=Bothrodendreae.=
_Bothrodendron._ Figs. 211–216.
Although in many respects the genus _Bothrodendron_ agrees very closely
in habit and in its anatomical features with _Lepidodendron_, there are
reasons for referring it to a distinct family of Palaeozoic Lycopods.
As the following description shows, the external features do not differ
in any essential points from those of certain types of the genus
_Sigillaria_, particularly such a species as _S. rimosa_, Gold.[608],
which has recently been refigured and described by Nathorst[609] from
Goldenberg’s type-specimen in the Stockholm Museum. The small size of
the leaf-scars is, however, a characteristic feature of _Bothrodendron_
(fig. 212, F); but a more important point is the fact that in a
recently described[610] English example of a cone of _Bothrodendron_
(fig. 216), the sporangia are very like those of recent Lycopods, and
differ from the radially elongated sporangia of _Lepidostrobus_. On the
other hand, a French cone described by Zeiller[611] as _Lepidostrobus
Olryi_, which is probably a strobilus of _Bothrodendron_, has the
radially elongated type of sporangium (fig. 212, E). The comparative
abundance of _Bothrodendron_ in Lower Carboniferous and Devonian rocks
points to the greater antiquity of this member of the Lycopodiales as
compared with _Lepidodendron_.
The name _Bothrodendron_ was instituted by Lindley and Hutton[612]
for impressions of stems from the English Coal-Measures, characterised
by two opposite rows of large depressions like those shown in fig.
211 and, in one of the specimens, by “a considerable number of
minute dots, arranged in a quincuncial manner.” The minute dots were
recognised as leaf-scars and the cup-like cavities were described
as probably connected with the occurrence of large cones. On very
slender evidence this Palaeozoic plant, which was named _Bothrodendron
punctatum_, was considered by these authors as probably a member
of the Coniferales. The large stem from the Coal-Measures in the
neighbourhood of Mons, Belgium, shown in fig. 211, affords a good
illustration of _Bothrodendron_ in a partially decorticated condition,
exhibiting a row of depressions similar to those on the Ulodendron
form of _Lepidodendron Veltheimianum_ (fig. 157), but distinguished by
the eccentric position of the scar at the bottom of each cup-shaped
cavity: in the Belgian specimen, which is partially decorticated and
shows the leaf-traces as small dots, the depressions have a diameter of
9 cm. It is believed by some authors that these Ulodendron shoots of
_Bothrodendron_ and _Lepidodendron_ owe their characteristic appearance
to the pressure of large cones, but, as I have already stated, there
are reasons for preferring the view that these crater-like hollows are
the scars of deciduous branches. Our knowledge of the strobili borne by
Bothrodendron stems is still meagre, but we have no reason to assume
the existence of any cones large enough to produce by the pressure
of their bases such depressions as those shown in fig. 211. In one
species at least the strobili were borne terminally on slender shoots
(fig. 213). The Ulodendron condition has so far been recognised in one
species only, _B. punctatum_.
In his catalogue of Palaeozoic plants, Kidston[613] included
_Bothrodendron punctatum_ as a synonym of _Sigillaria discophora_
König, a mistake which he afterwards rectified[614]: the generic name
_Bothrodendron_ was generally ignored by authors in the belief that
the specimens described by Lindley and Hutton were not generically
distinct from the fossils originally figured by Rhode as _Ulodendron_.
It was Prof. Zeiller who first demonstrated that the English authors
were justified in their choice of a new designation for stems with
large depressions in association with minute leaf-scars. In 1859
Haughton[615] proposed a new family name Cyclostigmaceae for some Upper
Devonian plants from County Kilkenny, Ireland: he described three
species of his new genus _Cyclostigma_, _Cyclostigma kiltorkense, C.
minutum_, and _C. Griffithsi_; these are now generally recognised as a
single species of _Bothrodendron_, though, as Nathorst suggests, the
Irish plant should perhaps be separated as a sub-genus _Bothrodendron_
(_Cyclostigma_) by reason of certain minor differences which
distinguish it from other species of the genus.
[Illustration: FIG. 211. _Bothrodendron punctatum._ Part of a specimen
from near Mons (Hainaut), in the Brussels Museum. (Reduced.)]
Another generic name, _Rhytidodendron_, was instituted by Boulay in
1876 for stems characterised by a finely wrinkled bark and small
spirally disposed leaf-scars. A short description of this type, which
occurs in the Middle and Lower Coal-Measures, may serve to illustrate
the external features of the commonest British example of the genus.
_a. Bothrodendron minutifolium_ (Boulay.) Figs. 212, A, C, D; 213.
1875. _Lycopodium carbonaceum_ (_Lycopodites carbonaceus_),
Feistmantel, Palaeontographica XXXIII., Pl. XXX. figs. 1, 2; p.
183.
1876. _Rhytidodendron minutifolium_, Boulay, Terr. Houill. Nord
France, p. 39, Pl. III. fig. 1.
1886. _Bothrodendron minutifolium_, Zeiller, Bull. Soc. Géol. France
[iii] XIV. p. 176, Pl. IX. figs. 1, 2.
1888. _Lepidostrobus Olryi_, Zeiller, Flor. Valenciennes, p. 502, Pl.
LXXVII. fig. 1.
1889. _Bothrodendron minutifolium_, Kidston, Trans. R. Soc.
Edinburgh, Vol. XXXV. Pt. ii.
1893. _Sigillaria_ (_Bothrodendron_) _minutifolia_, Weiss and
Sterzel, K. Preuss. Geol. Landesanstalt, Heft 2, p. 49, Pl. I.
figs. 3 and 4; Pl. II. figs. 8 and 9.
1904. _Bothrodendron minutifolium_, Zalessky, Mém. Com. Géol. Russie,
Pl. VI. fig. 6.
[Illustration: FIG. 212. _Bothrodendron._
A. _Bothrodendron minutifolium_, var. _rotundata_ Weiss. After
Weiss and Sterzel.
B. _B. punctatum._ After Zeiller.
C. _B. minutifolium._ After Weiss and Sterzel.
D. _B. minutifolium._ After Zeiller.
E. _Lepidostrobus Olryi._ After Zeiller.
F. _Bothrodendron punctatum._ After Zeiller.
G, H. _B. kiltorkense._ G, after Nathorst; H, after Weiss and
Sterzel.]
In habit a plant of _Bothrodendron_ recalls _Lepidodendron_ and recent
species of _Lycopodium_; the slender dichotomously branched twigs
bearing numerous leaves (fig. 212, D), have been mistaken for shoots
of _Lycopodium_, and fragments of branches might well be identified
as impressions of Mosses. The leaf-scars on the smaller shoots occur
on elongated cushions (fig. 212, C, D) with a transversely wrinkled
surface; on the older branches the leaf-scars are separated by fairly
large areas of bark characterised by sinuous transverse grooves and
narrow ridges bearing numerous small pits, as shown on an enlarged
scale in fig. 212, A. The original surface-features are shown on the
left of the drawing, and a slightly deeper level in the cortex is
represented on the right-hand side. The absence of leaf-cushions on the
older shoots is probably the result of secondary thickening, which also
alters the size and shape of the leaf-scars. Each scar has three pits
on its surface, as in _Lepidodendron_; a central leaf-trace scar and
lateral parichnos scars. The circular pit above the leaf-scars, which
occurs in most species, marks the position of the ligule. The relation
of the short leaves, 5 mm. long, to the leaf-cushions is shown in fig.
212, D. The absence of leaves, except in impressions of slender twigs,
may be interpreted as an indication that they were shed at an early
stage and did not persist many years. The leaf-cushions of the smaller
shoots of _Bothrodendron minutifolium_ closely resemble those figured
by Weiss on a Devonian plant, _Lepidodendron Losseni_[616].
One of the few examples so far discovered of a Bothrodendron cone is
shown in fig. 213; this specimen, at least 10 cm. long, was found by
Mr Hemingway in the Middle Coal-Measures of Yorkshire and described
by Dr Kidston. Numerous sporophylls are attached at right angles to
the axis, the surface of which is protected by their upturned distal
portions; the arrangement of the parts appears to be the same as in
_Lepidostrobus_. A specimen figured by Zeiller as _Lepidostrobus
Olryi_, which Kidston is probably correct in identifying with
_Bothrodendron minutifolium_, shows that each sporophyll carries a
horizontally elongated sporangium (fig. 212, E).
_b. Bothrodendron punctatum_ Lindley and Hutton[617]. Figs. 211, 212 B,
F.
This species, which is less abundant than _B. minutifolium_, in British
Coal-Measures, has been described by several authors as _Ulodendron_
on account of the occurrence of large depressions, like those shown
in fig. 211, on certain branches of the plant. At the suggestion of
Dr Kidston, Prof. Zeiller[618] figured an English specimen of this
species, presented to the Paris Museum by Mr Hutton, in which the
leaf-scars are preserved on the bark of a stem with Ulodendron scars.
The surface of the bark is characterised by numerous small pits and
discontinuous vertical lines in contrast to the transverse lines of _B.
minutifolium_ (cf. fig. 212, A and F). The leaf-scars on the smaller
shoots may have a diameter of only 0·3–0·5 mm., while on the larger
branches they reach a breadth of 1 mm. The ligule-pit may be in contact
with the upper edge (fig. 212, F) of the leaf-scar or separated from it
by a short distance.
[Illustration: FIG. 213. _Bothrodendron minutifolium_ Cone. From a
specimen in Dr Kidston’s Collection. (Slightly reduced. Kidston
(02) Pl. LIX.)]
_c_. _Bothrodendron kiltorkense_ (Haughton). Fig. 212, G, H.
1859. _Cyclostigma kiltorkense_, Haughton, Journ. R. Soc. Dublin,
Vol. II. p. 418, Pls. XIV.–XVII.
_C. minutum_, Haughton, Journ. R. Soc. Dublin, Vol. II. p. 418,
Pls. XIV.–XVII.
_C. Griffithsi_, Haughton, Journ. R. Soc. Dublin, Vol. II. p. 418,
Pls. XIV.–XVII.
1870. _Lepidodendron Veltheimianum_, Heer (_ex parte_), K. Svensk.
Vet. Akad. Handl. Vol. IX. Pl. IX. figs. 2–4.
_Cyclostigma kiltorkense_, _ibid_. Pl. XI. figs. 1–5.
_Calamites radiatus_ (_ex parte_), _ibid_. Pl. III. fig. 2_a_: Pl.
IX. fig. 2_b_.
_Stigmaria ficoides minuta_, _ibid_. Pl. IX. fig. 2_c_.
_Knorria imbricata_, _ibid_. Pl. X. fig. 4.
1889. _Bothrodendron kiltorkense_, Kidston, Ann. Mag. Nat. Hist.
[VI.], Vol. IV. p. 66.
1894. _Bothrodendron kiltorkense_, Nathorst, K. Svensk. Vet. Akad.
Handl. Vol. XXVI. No. 4, p. 65, Pls. XIV. XV.
1902. _Bothrodendron (Cyclostigma) kiltorkense_, _ibid_. Vol. XXXVI.
No. 3, p. 31, Pls. X.–XIV.
The specimens from the Upper Devonian rocks of Co. Kilkenny on which
Haughton founded this and two other species may be regarded as
representing one specific type. He described the circular leaf-scars
as arranged in alternating whorls. In habit the Irish species agrees
with _Bothrodendron minutifolium_, but the leaf-scars are more
elliptical (fig. 212, H) and the ligule-pit is usually absent. The
leaf-scar shown in fig. H is 1·2 mm. broad and 1·4 mm. in height. The
large collection obtained during the visit of a Swedish expedition
to Bear Island in 1898 under the leadership of Dr Nathorst has
materially increased our knowledge of this ancient type. The form
of the leaf-scars varies according to the age of the branch and
their disposition is far from constant even on the same specimen; in
some cases the scars are in fairly regular whorls (fig. 212, G; an
Irish specimen) while in others they are in regular spirals. This
irregularity of arrangement, which is well illustrated by Nathorst’s
figures of Bear Island and Irish specimens, finds its counterpart,
though in a less marked form, in recent species of _Lycopodium_, _e.g.
L. Selago_. Partially decorticated stems may present a superficial
resemblance to _Calamites_, the fissured bark simulating the ribs
of a Calamitean cast. Such stems, as Nathorst has pointed out, were
mistaken by Heer for _Calamites radiatus_. The smaller branches
are characterised by a smooth surface, and older shoots resemble
_Bothrodendron minutifolium_ in the presence of fine vertical lines.
The preservation of only one pit on the leaf-scars of many examples
led authors to conclude that the species is peculiar in this respect,
but Nathorst has shown that in more perfectly preserved specimens
each leaf-scar bears three small dots. A specimen from Ireland in the
British Museum[619] illustrates the dichotomous branching and the
longitudinal wrinkling of the bark; the leaf-scars are 2 mm. broad and
2·5 mm. deep.
Nathorst[620] has described some examples in which the leaf-scars
occur on the lower instead of on the upper end of the leaf-cushions;
these and other specimens with obscure surface-features he suggests
may be underground axes, comparable in habit with _Stigmaria_ though
not identical as regards details. It is pointed out that the absence
or scarcity of _Stigmaria_ in the Bear Island beds renders it unlikely
that _Bothrodendron_ bore typical Stigmaria branches. F. E. Weiss[621]
has recently described root-bearing organs possessing primary xylem
identical with that of _Bothrodendron mundum_; while closely resembling
_Stigmaria ficoides_ in certain anatomical characters, they clearly
represent a distinct type. This discovery of a Stigmaria-like axis
almost certainly belonging to _Bothrodendron_ is consistent with
Nathorst’s views on some of the Bothrodendron impressions from Bear
Island.
Information as to the cones of this species is restricted to a
description by Schimper[622] of a specimen in the Dublin Museum as
_Lepidostrobus Bailyanus_; this has sporophylls with a subtriangular
base bearing several megaspores and terminating distally in a slender
lamina 12 cm. in length.
An example of a _Bothrodendron_ with more prominent leaf-cushions than
those already mentioned is afforded by a species from Bear Island
described by Heer[623] as _Lepidodendron Wükianum_ and afterwards
referred by Nathorst[624] to _Bothrodendron_. The same type is recorded
also by Schmalhausen[625] from Lower Carboniferous or Devonian strata
of Siberia. Certain Scotch specimens from the Calciferous Sandstone,
which Kidston[626] referred to Heer’s species, are regarded by Nathorst
and, in part at least, by Weiss[627] and Sterzel as representing
a distinct species which these authors designate _Bothrodendron
Kidstoni_[628].
Without attempting the hopeless task of discriminating between
the various Carboniferous and Devonian specimens described under
the names _Cyclostigma_ or _Bothrodendron_, reference may be made
to the following records as illustrating the wide distribution of
the genus. Schmalhausen[629] records _Cyclostigma kiltorkense_
from Siberian rocks assigned to the Ursa stage (Devonian or Lower
Carboniferous). The fossil described by Dawson[630] from the Devonian
of Gaspé as _Cyclostigma densifolium_ probably represents a badly
preserved example of _Bothrodendron_: Weiss’s species _Cyclostigma
hercynium_[631] from Lower Devonian rocks of the Hartz district
may be identical with _Bothrodendron kiltorkense_. The supposed
identity of the latter species with _Dechenia Roemeriana_ Goepp., as
described by Potonié[632], appears to require confirmation[633], but
if this author is correct the connexion demonstrates the continuity
of Bothrodendron shoots and Stigmaria-like subterranean organs. The
specimens described from South Africa, from strata which may be
correlated with the Upper or possibly with the Lower Carboniferous
series of Europe, as _Bothrodendron Leslei_[634] in all probability
represents a species closely allied to the Irish and Bear Island
type. _Bothrodendron Leslei_ named after Mr Leslie whose discoveries
in the Carboniferous Sandstone of Vereeniging (Transvaal) have added
considerably to our knowledge of the South African Palaeozoic types,
is represented by imperfectly preserved casts characterised by more or
less circular scars displaying the same irregularity of arrangement
as in _Bothrodendron kiltorkense_. The leaf-scars appear to have
only one small pit, but this may not be an original feature. The
identification of this plant as _Bothrodendron_ receives support from
the discovery of rather more satisfactory specimens at Witteberg sent
to me for examination by Dr Schwarz[635]. These fossils bear a striking
resemblance to _B. kiltorkense_. _Cydostigma australe_[636] Feist.
described from the Lower Carboniferous rocks of New South Wales, though
too imperfectly preserved to refer with confidence to _B. kiltorkense_,
is no doubt a closely allied type.
[Illustration: FIG. 214. _Bothrodendron Leslei_ Seward.
_b._ Natural size.
_a, c._ Slightly enlarged.]
[Illustration: FIG. 215. _Bothrodendron mundum_ (Will.).
A, B. From a specimen (No. 26) in the Cambridge Botany School.
C. British Museum, Williamson Collection. (No. 416 b.)
D, E. From a section in Dr Kidston’s Collection.]
Reference was made in Volume I. (p. 133) to the so-called paper coal
of Carboniferous age from Central Russia, which consists of masses of
thin strips of cuticle of Bothrodendron stems. The figures published
by Zeiller[637] show that the plant possessed an epidermis consisting
of polygonal cells interrupted by spirally disposed gaps marking the
position of leaves; the gaps measure 0·5–1·5 mm. in breadth and agree,
therefore, with the size of the leaf-scars of the smaller forms of
_Bothrodendron_. The specimens from the Russian mines were first
figured by Trautschold and Auerbach[638] as _Lepidodendron tenerrimum_
and afterwards referred by Zeiller to _Bothrodendron punctatum_[639].
Nathorst[640], however, states that an examination of the Russian
material leads him to retain the name originally proposed; he records
the same type from Upper Devonian rocks of Spitzbergen. The chief
interest of these Russian specimens is their manner of preservation,
which Renault has described as the result of bacterial action; he
claims to have recognised the actual bacteria associated with the
cuticular membranes[641].
_Anatomy of vegetative shoots_ of Bothrodendron.
In 1889 Williamson[642] described several specimens of petrified
shoots from the Coal-Measures of Halifax which he named _Lepidodendron
mundum_: these are now known to be branches of a _Bothrodendron_. The
discovery was made by Mr Lomax[643] who found specimens showing the
external characters of _Bothrodendron_ and the anatomical characters
of _Lepidodendron mundum_. In some of the smaller twigs, the stele
consists of a solid core of xylem with external protoxylem; but in
the majority of specimens the centre of the xylem is replaced by
parenchymatous tissue, either as a small axial strand or, as in the
specimen shown in fig. 215, D, a wide pith, the elements of which
are arranged in regular vertical series. A diagrammatic section of
a small axis is represented in fig. 215, A: this branch, 2 mm. in
diameter, is composed of a broad outer cortex consisting exclusively of
primary tissue the outer cells of which are smaller and have thicker
walls than the more internal elements. The leaf-traces, _lt_, are
accompanied by a strand of delicate tissue, the parichnos. The stele
is almost solid; the tissues in contact with the xylem have not been
preserved but the inner cortex is represented by a few layers of small
parenchymatous cells, _c_¹. The larger section shown in fig. 215,
D, was cut from a specimen from Dulesgate of which the smooth surface
exhibits the characteristic leaf-scars of _Bothrodendron_. The section
measures 3 cm. in its longest diameter and the stele has a breadth of
3 mm. The outer cortex has a smooth surface and is composed of rather
thick-walled cells succeeded by a zone of secondary elements. The
middle cortex has disappeared and the space is partially occupied by
Stigmarian rootlets, _s_, and crushed patches of cortical tissue. The
position of a leaf-scar is seen at _a_; this is more clearly shown in
the enlarged drawing fig. E.
In his account of _Lepidodendron mundum_, Williamson[644] described
a section in which the primary wood is surrounded by a considerable
thickness of secondary xylem; a diagram of this is shown in fig. 215,
C. An examination of the section led me to compare the structure of
the outer cortical cells, characterised by radial rows of tangentially
elongated elements, with the outer cortex of _Stigmaria_. It has
recently been shown by Weiss[645] that this and other similar sections
present several points of agreement with _Stigmaria_, particularly
with _Stigmaria Brardi_ as described by Renault. At _s_ in fig. 215,
C, a vascular strand is seen passing through the outer cortex; this is
almost certainly the bundle of a rootlet: in the sections described by
Weiss rootlets are shown in a similar position. The chief anatomical
features of the Stigmaria-like organs of _Bothrodendron_ are:—the
considerable development of secondary xylem, the structure of the
outer cortex, which is practically identical with that of _Stigmaria
ficoides_, and the association of groups of short transfusion tracheids
with the bundles of the rootlets. It is very probable that the absence
of secondary xylem in the vegetative shoots of _Bothrodendron_ is
merely an accident and not a real distinction between the aerial and
subterranean branches of the plant; a supposition rendered probable by
the occurrence of secondary xylem in the axis of the cone described
by Watson. As Weiss points out, there are certain differences between
the true _Stigmaria_ and the corresponding organ of _Bothrodendron_;
the secondary xylem in _Bothrodendron_ is not broken up by broad
medullary rays as in the common _Stigmaria_, and in _Bothrodendron_ the
occurrence of a ring of primary xylem is another peculiarity.
In the vegetative shoots of _Bothrodendron mundum_ the stele differs
from those of _Lepidodendron_ in the narrower primary xylem ring and
in the large size of the metaxylem tracheae; from _Lepidodendron
Harcourtii_ and _L. fuliginosum_ the xylem is distinguished by its
smoother outer face which consists of numerous narrow xylem elements.
[Illustration: FIG. 216. _Bothrostrobus._ _l_, ligule. (After Watson.)]
_Cones_ of Bothrodendron (_Bothrostrobus_[646]).
The long and narrow cones referred to _Bothrodendron minutifolium_ from
English and French Coal-Measures are known only as impressions and it
is not possible to say whether they were heterosporous or homosporous;
the drawing given by Zeiller (Fig. 212, E) shows that the sporangia
were of the same form as those in _Lepidostrobus_, but we have no
more exact information as to their morphology. A recently published
description of a petrified strobilus by Mr Watson affords a welcome
addition to our knowledge. There is little doubt that this cone was
borne by a species of _Bothrodendron_; the evidence for this conclusion
is supplied by the agreement of the anatomical characters of the stele
with that of the vegetative shoots originally described by Williamson
as _Lepidodendron mundum_ and by the constant association of the cones
and vegetative shoots. In 1880 Williamson described a crushed cone
containing both megaspores and microspores which he spoke of as “a
diminutive organism, reminding us more of the dwarfed fruits of many
living Selaginellas than of the large _Lepidostrobi_[647].” Watson’s
specimens enable us to give a more complete account of this type. The
axis of the strobilus bears short sporophylls bent upwards into a
distal limb with a conspicuous ligule in a deep pit beyond the shortly
stalked sporangium. The length of the strobilus is estimated at 10 mm.;
the stele is of the same type as that of _Bothrodendron mundum_, but
it differs from the specimens of the vegetative shoots so far found
in having some secondary xylem. As shown in the sketch reproduced in
fig. 216 each sporophyll is characterised by two tangentially placed
grooves, _g_, on the lower face, and by numerous transfusion tracheids,
_tr_, above the vascular bundle, _vb_, immediately below the ligule,
_l_. Megasporangia and microsporangia occur on the same cone, the
megasporangia being on the lower sporophylls and containing a single
tetrad of megaspores. Fig. 219, E, shows a radial longitudinal section
of a microsporophyll bearing a sporangium on the adaxial side of the
ligule, _l_, below which is the single vascular bundle and a group of
short tracheids at _t_. The sporangia closely resemble those of species
of _Selaginella_ and _Lycopodium_ and, as pointed out by Watson[648],
they also recall the sporangia of the Palaeozoic genus _Spencerites_.
_Bothrostrobus_ is distinguished from _Spencerites_ by the presence
of a ligule, by the structure of the axis, and by the different
form of the sporophylls. The occurrence of four spores only in the
megasporangia is another character in which the extinct type resembles
recent Lycopods. It is impossible to decide whether Watson’s cone
represents a more or a less primitive type than _Lepidostrobus_: if we
accept Professor Bower’s views in regard to the evolution of vegetative
organs by the sterilisation of sporogenous tissue, we should probably
place _Lepidostrobus_ lower in the series than _Bothrostrobus_; but
the greater resemblance between the fertile and vegetative shoots of
_Bothrodendron_, as compared with the more pronounced difference in the
case of _Lepidodendron_, may be regarded as an argument in favour of
recognising _Bothrodendron_ as the more primitive type.
Another possible example of a _Bothrodendron_ cone has been described
by Nathorst from Spitzbergen as _Lepidostrobus_ _Zeilleri_[649]; this
appears to consist of an axis bearing spirally disposed sporangia
without any indication of sporophylls. This strobilus may belong to
_Bothrodendron tenerrimum_.
_Pinakodendron_.
The name _Pinakodendron_[650] was instituted by the late Prof. Weiss
for a type of stem closely resembling _Bothrodendron_ but differing in
the presence of a fine reticulation on the outer bark and in the form
of the leaf-scars. Weiss’s genus has been recognised by Kidston in
Dumfriesshire but our knowledge of the plant is as yet based solely on
a few small specimens.
_Omphalophloios_ (_a genus of uncertain systematic position_).
Figs. 193, C, 217.
This generic name was instituted by White[651] for certain specimens of
large stems originally described by Lesquereux from the Coal-Measures
of North America as _Lepidodendron mammillatum_ and _L. cyclostigma_.
The photograph reproduced in fig. 193, C, for which I am indebted to
Dr Kidston[652], represents a specimen described by him from the Upper
Coal-Measures of Somerset as _Omphalophloios anglicus_, and identified
with _Lepidodendron anglicum_ of Sternberg.
The surface of the impression shown in fig. 193, C, is characterised
by clearly defined rhomboidal areas or cushions (fig. 217, E) like
those of _Lepidodendron_, except in the absence of a median keel,
and similar to those on some forms of _Sigillaria Brardi_. A short
distance above the centre of each cushion is an oval or subcordate
region bounded by a rim-like margin and containing a small oval scar,
presumably that of a vascular strand. A triangular elevation which also
shows a small pit (Fig. 217, E, _a_) occurs below the oval area. The
appearance of the surface-features varies considerably on different
parts of a single specimen. Fig. 217, D, represents one of the numerous
figures published by White in his detailed account of the American
material. Each cushion bears a widely open =V=-shaped ridge, which is
described as a leaf-scar; above this is an oval area (2·5 mm. × 1·75
mm.), the surface of which is bounded by a narrow rim. Within the rim
is a smaller concave oval region with a small pit near its upper end.
[Illustration: FIG. 217. _Omphalophloios._
D. After White. E. After Kidston.]
We cannot, in the absence of petrified material, arrive at any
satisfactory conclusion as to the meaning of these surface-features.
White considers that _Omphalophloios_ is probably a rhizome of one of
the arborescent Lycopods, but whether or not this is its true nature
must be left for future discoveries. The fact that the rootlet bundles
of some Stigmarian axes are accompanied by a parichnos strand, as Weiss
has shown, may prepare us for the discovery of surface-features on
_Stigmariae_ not unlike those of _Omphalophloios_. (Fig. 193, C.)
A possible comparison may be suggested also with _Sigillaria Brardi_
as figured by Germar (fig. 196, A) in which circular scars, which may
be the scars of rootlets, occur below the leaf-base areas. It is not
impossible that in the surface-features of _Omphalophloios_ we have
both leaf and rootlet scars represented.
_General considerations._
The solid xylem core characteristic of the stele of some species of
Palaeozoic Lycopodiales (e.g. _Lepidodendron esnostense_ and _L.
rhodumnense_) may probably, as Tansley and Chick[653] point out, be
regarded as the lineal descendant of a primitive axial strand of
water-conducting elements. In the course of evolution the centre of the
tracheal column became partially converted into parenchymatous tissue,
as in _Lepidodendron vasculare_. The arrangement of the short cells
in regular vertical series is reminiscent of an early stage in the
development of tracheae: instead of forming tubular conducting elements
the central part of the stelar meristem acquired the short-celled form;
some of the cells became lignified as isodiametric storage tracheae
while others persisted as thin-walled parenchyma.
The production of secondary xylem and an increase in the girth of the
whole stem led to reduction in the amount of centripetally developed
conducting channels. Some of these assumed a new rôle and a shape in
harmony with their functions. A later stage is represented by a further
encroachment of the central parenchyma on the cylinder of centripetal
xylem, as seen in _Lepidodendron Harcourtii_ and other species. The
next stage is afforded by ribless species of _Sigillaria_ in which
the primary xylem is broken up into separate conducting strands. As
Kidston[654] reminds us, it is in the geologically more recent species
of _Sigillaria_, such as _S. Brardi_, which persist into the Permian
era, that this more extreme case of reduction occurs. The older genus
_Lepidodendron_ seems to have retained to the last the complete
cylinder of primary xylem. In the stele of _Stigmaria_, the rhizome of
_Sigillaria_ and of _Lepidodendron_, reduction of the centripetal xylem
has passed beyond the stage represented by the broken cylinder of the
ribless Sigillarias. With the exception of the examples described by
Renault[655] and by Weiss[656], _Stigmaria_ is characterised by little
or no centripetal primary xylem. It is, however, noteworthy that
Renault’s _Stigmaria_, in which centripetal xylem forms a prominent
feature, is attributed to _Sigillaria Brardi_, a species in which the
vascular cylinder of the aerial stem illustrates a later and not an
earlier phase in the replacement of centripetal by centrifugal wood.
It would seem, as Lady Isabel Browne[657] says, that most Stigmarian
axes had reached a more advanced stage in specialisation than is shown
in the stelar structure of the aerial shoots. The relatively greater
and probably the more precocious development of secondary xylem in
_Stigmaria_ than in _Lepidodendron_ or _Sigillaria_ may have some
significance in relation to the smaller amount of “old wood[658]” (in a
phylogenetic sense) in their steles.
As is pointed out in a later chapter, recent researches into the
anatomy of extinct members of the Osmundaceae by Kidston and
Gwynne-Vaughan have brought to light a striking parallelism in
evolutionary sequence between the Lepidodendreae and the ancestors of
_Osmunda_ and _Todea_, the two surviving genera of one of the most
ancient families of ferns.
There can be little doubt as to a very close relationship between
_Sigillaria_, _Lepidodendron_, and _Bothrodendron_. _Sigillaria_ seems
to have outlived _Lepidodendron_ and _Bothrodendron_. The two latter
genera are recorded from Upper Devonian rocks in several localities,
_Bothrodendron_ being particularly abundant in the pre-Carboniferous
floras of Bear Island and other parts of the world. A remarkable
stem described by Dr White[659] as _Archaeosigillaria primaeva_ from
Upper Devonian shales of New York is spoken of by him as “one of the
most highly developed representatives of a fairly distinct archaic
group foreshadowing the later genera _Bothrodendron_, _Sigillaria_,
_Lepidodendron_ and _Lepidophloios_.” The type-specimen, when first
discovered, consisted of an apparently unbranched stem reaching a
length of 5 metres. From the swollen basal part Stigmaria-like
rootlets spread into the surrounding shale. At a higher level the
fissured bark shows indistinctly defined leaf-cushions which pass
gradually upwards into cushions and scars arranged in closer order
on regular vertical ribs. The surface-features in this region are
practically those of a ribbed _Sigillaria_. Traced farther upwards the
vertical ribs die out and cushions of the Lepidodendroid form cover
the surface of the bark. The leaf-scars, with a supraposed ligular
pit and two vertically elongated parichnos-scars, are said to bear
a closer resemblance to those of _Sigillaria_ and _Bothrodendron_
than to the leaf-areas of _Lepidodendron_. Nothing is known as to the
anatomy of this stem, nor have fertile shoots been discovered. In the
absence of more trustworthy evidence than is available conclusions of a
phylogenetic nature must be accepted at their true value. It is however
legitimate to describe _Archaeosigillaria primaeva_ as one of the
oldest examples of a lycopodiaceous plant which shows well-preserved
external features, and these are of exceptional interest as indicating
a combination of generic characters. This Devonian type lends support
to the view that _Lepidodendron_ and _Sigillaria_ are offshoots,
differing from one another in comparatively unimportant points, from a
common ancestral type.
The generally accepted statement that arborescent Palaeozoic
Lycopodiales bore their sporangia on specially modified leaves
(sporophylls) grouped in cones which were usually produced at the tip
of slender branches, has recently shared the fate of most rules. Prof.
Bower in his _Origin of a Land Flora_ mentions a Belgian specimen of
_Pinakodendron musivum_ Weiss from the Westphalian series (Middle
Coal-Measures), to be described by Dr Kidston, which bore its sporangia
“associated with the leaves of certain portions of the stem, without
any cone-formation. The fertile and sterile portions are distinguished
only by the presence or absence of sporangia[660].”
_Lepidodendron_ and _Sigillaria_ can hardly be claimed as the direct
ancestors of any existing type of Lycopodiales, but while exhibiting
points of contact with _Lycopodium_, _Selaginella_, and _Psilotum_ they
are perhaps more closely allied to _Isoetes_.
Lady Isabel Browne[661], who has recently published an excellent
summary of the evidence on the relation of the Lepidodendreae to
_Isoetes_, concludes her examination of the arguments by expressing the
opinion that there is a strong probability of the correctness of the
view that _Isoetes_ may be derived “from the Lepidodendraceae in the
widest sense of the word.” This decision seems to me to accord best
with the facts.
The further question as to the relation of these Palaeozoic genera to
plants higher in the scale must be reserved for fuller consideration
in another volume. An attempt will also be made to consider how far
anatomical structure may be used as a guide to the conditions under
which _Lepidodendron_ and _Sigillaria_ as well as other members of
the Permo-Carboniferous floras passed their lives. The secondary
xylem of _Lepidodendron_ and _Sigillaria_ affords a striking example
of water-conducting tissue of homogeneous structure comparable
with the wood of Conifers rather than with that of Angiosperms. It
was presumably formed, for the most part, under uniform climatic
conditions: the absence of rings of growth points to uninterrupted
supply to evergreen shoots exposed to no alternation of activity and
arrested growth. Attention has already been called to the absence of
any tissue corresponding to secondary phloem. Even in young shoots of
_Lepidodendron_, no tissue has been found external to the meristematic
zone agreeing in the form of its elements with the channels through
which the elaborated food is conveyed from the leaves of recent plants
to the regions of cell-building. That the ‘secretory zone’ may have
served this purpose, at least in young stems, is not improbable. On
the other hand, it is difficult to understand why older Lepidodendron
stems show no indication of additions to the secretory zone. If this
tissue served for the transport of proteids we should expect to find
provision made for its constant renewal _pari passu_ with the secondary
growth of the xylem. The conclusion seems to me inevitable that the
supply of building-material was otherwise provided for than in recent
vascular plants. The physiological division of labour may have been
less complete in the tissue-systems of the Palaeozoic Lycopods than
in the more highly specialised organs of such an extinct genus
as _Lyginodendron_ or than in recent plants. Our knowledge of the
anatomical structure of many extinct types has already reached a stage
when we should take greater heed of the _modus operandi_ of the complex
machinery revealed by a study of petrified stems. From the known we
proceed to interpret the unknown; but there is a danger of neglecting
the possibilities of evolution during the countless ages which separate
the forests of the Coal period from those of the present era. We may
easily allow preconceived ideas to warp our judgment in attempting
to distribute the manifold activities which made up the life of a
_Lepidodendron_ among the structural units of the plant-body.