=Sigillaria.=
i. _General._
In view of the close resemblance between _Lepidodendron_ and
_Sigillaria_, another lycopodiaceous plant characteristic of
Carboniferous and Permian floras, a comparatively brief description of
the latter genus must suffice, more particularly as _Lepidodendron_
has received rather an undue share of attention. _Sigillaria_,
though abundantly represented among the relics of Palaeozoic floras,
especially those preserved in the Coal-Measures, is rare in a petrified
state, and our knowledge of its anatomy is far from complete. In
external form as in internal structure the difference between the two
genera are not such as enable us to draw in all cases a clearly defined
line of separation.
In the _Antediluvian Phytology_, Artis[448] figured a fossil from the
Carboniferous sandstones of Yorkshire which he called _Euphorbites
vulgaris_ on account of a superficial resemblance to the stems of
existing succulent Euphorbias. Rhode[449] also compared Sigillarian
stems with those of recent _Cacti_. The specimen described by Artis
is characterised by regular vertical and slightly convex ribs bearing
rows of leaf-scars in spiral series, like those on the cushions of
_Lepidodendron_. A few years earlier Brongniart[450] had instituted the
genus _Sigillaria_[451] for plants with ribbed but not jointed stems
bearing “disc-like impressions” (leaf-scars) disposed in quincunx; the
type-species named by the author of the genus _Sigillaria scutellata_
is identical, as Kidston[452] points out, with _Euphorbites vulgaris_
of Artis and with the plant afterwards figured by Brongniart as _S.
pachyderma_[453]. Brongniart in 1822 figured another type of stem
characterised by the absence of ribs and by prominent spirally arranged
cushions bearing relatively large leaf-scars like the upper part of
the specimen shown in fig. 203; this he named _Clathraria Brardii_, a
well-known and widely distributed Carboniferous and Permian species
now spoken of as _Sigillaria Brardi_ (figs. 196, A–C; 203). A third
type of stem figured by Brongniart as _Syringodendron striatum_[454]
agrees with _Sigillaria scutellata_ in having ribs, but differs in the
substitution of narrow oval ridges or depressions for leaf-scars; this
is now recognised as a partially decorticated _Sigillaria_, in which
the vascular bundle of each leaf is represented by a narrow ridge or
depression. The name _Syringodendron_, originally used by Sternberg, is
conveniently applied to certain forms of Sigillarian stems which have
lost their superficial tissues. A fourth generic name, _Favularia_, was
instituted by Sternberg[455] for Sigillarian stems with ribs covered
with contiguous leaf-scars of hexagonal form and prominent lateral
angles (fig. 193, A; fig. 200, G).
[Illustration: FIG. 193.
A. _Sigillaria elegans_ Brongn.
B. _Sigillaria rugosa_ Brongn. Middle Coal-Measures.
C. _Omphalophloios anglicus_ Kidst. Barnsley.
D. _Sigillaria elegans_ Brongn.
E. _Sigillaria tessellata_ Brongn.
(A, B, C, E, about ¾ nat. size. Dr Kidston’s Collection.)]
The generic or sub-generic title _Rhytidolepis_, also instituted
by Sternberg, is applied to ribbed Sigillarian stems such as _S.
scutellata_, _S. rugosa_ (fig. 193, B), _S. mammillaris_ (fig. 195),
or _S. laevigata_ (fig. 196, D). Goldenberg[456] proposed the name
_Leiodermaria_ for smooth Sigillarian stems with leaf-scars not in
contact with one another (fig. 196, C).
The shoot system of _Sigillaria_ consisted of a stout stem tapering
upwards to a height of 100 feet[457] or more as an unbranched column,
with its dome-shaped apex[458] covered with linear grass-like
leaves or, in some species, such as _Sigillaria Brardi_[459], _S.
Eugenii_[460], etc., the main trunk was occasionally divided by
apparently equal dichotomy. The younger portions of the stem or
branches were in some species clothed with leaves separated by a narrow
zigzag groove surrounding their hexagonal bases, while in other
forms each leaf was seated on a more or less prominent cushion having
the form illustrated by _Sigillaria McMurtriei_ (fig. 194) or by the
example represented in fig. 200, H; or as in the ribbed species shown
in figs. 193, B, and 195, the leaves in vertical series were separated
from one another by longer portions of the ribs. As in _Lepidodendron_
the cushions are frequently characterised by irregular transverse
wrinklings and other[461] surface-ornamentation which in some instances
at least may have been produced as the result of _post-mortem_
shrinkage of superficial tissue. From the rarity of shoots with the
foliage attached, it would seem that the leaves persisted for a
comparatively short time and were cut off by an absciss-layer leaving
behind a well-marked leaf-scar area. The linear leaves, reaching in
rare cases a length of one metre (e.g. _S. lepidodendrifolia_) but
usually much shorter, possessed a single median bundle, and the lower
face was characterised by two stomatal grooves and a median keel. It is
not uncommon to find leaf-bases of _Sigillaria_ detached from the stem
and preserved as separate impressions. The term _Sigillariophyllum_
used by Grand’Eury[462] may be applied to detached leaves, though it
is by no means easy to distinguish between the foliage of _Sigillaria_
and _Lepidodendron_. A comparison of a typical species of _Sigillaria_,
such as _S. rugosa_ (fig. 193, B) or _S. Brardi_ (fig. 196, A–C) with
a typical _Lepidodendron_ reveals obvious differences in the form of
the leaf-cushion, but in some cases the distinction becomes purely
arbitrary.
[Illustration: FIG. 194. _Sigillaria McMurtriei_ Kidst. From a
specimen from the Upper Coal-Measures of Radstock, in the British
Museum (V. 952). Nat. size.]
[Illustration: FIG. 195. _Sigillaria mammillaris._ (Rhytidolepis
form.) From a specimen in the Manchester Museum. _p_, parichnos;
_l_, ligule-pit; _t_, leaf-trace; _c_, cushion; _s_, leaf-scar.]
[Illustration: FIG. 196.
A–C. _Sigillaria Brardi._ (A after Germar; B, C after Zeiller.)
D. _Sigillaria laevigata._
E. _Lepidodendron Wortheni_ (D and E after Zeiller).]
Immediately above the centre of the upper boundary of a Sigillarian
leaf-scar a ligule pit may often be detected, as shown in fig. 195,
_l_, and in some cases, e.g. a specimen figured by Germar[463] (fig.
196, A) as _Sigillaria spinulosa_ (identical with _S. Brardi_), some
circular scars with a central pit surrounded by a raised rim occur
on the surface of the stem, either singly or in pairs, near the
leaf-scars; these, it is suggested, may represent the position of
adventitious roots or, as Germar thought, of some deciduous spinous
processes. The leaf-scars are frequently hexagonal in shape, with the
lateral angles either rounded (fig. 200, F) or sharply pointed (fig.
200, G, H); each scar bears three smaller scars as in _Lepidodendron_,
a central circular, oval or crescentic leaf-trace scar and larger oval
or slightly curved scars formed by the two parichnos arms (fig. 195,
_p_). The larger size of the parichnos arms, the individual cells of
which may often be detected as a fine punctation, is a distinguishing
feature of the genus, but otherwise the structure is very similar to
that in _Lepidodendron_. As shown in figs. 195, 200, F, G, the three
scars may occur nearer the upper than the lower margin of the leaf-base
area.
_Lepidodendron Wortheni_[464] (fig. 196, E), described from North
America by Lesquereux[465], by Zeiller[466] from France, and by
Kidston[467] from the Upper and Middle Coal-Measures of England,
may be quoted as a _Lepidodendron_ bearing a close resemblance to
_Sigillaria_. The shoots bear cushions two or three times as long
as broad and without the usual median division, but with numerous
irregular and discontinuous transverse wrinklings. _Lepidodendron
Peachii_ Kidston[468] affords another example of a form agreeing both
with _Sigillaria_ and with _Lepidodendron_. An Upper Devonian type
described by White[469] as _Archaeosigillaria primaeva_ affords a
striking instance of the combination on one stem of Sigillarian and
Lepidodendroid leaf-cushions.
The difference between the original surface of a Sigillaria stem and
that of partially decorticated specimens is seen in figs. 196, C and
D; in fig. C the bark of _Sigillaria Brardi_ shows the characteristic
wrinklings of the superficial tissue, while at a slightly lower level
the leaf-scars are replaced by the parichnos casts, a, and fine
longitudinal striations represent the elongated phelloderm cells laid
bare by the exfoliation of the surface-layers. Similarly, in the rib
of _Sigillaria laevigata_ (fig. 196, D) the parichnos arms, _p_, and
the longitudinal striations are exposed at the lower level, while the
surface is smooth and bears rows of widely separated leaf-scars.
[Illustration: FIG. 197. _Carica sp._ From the Royal Gardens, Kew.
(Much reduced.) M.S.]
The older part of a Sigillarian stem may present an appearance very
different from that of the younger shoots. The leaf-cushions may be
stretched apart as the result of elongation and increase in girth,
while in some cases the arrangement of the leaf-scars may vary on the
same axis as the result of inequalities in growth or changing climatic
conditions. The contiguous arrangement of the leaf-scars and narrow
cushions characteristic of the Clathrarian form of stem, as was first
demonstrated by Weiss[470], and afterwards illustrated by Zeiller[471]
and Kidston, may be gradually replaced (on the same specimen) by a more
distant disposition of the leaf-scars separated by a smooth intervening
surface of bark. The specimen of _S. Brardi_ reproduced in part in
fig. 203, and first figured by Kidston, affords an example of three
“species” on one piece of stem, _S. Brardi_ Brongn., _S. denudata_
Goepp. and _S. rhomboidea_ Brongn.[472]
The piece of _Carica_ stem, represented in fig. 197, illustrates the
danger of trusting to the disposition of leaves as a specific criterion.
Similarly, in the ribbed forms the degree of separation of the
leaf-scars is by no means uniform in a single species[473]. Some
authors have adopted a two-fold classification of Sigillarian stems
proposed by the late Prof. Weiss[474] of Berlin, who divided the
Sigillariae into (A) Sub-Sigillariae, comprising Leiodermariae and
Cancellatae, and (B) Eu-Sigillariae, including _Favulariae_ and
_Rhytidolepis_. Grand’Eury[475] adopts the terms _Rhytidolepis_ and
_Leiodermaria_ for ribbed and smooth stems respectively, the type to
which the name _Clathraria_ was applied by Brongniart being in some
cases at least the young form of Leiodermarian stems. While recognising
the artificial distinction implied by such terms as _Rhytidolepis_,
_Leiodermaria_, and other sub-generic titles, we may conveniently speak
of the two main types of _Sigillaria_ stems as ribbed and smooth.
Still older stems of _Sigillaria_ are not uncommon from which the
leaf-scars and other superficial tissues have been exfoliated,
leaving exposed a longitudinally fissured surface of secondary cortex
characterised by pairs of considerably enlarged parichnos strands
(fig. 198) which are sometimes partially or wholly fused into one
(_Syringodendron_ state of _Sigillaria_). The single or double nature
of the elliptical or circular parichnos areas is doubtless due to the
degree of exfoliation, which may extend sufficiently deep into the
cortex to reach the level of the parichnos before the single strand has
bifurcated (cf. _Lepidodendron_, p. 100). In the Museums of Manchester,
Newcastle, and other places casts of large Sigillaria stems may be
seen, which illustrate the differences in breadth and regularity of
the vertical ribs, and in the size and shape of the parichnos areas
in different regions of a partially decorticated stem. A cast of a
ribbed species in the Manchester Museum, having a length of 185 cm.
and a breadth of 56 cm., shows in the upper portion straight vertical
grooves and broad ribs bearing pairs of parichnos scars 11 mm. long;
in the lower portion the ribs tend to become obliterated and the
parichnos scars, 2 cm. in length, may be partially fused and arranged
in much less regular vertical series. A feature of these older ribbed
Sigillarian stems is the increase in the number of the ribs from below
upwards. Kidston[476] has described a specimen in the Sunderland
Museum, 6 feet 6 inches long, with a circumference at the slightly
bottle-shaped base of 5 feet. On the lower portion of the stem there
are 29 broad ribs; about one-third the height many of these bifurcate,
producing as many as 40 ribs in the upper part where the cast has a
circumference of 3 feet. The increase in number of the ribs is due in
part to bifurcation, but also to the intercalation of new ones. As
Kidston points out, this example shows that as a stem grew in length
additional leaves were developed at the apex. A similar stem, which
illustrates very clearly the increase in the number of ribs from below
upwards, may be seen in the Newcastle Museum.
Grand’Eury[477] has described an example of an old stem of a ribless
species of _Sigillaria_, _Syringodendron bioculatum_, bearing single
and double parichnos areas of nearly circular form and with a
diameter of 1–2 cm. In a specimen figured by Renault and Roche[478]
(_Syringodendron esnostense_) from the Culm strata in France, the
parichnos scars reach a length of 3 cm. As seen in the fragment of a
ribbed _Sigillaria_ represented in fig. 198, the large parichnos areas
exhibit a distinct surface pitting in contrast to the fine longitudinal
striation of the rib; the difference in surface-appearance is due to
the nature of the tissue, which in the parichnos consists of fairly
large parenchymatous elements with groups of secretory cells[479], and
in the exposed cortex of elongated elements. The vertical line in the
middle of fig. 198, which occurs in the middle of the rib, has probably
been formed by splitting of the bark.
[Illustration: FIG. 198. _Sigillaria_ with large parichnos areas. (⅓
nat. size.) M.S.]
Grand’Eury’s description of fossil forests of Sigillariae in the
rocks of the St Étienne[480] district affords a striking picture of
these arborescent Pteridophytes; he speaks of the stems of some of
the trees as swollen like a bottle at the base, characterised by the
Syringodendron features and terminating below in short repeatedly
forked roots of the type known as _Stigmariopsis_. Other specimens
of _Sigillaria_ stumps show a marked decrease in girth towards the
base; this tapered form is regarded by Grand’Eury as the result of the
development of aerial columnar stems from underground rhizomes.
The nature of the root-like organs of _Sigillaria_ is dealt with in the
sequel: a brief reference may, however, be made to the occurrence of
stumps of vertical trunks which pass downwards into regularly forked
and spreading arms. These arms lie almost horizontally in the sand or
mud like the underground rhizomes of _Phragmites_ and other recent
plants growing in swampy situations where water is abundant and where
deeper penetration of the soil would expose them to an insufficient
supply of oxygen[481]. It is certain that _Sigillaria_ had no tap-root,
but was supported on spreading subterranean organs bearing spirally
disposed long and slender rootlets which absorbed water from a swampy
soil.
[Illustration: FIG. 199. Partially decorticated stem of _Sigillaria_
showing two zones of cone-scars. From a cast in the Sedgwick
Museum, Cambridge. M.S. (⅕ nat. size.)]
The regularity of the leaf-scar series on a Sigillarian stem may be
interrupted by the occurrence of oval scars with a central scar and
surrounding groove (fig. 193, E); these occur in zones at more or less
regular intervals on the stem, as seen in the partially decorticated
cast represented in fig. 199. Zeiller has pointed out that the rows of
oval or circular scars, which mark the position of caducous stalked
strobili, may occur between the leaf-scars in vertical series, each of
which may include as many as 20 scars, while in other cases a single
series of such cone-scars may encircle the stem[482]. The zones are
usually of uneven breadth, as in _S. Brardi_, and their occurrence
produces some deformation of the adjacent leaf-scars.
By the earlier writers _Sigillaria_ was compared with succulent
Euphorbias, Cacti, and Palms; Brongniart[483] at first included
undoubted Sigillarian stems among Ferns, but after investigating
an agatized stem from Autun, he referred _Sigillaria_ to the
Gymnosperms[484] on the ground that it had the power of producing
secondary wood. It was then supposed that _Lepidodendron_ possessed
only primary xylem, and that the presence of a vascular meristem in
_Sigillaria_ necessitated its separation from the lycopodiaceous genus
_Lepidodendron_ and its inclusion in the higher plants. By slow degrees
it was recognised, as in the parallel case of the genus _Calamites_,
that the presence or absence of secondary vascular tissue is a
character of small importance. Williamson, whose anatomical researches
played the most important part in ridding the minds of palaeobotanists
of the superstition that secondary growth in thickness is a monopoly
of the Phanerogams, spoke in 1883 of the conflict as to the affinities
of _Lepidodendron_ and _Sigillaria_ as virtually over but leaving
here and there “the ground-swell of a stormy past[485].” In 1872
the same author had written: “If then I am correct in thus bringing
the Lepidodendra and Sigillariae into such close affinity, there is
an end of M. Brongniart’s theory, that the latter were gymnospermous
exogens, because the cryptogamic character of the former is disputed by
no one; we must rather conclude as I have done that the entire series
represents, along with the Calamites, an exogenous group of Cryptogams
in which the woody zone separated a medullary from a cortical
portion[486].”
In 1879 Renault[487] expressed the opinion that Brongniart by his
investigation of the anatomy of _Sigillaria elegans_ had established in
a manner “presque irréfutable” that _Sigillaria_ must be classed as a
Gymnosperm showing affinity with the Cycads.
In 1855 Goldenberg[488] described some strobili which he regarded as
those of _Sigillaria_ and recognised their close resemblance to a
fertile plant of _Isoetes_. He was led to the conclusion, which had
little influence on contemporary opinion, that _Sigillaria_ is related
to _Isoetes_ and must be classed among Pteridophytes. To these long
and narrow strobili Schimper gave the name _Sigillariostrobus_[489].
In 1884 Zeiller[490] supplied confirmation of Goldenberg’s view by
the discovery of cones borne on pedicels with Sigillarian leaf-scars,
thus demonstrating the generic identity of cones and vegetative
shoots, which Goldenberg had connected on the evidence of association.
Zeiller’s more recent work[491] and the still later researches of
Kidston[492] have added considerably to our knowledge of the morphology
of Sigillarian cones. Grand’Eury’s remark made so recently as 1890[493]
that opinion in regard to the Gymnospermous nature of _Sigillaria_ is
losing ground every day, bears striking testimony to the pertinacity
with which old beliefs linger even in the face of overwhelming proof of
their falsity.
It is remarkable, in view of the abundance of vegetative shoots,
how rarely undoubted Sigillarian strobili have been found; this
may, however, be in part due to a confusion with Lepidostrobi
which so far as we know do not differ in important respects from
Sigillariostrobi[494].
There can be no doubt that _Sigillaria_ usually produced its cones
on slender pedicels which bore a few leaves or bracts in irregular
verticils, or in short vertical series on comparatively stout stems, an
arrangement reminding us of the occurrence of flowers on old stems of
_Theobroma_ and other recent Dicotyledons. As Renault[495] pointed out
the fertile shoots are axillary in origin.
Dr Kidston[496] is of opinion that certain species of _Sigillaria_
bore cones sessile on large vegetative shoots characterised by two
opposite rows of cup-like depressions like those in the Ulodendron
form of _Lepidodendron Veltheimianum_ (fig. 157). He has described
the Ulodendron condition of two species, _Sigillaria discophora_
(König) and _S. Taylori_ (Carr.); the cup-like depressions may have a
diameter of several centimetres and are distinguished from those of
_Bothrodendron_ by the almost central position of the umbilicus. The
specimens which he figures as _S. discophora_ are identified by him
with the stem figured by König as _Lepidodendron discophorum_ and by
Lindley and Hutton[497] as _Ulodendron minus_. We have already dealt
with the nature of Ulodendron shoots, expressing the opinion that in
spite of the often quoted specimen described by D’Arcy Thompson[498],
in which a supposed cone occurs in one of the cups, there is no
satisfactory case of any undoubted cone having been found attached to
the large Ulodendron scars. It is more probable that the Ulodendron
depressions represent the scars of branches, either elongated axes,
or possibly in some cases deciduous tuberous shoots which served as
organs of vegetative reproduction. A specimen figured by Kidston as
_Sigillaria Taylori_ from the Calciferous sandstone of Scotland[499]
bears a row of slightly projecting “appendicular organs” attached to a
Ulodendron axis; but these furnish no proof of their strobiloid nature.
The main question is, are these Ulodendron shoots correctly identified
by Kidston as Sigillarian? The surface of the specimens shows crowded
rhomboidal scars surrounded in some cases by a very narrow border
or cushion; the general appearance is, as Kidston maintains, like
that of _Sigillaria Brardi_ in which the leaf-scars are contiguous
(e.g. fig. 203, upper part). None of the leaf-scars exhibit the three
characteristic features, the leaf-trace and parichnos scars, but only
one small scar appears on each leaf-base area. In a more recent paper
Kidston figures a small piece of a stem from Kilmarnock, which he
identifies as _Sigillaria discophora_, showing the three characteristic
scars on the leaf-base area. There is no doubt as to the Sigillarian
nature of this specimen, but it is not clear if the piece figured is
part of a Ulodendron shoot[500].
Prof. Zeiller[501] retains the older name _Ulodendron minus_ Lind.
and Hutt. in place of König’s specific designation and dissents
from Kidston’s identification of _Ulodendron minus_ and _U. majus_
of Lindley and Hutton as one species; he is also inclined to refer
these Ulodendron axes to _Lepidodendron_. In spite of the superficial
resemblance to _Sigillaria_ of the specimens described by Kidston,
and which I have had an opportunity of examining, I venture to regard
their reference to that genus as by no means definitely established.
We must recognise the difficulty in certain cases of drawing any
satisfactory distinction between _Sigillaria_ and _Lepidodendron_ based
on external features, and while giving due weight to the conclusions
of so experienced a palaeobotanist as my friend Dr Kidston, I venture
to think we are not in a position to state with confidence that
_Sigillaria_ possessed Ulodendron shoots.
ii. _Leaves._
The leaves of _Sigillaria_ agree closely with those of _Lepidodendron_;
they are either acicular (fig. 200, D) like Pine needles or broader
and flatter like the leaves of _Podocarpus_. Their attachment to
comparatively thick branches[502] shows that they persisted, in some
cases at least, for several years as in _Araucaria imbricata_. The
lower surface of the lamina was characterised by a prominent keel (fig.
142, A and C) which dies out towards the apex; on either side of it
are well-defined stomatal grooves (figs. 142, _g_, _g_; 143, A; 200,
D, _g_). The upper face may be characterised by another groove (fig.
142, B) but without stomata. The occurrence of the stomatal grooves,
the abundance of transfusion tracheae (fig. 142, _t_) surrounding
the vascular bundle, and the presence of strengthening hypodermal
tissue suggest that the leaves of _Sigillaria_ were of a more or less
pronounced xerophilous type and had a fairly strong and leathery
lamina. The mesophyll tissue consists either of short parenchymatous
cells or of radially elongated palisade-like elements and has the
loose or lacunar arrangement characteristic of the aerating system in
recent leaves; the slight development or absence of palisade-tissue may
indicate exposure to diffuse light of no great intensity.
In most species there is a single vein, but in others the xylem forms
a double strand (fig. 142, B). Sections of the lamina near the apical
region present a more circular form, owing to the gradual obliteration
of the upper groove and lower keel and to the dying out of the stomatal
grooves.
The transverse section of the leaf diagrammatically represented in
fig. 142, A, A′, shows the two stomatal grooves, _g_, and a prominent
keel; the single vein consists of a small group of primary tracheae,
_x_, some delicate parenchyma, and a brown patch of imperfectly
preserved tissues, _a_, resembling the secretory zone tissue of a
_Lepidodendron_. The whole is surrounded by a sheath of rather wide
and short thinner-walled spiral or reticulate tracheids, which may
be spoken of as transfusion tracheae, _t_, and compared with similar
elements in the leaves of many recent Conifers. To this tissue Renault
applies the epithet “water-bearing” and it is very likely that this
may have been its function. The shaded portions of the lamina, in fig.
142, A, represent the distribution of thicker-walled hypodermal tissue.
The section of a leaf 3 mm. wide shown in fig. 142, C, shows an almost
identical structure; the transfusion tracheae are richly developed
especially on the sides and lower surface of the vascular strand.
This leaf occurs in association with a petrified stem of _Sigillaria
scutellata_[503].
[Illustration: FIG. 200.
A. _Sigillaria Brardi_ [= _S. elegans_ Brongn. (39)]. Transverse
section of stem.
B. _Sigillaria Brardi_ [= _S. spinulosa_, Renault and Grand’Eury
(75)]: _c_³, outer cortex; _x_, _x_², xylem. (After Renault
and Grand’Eury.)
C. _S. Brardi_, primary xylem element. (A and C after Brongniart.)
D. Leaf of _Sigillaria Brardi_: _g_, _g_, stomatal grooves; _ep_,
piece of epidermis of stem. (After Renault.)
E. _Sigillaria Brardi._ Tangential section of leaf-bases: _p_,
parichnos. (After Renault.)
F, G, H. Sigillaria leaf-scars and cushions. (After Weiss.)]
Renault[504] has shown that the leaf-traces of _Sigillaria spinulosa_
(= _S. Brardi_) are accompanied in the outer cortical region of the
stem by a fairly large amount of secondary xylem; in sections of the
free lamina which he figures the secondary elements are much less
obvious and represented by a few tracheae only. Similarly, in the
leaf-base of _S. Brardi_ (fig. 200, E) the xylem consists of both
primary and secondary elements (_x_, _x_²), but in the lamina the
latter is poorly if at all represented. In the lamina of the leaves
of _S. Brardi_ the primary xylem forms a narrow slightly curved band
with two lateral groups of narrower, presumably protoxylem elements;
this is surrounded by delicate parenchyma styled by Renault, on very
slender evidence, phloem (“liber”). Some dark cells below the xylem are
described as sclerous tissue, and surrounding the bundle is a sheath of
transfusion tracheae (dotted area in fig. 200, E). It is possible that
the elements spoken of with hesitation by Renault as secondary xylem
are transfusion tracheae.
There has probably been some confusion in the minds of authors between
sclerous tissue and dark secretory tissue in Sigillarian leaves; the
crescentic band, _a_, shown in fig. 142, B, which corresponds in
position with the sclerous tissue of Renault in _S. Brardi_ leaves,
appears to be of the nature of secretory tissue.
The diagram shown in fig. 142, B, illustrates a type of leaf very like
those already described, except that there are two xylem strands, _x_.
The difference between the double strand and the single bundle seen in
figs. 142, A, C and 200 E, is comparatively small, but it is a real
distinction. This type of leaf (fig. 142, B) was originally described
by Renault[505] under the generic title _Sigillariopsis_. The genus was
founded on a French petrified specimen consisting of part of a ribbed
stem possessing a stele of the Sigillarian type and characterised by
separate primary xylem strands, like those of _S. Brardi_ described
by Brongniart in 1839. Renault considered the presence of two xylem
strands in the leaf a sufficient reason for the institution of a new
genus and named the specimen _Sigillariopsis Decaisnei_. Prof. Bertrand
of Lille kindly photographed for me Renault’s type-specimen and sent
several prints with explanatory notes. The transverse section of the
leaves shows very clearly the two xylem strands; each strand consists
of a triangular group of primary tracheae with the protoxylem apex
pointing towards the lower surface of the lamina. Below each primary
strand of centripetal xylem is an arc composed of a few small tracheae
which Renault and Bertrand describe as secondary xylem; it is, however,
not clear from the photomicrographs that these are of secondary origin,
their position and appearance reminding one of the primary centrifugal
xylem of a cycadean foliar bundle. Below this centrifugal xylem is
another arc of imperfectly preserved elements described by Renault as a
protective sheath and by Bertrand as glandular tissue; the latter term
is probably the more correct as the tissue may well correspond to the
secretory-zone tissue of Lepidodendron stems. Fairly large groups of
transfusion tracheids occur on the flanks of the xylem. Prof. Bertrand
points out that one of his sections, cut nearer the apex of a leaf than
that figured by Renault with a single xylem strand, contains a double
strand and thus shows the latter’s description to be an incorrect
interpretation of the imperfectly preserved tissues.
The _Sigillariopsis_ type of leaf was recognised by Scott[506] in
English material on which he founded the species _Sigillariopsis
sulcata_. In a section which he has recently figured[507] a lacuna
below the two xylem strands is described as “representing secretory
tissue”; a band of transfusion tracheae almost encircles the pair of
bundles.
In a note published in 1907, Kidston[508] demonstrated the association
of _Sigillariopsis_ leaves with an undoubted Sigillarian stem of the
Rhytidolepis type and expressed his conviction that Renault’s genus is
identical with _Sigillaria_. The correctness of Kidston’s conclusion
has been proved by Arber and Thomas[509] who found that the leaf-traces
of _Sigillaria scutellata_ bifurcate during their course through the
outer region of the cortex and enter the leaf as two distinct strands
of primary xylem. In the section from Dr Kidston’s collection shown in
fig. 142, B, the lamina, 4 mm. wide, consists mainly of thin-walled
assimilating tissue composed of radially elongated cells abutting at
the periphery on hypodermal mechanical tissue, except at the edges of
the stomatal grooves which are bounded by the small-celled epidermis.
A broad sheath of thicker-walled elements, _s_, surrounds numerous
scattered transfusion tracheae, _t_, and below the two xylem strands,
_x_, which are embedded in delicate parenchyma there is a crescentic
band of dark tissue, _a_, resembling the smaller strand, _a_, in fig.
142, A′, and the secretory zone tissue of a Lepidodendron stem.
iii. _Fertile shoots of_ Sigillaria.
Reference has already been made to the manner of occurrence of strobili
on Sigillarian stems; it remains to describe the structure of these
reproductive shoots. _Sigillariostrobus_, the name given to Sigillarian
strobili, may be defined in general terms as follows:
Cylindrical cones, rarely dichotomously branched[510] as in species
of _Lycopodium_ and _Selaginella_, which may reach a length of 30 cm.
(_e.g._ _Sigillariostrobus nobilis_ Zeill.[511]) and a diameter 2–5
cm.; peduncle long and slender, sometimes bearing acicular bracts or,
after leaf-fall, characterised by leaf-cushions and leaf-scars like
those on vegetative shoots (fig. 201, E). The stalked cones are borne
in irregular verticils and in some species in vertical series, the
fertile zones being separated by comparatively long sterile portions
of the stem (fig. 199). The cones were deciduous and, in certain cases
if not in all, the individual sporophylls became detached from the
cone-axis on maturity. The slender axis bore spiral or verticillate
imbricate sporophylls attached at right angles or more or less
obliquely. The basal rhomboidal portion bore spores on its upper
surface (fig. 201, F), presumably enclosed in a somewhat radially
elongated sporangium (fig. B) and was prolonged distally into a narrow
lanceolate free portion, in some species with a ciliate border (fig.
D). The sporangia probably produced megaspores and microspores, but
such spores as have been recognised appear to belong to the former
category. The designation _Triletes_ is applied to isolated spores of
_Sigillaria_ or to those of _Lepidodendron_.
_Sigillariostrobus Tieghemi_ Zeiller[512] (figs. 201, E, F). In
this species, from the Coal-field of Valenciennes, the pedicel
bore acicular leaves or bracts attached to the upper portion of
leaf-cushions arranged in vertical series (fig. E). The cones reached a
length of 16 cm. and a breadth of 2·5–5 cm.; the sporophylls are borne
in alternating verticils with 8–10 in each whorl. Several megaspores
(2 mm. in diameter) appear to have been produced in tetrads in each
sporangium.
[Illustration: FIG. 201. _Sigillariostrobus._
A, C. _Sigillariostrobus rhombibracteatus_ Kidst. (After Kidston.)
A. Portion of strobilus.
C. Megaspore.
B, D. _Sigillariostrobus ciliatus_ Kidst. (After Kidston.)
E, F. _Sigillariostrobus Tieghemi_ Zeill. (After Zeiller.)]
_Sigillariostrobus rhombibracteatus_ Kidston[513]. Fig. 201, A, C.
Kidston described this species from the Middle Coal-Measures of
England: it is similar in habit and in the form of the sporophylls
to _S. Tieghemi_, but rather smaller, and the more definitely
rhomboidal sporophylls have a ciliate margin. The cone was probably
heterosporous, but megaspores alone have so far been discovered. The
sporophylls bear a close resemblance to those of _Lycopodium cernuum_
(fig. 126, C). In some of the illustrations of this type given by
Kidston the naked cone-axis with its numerous sporophyll-scars is
clearly shown, reminding one of the naked axes of the cones of the
Silver Fir (_Abies pectinata_) or Cedar after the fall of the scales.
Our knowledge of Sigillarian cones is too incomplete to admit of a
detailed comparison with the strobili of _Lepidodendron_ or with those
of recent Pteridophytes. There can, however, be little doubt that
Goldenberg[514] was correct in his selection of _Isoetes_ as the most
nearly allied recent plant so far as the fertile leaves are concerned.
It would seem that the sporangia were comparatively delicate structures
which have left no clearly defined remains of their walls in the
carbonised specimens; Kidston, indeed, speaks of the hollow bases of
the sporophylls as holding the spores, but this is hardly likely to
have been the case. Our knowledge of the anatomy of _Sigillariostrobus_
is practically nil, but in one specimen of a _Sigillaria elegans_
stem Kidston[515] describes the structure of the tissues as seen
in a transverse section of a scar of a fertile shoot; from this we
learn that the stele was composed exclusively of primary tracheids
forming a solid strand without a pith. It is probable that the cones
of _Sigillaria_ were heterosporous, but in no instance have undoubted
microspores been discovered; the megaspores in each megasporangium
were fairly numerous as in _Isoetes_ (fig. 133, E). In one species,
_Sigillariostrobus major_ (Germar), from Permian rocks of France and
Germany, Zeiller[516] states that the whole of a single cone bore
megaspores (0·8–1 mm. in diameter) only; this is, however, not opposed
to the idea of heterospory, as we find instances in _Selaginella_ of
strobili bearing one kind of spore only (cf. p. 56).
In a few instances, it has been possible to correlate cones with
certain species of _Sigillaria_, but in most cases the strobili occur
as isolated fossils.
iv. _The structure of Sigillarian stems._
The first account of the anatomy of _Sigillaria_ we owe to
Brongniart[517] who published a description of the internal structure
of an agatised stem, about 4 cm. in diameter, from Autun, which he
referred to _Sigillaria elegans_. It has, however, been shown by
Zeiller[518] and by Renault that this petrified fragment belongs to
Brongniart’s species _S. Menardi_, which is probably a young form of
_S. Brardi_. Brongniart’s specimen, now preserved in the Paris Natural
History Museum, is a very beautiful example of a silicified plant: on
part of the surface are preserved the hexagonal contiguous leaf-scars,
like those shown in fig. 193, A, and on the polished transverse section
is seen a relatively large stele consisting of a ring of secondary
xylem surrounding a series of crescentic groups of primary xylem (fig.
200, A) enclosing a wide pith occupied by concentric layers of silica.
A portion of the outer cortex is preserved, and this is separated
from the stele by a broad space filled with siliceous rock. The main
features of this type may be described in a few words. The primary
xylem differs from that of such Lepidodendron stems as have been
described in being made up of groups of scalariform and occasionally
reticulate (fig. 200, C) tracheae, having a plano-convex or more or
less crescentic form as seen in transverse section. These primary
strands, in contact with one another laterally, have their narrowest
elements on the outer edge. The leaf-traces are given off from the
middle of the abaxial face of each xylem strand (fig. 202, C, _lt_);
these pass obliquely outwards through medullary rays and then, as in
_Lepidodendron_, turn sharply upwards before bending outwards again
on their way to the leaves. Each leaf-trace consists of a group of
primary tracheae to which a few secondary tracheae are added during
the passage through the secondary wood. The secondary xylem forms a
continuous cylinder of tracheae with scalariform bands on both radial
and tangential walls; the medullary rays are numerous and consist of
long and narrow series, usually one cell broad, of parenchymatous
cells with occasional short rays one or more cells in depth.
The slightly greater breadth of the rays between each primary xylem
strand tends to divide the secondary wood into bundles corresponding
in breadth to the primary groups. The outer cortex closely resembles
that of _Lepidodendron_; it consists internally of radial series
of secondary, elongated and rather stout, elements abutting on the
parenchymatous tissue of the leaf-cushions.
The next contribution to our knowledge of the anatomy of _Sigillaria_
was made by Renault and Grand’Eury[519] who described the structure of
_Sigillaria spinulosa_ Germar[520], a species now recognised as the
Leiodermarian condition of _S. Brardi_, and probably, therefore, not
specially distinct from the specimen described by Brongniart in 1839 as
_S. elegans_. In Brongniart’s fossil the leaf-cushions are in contact
(Clathrarian form of _S. Brardi_: fig. 203, upper part) whereas in
the specimen now under consideration the leaf-scars are further apart
(Leiodermarian form of _S. Brardi_, fig. 203, lower part, and fig. 196,
C). It may be, as Scott suggests, that these two specimens are not
specifically identical but closely allied, an opinion based on certain
anatomical differences[521]; we may, however, include both under the
comprehensive name _S. Brardi_.
The primary xylem (fig. 200, B, _x_), is in some regions separated
into distinct strands, in others it forms a continuous band equal in
length to several of the separate groups. This type of stele, in which
the primary xylem consists in part of separate strands and in part of
a continuous cylinder, forms a transition between that represented
in fig. 200, A, and the steles of _Sigillaria elegans_ (fig. 202,
A) and most species of _Lepidodendron_. The tendency of the primary
xylem strands to become united laterally, forming broader bands,
was first described by Solms-Laubach[522] in a French specimen of
_Sigillaria spinulosa_ in the Williamson collection. The leaf-traces
arise from the middle of the concave outer face of the primary xylem
groups. The inner cortex is composed of small parenchymatous cells as
in _Lepidodendron_, and it is noteworthy that traces of partially
disorganised tissue, described as large canals, in the region external
to the secondary wood, bear a resemblance[523] to the secretory tissue
of _Lepidodendron_.
Other interesting features are presented by the structure of the outer
cortex and the parichnos. The outer cortex in the leaf-scar region is
composed of parenchyma, but for the most part it consists of radially
elongated groups of thin-walled parenchyma enclosed in a framework
of thicker-walled and elongated elements (fig. 200, B, _c_³). This
type of cortex, to which Brongniart applied the name _Dictyoxylon_,
would produce a cast in the case of a partially decorticated stem
characterised by a surface formed of irregularly oval and raised areas
bounded by narrow grooves; the greater prominence of the former being
due to the more rapid decay of the softer tissue, which would produce
depressions on the exposed face of the dead stem. Casts of this type
are not uncommon in Carboniferous rocks, and while some may belong to
the Pteridosperm _Lyginodendron_, others may be those of Sigillarian
stems.
[Illustration: FIG. 202.
A. _Sigillaria elegans_ Brongn. (Section in Dr Kidston’s Collection
cut from the specimen shown in fig. 193, D.)
B, C. _Sigillaria elongata_ Brongn. _lt_, leaf-trace. (From
specimens in the collection of Prof. Bertrand.)]
The large parichnos-strands, produced as in _Lepidodendron_, by the
forking of a single strand arising in the middle cortical region,
consist in part of tissue containing secretory canals, a structure like
that recently described by Miss Coward[524] in the large parichnos
strands of _Syringodendron_ stems.
An example of a decorticated specimen is described by Renault[525] as
_Sigillaria xylina_. This stem is presumably referred to _Sigillaria_
because the primary xylem consists of separate strands. It is
characterised by the unusually large development of secondary wood and
by the relatively small size of the pith. The xylem cylinder has a
diameter of 4–5 cm. and the pith is only 4–5 mm. in breadth.
Another example of a petrified Sigillaria stem has been described by
Kidston[526] as _S. elegans_ Brongn.[527] (fig. 193, D), a species
characterised by vertical rows of sub-hexagonal and contiguous
leaf-scars and by the presence of verticils of cone-scars. Fig. 193, D,
represents Kidston’s specimen in surface-view; one row of leaf-scars
is shown, but most of the superficial tissues have been destroyed.
The crushed stele, 13 mm. in its longest diameter, has a continuous
cylinder of primary xylem, (fig. 202, A, _x_) characterised by a
regularly crenulate outer margin with the smallest elements at the
edge; the prominent ridges separating the sinuses are rounded. The
leaf-traces arise from the bottom of each sinus; the leaf-bundles are
mesarch, and consist exclusively of primary elements. The secondary
xylem, _x²_, like that of the primary xylem, has a crenulate outer
edge. The most interesting feature of the outer cortex is afforded by a
tangential section which, in addition to the leaf-scars, cuts through a
cone-scar showing a solid primary stele surrounded by the cortex of the
cone-peduncle.
Another type of _Sigillaria_, probably _S. elongata_ Brongn. (fig.
202, B, C), which is very similar to _S. scutellata_ has been briefly
described by Prof. Bertrand[528], to whom my thanks are due for the
two photographs reproduced in fig. 202, B., C. His specimen, from the
Pas de Calais Coal-field, shows a ribbed Rhytidolepis form of surface
(fig. 202, B). The stele (fig. 202, C) agrees closely with that of _S.
elegans_ as described by Kidston, but the ridges on the fluted surface
of the primary xylem are more pointed. “In the immediate neighbourhood
of the origin of a leaf-trace, the spiral elements form a median band
in the middle of a sinus” and from this the leaf-traces are given off.
No secondary xylem was found in the leaf-traces at any part of their
course.
Bertrand compares the stele of _S. elongata_ with that of the type
of _Lepidodendron_ represented by the Burntisland species named by
Williamson _L. brevifolium_ (fig. 186) and now usually referred to
_L. Veltheimianum_; the chief distinguishing features are the greater
prominence in the French species of the surface-ridges or teeth of the
primary xylem, a feature which occurs in _L. Wünschianum_, and the
detachment of the leaf-traces from the bottom of each sinus (fig. 202,
C, _lt_) instead of from the sides of the sinus. It is, however, not
clear how far this latter distinction is a real one; in _Lepidodendron
Wünschianum_ the leaf-traces appear to arise, as in _Sigillaria_, from
the middle of each sinus.
Other types of ribbed Sigillaria stems have been briefly described by
Scott[529], Kidston[530], and more recently, by Arber and Thomas[531].
The specimen described by Scott agrees in the main with _S. elegans_ of
Kidston and with _S. elongata_ of Bertrand.
Kidston’s sections of _S. scutellata_ show a continuous primary xylem
cylinder with a slightly and irregularly crenulate outer margin. It
would seem that one important diagnostic character in Sigillarian stems
is afforded by the degree and form of the crenulations on the outer
surface of the primary xylem. _S. scutellata_ has been described also
by Arber and Thomas; these authors were the first to demonstrate the
presence of a ligule and ligular pit on the leaf-base in a petrified
stem, and they also contribute the important fact that the leaf-traces
in passing through the phelloderm bifurcate and enter the leaf as two
distinct vascular strands. This double bundle has been referred to in
the description of Sigillaria leaves. (Page 214.)
Although our knowledge of the anatomy of _Sigillaria_ has been
considerably extended since Williamson[532] drew attention to our
comparative ignorance of the subject, there are several points on which
information is either lacking or very meagre. As regards the stele,
it is in all types so far investigated, of the medullated type and
constructed on the same plan as that of _Lepidodendron Wünschianum_,
_L. Veltheimianum_, and other species. Secondary xylem was developed
at an early stage of growth, and its relation to the primary xylem,
from which as Kidston points out in his description of _S. elegans_,
it may be separated by a few parenchymatous elements, is like that
in _Lepidodendron_. The tendency of the outer face of the secondary
xylem to present a crenulate appearance in transverse sections may,
as Scott thinks[533], be a feature of some diagnostic importance,
but this is not a constant character in the genus. In origin and
in their mesarch structure, the leaf-traces closely resemble those
of _Lepidodendron_. The earlier account of the structure of the
leaf-traces of _Sigillaria_, which were described as possessing both
centrifugal and centripetal wood, led Mettenius[534] to draw attention
to an important anatomical resemblance between this genus and modern
Cycads. This comparison was, however, based on a misconception; the
Cycadean leaf-trace, consisting solely of primary wood, is not strictly
comparable with those of some species of _Sigillaria_, in which one
part of the xylem is primary and another secondary. The occasional
presence of secondary xylem in Sigillarian leaf-traces is matched in
some _Lepidodendra_[535], and cannot be accepted as a distinguishing
feature.
The origin of the leaf-traces from the middle of the sinuses on
the edge of the primary xylem is regarded as a difference; in
_Lepidodendron_ the leaf-traces are said to arise in some species
from the sides of the crenulations; but, as already pointed out,
this is a distinction of doubtful value. The division of the primary
xylem into separate strands in some stems of _Sigillaria_ of the
Clathrarian and Leiodermarian forms is a characteristic peculiarity;
but _S. spinulosa_ forms a connecting link between this type and the
continuous arrangement of the xylem in _S. elongata_ and _S. elegans_.
Kidston[536] has shown that the discontinuous primary xylem occurs in
Lower Permian species, a fact consistent with the view that the greater
abundance of the centripetally developed wood, characteristic of the
older species, represents a more primitive feature. This is not merely
a conclusion drawn from a consideration of geological age, but it is in
harmony with the view expressed by Scott[537] that as plants achieved
greater success in producing secondary centrifugal wood, the retention
of any considerable quantity of primary xylem became superfluous. As
yet we know very little of the structure of the perixylic tissues of
_Sigillaria_, but there is no sufficient reason for supposing that
these differ in essentials from those in _Lepidodendron_. The middle
and outer cortical tissues are practically identical in the two genera.
The parichnos is of the same type, except that in _Sigillaria_ it
reached greater dimensions in the outer part of its course.
V. _Sigillaria Brardi_[538] Brongniart.
Figs. 196, A–C; 200; 203.
1822. _Clathraria Brardi_, Brongniart, Classif. Vég. foss., Pl. XII.
fig. 5.
1828. _Sigillaria Brardi_, Brongniart, Hist. Vég. foss. p. 430, Pl.
CLVIII. fig. 4.
_S. Menardi_, _ibid._ Pl. CLVIII.
1836. _Lepidodendron Ottonis_, Goeppert, Fossil Farnkr. Pl. XLII.
1839. _S. elegans_, Brongniart, Arch. Mus. Nat. Hist. Paris, Vol. I.
p. 406, Pl. XXV.
1849. _S. spinulosa_, Germar, Verstein. Wettin und Löbejün, p. 59,
Pl. XXV.
1893. _S. mutans_, Weiss, Abhand. Preuss. Geol. Anst. [N.F.] Heft 2,
Pl. VIII.
[Illustration: FIG. 203. _Sigillaria Brardi_ Brongn. (¾ nat. size).
From a photograph of a specimen in Dr Kidston’s collection, from
the Upper Transition Series of Staffordshire. Published by Kidston
(02) Pl. LIX. fig. 1.]
The aerial shoots of this species are occasionally branched
dichotomously[539], the apical portions bearing short crowded
leaves[540]; the surface of the bark is either completely covered with
contiguous leaf-scars without definite leaf-cushions or with projecting
cushions forming a narrow sloping surface surrounding each leaf-scar.
Other parts of the plant may possess cushions similar in their
kite-shaped form to those of _Lepidodendron_, but without a median
vertical groove, or the leaf-scars may be spirally disposed at varying
distances apart on a comparatively smooth and longitudinally wrinkled
bark. The species exhibits striking instances of a transition between
the Favularian, Clathrarian, and Leiodermarian forms of stems. The
leaf-scars, which are hexagonal in outline,—the lateral angles pointed
and transversely elongated, the upper and lower angles rounded,—bear
three scars, the central leaf-trace and two straight or curved lateral
parichnos scars; a ligular pit occurs immediately above the centre of
the upper edge of the leaf-scar and occasionally circular elevations
with a central pit occur singly or in pairs below a leaf-scar (fig.
196, A). The linear leaves, which may persist on shoots having a fairly
large diameter[541], have a single median vein and two stomatal grooves
on the lower surface[542] (fig. 200, D).
Partially decorticated and younger shoots are characterised by the
occurrence of pairs of elliptical parichnos areas and a smaller median
leaf-trace scar. The surface of older stems, which may show signs
of longitudinal splitting (_Syringodendron_ state), bears pairs of
parichnos scars reaching a length of 2–2·5 cm. and a breadth of 10–13
mm. The regularity of the leaf-scar arrangement is interrupted at
intervals by the occurrence of more or less regular verticils of scars
marking the position of deciduous shoots. Grand’Eury[543] has figured
cones which he believes to be those of this species, and Zeiller
refers the large strobili, _Sigillariostrobus major_, to _Sigillaria
Brardi_[544].
The subterranean axes were characterised by spirally disposed
rootlet-scars like those of _Stigmaria ficoides_ (figs. 204, 205) and
by a cortical surface with the features of _Stigmaria rimosa_ Gold.[545]
The anatomy of the stele and leaves has already been described (p.
219). The stele of the Stigmarian portion of the plant consists of
a band of centripetal primary xylem and a cylinder of centrifugally
formed secondary wood with medullary rays containing vascular bundles
passing out to the rootlets[546].
_Sigillaria Brardi_ occurs not uncommonly in Permian rocks; it is
recorded from France[547], Germany[548], Pennsylvania[549], and
elsewhere. It is found in the Upper, Middle, and Lower Coal-Measures
of England[550] and in Permo-Carboniferous strata in Africa[551] and
Brazil[552].