristematic zone tissue. This section together
with the similar section of _Lepidodendron vasculare_ described on a
previous page demonstrates that the projecting ridges of the primary
xylem form apparently vertical bands: they are not characterised by a
lattice-work arrangement as described by Bertrand and by other authors
who have accepted his conclusions. If a reticulum of intersecting
ridges were present on the face of the xylem cylinder its existence
would be revealed by such a section as that represented in fig. 179, D.
[Illustration: FIG. 180. _Lepidodendron Wünschianum._ From Arran. (⅕
nat. size.) (Sedgwick Museum, Cambridge.)]
6. _Lepidodendron Wünschianum_ (Williamson). Figs. 180–184.
Reference was made in Volume I. to the occurrence of large stems of
a Lepidodendron in volcanic beds of Calciferous sandstone age in the
island of Arran[350]. These were discovered and briefly described by Mr
Wünsch in 1867[351] and afterwards named by Carruthers _Lomatophloyos
Wünschianus_[352]. Mr Carruthers visited the locality and published
an account of the peculiar method of preservation of the plant
remains[353]. It is, however, to Williamson[354] that we owe the more
complete description of these Arran stems. Portions of large stems
from the Arran beds are preserved in the British Museum, the Sedgwick
Museum, Cambridge, and in the Manchester Museum. The section of one of
these is shown in fig. 180; an outer shell of bark encloses a mass of
volcanic ash in which are embedded several woody cylinders originally
described as “internal piths[355],” and by Carruthers as young stems
produced from spores which had germinated in the hollow trunk of a
large tree. The true interpretation was supplied by Williamson who
showed that a stem of the dimensions of that represented by the outer
cortex, _e_, fig. 180, must have possessed a single stele of the size
of those seen in the interior of the hollow trunk. The additional woody
cylinders, or steles, were derived from other stems, and carried,
probably by water, into the partially decayed trunk. In addition to
large Lepidodendron stems Williamson described smaller shoots as well
as an Halonial branch and made brief reference to some cones described
by Binney[356] in 1871 from the same locality.
The following account of _Lepidodendron Wünschianum_ is based on an
exceptionally fine specimen discovered by Mr T. Kerr of Edinburgh in
Calciferous sandstone volcanic ashes at Dalmeny in Linlithgowshire. The
material from this locality described by Mr Hill and myself[357] was
generously placed in my hands by Dr Kidston of Stirling. Fig. 181, A,
shows a transverse section, 33 cm. in diameter, consisting of a shell
of outer cortical tissue enclosing a core of light-coloured volcanic
ash; on the decay of the more delicate middle cortex the cylindrical
stele dropped to one side of the hollow trunk. The stele, fig. 182, has
a diameter of 6·5 cm.; the centre is occupied by concentric layers of
silica, _s_, surrounded externally by the remains of a parenchymatous
pith, _p_, made up of isodiametric and sinuous hypha-like elements
like those in the middle cortex of Lepidodendron shoots. On the inner
edge of the primary xylem, _x′_, occur several isodiametric tracheae
with fine scalariform and reticulate thickening bands like those in
the central region of the stele of _Lepidodendron vasculare_: it is
probable that these elements are vestiges of conducting tissue which in
ancestral forms formed a solid and not a medullated stele.
[Illustration: FIG. 181. _Lepidodendron Wünschianum._ Calciferous
Sandstone, Dalmeny. (A, Sedgwick Museum, Cambridge. B–F, Botany
School, Cambridge.)]
[Illustration: FIG. 182. _Lepidodendron Wünschianum._ The stele of
the stem shown in fig. 181, A. (Cambridge Botany School.)]
The primary xylem is limited externally by an unequally fluted surface
with exarch protoxylem elements; it is, however, noteworthy that there
is not always a very clearly defined difference between the small
protoxylem and the large centripetally developed tracheae. Immediately
beyond the primary xylem occur numerous thin-walled parenchymatous
cells with spiral and reticulate pitting; beyond these is the broad
zone of secondary xylem, _x_², composed of scalariform tracheae and
numerous medullary rays consisting of one, two, or several rows of
radially elongated elements with spiral and reticulate pitting. In
tangential sections the rays are seen to vary considerably in size,
some being made up of a single row of cells while others are longer and
broader; through the latter leaf-traces pass horizontally. Portions of
medullary rays are seen at _mr_ in fig. 181, C and E.
The leaf-traces given off from projecting ridges on the outer edge
of the primary xylem pass upwards for a short distance and then bend
outwards through a broad medullary ray; on reaching the limit of
the secondary xylem they again bend sharply upwards, appearing in
transverse section at _lt_ fig. 181, B. Each leaf-trace consists at
first of long tracheae accompanied by numerous thin-walled spiral
and reticulate parenchymatous cells derived from the tissue in
contact with the outer edge of the primary wood. Fig. 181, B, shows
a leaf-trace near the edge of the secondary xylem; it consists of a
group of primary tracheae, with narrower protoxylem elements, _px_,
near the outer margin, almost completely enclosed by radially disposed
series of smaller and more delicate tracheae. These secondary elements
of the leaf-trace are apparently added during its passage through
the medullary ray, but additions are also made to this tissue by
the meristematic zone, _m_, fig. 181, B and E. In contact with the
outermost tracheae of normal size at the edge of the secondary xylem
there are some smaller lignified elements, as at _a_, fig. 181, E, and
at T, fig. 183; this juxtaposition of large and small tracheae has been
referred to in the description of _L. vasculare_.
Prof. Williamson[358], in his account of the Arran specimens of this
species, expressed the opinion that the trees probably perished “in
consequence of the mephitic vapours which filled the atmosphere”; it
maybe that in the striking difference in the diameter of the conducting
elements on the margin of the wood we have evidence of approaching
death.
Beyond the most recently formed tracheae we have a band of delicate
parenchymatous cells (_m_, figs. B and E, 181; C, figs. 183, 184)
forming the meristematic zone[359]. The longitudinal section
represented in fig. 184 shows some recently formed narrow tracheae, T,
and beyond these the meristematic zone composed of thin-walled short
cells, C, arranged in horizontal rows. It is this small-celled tissue
to which the name phloem has been applied by some authors[360], a
term which seems to me to be misleading and inappropriate. In passing
through this zone of dividing cells the leaf-traces become surrounded
by an arc of meristem from which elements are added to the radially
placed rows of secondary tracheae. Beyond the meristematic region
portions of the secretory zone are preserved, consisting of large
sacs or spaces and small dark cells as seen in figs. 181, B, E, _sc_,
F; 183, 184. This tissue has the same structure as in _L. vasculare_
and in _L. fuliginosum_: it is a striking fact that there are no
indications of any additions to the secretory zone even in stems with
such a large amount of secondary xylem as in the Dalmeny specimen (fig.
182, _x_²). If the secretory zone were of the nature of phloem we
should expect to see signs of additions made to it in the course of
growth. In this connexion it is worth mentioning that in the recent
fern _Botrychium_ (Ophioglossaceae) secondary xylem is formed in the
stem, but apparently no additions are made to the phloem. The structure
of the secretory zone tissue as seen in the longitudinal section
fig. 184, S, is also a serious difficulty in the way of accepting
the designation phloem as employed by Scott and Weiss. Between the
secretory zone and the outer cortical region, no tissues have been
preserved. The shell of bark consists chiefly of radial rows of
elongated cells with rather thick walls characterised by the occurrence
of small intercellular spaces and by tangentially placed bands of
secretory cells and sacs (fig. 181, D, _s_). Immediately internal to
the secondary cortex or phelloderm occur groups of secretory tissue as
shown in the section of _L. Harcourtii_ (fig. 179, B).
[Illustration: FIG. 183. _Lepidodendron Wünschianum._ From the
specimen shown in fig. 181, S, secretory zone; C, meristem; T,
immature tracheae.]
[Illustration: FIG. 184. _Lepidodendron Wünschianum._ Longitudinal
section of the specimen represented in transverse section in fig.
183.]
The large tree shown in transverse section in fig. 181, A, has
lost its leaf-cushions; the bark, as seen in the lower part of the
photograph, presents a fissured appearance like that with which we
are familiar on an old Oak or Elm stem. A radial longitudinal section
through the phelloderm revealed the existence of a crushed leaf-trace
passing outwards in an approximately horizontal course accompanied
by a strand of parenchymatous tissue[361] having the characteristic
structure of a parichnos. It is probable that the surface of this
partially decorticated stem differed in appearance from that of an old
_Sigillaria_ (cf. fig. 198) in the much smaller and less conspicuous
parichnos strands.
In addition to the large stems of _L. Wünschianum_ from Arran and
Dalmeny numerous examples of smaller axes from the former locality are
represented in the Williamson collection (British Museum). Some of
the twigs are characterised by a solid stele (protostele) giving off
numerous leaf-traces accompanied by short spirally thickened tracheids
like those which occur at the outer edge of the primary xylem in
the larger stem: these extend into the leaf where they are arranged
round the vascular bundle like the transfusion tracheids[362] in many
recent conifers. The surface of these smaller shoots bears large
leaf-cushions which are seen in longitudinal section to have the form
characteristic of _Lepidophloios_. It is worthy of note that a section
of a bifurcating axis of this species from the Calciferous Sandstone of
Craigleith (British Museum Collection[363]), although its diameter is
19 × 14 cm., shows no signs of secondary wood. This late appearance of
secondary xylem and other anatomical features suggest the possibility
of the specific identity of _L. Wünschianum_ and _L. Harcourtii_[364].
In 1871 Binney[365] described a specimen of a heterosporous cone,
_Lepidostrobus Wünschianus_, from Arran exhibiting the ordinary
features of lepidodendroid strobili; this was probably borne by
_Lepidodendron Wünschianum_.
7. _Lepidodendron macrophyllum_ (Williamson). Fig. 186, C.
The diagrammatic sketch reproduced in fig. 186, C, was made from the
transverse section of a small twig, slightly less than 2 cm. in its
longest diameter, originally figured by Williamson[366] in 1872.
Earlier in the same year Carruthers[367] published a short account
of the same form based on specimens collected by Mr Butterworth
from the Coal-Measures of Lancashire near Oldham, but both authors
refrained from instituting a new specific name. In a later publication
Williamson spoke of the type as _Lepidodendron macrophyllum_[368].
Williamson’s species has nothing to do with _Lycopodites macrophyllus_
of Goldenberg[369]. The most striking feature of this rare form is
the large size of the leaf-cushions, which are of the _Lepidophloios_
type, in proportion to the diameter of the shoot. The stele consists
of a ring of xylem, all of which is primary in the sections so far
described, enclosing a parenchymatous pith: a Stigmarian rootlet is
shown at _s_.
8. _Lepidodendron Veltheimianum_ Sternb. (General account). Figs. 157,
185, 186, A, B.
1820. “Schuppenpflanze,” Rhode, Beit. zur Pflanzenkunde der
Vorwelt, Pl. III. fig. 1.
1825. _Lepidodendron Veltheimianum_, Sternberg, Flora der Vorwelt,
Pl. LII. fig. 5.
1836. _Pachyphloeus tetragonus_, Goeppert, Die fossilen
Farnkräuter, Pl. XLIII. fig. 5.
1852. _Sagenaria Veltheimiana_, Goeppert, Foss. Flora des
Übergangsgebirges, Pls. XVII–XXIV.
1875. _Lepidodendron Veltheimianum_, Stur, Culm Flora, p. 269, Pls.
XVIII–XXII.
1886. _Lepidodendron Veltheimianum_, Kidston, Catalogue of
Palaeozoic plants, British Museum, p. 160.
1901. _Lepidodendron Veltheimianum_, Potonié, Silur und Culm Flora,
p. 116, figs. 72–76.
1904. _Lepidodendron Veltheimianum_, Zalessky, Mém. Com. Géol.
Russie, Pl. IV. figs. 4, 5.
1906. _Lepidodendron Veltheimi_, Potonié, Königl. Preuss. geol.
Landesanstalt, Lief. III.
The above list may serve to call attention to a few synonyms[370]
of this plant, and to a selection of sources from which full
information may be obtained as to the history of our knowledge of this
characteristic and widely spread Lower Carboniferous type.
_Lepidodendron Veltheimianum_ is represented by casts of stems,
the largest of which hitherto described reaches a length of 5·22
metres with a maximum diameter of 63 cm.; this specimen, figured
by Stur[371], consists of a tapered main axis giving off smaller
lateral shoots, some of which exhibit dichotomous branching. Fig.
185, C and D, represent the external features of a well-preserved
cast and impression respectively. Oblique rows of prominent cushions
wind round the surface of the stem and branches: each cushion is
prolonged upwards and downwards in the form of a narrow ridge with
sloping sides which connects adjacent cushions by an ogee curve. At
the upper limit of the broader kite-shaped portion of the cushion
the ligular pit forms a conspicuous feature; immediately below this
is the leaf-scar with its three small scars,—the lateral parichnos
strands and the central leaf-trace. The two oval areas shown in fig.
185, D, just below the lower edge of the leaf-scars, represent the
parichnos arms which impinge on the surface of the cushions on their
way to the leaves, as explained on a previous page. It is possible
that these areas were visible on the living stem as strands of loose
parenchyma comparable with the lenticel-like pits on the stipules of
_Angiopteris_[372] and the leaf-bases of Cyatheaceous ferns, or it may
be that their prominence in the specimen before us is the result of
the decay of a thin layer of superficial cortex which hid them on the
living tree. Fig. 185, B, illustrates the appearance of a stem in a
partially decorticated condition (_Bergeria_ state). A further degree
of decortication is seen in fig. 185, A, which represents the _Knorria_
condition.
[Illustration: FIG. 185. _Lepidodendron Veltheimianum._ From specimens
in Dr Kidston’s Collection. (Approximately nat. size.)]
Fig. 157 shows a Ulodendron axis of this species; in the lower part the
specimen illustrates the partial obliteration of the surface features
as the result of the splitting of the outer bark consequent on growth
in thickness of the tree. By an extension of the cracks, shown in an
early stage in fig. 157, the leaf-cushions would be entirely destroyed
and the surface of the bark would be characterised by longitudinal
fissures simulating the vertical grooves and ridges of a Sigillarian
stem. The large stumps of trees shown in the frontispiece to Volume I.
are probably, as Kidston[373] suggests, trunks of _L. Veltheimianum_ in
which the leaf-cushions have been replaced by irregular longitudinal
fissures. In old stems of _Sigillaria_ the enlarged parichnos areas
constitute a characteristic feature (p. 205), but it does not follow
that the absence of large parichnos scars is a distinguishing feature
of all _Lepidodendra_.
In this species, as in others, the form of the leaf-cushion exhibits
a considerable range of variation dependent on the thickness of the
shoot; the contiguous cushions of young branches become stretched apart
as the result of increasing girth of the whole organ, and casts of
still older branches may exhibit very different surface-features[374].
The leaves as seen on impressions of slender branches are comparatively
short, reaching a length of 1–2 cm. It is important to notice that
leafy twigs of this species may bear terminal cones[375] resembling
in form those of _Picea excelsa_ and other recent conifers, though
differing essentially in their morphological features.
The fossil stumps of trees represented in the frontispiece to Volume
I. bear horizontally spreading and dichotomously branched root-like
organs having the characters of _Stigmaria ficoides_[376]. Geinitz has
suggested that _Stigmaria inaequalis_ Göpp. may be the underground
portion of _Lepidodendron Veltheimianum_.
It is unfortunately seldom possible to connect petrified
_Lepidodendron_ cones with particular species of the genus based on
purely vegetative characters, but it is practically certain that we are
justified in recognising certain strobili described by Williamson[377]
from the Calciferous Sandstone series of Burntisland on the Firth of
Forth as those of _Lepidodendron Veltheimianum_. Williamson believed
that the cone which he described belonged to the plant with shoots
characterised by the anatomical features of his species _Lepidodendron
brevifolium_ (= _L. Veltheimianum_), a conclusion which is confirmed by
Kidston[378]. The cone of _L. Veltheimianum_, which reached a diameter
of at least 1 cm. and a length of 4 cm., agrees in essentials with
other species of _Lepidostrobus_; the axis has a single medullated
stele of the same general type as that of the vegetative shoots of
_Lepidodendron fuliginosum_ and _L. Harcourtii_. The sporophylls are
described by Williamson as spirally disposed, and Scott notices that
in some specimens they are arranged in alternate whorls; as in recent
Lycopods both forms of phyllotaxis may occur in the same species. The
heterosporous nature of this strobilus, to which Scott first applied
the name _Lepidostrobus Veltheimianus_, is clearly demonstrated by the
two longitudinal sections contributed by Mr Carruthers and figured by
Williamson in 1893[379].
Each sporophyll, attached almost at right angles to the cone-axis,
bears a radially elongated sporangium seated on the median line of
its upper face; its margins are laterally expanded as a thin lamina;
from the middle of the lower face a narrow keel extends downwards
between two sporangia belonging to a lower series. From the base of a
sporangium a mass of sterile tissue penetrates into the spore-producing
region as in the large sporangia of _Isoetes_ (cf. fig. 191, H, _a_,
and fig. 133, H). The distal and free portion of the sporophylls is
bent upwards as a protecting bract. Some of the sporangia in the upper
part of the cone produced numerous microspores, while 8–16 megaspores
occur in the lower sporangia. The megaspores, having a mean diameter
of 0·8 mm. “quite 40 times the size of the microspores[380],” are
characterised by tubular capitate appendages, and by a conspicuous
three-lobed projection (fig. 191, E)[381] which, as Scott suggests,
may represent the outer spore-wall which has split as the result of
germination. It is not improbable, as shown in fig. 191, I, that this
cap was present before germination. The megaspores represented in fig.
191, I, illustrate their characteristic form as seen in a section of a
megasporangium, _Sm_; the open beak-like portion of the larger spore is
probably the apical region which has split along the three-rayed lines.
These lines form a characteristic feature of both recent and extinct
spores and denote their origin in tetrads. The spore shown in fig. 191,
E[382], illustrates the external features. The apical region of the
prothallus of a megaspore of _Lepidodendron Veltheimianum_ described
by Mr Gordon[383] consists of smaller cells than those occupying the
greater part of the spore-cavity, a differentiation which he compares
with that of the prothallus of _Selaginella_.
[Illustration: FIG. 186.
A, B. _Lepidodendron Veltheimianum._ (Botany School, Cambridge.)
C. _Lepidodendron macrophyllum._ (British Museum. No. 377.)
_x_, Primary xylem; _x_², secondary xylem; _s_, Stigmarian rootlet.]
There can be little doubt that the petrified shoots described by
Williamson[384] from the Calciferous Sandstone beds of Burntisland as
_Lepidodendron brevifolium_ are identical with specimens possessing
the external features of _L. Veltheimianum_. In 1872 Dawson expressed
the opinion that Williamson’s species should be referred to _L.
Veltheimianum_, and evidence subsequently obtained confirms this
view. The stele of this species is of the medullated type, differing
from that of _L. fuliginosum_ and _L. Harcourtii_ in the absence of
prominent ridges on the external surface of the primary xylem, and from
_L. vasculare_ in the possession of a parenchymatous pith. In younger
twigs the cortex consists of fairly homogeneous tissue, but in older
branches there is a greater distinction between a delicate middle
cortex and a stronger outer cortex. Fig. 186, A, represents a stem
in which the vascular cylinder is composed of a primary xylem ring,
_x_, 1·5 mm. broad, succeeded by a zone of secondary wood 1·2 cm. in
breadth. The junction between the primary and secondary xylem is shown
on a larger scale in fig. 186, B. The tissues abutting on the secondary
xylem have not been preserved; the outer cortex, which consists
chiefly of secondary elements, is divided superficially into unequal
ridges corresponding to the leaf-cushions which have been more or less
obliterated as the result of growth in thickness of the stem.
9. _Lepidodendron Pedroanum_ (Carruthers).
In 1869 Mr Carruthers described some specimens of vegetative stems and
isolated sporangia, collected by Mr Plant in Brazil, as _Flemingites
Pedroanus_[385]. From a more recent account published by Zeiller[386]
it is clear that Carruthers’ species is a true _Lepidodendron_; an
examination of the type-specimens in the British Museum confirms this
determination. The contiguous leaf-cushions have rounded angles similar
in form to those of _Lepidodendron Veltheimianum_ and _L. dichotomum_,
but it is not unlikely that the Brazilian plant is specifically
distinct from European species. A figure of one of the specimens on
which Carruthers founded the species is given by Arber[387] in his
_Glossopteris Flora_. The Brazilian plant is chiefly interesting as
affording proof of the existence of _Lepidodendron_ in the southern
hemisphere; the species has also been recognised in South Africa from
material collected by Mr Leslie at Vereeniging[388].
As Zeiller[389] has suggested, it is not improbable that the fossils
described by Renault[390] from Brazil as _Lycopodiopsis Derbyi_ may be
the petrified stems of _Lepidodendron Pedroanum_. The structure of the
central cylinder of Renault’s species is of the type represented by _L.
Harcourtii_; the xylem forms a continuous ring and does not consist of
separate strands of tracheae as Renault believed.
10. _Lepidodendron australe_ (M’Coy). Figs. 187, A–C.
Specimens described under this name are interesting rather on account
of their extended geographical range and geological antiquity than
on botanical grounds. The drawings reproduced in fig. 187 illustrate
the characteristic appearance of this Lower Carboniferous and Upper
Devonian type, as represented by a specimen recently described[391]
from the Lower Karroo (Dwyka) series, which is probably of
Carboniferous age, near Orange River Station, South Africa. The surface
is divided into polygonal or rhomboidal areas (figs. A and B) 8–9 mm.
long and 7–8 mm. broad, arranged in regular series and representing
leaf-scars, comparable with those of _Sigillaria Brardi_ and other
species, or possibly partially decorticated leaf-cushions. A short
distance below the apex of each area there is a more or less circular
prominence or depression (fig. 187, B) and on a few of the areas there
are indications of a groove (fig. A, _g_) extending from the raised
scar to the pointed base, as at _g_, _g_.
[Illustration: FIG. 187. _Lepidodendron australe._ Fig. A, nat. size.]
In examining the graphitic layer on the surface of the South African
specimen shown in fig. 187, A, use was made of a method recently
described by Professor Nathorst[392]. A few drops of collodion were
placed on the surface, and after a short interval the film was removed
and mounted on a slide. The addition of a stain facilitated the
microscopic examination and the drawing of the collodion film. The
cell-outlines (fig. 187, C) on the surface of the polygonal areas
may be those of the epidermis, but they were more probably formed by
a subepidermal tissue; the scar, which interrupts the continuity of
the flat surface, may mark the position of a leaf-base, or, assuming
a partial decortication to have occurred prior to fossilisation, it
may represent a gap in the cortical tissue caused by the decay of
delicate tissue which surrounded the vascular bundle of each leaf in
its course through the cortex of the stem. If the impression were
that of the actual surface of a _Lepidodendron_ or a _Sigillaria_,
we should expect to find traces of the parichnos appearing on the
leaf-scar as two small scars, one on each side of the leaf-bundle.
In specimens from Vereeniging described in 1897[393] as _Sigillaria
Brardi_, which bear a superficial resemblance to that shown in fig. A,
the parichnos is clearly shown. On the other hand, an impression of a
partially decorticated Lepidodendroid stem need not necessarily show
the parichnos as a distinct feature: owing to its close association
with the leaf-trace in the outer cortex, before its separation in the
form of two diverging arms, it would not appear as a distinct gap apart
from that representing the leaf-bundle. The absence of the parichnos
may be regarded as a point in favour of the view that the impression
is that of a partially decorticated stem. Similarly, the absence of
any demarcation between a leaf-cushion and a true leaf-scar such as
characterises the stems of Lepidodendra and many Sigillariae is also
favourable to the same interpretation.
In 1872 Mr Carruthers[394] described some fossils from Queensland,
some of which appear to be identical with that shown in fig. 187
under the name _Lepidodendron nothum_, Unger[395], a species founded
on Upper Devonian specimens from Thuringia. The Queensland plant is
probably identical with Dawson’s Canadian species, _Leptophloeum
rhombicum_[396]. In 1874 M’Coy[397] instituted the name _Lepidodendron
australe_ for some Lower Carboniferous specimens from Victoria,
Australia: these are in all probability identical with the Queensland
fossils referred by Carruthers to Unger’s species, but as the identity
of the German and Australian plants is very doubtful[398] it is better
to adopt M’Coy’s specific designation.
Krasser[399] has described a similar, but probably not specifically
identical, type from China; from Devonian rocks of Spitzbergen
Nathorst[400] has figured, under the name _Bergeria_, an example of
this form of stem, and Szajnocha[401] has described other specimens
from Lower Carboniferous strata in the Argentine.
_Lepidodendron australe_ has been recorded from several Australian
localities[402] from strata below those containing the genus
_Glossopteris_ and other members of the Glossopteris, or, as it has
recently been re-christened, the Gangamopteris[403] Flora.
viii. _Fertile shoots of_ Lepidodendron.
A. _Lepidostrobus._
The generic name _Lepidostrobus_ was first used by Brongniart[404]
for the cones of _Lepidodendron_, the type-species of the genus being
_Lepidostrobus ornatus_, the designation given by the author of the
genus to a Lepidostrobus previously figured by Parkinson[405] in his
_Organic Remains of a Former World_. The generic name _Flemingites_
proposed by Carruthers[406] in 1865, under a misapprehension as to
the nature of spores which he identified as sporangia, was applied
to specimens of true _Lepidostrobi_. Brongniart also instituted the
generic name _Lepidophyllum_ for detached leaves of _Lepidodendron_,
both vegetative and fertile; the specimen figured by him in 1822 as
_Filicites_ (_Glossopteris_) _dubius_[407], and which was afterwards
made the type-species of the genus, was recognised as being a portion
of the lanceolate limb of a large single-veined sporophyll belonging to
a species of _Lepidostrobus_.
In an unusually large _Lepidophyllum_, or detached sporophyll of
_Lepidostrobus_, in the Manchester University Museum, the free laminar
portion reaches a length of 8 cm.
It is not uncommon to find _Lepidodendron_ preserved in the form of a
shell of outer cortex, which has become separated along the phellogen
from the rest of the stem; as the result of compression the cylinder
of bark may assume the appearance of a flattened stem covered with
leaf-cushions. A specimen preserved in this way was described by
E. Weiss as a cone of _Lomatophloios macrolepidotus_ Gold., and is
quoted by Solms-Laubach and other authors[408] as an example of an
unusually large _Lepidostrobus_. An examination of the type-specimen
in the Bergakademie of Berlin convinced me that Weiss had mistaken
the partially destroyed leaf-cushions for sporophylls, and Stigmarian
rootlets, which had invaded the empty space, for sporangia[409].
In external appearance some species of _Lepidostrobus_ bear a
superficial resemblance to the cone of a Spruce Fir (_Picea excelsa_),
but the surface of a lycopodiaceous strobilus is usually covered by
the overlapping and upturned laminae which terminate the more or less
horizontal sporangium-bearing portion of the sporophyll.
Fig. 188 affords a good example of a long and narrow _Lepidostrobus_.
This specimen from the Middle Coal-Measures of Lancashire has a length
of 23 cm.; like other _Lepidostrobi_ it is borne at the tip of a
slender shoot. The fossil is sufficiently well preserved to show the
characteristic radially elongated form of the large sporangia and the
long and upturned distal portions of the sporophylls.
We may briefly describe _Lepidostrobus_ as follows:—Cylindrical
strobili consisting of an axis containing a single cylindrical stele
which agrees generally with that of the vegetative shoots of _L.
Harcourtii_ and other species. The amount of parenchymatous pith
varies in different forms; in some the primary xylem is almost solid.
The middle cortical region, which has usually been destroyed before
fossilisation, possesses the loose lacunar structure characteristic
of this region in the vegetative branches. The thicker walled outer
cortex is continued at the periphery into crowded, usually spirally
disposed sporophylls, each of which consists of a more or less
horizontal pedicel, which may be characterised by a keel-like median
ridge on its lower surface, while to the central region of the upper
face is attached a large radially elongated sporangium. One of the
chief differences between a _Lepidodendron_ cone and those of the
recent genus _Lycopodium_ is the greater radial elongation of the
sporangia in the former. Some species of _Lepidostrobus_ may have been
homosporous; some are known to be heterosporous. In the latter the
megasporangia borne on the lower sporophylls usually contain several
megaspores as in _Isoetes_ (cf. fig. 133, E). Beyond the distal end of
the sporangium the sporophyll becomes broader in a horizontal plane and
is bent upwards as a lanceolate limb; it may also be prolonged a short
distance downwards as a bluntly triangular expansion.
[Illustration: FIG. 188. _Lepidostrobus._ Middle Coal-Measures,
Bardsley, Lancashire. From a specimen in the Manchester Museum.
(½ nat. size.)]
There can be little doubt that the Palaeozoic _Lepidodendra_, like
_Lycopodium cernuum_ (fig. 123) and other recent Lycopods, usually
bore their cones at the tips of slender shoots. The fertile shoot of
_Lepidophloios scoticus_ shown in fig. 160, B, affords one of several
instances supporting this statement; similar examples are figured
by Brongniart[410], Morris[411], and by more recent writers. The
apparently sessile cone figured by Williamson[412] from a specimen in
the Manchester Museum is certainly not _in situ_, but is accidentally
associated with the stem.
The general absence of secondary wood in the steles of _Lepidostrobi_
is, as Dr Kidston[413] points out, consistent with the view that the
cones were shed on maturity and that fertilisation probably took place
on the ground, or perhaps on the surface of the water where the slender
hairs of the megaspores (fig. 191, F, I) may have served to catch the
microspores.
[Illustration: FIG. 189. _Lepidostrobus._ Section through the apical
region of a cone above the axis. (Manchester University
Collection.)]
Fig. 189 is an accurate representation of a transverse section,
6 mm. in diameter, of what is no doubt the apical portion of a
_Lepidostrobus_ from the Coal-Measures of Shore, Lancashire. The
section cuts across the upturned free laminae above the level of the
apex of the cone-axis. Each lamina contains a small vascular bundle
composed of a few tracheae and some thin-walled cells surrounded by
delicate mesophyll tissue. Immediately in front of the distal end
of a sporangium a small ligule is borne on the upper face of the
sporophyll (fig. 191, A, B, _l_) occupying the same position as in
_Selaginella_ (cf. fig. 131, F). Strands of vascular tissue pass in a
steeply ascending course from the xylem to the pedicels of sporophylls,
finally curving upwards and ending in the upper limb. Each vascular
bundle consists of a strand of xylem, apparently of mesarch structure,
accompanied by a few layers of parenchyma on its outer face and by
a group of cambiform elements, the whole being enclosed in a sheath
of parenchyma continuous with the inner cortex of the cone axis. The
vascular bundle is accompanied by a parichnos in the outer cortex and
in the sporophyll.
Reference has already been made to the belief on the part of some
palaeobotanists that the large scars of _Ulodendron_ represent
attachment-surfaces of sessile cones, and reasons have been given
against the acceptance of this view.
There is considerable range in the size of _Lepidostrobi_. An
incomplete specimen, 33 cm. long and 6 cm. broad, which may have been
50 cm. in length, is described by Renault and Zeiller[414] from the
Commentry Coal-field. The larger cones afford a striking demonstration
of the enormous spore-output of some species of _Lepidodendron_.
Among the earliest accounts of the anatomy of _Lepidostrobus_ are those
by Hooker[415] and Binney[416]. One of the specimens described by the
former author (fig. 190) affords an interesting example of an unusual
manner of fossilisation; a hollow stem or _Lepidodendron_ is filled
with sedimentary material containing several pieces of _Lepidostrobi_
in an approximately vertical position.
[Illustration: FIG. 190. _Lepidodendron_ stem with _Lepidostrobi_.
(After Hooker.)
A. Side-view showing leaf-cushions on the left-hand side and the
_Knorria_ condition on the right.
B. View of transverse section; _s_, sections of _Lepidostrobi_.]
The fact that _Lepidostrobi_ usually occur as isolated specimens
renders it impossible in most cases to refer them to particular species
of _Lepidodendron_. Neither external features nor anatomical characters
afford satisfactory criteria by which to correlate vegetative and
fertile shoots; in some measure this is due to the imperfection of
our knowledge as regards the range of structure within the limits of
species; it is also due to lack of information as to the extent to
which the transition from sterile to fertile portions of a shoot is
accompanied by anatomical differences. Prof. Williamson wrote: “I have
for many years endeavoured to discover some specific characters by
which different _Lepidostrobi_ can be distinguished and identified, but
thus far my efforts have been unsuccessful[417].” In a few cases, such
as those mentioned in the description of _Lepidodendron Veltheimianum_
and _L. Wünschianum_, it has been possible to correlate cones and
vegetative shoots.
The most complete account we possess of the anatomy of _Lepidodendron_
cones is that by Mr Maslen[418], who first demonstrated the occurrence
of a ligule on the sporophylls, and thus supplied a missing piece of
evidence in support of the generally accepted view as to the homology
of the sporangium-bearing members and foliage leaves.
i. _Lepidostrobus variabilis_ (Lindley and Hutton).
1811. “Strobilus,” Parkinson, Organic Remains, Vol. I. p. 428, Pl.
IX. fig. 1.
1828. _Lepidostrobus ornatus_, Brongniart, Prodrome, p. 87.
1831. _L. variabilis_, Lindley and Hutton, Foss. Flora, Pls. X. XI.
1831. _L. ornatus_, Lindley and Hutton, Foss. Flora, Pl. XXVI.
1837. _L. ornatus_ var. _didymus_, _Ibid._ Pl. CLXIII.
1850. _Arancarites Cordai_, Unger, Genera et Spec. Plant. foss. p.
382.
1875. _Lepidostrobus variabilis_, Feistmantel, Palaeontographica,
Vol. LXIII. Pl. XLIV.
1886. _L. variabilis_, Kidston, Cat. Palaeozoic Plants, p. 197.
1890. _L. ornatus_, Zeiller, Flor. Valenciennes, p. 497, Pl. LXXVI.
figs. 5, 6.
—— _L. variabilis_, Zeiller, Flor. Valenciennes, p. 499, Pl.
LXXVI. figs. 3, 4.
Under this specific name are included strobili from Upper Carboniferous
rocks which, in spite of minor differences, may be considered as one
type. The cylindrical cones vary considerably in size, some reaching a
length of 50 cm. or more. The sporophylls are attached by a pedicel,
4–8 mm. long, at right angles to the axis, while the distal portion
forms an oval lanceolate limb 10–20 mm. in length. The sporangia are
4–8 mm. long.
The branched example figured by Lindley and Hutton[419] as a variety
(L. _ornatus_ var. _didymus_) illustrates a phenomenon not uncommon in
both Palaeozoic and recent lycopodiaceous strobili.
[Illustration: FIG. 191. _Lepidostrobus._
A–D. _L. oldhamius._
B, C, D. From sections in the Binney Collection, Cambridge.
E. Megaspore. (After Kidston.)
F. Megaspore (Coal-Measures, Halifax). (After Williamson.)
G. Megaspore of _Lepidostrobus foliaceus_. (After Mrs Scott.)
H. Tangential section of sporangium. (After Bower.)
I. Part of sporangium wall, _Sm_, of the cone of _Lepidodendron
Veltheimianum_, enclosing two megaspores. (Cambridge Botany
School.)]
ii. _Lepidostrobus oldhamius_ Williamson[420]. Fig. 191, A–D.
Williamson[421] instituted this term for strobili previously
described by Binney[422], without adequate evidence, as the cones of
_Lepidodendron Harcourtii_. In shape and in the main morphological
features this type resembles _L. variabilis_, which is however known
only in the form of casts and impressions. A cone of _L. oldhamius_,
2–3 cm. in diameter, possesses a medullated stele consisting of a
ring of primary xylem (fig. 191, D, _x_) with exarch protoxylem and
no secondary elements. Maslen found several short tracheae at the
periphery of the xylem and states that these led him to compare the
cone with the vegetative shoots of _Lepidodendron vasculare_, but
the common occurrence of such elements in different types of shoot
renders them of little or no specific value. The inner cortex is like
that of vegetative shoots of _Lepidodendron_ and the middle cortex,
which was no doubt of the type described in _Lepidostrobus Brownii_,
is represented by a gap in the sections, beyond which is the stronger
outer cortex (fig. 191, D) passing into the horizontal pedicels of the
sporophylls. The section of the axis reproduced in fig. 191, D, was
figured by Binney[423] as _Lepidodendron vasculare_. The leaf-traces,
several of which are seen in the middle cortical region in fig. D,
_lt_, consist of a strand of scalariform tracheae, with a mesarch
protoxylem, succeeded by a few parenchymatous cells; beyond these there
is usually a small gap which was originally occupied by a strand of
thin-walled cells. It is important to note that in one sporophyll-trace
figured by Maslen[424] there is a strand of thin-walled elongated
elements abutting on the xylem, which he describes as phloem. This
tissue is certainly more like true phloem than any which has hitherto
been described in the leaf-traces of vegetative shoots. The state
of preservation is not, however, sufficiently good to enable us to
recognise undoubted phloem features.
In such cones as I have examined no tissue has been seen which shows
the histological features characteristic of the secretory zone of
vegetative shoots: the “phloem” (Maslen) occupies the position in
the sporophyll bundle which in the vascular bundles of foliage
leaves is occupied by a dark-celled and partially disorganised
tissue in continuity with the secretory zone of the main stele. It
may be that in the strobili this tissue occurred in a modified
form, but even assuming that the section figured by Maslen shows
true phloem, an assumption based on slender evidence, this is not
sufficient justification for the application of the term phloem to a
tissue occupying a corresponding position in vegetative shoots and
distinguished by well-marked histological features.
The sporophyll-traces, as seen in the outer cortex in fig. 191, D,
are partially surrounded by a large crescentic space, _p_, which was
originally occupied by the parichnos. The sporangia are attached along
the middle line of the sporophyll and, as in _Lepidostrobus Brownii_,
a cushion of parenchyma projects into the lower part of the sporangial
cavity (fig. 191, A, _a_; C, _a_).
The diagrammatic sketch of part of a section in the Binney Collection
reproduced in fig. 191, B, shows the position of the ligule, _l_. No
megaspores have been discovered in any specimens of this type; the
microspores, which occur both singly and in tetrads, have a length of
0·02–0·03 mm.
The drawing shown in fig. 191, A, based on a section in the Binney
Collection, illustrates the general arrangement of the parts of a
typical _Lepidostrobus_. I have made use of this sketch instead of that
given by Maslen, as his figure conveys the idea that the sporophylls
are superposed, whereas, whether they are verticillate or spiral, a
radial longitudinal section would not cut successive sporangia in the
same plane.
iii. _Lepidostrobus Brownii_ (Brongn.).
In 1843 a specimen of a portion of a petrified cone was purchased by
the British Museum, assisted by the Marquis of Northampton and Robert
Brown, for £30 from a French dealer. This fossil, from an unknown
locality, was briefly described by Brown in 1851[425] and named by him
_Triplosporites_, but in a note added to his paper he expressed the
opinion that the generic designation _Lepidostrobus_ would be more
appropriate. Brongniart afterwards named the cone _Triplosporites_
_Brownii_[426], and Schimper[427] described it in his _Traité_ as
_Lepidostrobus Brownii_. The type-specimen is preserved in the British
Museum and the Paris Museum possesses a piece of the same fossil.
The central axis of the cone has a stele of the type characteristic
of _Lepidodendron fuliginosum_ and _L. Harcourtii_, and the xylem
is surrounded by a thin-walled tissue described by Bower[428] as
possibly phloem; but in the absence of longitudinal sections it is
impossible to say how far the tissue external to the xylem agrees with
that in Lepidodendron stems. The sporophylls consist of a horizontal
portion, to the upper face of which the radially elongated sporangia
are attached, one to each sporophyll; beyond the distal end of the
sporangium the sporophyll bends sharply upwards as a fairly stout
lamina. The wall of the sporangium is composed of several layers of
cells, as shown in a drawing published by Bower[429]; in the interior
occur groups of microspores, and from a ridge of tissue which extends
along the whole length of the sporangium irregular trabeculae of
sterile tissue project into the sporangial cavity, as in _Isoetes_
(fig. 191, H: cf. fig. 133, H).
Further information in regard to _Lepidostrobus Brownii_ has recently
been supplied by Prof. Zeiller[430], who recognises the existence of a
ligule, and draws attention to some interesting histological features
in the tissue of the sporophylls[431].
_Spores of Palaeozoic Lycopodiales._
The calcareous nodules from the Coal seams of Yorkshire and Lancashire
are rich in isolated spores, many of which are undoubtedly those of
_Lepidostrobi_. Examples of spores were figured by Morris[432] in 1840,
and their occurrence in coal has been described by several authors,
one of the earliest accounts being by Balfour[433]. The drawings of
Palaeozoic and recent spores published by Kidston and Bennie[434]
demonstrate a striking similarity between the megaspores of existing
and extinct Lycopods, the chief difference being the larger size of the
fossils.
The general generic name _Triletes_, originally used by Reinsch[435],
is a convenient term by which to designate Pteridophytic spores which
cannot be referred to definite types.
It is usual to find more than four megaspores in each megasporangium
in Palaeozoic and not infrequently, as we have seen, in Mesozoic
lycopodiaceous strobili, but in some Palaeozoic cones, e.g.
_Bothrostrobus_ (fig. 216) and _Lepidostrobus foliaceus_[436], a single
tetrad only appears to have reached maturity.
The occurrence of long simple or branched and sometimes capitate hairs
is a common feature of Carboniferous megaspores (fig. 191, E, F, I).
It is possible that these appendages served to catch the microspores,
thus facilitating fertilisation. A peculiar form of megaspore has
been described by Mrs Scott[437], and assigned by her to _Lepidostrobus
foliaceus_, the megasporangium of which apparently contained only
four spores. As shown in fig. 191, G, a large bladder-like appendage
characterised by radiating veins is attached to the thick spore-coat;
it is suggested that this excrescence may be compared with the
“swimming” apparatus of the recent water-fern _Azolla_. The epithet
swimming which it is customary to apply to the appendages of _Azolla_
megaspores would seem to be inappropriate if Campbell[438] is correct in
stating that spores of _Azolla_ are incapable of floating.
B. _Spencerites._
_Spencerites insignis_ (Williamson). Fig. 192.
1878. _Lepidostrobus sp_., Williamson, Phil. Trans. R. Soc., p.
340, Pl. XXII.
1880. _Lepidostrobus insignis_, Williamson, Phil. Trans. R. Soc.,
p. 502, Pl. _XV_. figs. 8–12.
1889. _Lepidodendron Spenceri_, Williamson, Phil. Trans. R. Soc.,
p. 199, Pl. VII. figs. 20–22; Pl. VIII. fig. 19.
1897. _Spencerites insignis_, Scott, Phil. Trans. p. 83, Pls.
XII–XV.
Another type of lycopodiaceous strobilus, differing sufficiently from
_Lepidostrobus_ to deserve a special generic designation, is that
originally described by Williamson[439], from the Lower Coal-Measures
of Yorkshire, as a type of _Lepidostrobus_, _L. insignis_, but
afterwards[440] more fully investigated and assigned to a new genus
by Scott[441]. It should be pointed out that in a later publication
Williamson spoke of the lycopodiaceous axis, which he suspected might
belong to his _L. insignis_, as possibly worthy of recognition as a
distinct generic type.
[Illustration: FIG. 192. _Spencerites insignis_ (Williamson). (After
Miss Berridge.)]
Of the two species included by Scott in his genus _Spencerites_ only
one, _S. insignis_, need be considered. Since the publication of
Scott’s paper our knowledge of this type has been extended by Miss
Berridge[442] and by Prof. Lang[443].
The axis of the strobilus has a stele characterised by a pith
of elongated elements, most of which have thin walls; the xylem
cylinder possesses about twenty protoxylem strands forming more or
less prominent exarch ridges. The cortex exhibits a differentiation
comparable with that in the shoots of _Lepidodendron_. The sporophylls
are arranged in alternating verticils, each whorl consisting of ten
members: the narrow horizontal pedicel of a sporophyll, containing a
single vascular bundle, as shown in fig. 192, is expanded distally
into a prominent upper lobe bearing a cushion of small and delicate
cells, to which the sporangium is attached, and prolonged obliquely
upwards as a free leaf-like lamina. The lower blunt prolongation of
the sporophylls appears to form a thick dorsal lobe, but, as Lang has
pointed out, it is highly probable that the present form of the dorsal
lobe is of secondary origin, and is “due to the disappearance of a
mucilage cavity from a large sporophyll base[444].” As Miss Berridge
remarks, the vascular bundle of the sporophyll does not give off a
branch to the ventral lobe and sporangium. In attachment, in shape, and
in the structure of the wall the sporangia differ markedly from those
of _Lepidostrobi_. The spores, which also constitute a characteristic
feature of the genus, have a maximum diameter of 0·14 mm.; they are
described as oblate spheroids with a broad hollow wing running round
the equator (fig. 192) comparable with the air-sacs of the pollen
of _Pinus_. Scott points out that the spores of _Spencerites_ are
intermediate in size between the microspores of _Lepidodendron_ and
the megaspores of _Lycopodium_; it is difficult therefore to decide
to which category they should be referred. _Spencerites_ is clearly
distinct from _Lepidostrobus_; the absence of a ligule, the manner of
attachment of the sporangia, and the form and size of the spores, are
characteristic features.
A comparison of _Spencerites_ with the strobili of _Lycopodium cernuum_
(figs. 123, 126–129) has recently been made by Lang, who draws
attention to the striking agreement as regards general plan and even
detailed structural features between the Palaeozoic and the recent
type of strobilus. It is interesting to find, as Lang points out, that
in the original account of the fossil cone by Williamson, the view
is expressed that the sporangiophores were confluent. An examination
of the section figured by Williamson[445] led Lang to confirm this
opinion. It would be out of place to enter here into a detailed
comparison of _Spencerites insignis_ and the cone of _Lycopodium_,
but the resemblances are considered by Lang to be sufficiently
close to suggest that the striking similarity may be indicative of
relationship[446].
It is worthy of notice that the radial section of _Spencerites_ (fig.
192) presents a fairly close resemblance to a corresponding section
through a cone-scale of _Agathis_ (Kauri Pine)[447]. In each case the
megasporangium is attached by a narrow pedicel to the sporophyll and
the latter has a similar form in the two plants, though the extent of
the resemblance is somewhat lessened by Lang’s more complete account
of the Palaeozoic type. If the _Spencerites_ sporangia possessed an
integument the similarity with the _Agathis_ ovule would of course be
much closer: recent palaeobotanical investigations have shown that
ovules and sporangia are not separated by impassable barriers.
[Since this Chapter was set up in type a paper has appeared by Dr Bruno
Kubart on a new species of _Spencerites_ spore, _S. membranaceus_,
from the Ostrau-Karwiner Coal-basin (Austria). The spores are larger
than those of _S. insignis_ and in some the cells of a prothallus are
preserved. Kubart figures a section of a spore containing a group
of seven cells, a central cell, which he regards as an antheridial
mother-cell, surrounded by six wall-cells. Kubart (90).]