UNDERGROUND RHIZOMES AND ROOTS OF PALAEOZOIC
LYCOPODIACEOUS PLANTS.
=Stigmaria.=
_Stigmaria ficoides_ is the name given to cylindrical casts met with
in Palaeozoic rocks, from the Devonian[553] to the Permian[554],
characterised by a smooth or irregularly wrinkled surface bearing
spirally disposed circular scars bounded by a raised rim and containing
a small central pit. It is not uncommon to find evidence of a partial
collapse of the substance of the plant as seen in fig. 204; this
is doubtless the expression of a shrinkage of the middle cortical
region, which was composed of a delicate and lacunar system of cells.
There can be no reasonable doubt that Stigmaria grew in water or
in swampy ground. Specimens are occasionally met with in which the
cast terminates in a bluntly rounded apex; such are, perhaps, young
branches which have not grown far from the base of the aerial stem
from which they arose (cf. fig. 207, B, C). Other examples occur,
such as Goeppert[555] figured and Gresley[556] has more recently
described, which are twisted and distorted as though obstacles had been
encountered in the ground in which they grew.
[Illustration: FIG. 204. _Stigmaria ficoides_ Brongn. M.S. (See Vol. I.
p. 73.)]
[Illustration: FIG. 205. _Stigmaria ficoides._ From a specimen in the
York Museum, from Bishop Auckland. _a_, base of rootlet showing
vascular bundle scar. M.S.]
The circular scars mark the bases of long single and occasionally
forked appendages (rootlets) which spread on all sides into the
surrounding medium (figs. 205, 208). The occurrence of rootlets
radiating through the shale or sandstone affords proof that the
Stigmarias are often preserved in their position of growth. This
was recognised by Steinhauer[557] and Logan[558], and has been more
recently emphasised by Potonié[559] as an argument in favour of the
view that the beds containing such specimens are old surface-soils.
_Stigmaria_ usually shows regular dichotomous branching, the arms
spreading horizontally or slightly downwards and always arising
from four main branches in the form of a cross (fig. 207). The most
remarkable specimens found in England are described by Williamson[560]
in his monograph of _Stigmaria_. One of two large casts found near
Bradford in Yorkshire, and now in the Manchester Museum, shows four
large primary arms radiating from the base of an erect stump 4 feet in
diameter. Each arm divides a short distance from its base into two, and
the smaller branches extend almost horizontally for several feet[561].
An illustration published by Martin in 1809[562] shows a characteristic
feature of Stigmarian casts, namely the presence of a smaller axis,
usually occupying an eccentric position inside the larger. This
represents the cast of the fairly broad parenchymatous pith which,
on decay, left a space subsequently filled by sand or mud: at a
later stage the surrounding wood and cortex were removed and the
cavity so formed was similarly filled. A thin layer of coal formed
by the carbonisation of some of the tissues frequently surrounds the
medullary cast, and Steinhauer, whose account of the genus is much
fuller and more scientific than those of other earlier and many later
writers, recognised the true nature of this internal cast. Artis[563]
regarded it as the remains of a young plant, which he described as
“perforating its parent,” at length bursting it and assuming its place,
a gratuitously drastic interpretation.
In 1838[564] Lindley and Hutton figured a partially petrified specimen
of _Stigmaria_ obtained by Prestwich from Carboniferous rock of
Shropshire. This example showed a fairly broad cylinder of secondary
wood penetrated by medullary rays. The medullated stele consisted of a
pith surrounded by a small amount of primary xylem and by a cylinder
of secondary scalariform tracheae. The preservation of the tissues
abutting on the edge of the wood is usually very imperfect, and the
middle cortex of lacunar parenchyma has practically in every case
eluded the action of mineralising agents; the outer cortex, on the
other hand, consists of more resistant elements and is frequently well
preserved. As in _Lepidodendron_ and _Sigillaria_ stems, meristematic
activity produced a broad band of secondary cortex; and beyond this
were attached to cushion-like pads the numerous appendages, each
supplied with a single vascular bundle which arose from the primary
xylem and passed outwards through a medullary ray. There is abundant
evidence that the appendages were hollow, a fact in striking accord
with the aquatic and semi-aquatic habitat (cf. _Isoetes_ root, fig.
133, G).
[Illustration: FIG. 206. _Cyperus papyrus._ Piece of rhizome showing
rootlet-scars. Nat. size. M.S.]
The piece of dried rhizome of _Cyperus papyrus_ shown in fig. 206 is
an almost exact counterpart of _Stigmaria ficoides_; the wrinkled and
shrivelled surface and the circular root-scars containing the remains
of a vascular bundle are striking features in common and, it may be
added, the two plants, though very different in structure and in
systematic position, illustrate anatomical adaptations to a similar
environment.
_Stigmaria ficoides_ Brongniart[565]. Figs. 204, 205, 207, 208.
1809. _Phytolithus verrucosus_, Martin, Petrifact. Derb. Pls. XI–XIV.
1818. _Phytolithus verrucosus_, Steinhauer, Trans. Phil. Soc.
America, [N.S.] Vol. I. p. 268, Pl. IV.
1820. _Variolaria ficoides_, Sternberg, Flora der Vorwelt, p. 22, Pl.
XII.
1822. _Stigmaria ficoides_, Brongniart, Mem. Mus. d’hist. nat. Paris,
Pl. XII. fig. 7, p. 228.
1825. _Ficoidites verrucosus_, Artis, Antediluvian Phytology, Pl. X.
1840. _Stigmaria anabathra_, Corda, Flor. der Vorwelt, Pl. XIV.
The first figure of _Stigmaria_ is said to be by Petver in 1704;
Volkmann published illustrations of this common fossil in 1720 and
Parkinson in 1804[566]. Binney, whose researches may be said to have
inaugurated a new era in the investigation of fossil plants, wrote
in 1844: “Probably no fossil plant has excited more discussion among
botanists than the _Stigmaria_. It is the most common of the whole
number of plants found in the Coal-Measures, but there has hitherto
been the greatest uncertainty as to its real nature[567].” This
uncertainty still exists, at least in the minds of some who know enough
of the available data to realise that our knowledge is imperfect.
To pass to the questions of the affinity and nature of _Stigmaria_:
Brongniart[568] at first compared his genus with recent Aroideae, but
he afterwards[569] spoke of it as probably the root of _Sigillaria_.
Other writers regarded _Stigmaria_ as a dicotyledonous plant comparable
with Cacti and succulent Euphorbias. For many years opinion was divided
as to whether _Stigmaria_ represents an independent and complete plant
or the underground system of _Sigillaria_.
Artis[570], Lindley and Hutton[571], as well as Goldenberg[572],
believed it to be a prostrate plant unconnected with any erect aerial
stem. Goldenberg figured one of the slender rootlets terminating in an
oval body described as a reproductive organ. This seed-like impression
is either some extraneous body or an abnormal development at the end
of a rootlet. In 1842 Logan drew attention to the almost complete
monopolisation by _Stigmaria_ of the underclays, the rock which as
a general rule occurs below a seam of coal. He wrote: “The grand
distinguishing feature of the underclays is the peculiar character of
the vegetable organic remains; they are always of one kind (_Stigmaria
ficoides_) and are so diffused throughout every part of the bed, that
by their uniform effect alone the clay is readily recognised by the eye
of the miner[573].” This fact, which has played a very conspicuous part
in the perennial discussions on the origin of coal, led to the almost
general recognition of the underclays as surface-soils of the Coal
period forests.
The next step was the discovery of _Stigmaria_ in the Coal-Measures of
Lancashire and in the Carboniferous rocks of Cape Breton, Nova Scotia,
forming the basal branches of erect stems identified by Binney[574],
Bowman[575] and Richard Brown[576] as undoubted Sigillariae. In one
case Brown found what he considered to be convincing evidence of the
continuity between _Stigmaria_ and _Lepidodendron_.
In 1842 Hawkshaw[577] described certain fossil trees, the largest of
which had a circumference at the base of 15 ft., discovered, in the
course of excavations for a railway in Lancashire, in soft shale at
right angles to the bedding. The surface features were not sufficiently
clear to enable him to decide with certainty between _Sigillaria_ and
_Lepidodendron_, but while inclining to the former, it is interesting
to note that the occurrence of numerous Lepidostrobi near the root led
him to recognise the possibility of a connexion between the Stigmarian
roots and Lepidodendron stems. In 1846 Binney gave an account of
similar trees found at Dukinfield near Manchester: he spoke of one
stem as unquestionably a _Sigillaria_ with vertical ribs, furrows, and
scars, about 15 inches high and 4 ft. 10 inches in circumference. He
expressed his conviction that “_Sigillaria_ was a plant of an aquatic
nature[578].” Similar descriptions of rooted stems in the Coal-Measures
of Nova Scotia were published by Brown in 1845, 1846 and 1849; in the
last paper he figured a specimen, which has become famous, showing a
Syringodendron stem terminating in branching Stigmarian (or possibly
Stigmariopsis) roots bearing on the lower surface a series of what he
called conical tap roots[579]. A similar specimen discovered in Central
France nearly fifty years later demonstrated the accuracy of Brown’s
description.
Despite these discoveries the root-like nature of _Stigmaria_ was
not universally accepted. It was, however, generally agreed that
_Stigmaria_ formed the roots of _Sigillaria_; it was, moreover, held by
some that _Lepidodendron_ stems also possessed this type of root, an
opinion based on Brown’s record and on the occurrence of _Stigmaria_ in
beds containing _Lepidodendron_ but no _Sigillaria_ stems, as in the
volcanic beds of Arran and elsewhere, and on observations of Geinitz
and others[580]. There is now general agreement that _Lepidodendron_
and _Sigillaria_ had the same type of “root,” though the connexion of
_Stigmaria_ with the former was not so readily admitted, and indeed
the evidence in support of it is still very meagre. Goeppert and
other authors were unable to believe that the numerous species of
_Sigillaria_ possessed roots of so uniform a type, but Goeppert, by his
recognition of several varieties of _Stigmaria_, supplied a partial
answer to this objection.
Messrs Mellor and Leslie[581] have described and figured some large
casts of roots exposed in Permo-Carboniferous rocks in the bed of the
Vaal river at Vereeniging (Transvaal) which exhibit certain features
suggesting comparison with _Stigmaria_. Some of these reach a length
of 40–50 feet and, when complete, were probably not less than 100
feet long: in some of them the centre of the cast from which forked
arms spread almost horizontally shows a depression in the form of
a cross indicating a regular dichotomous branching like that of
_Stigmaria_. The authors incline to the belief that the roots belong
to _Noeggerathiopsis_ and not to a lycopodiaceous plant, though
Lepidodendroid stems are abundant in the sandstone a few feet higher
in the series. Despite the absence of any Stigmarian scars on the
surface of the fossil it is probable that these fine specimens are the
rhizomes of some lycopodiaceous plant, possibly _Bothrodendron_, which
is not uncommon in the Vereeniging beds.
Admitting that _Stigmaria_ is part of _Sigillaria_, the next question
is, is _Stigmaria_ a root in the ordinary sense, the underground
system formed on germination of the spore and of equal age with the
shoot, or did it bear a different relation to the Sigillarian stems?
To this question different answers would still be given. Goeppert[582]
discussed evidence in favour of the view that aerial Sigillarian shoots
were produced as vegetative buds on pre-existing Stigmarian axes,
like young moss plants on a protonema. At a later date Renault[583]
developed a similar view as regards _Sigillaria_; but we may pass on to
consider the more recent and complete observations of Grand’Eury[584]
and Solms-Laubach[585].
The recognition of two distinct types of Stigmariae in the
Coal-Measures of Central France led Grand’Eury[586] to institute a
new genus, _Stigmariopsis_. This type, which is characterised by a
difference in habit as well as by other distinguishing features,
is represented by such specimens as those figured by Goldenberg as
_Stigmaria abbreviata_, bearing lenticular scars spirally disposed on
a cortical surface characterised by irregular longitudinal wrinklings.
_Stigmariopsis_ has frequently been found in direct continuity with
Sigillarian stems of the Leiodermarian-Clathrarian type, spreading
obliquely downwards in the form of rapidly narrowing arms clothed with
slender and usually simple appendages; and from the under surface of
these arms short conical outgrowths are given off. It is probable,
as Solms-Laubach believes, that _Stigmariopsis_ was represented also
by long horizontally creeping rhizomes[587] of uniform breadth from
which ribless Sigillarian aerial shoots arose as bud-like outgrowths.
Grand’Eury, the author of the genus, confined the term to the shorter
and more rapidly tapered organs spreading from the base of erect
stems; the horizontal rhizomes of all Sigillarian stems he refers to
_Stigmaria_. The pith-casts of _Sigillariopsis_ may be recognised by
their long vertical ridges and grooves, a feature readily understood by
reference to the stem structure. The _Stigmariopsis_ rhizomes though
rare in England have been recognised by Dr Kidston[588] in the Middle
Coal-Measures of Yorkshire; he has figured a pith-cast very like that
illustrated in Solms-Laubach’s Memoir as _Stigmariopsis anglica_.
The surface-features of a _Stigmariopsis_ pith-cast are clearly shown
on a specimen from St Étienne in the Williamson collection[589].
[Sidenote: STIGMARIOPSIS]
The most complete account of Grand’Eury’s views in regard to the
anchoring and absorbing organs of _Sigillaria_ is given in his
monograph on the Coal-field of Gard[590], St Étienne, and these are
clearly stated also by Solms-Laubach[591] who confirms the conclusions
of the French author as to the manner of development of the aerial
shoots. Grand’Eury believes that both _Stigmaria_ and _Stigmariopsis_
are rhizomes and not true roots. The surface-features of _Stigmaria_
have already been described. This type Grand’Eury speaks of as
characterised by the uniform diameter and considerable horizontal
elongation of the bifurcated axes; he thinks they grew both as
floating rhizomes and on the ground: they may frequently be traced for
a considerable distance without showing any signs of connexion with
aerial shoots, but occasionally they have been seen in organic union
with Sigillarian stems. He believes that these rhizomes were produced
as the result of germination under water of the spores of _Sigillaria_
or _Lepidodendron_ and developed as long and branched aquatic
rhizomes capable of independent existence. Under certain conditions,
as he thinks in shallower water, the rhizomes produced bulb-like
outgrowths which grew into erect stems having the surface-features
of _Sigillaria_. This method of origin is practically the same as
that described by Goeppert in 1865. The vascular medullated cylinder
of these erect branches was in direct continuity with that of the
Stigmarian rhizomes.
[Illustration: FIG. 207. An early stage in the development of
_Sigillaria_.
A. Surface-features enlarged. (After Grand’Eury.)]
[Illustration: FIG. 208. Later stage in the development of
_Sigillaria_; _Syringodendron_ with _Stigmariopsis_. (After
Grand’Eury.)]
The next stage is that in which the undifferentiated bulb becomes
swollen at the base and develops four primary roots (fig. 207 B, C)
which grow obliquely downwards and produce numerous rootlets. Meanwhile
the parent rhizome gradually decays, finally setting free the aerial
stems which are now provided with spreading and forked roots (fig.
208) such as we are familiar with in English specimens as _Stigmaria
ficoides_, but which in the French specimens show the features of
_Stigmariopsis_. At this later stage conical outgrowths are formed
from the under surface of the Stigmariopsis arranged in a more or less
regular series surrounding the centre of the forked and spreading roots
(fig. 209). These conical and positively geotropic organs were long
ago described by Richard Brown as tap-roots. Grand’Eury’s conclusions
are briefly as follows: _Sigillaria_, and we may add _Lepidodendron_,
had no true roots and in this respect are comparable with _Psilotum_
(fig. 118): the organs which are described by Grand’Eury as roots
are correctly so named in a physiological sense, but morphologically
they do not strictly conform, either in origin or in the arrangement
of their appendages, to true roots. The question as to whether they
are entitled to the designation root is one which it is needless and
indeed futile to discuss in detail; it would be conceding too much to
a formal academic standpoint to refrain from applying to them the term
root, as that best describes their share in the life of the Sigillarian
stems. The horizontal Stigmarian axes are rhizomes in the ordinary
sense of the term and from these were developed Sigillarian shoots,
characterised in the lower portions by large parichnos strands. From
the base of the young bulbous shoots roots were formed: these roots
being, in the French specimens, of the _Stigmariopsis_ type.
[Illustration: FIG. 209. _Stigmariopsis_ and “tap-roots.” (After
Grand’Eury.)]
These conclusions require some modification when applied to British
representatives of the arborescent Lycopodiales. The long spreading
and dichotomously branched root-like organs attached to the base of
Sigillarian and Lepidodendron stems are true examples of _Stigmaria
ficoides_ or other species. _Stigmariopsis_ occurs but rarely. This
marked difference between French and English specimens may be explained
if we adopt the opinion of Solms-Laubach, who believes that the true
_Stigmaria_ represents both the parent rhizome and the later-formed
roots of the Rhytidolepis Sigillarian species and of _Lepidodendron_,
the _Stigmariopsis_ form having the corresponding relation to the
Leiodermarian-Clathrarian species.
The opinion expressed by Williamson[592] in 1892 that Grand’Eury’s
hypothesis “appears to be identical with the vague and speculative
guesses that were prevalent among us in the early years of the present
[nineteenth] century” illustrates the strength of conviction based on
English specimens as to the root-nature of _Stigmaria_.
There is undoubtedly considerable confusion, which can be cleared
up only by further research, as to the precise relation between
_Stigmaria_ and _Stigmariopsis_ on the one hand and the different
types of _Sigillariae_ on the other. The main contention, and this is
the most important point, of Renault, Grand’Eury and Solms-Laubach as
to the manner of formation of the aerial shoots from rhizomes and the
subsequent production of forked roots and their ultimate separation
from the parent rhizome is, as I believe, correct. Williamson held
that _Stigmaria_ must be regarded as a true root; he found no evidence
to support the view that the large rooted stem discovered by Hawshaw,
Binney, and others had been originally produced from aquatic rhizomes.
It must, however, be remembered that Grand’Eury’s opinion is based
on evidence afforded by the exceptionally well displayed Sigillarian
forests of St Étienne, on a scale such as English strata have not
as yet afforded. Moreover, the absence of any parent-rhizome in
association with the rooted stumps described by Williamson and by
others is not a serious argument against their rhizome origin.
The specimen represented in fig. 209, which was examined _in situ_
by Solms-Laubach and Grand’Eury, shows a Sigillarian stem in the
Syringodendron condition bearing rows of paired parichnos scars;
from the base forked and rapidly tapering arms radiate through the
surrounding rock and, as shown by other specimens, these bear numerous
appendages like those of the English Stigmarias. The surface-features
of the arms are those of _Stigmariopsis_ and the centre of each, as
seen on the broken face, is occupied by a pith-cast characterised by
parallel longitudinal ridges resembling those on the medullary casts of
_Calamites_. It is noteworthy that the petrified rhizome originally
described by Renault as _Stigmaria flexuosa_, and afterwards identified
by him as the subterranean system of _Sigillaria Brardi_, possesses
a vascular cylinder composed of primary xylem strands of crescentic
transverse section lining the pith; a cast of the pith, after the
removal by decay of its delicate parenchymatous tissue, would exhibit
the surface-features of _Stigmariopsis_. _Stigmaria flexuosa_ no
doubt represents a true _Stigmariopsis_ rhizome. On the other hand,
as Williamson has shown, the inner surface of the wood of _Stigmaria
ficoides_ consists of a reticulum of xylem with meshes of medullary-ray
tissue; a cast of such a surface presents a very different appearance
from that of _Stigmariopsis_.
Returning to fig. 209: from the lower surface of the _Stigmariopsis_
arms numerous conical outgrowths, reaching a length of several
centimetres, project vertically downwards; these also possess
Stigmariopsis pith-casts and are identical with the “tap-roots” of
Richard Brown. The stump seen in fig. 209 shows the characteristic
hollow base of the erect stem: this is the region which, it is
believed, represents the position of the Stigmarian rhizome from which
the aerial shoot was developed. Although no remains of the parent
rhizome were found, traces of the rootlets which probably belonged to
it were found in the neighbourhood. The absence of the actual rhizome
is, however, not surprising as it would not persist after its aerial
Sigillarian branches had attained independence by the development of
their own dichotomously branched absorbing and holdfast organs.
The Stigmarian axes of Palaeozoic Lycopods are compared by Miss
Thomas[593] with the prop-roots of certain recent flowering plants
which grow in tropical tidal swamps; their roots grow downwards from
the stem at an angle of 50°-60° before spreading out horizontally.
This author also makes some interesting suggestions in regard to
the evidence afforded by anatomical structure as to the habitat of
_Sigillaria_ and _Lepidodendron_.
_Anatomy._
The more important anatomical features of _Stigmaria_ must be dealt
with briefly. Williamson’s monograph, published in 1887[594], is
considerably in advance of the work of that of any of the numerous
writers who had previously dealt with the subject. The diagrammatic
transverse section reproduced in fig. 210, H, illustrates the general
arrangement of the tissues. The medullated stele was described by
Williamson as consisting entirely of centrifugally developed secondary
xylem and distinguished, therefore, from the stele of a _Lepidodendron_
or _Sigillaria_ by the absence of a centripetally produced primary
xylem zone. The secondary xylem tracheae are characterised by
scalariform pits on both radial and tangential walls and, as shown in a
figure given by Solms-Laubach[595], the spaces between the transverse
bars are bridged across by fine threads, as in the tracheae of
_Lepidodendron_.
One of the largest specimens of a petrified _Stigmaria_ which I have
seen is one lent to me by Mr Lomax from the Coal-Measures of Halifax in
which the flattened transverse section measures 18 cm. × 3·5 cm., the
cylinder of wood being 1·1 cm. × 7 mm. in diameter.
In French examples of _Stigmaria_ or _Stigmariopsis_ it has been
demonstrated by Renault[596] that primary xylem strands occur very like
those in the stem of some species of _Sigillariae_ (see p. 219). If a
well-preserved section of an English _Stigmaria_ is examined it will
be seen that the edge of the secondary wood consists of a few narrower
elements which do not exhibit the radial seriation characteristic of
secondary elements.
A type of _Stigmaria_ characterised by centripetal primary wood has
been described by Weiss[597] and referred by him to _Bothrodendron
mundum_; the main results of his observations are stated in the
account of _Bothrodendron_ on a subsequent page. This discovery is
of considerable interest not only as rendering our knowledge of
_Bothrodendron_ remarkably complete but as confirmatory of Renault’s
account of French Stigmarian axes in which centripetal primary wood
is well developed between the secondary xylem and the centre of the
stele. The Stigmarian axis of Bothrodendron was originally figured
by Williamson as _Lepidodendron mundum_[598]. The chief difference
between Weiss’s specimen and those described by Renault[599] as the
Stigmarian axes of _Sigillaria Brardi_, is that in the English plant
the centripetal wood forms a cylinder of uniform breadth instead of a
band with a crenulated inner margin as figured by Renault.
[Sidenote: STIGMARIA]
[Illustration: FIG. 210. _Stigmaria._
A. Transverse section of vascular bundle of rootlet and part of
outer cortex. _t_, tracheae. (After F. E. Weiss.)
B, C. Vascular bundle of rootlet; in C a series of small tracheae
are shown extending from the protoxylem.
D. Rootlets from the outer cortex of E.
E. Part of a large _Stigmaria_: _St_, stele; _s_, intruded rootlet.
F. Vascular bundle and tracheae passing obliquely towards the outer
cortex, _c_³.
G. Outer cortex of _Stigmaria_.
H. Diagrammatic section of _Stigmaria_: _p_, phelloderm; _r_,
rootlets.]
An interesting agreement between the French and English specimens
is the occurrence in the cortex of groups of reticulate elements:
in Weiss’s section these are short and wide and occur in the middle
cortex; in Renault’s plant they are more fusiform and occur in the
secondary cortical tissue. These elements appear to have been arranged
as an interlacing network in the middle cortex and were in close
connexion with the rootlet-bundles, comparable, as Weiss points out,
with the transfusion tracheids accompanying Lepidodendron leaf-traces.
It is probable that these short and wide tracheal elements served for
water-storage and thus afford another indication of the xerophilous
character of the Carboniferous Lycopods, a feature possibly connected
with a salt-marsh habitat.
The presence of conspicuous medullary rays gives the secondary xylem
of _Stigmaria_ the appearance of being divided into several more or
less distinct groups (fig. 210, E, _St_). In tangential longitudinal
section the xylem assumes the form of a broad reticulum with lenticular
meshes filled with medullary-ray tissue through which strands of xylem
are cut across in a transverse direction as they pass outwards from
the inner edge of the wood to supply the rootlets. In addition to
these broader or primary medullary rays, there were numerous secondary
rays composed of narrow plates of parenchymatous cells one or several
elements in depth. As Williamson pointed out, the medullary-ray tissue
consists in part of radially elongated tracheal elements with spiral or
scalariform thickening bands like those described in the same position
in Lepidodendron stems.
Our knowledge of the minute structure of the tissues abutting on the
secondary xylem is far from complete.
The xylem is succeeded by a zone of delicate cells which was the
seat of meristematic activity. It is noteworthy that in a section
figured by Williamson[600] there is the same disparity in size between
the outermost elements of the xylem and the adjacent cells of the
meristematic zone as in Lepidodendron stems. Beyond this region an
imperfectly preserved lacunar tissue occurs like that which I have
called the secretory zone in Lepidodendron stems; but information as
to the structure of this part of _Stigmaria_ is much more incomplete
than in the case of the aerial shoots. The middle cortex was of the
same lacunar type as in the stems, and the fact that it is never well
preserved in large Stigmarian axes suggests that it may have been
even more richly supplied than in the aerial stems with an aerating
system of spaces. The outer cortex, consisting in young examples of
large-celled parenchyma, became at an early stage of growth the seat of
cambial activity which resulted in the production of radially placed
series of secondary elements (fig. 210, H, _p_). The outer and older
elements of this secondary cortex are more tangentially stretched than
the inner cells, a necessary result of the position of the phellogen on
the internal edge of the tissue and of the increasing girth of the axis.
In comparatively young Stigmarian axes the outer cortex already
possesses a band of secondary radially disposed cells characterised
by the greater tangential extension of the more external elements;
usually this tissue terminates abruptly on the inner edge and the
line of separation no doubt marks the position of the phellogen.
Occasionally some delicate secondary elements are preserved internal
to the phellogen, and these in young specimens form a narrow cylinder
composed in part of radially elongated cells showing signs of recent
tangential divisions. In its earlier stage of activity the phellogen
seems to form a greater amount of secondary tissue on the outside, but
this is clearly not of the nature of cork, the tissue which occupies a
corresponding position in recent plants. The primary cortex shows no
signs of shrinkage or collapse as would be the case were it cut off
from the vascular system by a zone of impermeable cork.
Fig. 210, G, represents a piece of the external tissue of a specimen in
which the slightly flattened xylem cylinder measures 1·4 × 1 cm.; the
inner cortex has disappeared and fragments only of the middle cortex
are preserved. The outer cortex, with an average breadth of 2 mm.,
consists superficially of primary parenchyma with a somewhat uneven
surface and with a rootlet attached here and there; a short distance
below the surface is a band of conspicuous cells, _b_, characterised
by dark contents suggesting very imperfectly preserved fungal hyphae,
but the nature of the substance filling the cells cannot be made out
with certainty. It is, however, interesting to find that this dark
band constitutes an obvious feature (fig. 210 H, _b_); its position is
comparable with that of the dark-walled cells in the outer cortex of
rootlets. A short distance internal to this dark band tangentially
elongated cells form the outermost elements of the secondary cortex;
these become gradually narrower towards the interior and pass into
radial series of smaller cells of uniform size, as seen on the inner
edge of fig. 210, G. At the inner boundary of this tissue, just below
the region shown at the bottom of the drawing, was situated the
phellogen. Such traces of tissue as occur on the inner side of the line
where splitting has usually occurred, consist of thinner elements with
recently formed tangential walls and probably represent an early stage
in the development of phelloderm.
A much older section is shown in part in fig. 210, E. The secondary
xylem cylinder, _St_, is shown in the lower part of the section; beyond
this is a band of secondary tissue which reaches in some places a
breadth of 6 cm. The greater part of this tissue consists of phelloderm
of very uniform structure made up of radial series of cells: this
is interrupted in most parts of the section by a gap crowded with
intruded rootlets (a portion of this is enlarged in fig. 210, D).
Beyond this gap the secondary tissue consists of radial series of cells
characterised by the considerable tangential elongation of many of
the elements, precisely like the tissue figured by Williamson. In all
probability the gap represents a line of weakness due to the phellogen,
and if this is the case it is clear that in an old _Stigmaria_ the
phelloderm exceeded in amount the tissue formed external to the
phellogen. The secondary tissue on the inner side of the phellogen is
characterised by numerous irregular concentric lines superficially
resembling rings of growth in the wood of a Conifer: these are,
however, not the result of any periodic change in external conditions,
but are apparently due to crushing of the tissue and are possibly,
to some extent, the result of the presence of secretory strands like
those in the phelloderm of _Lepidodendron_. The surface of this
older rhizome retains patches of primary tissue, and an occasional
rootlet, as at _r_, fig. 210, E, is seen in connexion with the cortex;
the cortex has been vertically fissured as the result of secondary
growth and presents an appearance like that shown in _Lepidodendron
Wünschianum_ and _L. Veltheimianum_ (figs. 181, A, and 186, A).
The form in which a Stigmarian rootlet is usually preserved is shown
in fig. 210, D; the single vascular bundle strand with its endarch
protoxylem (fig. 210, B, _px_) is enclosed by a ring of inner cortical
parenchyma (fig. 210, F, _c_¹); the cells in immediate contact with
the xylem having usually disappeared. Beyond the middle cortical
space a second cylinder of parenchyma represents the outer cortex (F,
_c_³) in which a layer of dark-walled cells (_b_, fig. 210, F) may be
compared with the hypodermal band in the main Stigmarian axis (G, _b_).
These Stigmarian rootlets, usually less than 1 cm. in diameter, are the
commonest objects in sections of the calcareous nodules from English
coal-seams. A good example of their abundance is shown in fig. 210, D
and E; here they have invaded the space formed by the splitting of the
secondary cortical tissues along the line of the phellogen and a few
are seen here and there in the deeper layers of the phelloderm (_s_,
fig. 210, E). Not infrequently the close contact of these ubiquitous
rootlets with the tissues of the plant which they have invaded leads to
confusion between invader and invaded. Partially decayed tissues lying,
probably, under water were penetrated by Stigmarian rootlets in exactly
the same way as the roots of recent plants bore through vegetable
substances which happen to be in their path. The rootlet bundles are
in the first instance composed of the primary tracheae which line
the inner edge of the secondary xylem; these receive additions from
the meristematic zone, and thus, when seen in the cortex outside the
stelar region, are found to consist in part of primary and in part of a
fan-shaped group of secondary tracheae. On the other hand, the monarch
bundle as it appears in a free rootlet is usually composed entirely of
primary elements (fig. 210, A–C, F). It has been shown by Weiss[601]
that in the Stigmarian rhizome of what is probably _Lepidodendron
fuliginosum_, the rootlet bundle is accompanied by a parichnos strand,
but this has not been detected in the ordinary _Stigmaria ficoides_.
When free from the parent axis a rootlet usually consists of an
outer cylinder of cortex enclosing a broad space in which remnants
of lacunar tissue are sometimes seen. The relation of the external
features of a well-preserved Stigmarian rootlet-scar to the internal
structure of a petrified rootlet is very clearly seen on comparing
such sections as those represented in fig. 210, D, with the form of
the scar on a Stigmarian cast. A specimen figured by Hooker[602] in
1848 affords a good illustration of the structure of a rootlet-base as
seen in an unusually complete cast; this correlation of anatomical and
surface features is clearly described also by Williamson[603] and by
Solms-Laubach[604]. It is probable that even during life the rootlets
were hollow for a part at least of their length as are the roots of
_Isoetes_ (fig. 133, G).
An interesting discovery was made a few years ago which confirmed a
statement by Renault which Williamson was unable to accept, namely
that the xylem bundle of a rootlet occasionally gives off a delicate
tracheal strand at right angles to the long axis of a rootlet. In some
rootlets Weiss[605] found obliquely running delicate strands of xylem,
surrounded by a layer of parenchymatous tissue, in the space between
the vascular bundle and the outer cortical cylinder. It is clear that a
few spiral tracheids are occasionally given off from the protoxylem of
a rootlet bundle: these follow an oblique course to the outer cortex,
where in some cases they have been traced into connexion with short
and spirally marked cells resembling transfusion tracheae (fig. 210,
A). This arrangement may serve as a means of facilitating the passage
of water absorbed by the superficial cells into the xylem strand. It
should be noticed that, like roots of recent water-plants, the rootlets
of _Stigmaria_ had no root-hairs. Fig. 210, F, shows a transverse
section of part of a rootlet in which the outer cortical cylinder,
_c_³, is connected, as in the roots of _Isoetes_, with the sheath
surrounding the vascular bundle. A few obliquely cut tracheae are seen
in this section traversing the connecting band of parenchyma _t_, fig.
210, A.
A point of biological interest in connexion with Stigmaria rootlets is
the occasional presence of hypertrophied cells, the large size of which
is due to the attacks of a fungus named by Weiss[606] _Urophlyctites
stigmariae_.
In addition to _Stigmaria ficoides_, which is by far the commonest
form, a few other species have been founded on external characters.
One of these is represented by _Stigmaria stellata_, Goepp.[607],
characterised by the presence of radially disposed ridges and small
tubercles surrounding each rootlet-scar. Kidston refers to Goeppert’s
species as a Lower Carboniferous type. We have no evidence as to the
meaning of the stellate ridges and tubercles, nor have we any reason
to suppose that this form differed essentially in structure from
_Stigmaria ficoides_.