Arborescent Lycopodiales.
Among the best known plants in the Palaeozoic floras are the genera
_Lepidodendron_ and _Sigillaria_, types which are often spoken of as
Giant Club-Mosses or as ancestors of existing species of _Lycopodium_
and _Selaginella_. Of these genera, but more particularly of
_Lepidodendron_, we possess abundant records in a condition which
have made it possible to obtain fairly complete information not only
in regard to habit and external features but as to the anatomical
characters of both vegetative and reproductive shoots. The structure
of _Lepidodendron_ differs too widely from that of recent Club-Mosses
(species of _Lycopodium_) to justify the statement that this
prominent member of the Palaeozoic vegetation may be regarded as a
direct ancestor of any living plant. There is at least no doubt that
_Lepidodendron_ and _Sigillaria_ must be included in the Pteridophyta.
The description by Dr Scott[236] of the genus _Lepidocarpon_, founded
on petrified specimens of strobili, demonstrated the existence of a
type of lycopodiaceous plant in the Carboniferous period distinguished
from all living representatives of the group by the possession
of integumented megaspores, which may fairly be styled seeds.
_Lepidocarpon_ and another seed-bearing plant _Miadesmia_ are described
under a separate heading as lycopodiaceous types characterised by an
important morphological feature, which among recent plants constitutes
a differentiating character between the Pteridophytes and the
Phanerogams.
Lepidodendron.
i. _General._
The genus _Lepidodendron_ included species comparable in size with
existing forest trees. A tapered trunk rose vertically to a height of
100 feet or upwards from a dichotomously branched subterranean axis
of which the spreading branches, clothed with numerous rootlets, grew
in a horizontal direction probably in a swampy soil or possibly under
water. A description by Mr Rodway[237] of Lycopods on the border of a
savannah in Guiana forming a miniature forest of Pine-like Lycopodiums
might, with the omission of the qualifying adjective, be applied with
equal force to a grove of Lepidodendra. The equal dichotomy of many of
the branches gave to the tree a habit in striking contrast to that of
our modern forest trees, but, on the other hand, in close agreement
with that of such recent species of _Lycopodium_ as _L. cernuum_ (fig.
123), _L. obscurum_ (fig. 124) and other types. Linear or oval cones
terminated some of the more slender branches (fig. 188) agreeing in
size and form with the cones of the Spruce Fir and other conifers or
with the male flowers of species of _Araucaria_, _e.g. A. imbricata_.
Needle-like leaves, varying considerably in length in different
species, covered the surface of young shoots in crowded spirals and
their decurrent bases or leaf-cushions formed an encasing cylinder
continuous with the outer cortex. The fact that leaves are usually
found attached only to branches of comparatively small diameter would
seem to show that _Lepidodendron_, though an evergreen, did not retain
its foliage even for so long a period as do some recent conifers.
By the activity of a zone of growing tissue encircling the cylinder of
wood the main trunk and branches grew in thickness year by year: the
general uniformity in size of the secondary conducting elements affords
no indication of changing seasons. As the branches grew stouter and
shed their leaves the surface of the bark resembled in some degree that
of a Spruce Fir and other species of _Picea_, in which the leaf-scars
form the upper limit of prominent peg-like projections, which, at
first contiguous and regular in contour, afterwards become less regular
and separated by grooves (fig. 140) and at a later stage lose their
outline as the bark is stretched to the tearing point (fig. 140, C).
The leafless branches of _Lepidodendron_ were covered with spirally
disposed oval cushions less peg-like and larger than the decurrent
leaf-bases of _Picea_, which show in the upper third of their length
a clean-cut triangular area and swell out below into two prominent
cheeks separated by a median groove and tapering with decreasing
thickness to a pointed base, which in some forms (_e.g. Lepidodendron
Veltheimianum_, fig. 185, C, D), is prolonged as a curved ridge to the
summit of a lower leaf-cushion.
[Illustration: FIG. 140. _Picea excelsa._ Shoots of different ages
showing changes in the appearance of the leaf-cushions: a leaf
attached to a cushion in fig. A. (Slightly enlarged.)]
A portion of the cushion below the triangular leaf-scar often shows
transverse gaping cracks or depressions (fig. 185, C) such as occur
on a smaller scale on the older cushions of a Fir twig (fig. 140).
Secondary thickening, as in recent trees, is not confined to the
vascular cylinder but at an early stage, frequently before there are
any signs of secondary wood, the outer region of the broad cortex
becomes the seat of active cell-formation which results in the addition
of a considerable thickness to the bark. At a later stage of increase
in girth, the leaf-cushions are stretched apart and the original
surface-features become obliterated by vertical cracks and by the
exfoliation of the superficial tissues[238].
Some species of _Lepidodendron_ produced branches characterised by
spiral or vertical series of scars; these in older shoots were replaced
by depressions having a diameter of several inches and comparable in
appearance, as also perhaps in manner of formation, with the scars
left on the stem of a Kauri Pine (_Agathis australis_)[239] on the
abscission of lateral branches by a natural process. These shoots,
known as _Ulodendron_, are described in a subsequent section. (Page
128.)
A fully-grown _Lepidodendron_ must have been an impressive tree,
probably of sombre colour, relieved by the encircling felt of green
needles on the young pendulous twigs. The leaves of some species were
similar to those of a fir while in others they resembled the filiform
needles of the Himalayan Pine (_Pinus longifolia_). The occasional
presence of delicate hyphae in the tissues of _Lepidodendron_
demonstrates susceptibility to fungal pests.
Architecturally, if one may use the term, _Lepidodendron_ owed its
power of resistance to the bending force of the wind to its stout
outer bark formed of thick-walled elements produced by the activity of
a cylinder of cortical meristem (figs. 148, 172, etc.). The vascular
axis, of insignificant diameter in proportion to the size of the stem
(figs. 152, 153, 172, 181, A), must have played a subordinate part,
from a mechanical point of view, as compared with the solid mass of
wood of a Pine or an Oak.
• • • • •
Within the compass of a text-book it is impossible, even if it were
desirable, to include an account of the majority of the species of the
widely distributed Palaeozoic genus _Lepidodendron_. In spite of the
great number of known species of this common member of Carboniferous
floras, our knowledge of the type as a whole is deficient in many
points, and such information as we possess needs systematising and
extending by comparative treatment based on a re-examination of
available data.
In order to appreciate the meaning of certain external features
characteristic of Lepidodendron stems it is essential to have some
knowledge of the internal structure.
A dual system of terminology has been unavoidably adopted for
species of _Lepidodendron_: the majority of specific names have been
assigned to fossils known only in the form of casts or impressions,
while petrified fragments, which unfortunately seldom show the
surface-features, have received another set of names. A glance at the
older palaeobotanical literature reveals the existence of several
generic designations, which fuller information has shown to have been
applied to lepidodendroid shoots deprived of some of their superficial
tissues before fossilisation and differing considerably in appearance
from the more complete branches of the same species[240]. It has in
some instances been possible to correlate the two sets of specimens,
casts or impressions, showing external features, and petrified
fragments. We may reasonably expect that future discoveries will enable
us to piece together as definite specific types specimens at present
labelled with different names.
A well-preserved leaf-cushion of a _Lepidodendron_—the most obvious
distinguishing feature of the genus—is rhomboidal or fusiform and
vertically elongated (fig. 146, C, E; fig. 185, C, D): in exceptional
cases it may reach a length of 8 cm. and a breadth of 2 cm. The
cushion as a whole represents a prominent portion of the stem or
branch comparable with the elevation on the twig of a Spruce Fir and
the leaf-base of a _Lycopodium_ (cf. fig. 121, A, lower portion) which
appears in a transverse section of a branch as a rounded prominence
(cf. _Lycopodium_, fig. 125, A and H). Disregarding differences in
detail, a typical Lepidodendron leaf-cushion is characterised by a
clearly defined smooth area often situated in the middle region (fig.
146, C, _s_). This is the leaf-scar or place of attachment of the base
of the leaf which was cut off by an absciss-layer while the branch was
comparatively young, as in recent forest trees and in some species of
Ferns. On the leaf-scar are three smaller scars or cicatricules, the
central one is circular or more or less triangular in outline, the two
lateral scars being usually oval or circular. The central pit marks
the position of the single vascular bundle which constituted the
conducting tissue connecting the leaf with the main vascular system of
the stem. The two lateral scars (figs. 145, A, _p_; 146, C, _s_; 147,
_p_) represent the exposed ends of two strands of tissue, the forked
branches of a strand which pass from the middle cortex of the stem into
the leaf; this is known as the parichnos, a name proposed by Professor
Bertrand in 1891[241].
The specimen shown in fig. 141 shows the linear leaves attached to
their respective cushions.
[Illustration: FIG. 141. _Lepidodendron Sternbergii._ From a specimen
in the British Museum (No. v. 1235) from the Coal-Measures of
Shropshire. (Nat. size.)]
The lamina has a well-defined median keel on the lower surface and
on either side a groove in which sections of petrified leaves have
demonstrated the occurrence of stomata (cf. fig. 142).
ii. _Leaves and Leaf-cushions._
All Lepidodendron leaves, so far as we know, possessed a single
median vein only. In some species, as for example in _Lepidodendron
longifolium_ Brongn., they have the form of long and slender
acicular needles very similar to those of _Pinus longifolium_; in
_L. Sternbergii_ (fig. 141) they are much broader and shorter. In
external form as in internal structure it is often impossible to
distinguish between the leaves of _Lepidodendron_ and _Sigillaria_.
The distinguishing features enumerated by the late M. Renault cannot
be employed, with any great degree of confidence, as diagnostic
characters. In transverse section the lamina of a Lepidodendron
leaf presents the same appearance as that of the Sigillarian leaves
represented in fig. 142. Near the base the free part of the leaf is
usually sub-rhomboidal in section with short lateral wings, a ventral
keel and two stomatal grooves (fig. 142, A, B, _g_). The form and
arrangement of stomata are shown in fig. 143, A, which was drawn from
a piece of a leaf shown in surface-view in a section lent to me by
Professor Weiss. It should, however, be pointed out that the leaf
cannot be certainly identified with _Lepidodendron_ rather than with
_Sigillaria_, but as the leaves of these two genera are constructed on
the same plan the identification is of secondary importance.
[Illustration: FIG. 142. Leaves of _Sigillaria_ in transverse
section. A, A′. Section in the Manchester University Museum (Q.
631). B, C. Sections in Dr Kidston’s Collection.]
The single xylem bundle consists of primary tracheae only, at least in
such laminae as have been identified as Lepidodendroid. Surrounding
the xylem strand occur delicate parenchymatous cells in some cases
accompanied by darker and thicker-walled elements. As in _Sigillaria_,
the leaves of which are more fully described on page 210, a fairly
broad sheath of wider and shorter scalariform or spiral transfusion
tracheids surrounds the conducting strand (figs. 142, _t_; 143, B, C,
_t_). As Renault shows in the case of _Lepidodendron esnostense_[242],
the small leaves of which are 1·5–2 mm. broad at the base and several
centimetres long, the stomatal grooves and keel die out towards the
apex when the lamina assumes a more nearly circular form (fig. 143, C).
[Illustration: FIG. 143.
A. Stomata in surface-view (_Lepidodendron_?). _a_, parenchyma;
_t_, transfusion tracheae; _x_, xylem. (Manchester University
Collection R. 723).
B, C. _Lepidodendron esnostense_ Ren. (After Renault.)]
The area of the cushion excluding the leaf-scar is spoken of by some
writers as the field. Below the leaf-scar the kite-shaped cushion
tapers to a gradually narrowing basal position: in _Lepidodendron
Veltheimianum_, a species characteristic of Lower Carboniferous
strata, it is seen to be continuous, as a ridge with sloping sides,
with a lower cushion (fig. 185).
Below a leaf-scar the cushion frequently shows a pair of oval areas on
which a fine pitting may be detected in well-preserved impressions,
these oval scars, as seen in fig. 185, D, are practically continuous
at the upper end with the parichnos scars on the leaf-scar area;
this is explained by the fact that these infra-foliar scars also owe
their existence to patches of lacunar, aerenchymatous tissue in close
connexion with the parichnos[243].
Shortly before entering the base of the leaf-lamina the parichnos
divides into two arms which diverge in the outer cortical region right
and left of the vascular bundle, and passing obliquely upwards they
come close to the surface of the leaf-cushion just below the leaf-scar.
The diagram—fig. 144, B—shows a leaf-trace, _lt_, in the leaf-cushion,
as seen in a diagrammatic drawing of a vertical radial section of a
stem, the dotted lines, _p_, _p′_, show the two parichnos arms which
are represented as impinging on the surface of the leaf-cushion at
_p′_, and then bending upwards to pass into the leaf-base right and
left of the vascular bundle or leaf-trace. For convenience the arms of
the parichnos are represented in one plane though actually in different
vertical planes.
Fig. 144, A, shows the difference between a view of the original
surface of a _Lepidodendron_, as at _a_, where a leaf-cushion with a
leaf-scar is seen, and a view of an impression representing the outer
cortex, _b_, a short distance below the surface. The surface _b_,
in fig. 144, A, corresponds to the face _d_-_e_ in the diagrammatic
longitudinal section fig. 144, B: the outline of each cushion is
clearly visible and in the centre is seen the leaf-trace, _lt_, with
its parichnos.
The surface-features, _a_ (fig. 144, A), have been impressed on the
rock, _c_, (fig. 144, B) in which the specimen was entombed and by the
removal of the cast of the stem, that is the thickness _b_ to _e_ in
fig. 144, B, the form of the leaf-cushion is revealed. The presence of
the two infra-foliar parichnos scars at _p′_ (fig. 144, A) is explained
by the diagram, fig. 144, B, _p′_.
The relation of the parichnos to the oval scars below a Lepidodendron
leaf-cushion has been worked out in detail by Weiss who shows that, at
least in some species, the two arms do not bend downwards as shown in
the diagram, fig. 144, B, but pursue a straight gradually ascending
course as seen in fig. 145, A. Just below the leaf-scar region of
the cushion each arm comes into association with a group of lacunar,
aerenchymatous tissue, such as occurs in the roots of certain Mangrove
plants, and it is this aerenchyma which is exposed on the two oval
depressions below the leaf-scar. The structure of this aerenchyma
is shown in fig. 145, B; it consists in this species (_L. Hickii_
Wats.) of stellate cells which would constitute an efficient aerating
system. Probably, as Weiss suggests, these patches of aerenchyma were
originally covered by an epidermis provided with stomata, and it is
owing to the destruction of this superficial layer that the two oval
scars often form a prominent feature on Lepidodendron leaf-bases[244].
The diagram reproduced in fig. 144, B, may be taken as practically
correct, as the patches of aerenchyma described by Weiss do not differ
essentially from the parichnos tissue.
[Illustration: FIG. 144. _Lepidodendron Veltheimianum_ Sternb.
A. Leaf-cushion and leaf-scar seen in surface-view at _a_; on the
rest of the specimen a slightly lower surface is exposed. (After
Stur.)
B. Diagrammatic longitudinal section to explain the differences
between its two surfaces _a_ and _b_ shown in fig. A.
The shaded portion _c_ represents the rock matrix, the surfaces
_ab_, _ed_, mark the outer and inner edge of the outer portion of
the bark of the Lepidodendron stem. _lt_, leaf-trace; _p_, _p′_,
parichnos.]
[Illustration: FIG. 145.
A. Diagrammatic surface-view and longitudinal section of a
Lepidodendron leaf-cushion.
B. Aerenchyma below the leaf-scar. (After F. E. Weiss.)]
The parichnos scars are shown on the leaf-scar and cushion in fig. 146,
C. In the lower leaf-cushion shown in fig. 146, E, the infra-foliar
parichnos scars, _p_, are clearly seen, but the preservation of
the leaf-scar is not sufficiently good to show them on that part
of the fossil. In the upper cushion (fig. 146, E) the position of
the parichnos arms is shown on the leaf-scar, but the infra-foliar
parichnos scars are hidden by two small spiral shells. The genus
_Spirorbis_, to which these shells are referred, appears to have
persisted from the Silurian epoch to the present day. The comparatively
frequent occurrence of _Spirorbis_ shells on the leaves and other
parts of Palaeozoic plants, has recently been dealt with in a paper
by Barrois[245] who discusses in detail the habitats of these small
animals from the point of view of the conditions under which the plants
were preserved. In a note by Malaquin appended to Barrois’ paper the
belief is expressed that _Spirorbis_ lived on pieces of Palaeozoic
plants which lay under water.
The fact that with one exception all the Spirorbis shells on the
specimen of _Lepidodendron_, of which two leaf-cushions are shown in
fig. 146, E, occur on the large parichnos scars on the cheeks of the
cushions, suggests the possibility that the escape of gases from the
parichnos tissue may have rendered the position attractive to the
_Spirorbis_. It can hardly be accidental that the shells occur on the
parichnos strands. This fact recalls the view held by Binney[246] and
accepted with favour by Darwin[247] that _Lepidodendron_ and other
coal-forest trees may have lived with the lower parts of the stems in
sea water.
Above the leaf-scar is a fairly deep triangular or crescentic pit
(fig. 146, C, _l_) known as the ligular pit from the occurrence on
younger shoots of a delicate organ like the ligule of _Isoetes_ (fig.
132) embedded in a depression in the upper part of the leaf-cushion.
The ligule was first figured in _Lepidodendron_ by Solms-Laubach[248]
and described in English material by Williamson under the name of the
adenoid organ[249].
In some Lepidodendron stems a second triangular depression may occur
above the ligular pit, the meaning of which is not clear: this has been
called the triangulum by Potonié[250]. Stur[251] suggested that it may
represent the position occupied by a sporangium in Lepidodendron cones.
It is important to remember that as a branch increases in girth the
leaf-cushions are capable of only a certain amount of growth: when
the limit is reached they are stretched farther apart and thus the
narrow groove which separates them is converted in older stems into
a comparatively broad and flat channel, thus altering the surface
characters.
[Illustration: FIG. 146. Lepidophloios and Lepidodendron leaf-cushions.
A, B, D, F, G, H, I. _Lepidophloios_. (Fig. A should be reversed.)
C, E. _Lepidodendron aculeatum_.
A, B. From a specimen in the Sedgwick Museum, Cambridge
(leaf-cushion 3 cm. broad).
C. From a specimen in the Sedgwick Museum, Cambridge (leaf-cushion
4 cm. long).
D. From a section in the Cambridge Botany School Collection.
E. From a specimen in the Bunbury Collection, Cambridge Botany
School, showing _Spirorbis_ shells (leaf-cushion 2 cm. long).
F. From a section in the Williamson Collection, British Museum No.
1, 973.
G, H, I. From sections in the Cambridge Botany School Collection.]
Another feature worthy of notice in reference to the leaf-cushions
of _Lepidodendron_ is the occurrence in rare instances of alternate
zones of larger and smaller cushions. This variation in the size of
the leaf-cushions is by no means uncommon in the closely allied genus
_Sigillaria_; in _Lepidodendron_ it has been described by Potonié[252]
in _L. volkmannianum_ and more recently by Mr Leslie and myself[253] in
a South African species _L. vereenigense_.
Owing to the natural exfoliation of the superficial layers of the
outer bark at a certain stage in the growth of the plant, or in
some instances no doubt as the result of _post-mortem_ decay, which
destroys the delicate cells of the meristematic zone in the outer
cortex, isolated leaf-cushions and strips of the external surface are
occasionally met with as carbonised impressions.
The appearance presented by a Lepidodendron stem which has been
deprived of its superficial tissues may be dealt with more intelligibly
after we have become familiar with the anatomical characters.
iii. _Lepidophloios_.
Before proceeding further with the genus _Lepidodendron_ a
short account may be intercalated of the external features of a
lepidodendroid type of stem which it is customary to describe under a
distinct generic title _Lepidophloios_. This name is convenient for
diagnostic purposes though it seems clear that apart from the form of
the leaf-cushion (fig. 146, A) we are at present unable to recognise
any well-defined differences between the two forms _Lepidodendron_ and
_Lepidophloios_. For general purposes the name _Lepidodendron_ will be
used as including plants possessing leaf-cushions of the type already
described as well as those with the Lepidophloios form of cushion.
The generic name _Lepidophloios_ was first used by Sternberg[254]
for a Carboniferous species which he had previously described as
_Lepidodendron laricinum_. In 1845 Corda[255] instituted the name
_Lomatophloios_ for specimens possessing the same external characters
as those for which Sternberg had chosen the name _Lepidophloios_.
The leaf-cushions of _Lepidophloios_ differ from those of the true
_Lepidodendron_ in their relatively greater lateral extension (cf.
fig. 146, A and C), in their imbricate arrangement and in bearing
the leaf, or leaf-scar, at the summit. In some species referred to
_Lepidophloios_ the cushions are however vertically elongated and
in this respect similar to those of _Lepidodendron_: an example of
this type is afforded by _Lepidophloios Dessorti_ a French species
described by Zeiller[256]. In younger branches the cushions may be
directed upwards having the leaf-scar at the top; but in the majority
of specimens the cushions are deflexed as in figs. 146, D; 160, A. The
shoot of _Lycopodium dichotomum_ shown in fig. 121, B, with the leaves
in the reversed position bears a close resemblance to a branch of
_Lepidophloios_.
The photograph of _Lepidophloios scoticus_ Kidst.[257] reproduced in
fig. 160, A, illustrates the dichotomous branching of the stem and the
form of the cushions with the leaf-scars pointing downwards. In the
fertile branch of the same species shown in fig. 160, B, the leaf-scars
face upwards.
In most species the cushions are simply convex without a median keel,
but in some cases a median ridge divides the cushion into two cheeks as
in the genus _Lepidodendron_. The leaf-scar bears three small scars,
the larger median scar marking the position of the leaf-trace, while
the lateral scars are formed by the two arms of the parichnos: in some
examples of deflexed cushions, though not in all, a ligular pit occurs
on the cushion a short distance above the leaf-scar.
The drawing reproduced in fig. 146, A, showing the leaf-scar on the
upper edge of the cushion should have been reversed with the leaf-scars
pointing downwards. This figure represents part of the surface of a
specimen consisting of the outer cortex of a stem with leaf-cushions 3
cm. broad. The thickness of this specimen is 4 cm.: a section through
the line _ab_ is represented in fig. 146, D (reproduced in the correct
position, with the leaf-scars, _sc_, pointing downwards): internal
to the cushions is a band of secondary cortex (the shaded strip on
the outer edge of the section) which was formed on the outside of
the phellogen. The phellogen is a cylinder of actively dividing cells
in the outer part of the cortex of the stem, often spoken of as the
cork-cambium or cortical meristem, which produces a considerable amount
of secondary cortical tissue on its inner face and a much smaller
amount towards the stem surface. This delicate cylinder frequently
forms a natural line of separation between the outer shell of bark and
the rest of the stem. In the specimen before us, the thin-walled cells
of the phellogen were ruptured before petrification and the outer shell
of bark was thus separated as a hollow cylinder from the rest of the
stem: this cylinder was then flattened, the two inner surfaces coming
into contact. Fig. 146, D, represents a section of one half of the
thickness of the flattened shell.
This separation of the outer cortex, and its preservation apart
from the rest of the stem, is of frequent occurrence in fossil
lycopodiaceous stems. The flattened outer cortical shell of a
_Lepidophloios_, specifically identical with that shown in fig. 146, A
and D, was erroneously described by Dr C. E. Weiss in 1881 as a large
lepidodendroid cone[258].
Fig. 146, B, affords a view of the inner face of the specimen of which
the outer surface is seen in fig. 146, A: the surface shown in the
lower part of the drawing, on which the boundaries of the cushions are
represented by a reticulum, corresponds to the inner edge of the strip
of secondary cortical tissue represented by the vertically shaded band
in fig. 146, D.
The shaded surface in fig. 146, B, represents a slightly deeper level
in the stem which corresponds to the outer edge of the vertically
shaded band of fig. 146, D: the narrow tapered ridges (fig. 146, B)
represent the leaf-traces passing through the secondary cortex, and the
fine vertical shading indicates the elongated elements of which this
strip of secondary cortex is composed.
In the longitudinal section diagrammatically reproduced in fig. 146,
D, cut along the line _ab_ of fig. 146, A, the parenchymatous tissue
of the stout cushions has been partially destroyed, as at _a_; at
_s_ is seen the section of a Stigmarian rootlet which has found its
way into the interior of a cushion. Each leaf-trace is accompanied
by a parichnos strand as in the true _Lepidodendron_; at the base of
the leaf-cushion the parichnos branches into two arms which diverge
slightly right and left of the leaf-trace, finally entering the base of
the leaf lamina as two lateral strands (fig. 147, _p_). At one point
in fig. 146, D the section has shaved a leaf-trace represented by a
black patch resting on the parichnos just above the line _ef_, but it
passes through one of the parichnos arms _p′_ which debouches on to the
leaf-scar _sc_ at _p_. Had the section been cut along the line _cd_ of
fig. 146, A the leaf-trace would have been seen in a position similar
to that occupied by the parichnos _p′_ in fig. 146, D.
[Illustration: FIG. 147. _Lepidophloios_ leaf-cushion in tangential
section. (From a section in the Williamson Collection, British
Museum, No. 1973.)]
Fig. 147, A, affords a good example of a tangential section through
a _Lepidophloios_ leaf-cushion, 1 cm. broad, like that represented
in fig. 146, A, showing the vascular bundle _lt_, the two parichnos
strands, _p_, composed of large thin-walled cells (cf. _Isoetes_, fig.
133, H, I), and the ligular pit near the upper edge of the section
enclosing the shrunken remains of the ligule (fig. 147, B, _l_).
[Sidenote: LEPIDODENDRON]
Fig. 147, B, shows the form of the tangentially elongated leaf-cushions
of _Lepidophloios_ and their spiral disposition.
Fig. 146, F, represents a section similar to that shown in figs. 147,
A and B, but in this case the leaf-trace, _lt_, and the parichnos
strands, _p_, lie in a cavity formed by the destruction of some of the
leaf-cushion tissue. It is worthy of notice that the parichnos cells
have resisted decay more successfully than the adjacent tissue of the
cushion.
The diagrammatic sketches reproduced in fig. 146, H and I, were
made from a transverse section similar to one originally figured by
Williamson[259]: fig. 146, H, corresponding in position to the line
_gh_ in fig. 146, A, passes through the ligular pit, _l_, and cuts
across the parichnos in the act of branching; the leaf-trace passes
outwards beyond the =Y=-shaped parichnos strand. In the other section,
fig. 146, I, the parichnos is shown in a horizontal plane and the
leaf-trace, _lt_, appears in oblique transverse section. In both
sections and in fig. 146, G the shaded band at the base represents the
secondary cortical tissue external to the phellogen.
The transverse section represented in fig. 146, G, shows in the
left-hand cushion, _a_, the exit of the two parichnos arms and the
leaf-trace between them: it illustrates also the various forms assumed
by lepidodendroid leaf-cushions when cut across at different levels.
iv. _The Anatomy_ of Lepidodendron vasculare _Binney_[260].
Figs. 148–155, 168, A.
In the earlier literature dealing with the anatomy of _Lepidodendron_
and _Sigillaria_ the presence or absence of secondary vascular tissue
was made the criterion of generic distinction and the distinguishing
feature between the classes Pteridophytes and Gymnosperms,
_Lepidodendron_ being relegated to the former class because it
was supposed to have no power of forming secondary wood, while
_Sigillaria_, characterised by a considerable development of such
tissue, was classed by Brongniart and afterwards by Renault as a
Gymnosperm. Binney[261] in 1865 recognised that the two types of stem
pass into one another, but it was Williamson[262] who provided complete
demonstration of the fallacy of the Brongniartian view.
These two undoubted Pteridophytes agree very closely in anatomical
structure and both are now recognised as arborescent genera of
Lycopodiaceous plants. In a paper published by Lomax and Weiss
in 1905[263] a specimen is described from the Coal-Measures of
Huddersfield, in which a decorticated stem with the anatomical
characters of Binney’s _Sigillaria vascularis_ gives off a branch
having the anatomical structure which it has been customary to
associate with the species _Lepidodendron selaginoides_, so-called by
Sternberg and founded by him on impressions showing well-preserved
external characters.
In 1862 Binney[264] described petrified specimens of vegetative shoots
from the Lower Coal-Measures of Lancashire under the names _Sigillaria
vascularis_ and _Lepidodendron vasculare_. These were afterwards
recognised as different states of the same species. A few years after
the publication of Binney’s paper Carruthers[265] identified Binney’s
species _Lepidodendron vasculare_ with Sternberg’s _L. selaginoides_.
The evidence on which this identification rests has not been stated,
but many writers have retained this specific designation for the
well-defined type of anatomical structure first described by Binney
as _L. vasculare_. The use of the specific name _selaginoides_ is,
however, open to objection. The species _Lepidodendron selaginoides_,
as pointed out by Kidston[266], is probably identical with the plant
which Brongniart had named _L. Sternbergii_ before the institution
of Sternberg’s species, and we are not in possession of convincing
evidence as to the connection of _L. Sternbergii_ (= _L. selaginoides_)
with specimens possessing the anatomy of Binney’s type. Binney’s
designation is therefore retained for the anatomical type described in
the following pages[267].
The most detailed account hitherto published of the anatomy of
_Lepidodendron vasculare_ is that by the late M. Hovelacque[268], based
on material from the Lower Coal-Measures of England.
[Illustration: FIG. 148. _Lepidodendron vasculare_ Binney.
A. Transverse section. (Based on a section 2·5 cm. in diameter, in
the Cambridge Botany School Collection.)
B. Longitudinal section. (Drawn from a section in Dr Kidston’s
Collection.)]
The small shoot, represented somewhat diagrammatically in fig. 148,
A, illustrates the anatomical features of a typical example of the
species: the shoot has a diameter of 2·5 cm. and its central cylinder
(_x_-_sc_) is 2·5 mm. in width.
Noticeable features are (i) the small size of the central cylinder
(or stele) in proportion to the diameter of the branch, (ii) the
production at a comparatively early stage of growth of a zone of
secondary wood, _x_², which gradually assumes the form of a complete
cylinder of unequal breadth, surrounding the primary xylem, _x_,
(iii) the formation of a secondary cortical tissue by a meristematic
cylinder (phellogen, _pl_) situated close to the leaf-cushion region
of the outer cortex. On the outer edge the stele consists of narrow
tracheae some of which show in longitudinal section the spiral form
of thickening characteristic of most protoxylem elements: towards
the centre of the stele the diameter of the tracheae gradually
increases and parenchymatous cells become associated with the
elongated scalariform elements. In the central region the stele is
composed of parenchymatous tissue arranged in vertical series of short
cells, interspersed with short tracheae distinguished by the greater
thickness of their walls and by their scalariform and reticulate
thickening bands. Some of these short tracheae are shown in vertical
section in fig. 149, B: the fine and broken lines connecting adjacent
thickening bands probably represent the remains of the original wall.
These delicate bands, which have been figured in various species of
lepidodendroid plants[269], are worthy of notice in connexion with the
recent work of Mr Gwynne-Vaughan[270] who has shown that in many recent
ferns the scalariform bands in the xylem elements are not connected
by a thin pit-closing membrane, but are separated from one another by
open spaces. In the Lepidodendron tracheae we seem to have a stage in
which the intervening membrane is in process of absorption. It is,
however, possible that the threads may be the result of contraction and
splitting of the membrane during drying or decay.
[Illustration: FIG. 149. _Lepidodendron vasculare_. _a_, immature
tracheae; _m_, meristem; _mr_, medullary ray; _x_, xylem.
A. Longitudinal section through the edge of the secondary wood.
B. Short tracheae in the centre of the stele. (From a specimen from
the Halifax Hard bed in Dr Kidston’s Collection.)]
The stele of _Lepidodendron vasculare_, before the addition of any
secondary xylem, may be described as a protostele, a term originally
proposed by Professor Jeffrey[271], in which the central part of the
conducting strand of xylem elements has been converted into rows of
parenchyma and short tracheids, the latter being better adapted to
storage than to conduction. It is probable that this type of stelar
anatomy, which distinguishes _L. vasculare_ from other species,
represents a comparatively primitive arrangement forming a transition
between the stele of _L. esnostense_, which consists of a solid rod
of tracheids, and the stele of _L. Harcourtii_ (fig. 179, A) and
other species in which the xylem forms a cylinder enclosing a large
parenchymatous pith.
Parenchymatous cells occur in contact with the outer edge of the
xylem-cylinder some of which are distinguished by an irregular
reticulate pitting. The tangential section represented in fig. 148, B,
illustrates the appearance of a shoot of _L. vasculare_ in which no
secondary xylem is present: the central strand of tissue consists of
the parenchyma abutting on the xylem with several leaf-traces (_lt_)
passing upwards in an almost vertical course from the outer edge of the
stele.
The secondary xylem (fig. 148, A, _x_²) consists of radially arranged
scalariform tracheae with associated rows of parenchymatous cells which
form medullary rays (fig. 149, _mr_). Leaf-traces pass through the
medullary rays in the secondary xylem cylinder in a direction at right
angles to the primary xylem stele from which they are given off, but at
the outer edge of the secondary xylem they bend suddenly upwards and
for a time follow a steep and almost vertical course.
In well-preserved longitudinal sections the outermost secondary xylem
tracheae are seen to be succeeded by a few narrow and vertically
elongated elements (fig. 149, A, _a_), which represent young
unlignified tracheae: these are followed by shorter parenchymatous
cells (_m_) forming part of a meristematic zone from which the
secondary xylem receives additions.
Returning to fig. 148, A; the zone of secondary wood, _x_², composed
of scalariform tracheids and medullary rays, is succeeded by a few
layers of parenchymatous cells and beyond this is a broader zone, _sc_,
to which the term secretory zone has been applied[272]; this is made
up of small parenchymatous cells varying in size and of larger spaces
which appear to have been formed by the disorganisation of thin-walled
elements. The whole zone presents a characteristic appearance due to
the association of small cells, large clear spaces, and a certain
amount of dark-coloured material suggestive of tissue disorganisation
and secreted products. The anatomical characters of the secretory zone
are shown in the photograph, fig. 168, A, _sc_. Several leaf-traces
are seen in transverse section in the secretory zone (black dots in
fig. 148, A, _sc_; fig. 154, C, _lt_): each trace consists of a strand
of narrow tracheae accompanied by a few encircling layers of small
parenchymatous cells. As a trace continues its steeply ascending course
through the secretory zone, it becomes associated with a strand of
that tissue and assumes the form of a collateral vascular bundle, the
outer part of which does not consist of typical phloem but of shorter
elements derived from the secretory zone. Beyond the secretory zone
we find a more homogeneous tissue composed of parenchymatous elements
slightly extended tangentially (figs. 148, A, _c_¹; fig. 168, A, _c_);
this is spoken of as the inner cortical region. In the great majority
of sections of _L. vasculare_ as of other species of the genus, the
broader middle cortex (fig. 148, _c_²) is occupied by mineral matter,
introduced subsequent to decay of the tissue; or it is represented by
patches of delicate tissue composed of loosely arranged parenchymatous
cells varying considerably in size and shape, some being small, oval
or polygonal elements while others have the form of sinuous hypha-like
tubes.
In this middle cortical region may be seen leaf-traces passing outwards
in an almost horizontal course (fig. 148, A, _lt_): after leaving the
inner cortex the leaf-traces bend somewhat abruptly outwards to follow
a more direct path through the middle and outer cortex. The ring of
tissue, _s_, seen in the middle cortex of fig. 148, A, belongs to a
Stigmarian rootlet.
The outer cortex (fig. 148, A and B, _c_³) consists of homogeneous
parenchyma which is stronger and more resistant to decay than the
looser middle cortex. The leaf-traces, as shown in fig. 148, B, pass
through this region in a rather steeply ascending direction: each is
seen to be enclosed by a space originally occupied by a strand of
middle cortical tissue which accompanies lepidodendroid leaf-traces on
their under side and has already been described as the parichnos, (pp.
97, 100–103; figs. 146, 147).
The surface of the stem shown in section in fig. 148, A, is composed of
broad leaf-cushions. A single leaf-trace with its parichnos passes into
each cushion, but in the neighbourhood of the base of a cushion the
parichnos bifurcates (cf. fig. 146, H, I) and the arms diverge slightly
to the right and left finally passing beyond the cushion into the
lamina of the leaf, their position being shown, as already explained,
by the two small lateral scars on the leaf-scar area.
The diagrammatic sketch of a radial longitudinal section through a
leaf-cushion represented in fig. 150 illustrates the relation of the
leaf-trace to the leaf-cushion. The trace consists of xylem, _x_, above
and a strand of the secretory zone, _st_, below; the parichnos tissue
was originally present on the under side of the leaf-trace at _a_. The
external surface, _bc_, marks the limit of the leaf-scar through the
middle of which passes the vascular strand _lt_.
The lower gap _a_ has been formed by the tearing of thin-walled cells
of the phellogen, the meristematic tissue from which a considerable
amount of secondary cortical tissue or phelloderm has been produced
at _pd_. On the outside of the cushion, _c_, the cells are somewhat
crushed and distinguished by their darker colour from the bulk of the
parenchymatous tissue _d_.
This section also illustrates another characteristic feature of
_Lepidodendron_, namely the presence of a ligule and a ligular pit:
the former is represented by a carbonised patch of tissue and the
latter extends from the surface of the cushion at _b_, just above the
leaf-scar, almost to the level of the leaf-trace, _lt_. A comparison of
this section with figs. 146 and 147 will make clear the relation of the
several parts of the cushion and leaf-scar.
The gaps _gg_, seen in fig. 148, A and B, mark the position of the
delicate meristematic zone or phellogen which arises close to the bases
of the leaf-cushions; the phellogen has already produced a few rows of
radially disposed elements, represented by short radial lines in the
drawing, which constitute secondary cortical tissue.
[Illustration: FIG. 150. _Lepidodendron vasculare_. Leaf-cushions in
longitudinal section. (From a specimen in Dr Kidston’s Collection.)]
In older shoots the amount of the secondary cortical tissue developed
on the inner side of the phellogen is considerable (cf. figs. 152, 153).
The structure of the cortex of a shoot in which secondary growth, both
in the stele and in the outer cortex, has progressed further than in
the specimen shown in fig. 148 is represented in fig. 151.
[Illustration: FIG. 151. _Lepidodendron vasculare_. An older stem than
that shown in fig. 148. (From a section in the Manchester Museum.
No. 351.)]
The section (fig. 151, A) measures 7 × 3·8 cm. in diameter; the
primary xylem is surrounded by a fairly broad cylinder of secondary
wood (fig. 151, E, _x_ and _x²_). The almost smooth surface of the
primary wood (fig. 151, E, _x_) is succeeded by the secondary xylem,
_x²_, characterised at its inner edge by the tapered ends of the
radial rows of scalariform tracheids between which occur several
delicate parenchymatous cells (fig. 151, E, _a_). The occurrence
of such isodiametric elements, often exhibiting a delicate spiral
thickening band, is a characteristic feature of the boundary between
primary and secondary wood in lepidodendroid stems. The secondary wood
is penetrated by numerous medullary rays and in some of them are seen
strands of narrow spirally thickened tracheae—the leaf-traces—which
are in organic continuity with the exarch protoxylem of the primary
wood. The leaf-traces are oval and mesarch. The space, _c²_, (fig.
151, A) originally occupied by the delicate middle cortex, is succeeded
by a shell of outer cortex composed chiefly of secondary tissue
(phelloderm, _pd_) passing towards the inner boundary of this region
into the primary outer cortex _g_ (fig. 151, A and C). The radially
disposed elements which make up the bulk of the phelloderm are
associated with concentric rows of secretory strands, represented by
tangentially arranged dots in fig. 151, A: on the outer edge of the
phelloderm a few patches of primary cortex are still preserved, as at
_c_, fig. A. One of these is shown on a larger scale in fig. B; at _m_
the phelloderm is interrupted by a gap beyond which the cells have
thinner walls and show signs of recent division; this is probably the
position of the phellogen. The tissue _b_, fig. 151, B, consists of
secondary cortex succeeded beyond _d_ by the parenchymatous tissue of
the leaf-cushion, in which the remains of a ligule, _l_, are seen in
the ligular pit. This section corresponds in position to a line drawn
across fig. 150 at the level of _b_. In this specimen we have two kinds
of secondary cortical tissue: that formed external to the phellogen,
from _m_ to _d_ in fig. 151, B, is less in amount than that produced
internal to the phellogen. We cannot make any satisfactory statement
as to the nature of this secondary tissue, whether or not any of it
agreed in composition with the cork which is usually formed external to
the phellogen in recent plants. As the stem of a _Lepidodendron_ grew
in girth the leaf-cushions became separated by intervening depressions
composed of the secondary cortex formed external to the phellogen, but
at a later stage the cushions were thrown off, leaving the outer edge
of the phelloderm as the superficial tissue. This exposed tissue became
fissured as growth and consequent stretching continued, producing the
appearance seen on the surface of the still older stem represented in
fig. 153.
The inner edge of the phelloderm seen at _e_ in fig. 151, C, passes
suddenly into the inner primary region of the outer cortex (fig. 151, A
and C, _g_) which comprises two types of parenchymatous tissue, patches
of isodiametric cells, _g_, _g_, alternating with radially arranged
areas consisting of tangentially elongated elements (fig. C, _f_, _f_;
fig. D) which extend as wedges into the phelloderm.
The longitudinal section represented in fig. 152, B, shows an equal
bifurcation of a stem in which no secondary xylem is present; in the
lower part of the section the xylem and the outgoing leaf-traces are
seen in radial section and at the upper end of each arm the leaf-traces
alone, _lt_, are exposed, as in fig. 148, B. It is interesting to
notice the large amount of phelloderm which has been produced in the
fork of the branch, at _pd_, where greater strength is required.
[Illustration: FIG. 152. _Lepidodendron vasculare._ Sections of
dichotomously branched shoot.
A. From a section (10·5 × 9 cm.) in the Cambridge Botany School
Collection.
B. From a section (8 cm. long) in the Cambridge Collection.]
The section represented diagrammatically in fig. 152, A, has lost the
outermost part of the cortex together with the leaf-cushions; it
consists largely of secondary cortex composed of radially disposed
phelloderm cells and tangentially placed secretory strands (represented
by the discontinuous black lines in the drawing): the dotted region
in the central part of the axis is composed of primary cortical
parenchyma, and the two spaces surrounding the steles contain portions
of the lacunar middle cortex. Each stele possesses a narrow crescentic
zone of secondary xylem; the amount is greater in the case of the
right-hand stele, of which a small piece is shown on a larger scale;
the striking contrast in size between the outer and more internal
secondary tracheae is no doubt the expression of some unfavourable
condition of growth. The position of the secretory zone beyond the
secondary xylem is shown at _sc_, fig. 152, A.
[Illustration: FIG. 153. _Lepidodendron vasculare._
(From a specimen (16 × 7·5 cm.) in the Manchester Museum.)]
An example of a large and partially decorticated stem is afforded by
the specimen (16 × 7·5 cm.) shown in fig. 153. The irregularly ribbed
surface is formed of rather thick-walled phelloderm, in which occur
tangentially arranged rows of secretory strands. The tapered form of
the secondary cortex as it abuts internally on the primary cortex is
shown very clearly in the drawing (cf. fig. 151, C). The stele in this
much older stem consists mainly of secondary wood.
[Illustration: FIG. 154. _Lepidodendron vasculare._ Shoot (2·8 cm.
diam.) with two steles.
(From a specimen from Halifax in the Williamson Collection, British
Museum, No. 340.)]
An interesting example of a small shoot, the largest diameter of which
is 2·8 cm., is shown in fig. 154, A: the section was cut a short
distance above the bifurcation of the stele into two approximately
equal branches. The outer part of the cortex consists of phelloderm,
_pd_, with the usual rows of secretory tracts, and primary outer
cortex _g_; the middle cortex is represented by patches of parenchyma
with a few leaf-traces. To one of the steles, _s′_ (fig. 154, A), a
crescent-shaped band of secondary xylem has been added; the other
stele, _S_, possesses no fully developed secondary elements.
Fig. 154, B and C, illustrates the anatomical features immediately
external to the primary xylem of the smaller stele, _s_. The
comparatively broad band of radially disposed parenchyma, _m_, is
connected with the outermost elements of the xylem by a few rather
dark and small crushed parenchymatous cells. The band _m_, which we
may speak of as the meristematic zone, clearly consists of cells in a
state of division; it is in this region that the secondary xylem is
produced. Beyond the leaf-trace, (fig. 154, C, _lt_), occurs a portion
of the secretory zone, some of the smaller cells of which show signs
of disorganisation; but most of this tissue has been destroyed (fig.
154, B, _sc_). The outer edge of the secretory zone is shown in fig.
154, D abutting on the cells of the inner cortex, _c′_. The leaf-trace
shown in the inner cortex in fig. 154, B illustrates the more oval or
tangentially extended form of the xylem in this region, in contrast to
the more circular outline which it exhibits on the inner side of the
secretory zone.
[Illustration: FIG. 155. _Lepidodendron vasculare_. Outer edge of
secondary xylem: _m_, meristematic zone; _mr_, medullary ray.
(Drawn from the section shown in fig. 168, A).]
The transverse section, part of which is reproduced in fig. 168, A,
illustrates a characteristic feature, namely the juxtaposition of the
outermost tracheae of the secondary xylem and much smaller cells of
the meristematic zone. This is seen in fig. 155, which shows a small
piece of fig. 168, A, on a larger scale. In plants with a normal
cambium the segments cut off from the initial layer fit on to the
elements of the xylem or phloem to which they are to form additions,
but in _Lepidodendron_ it seems to be a general rule to find each
of the most external lignified elements abutting on a group of two
or three much smaller cells. It is difficult to believe that the
meristem shown in fig. 155, _m_, could produce secondary xylem elements
equal in size to those already formed: in all probability had growth
continued there would have been a marked difference between the size
of the secondary tracheids, as in fig. 152, A, _x²_, where there was
no doubt some cause which interfered with normal cambial activity.
This disparity in size between the secondary xylem elements and the
adjacent parenchymatous tissue of the meristematic zone is by no means
exceptional and may be described as the general rule. It is at least
certain that in _Lepidodendron vasculare_, as in other species, the
secondary xylem was succeeded by a broad band of parenchymatous tissue,
from which new tracheae and medullary-ray elements were produced, and
not by a narrow cambium such as occurs in recent plants.
v. Lepidodendron _stems as represented by casts and
impressions of partially decorticated specimens._
The differentiation of the outer cortex of a _Lepidodendron_ into
comparatively thin-walled and more resistant tissue has been the cause
of unequal decay and the consequent formation of shrinkage cavities. In
addition to the unequal resisting power of contiguous tissues, another
important factor in determining the nature of casts and impressions is
the existence of the cylinder of delicate cells in the outer cortex of
stems and branches. As already pointed out, this meristematic cylinder
or phellogen constitutes a natural line of separation, as in the case
of the cambium layer between the wood and the external tissues in a
fresh Sycamore twig. The result of the separation of an outer shell of
bark from the rest of the stem and the results of unequal decay in the
more superficial tissues, have necessarily led to the preservation of
the same specific type under a variety of forms.
Our knowledge of the anatomy of Lepidodendron stems enables us to
recognise in fossils of very different appearance specimens in various
conditions of preservation of one and the same type. Such names as
_Knorria_, _Bergeria_ and _Aspidiaria_ are examples of generic titles
instituted before any adequate knowledge of Lepidodendron anatomy was
available.
Differences in age as well as different degrees of decortication have
contributed in no small measure to the institution of generic and
specific names which more recently acquired knowledge has shown to be
superfluous.
_a._ _Knorria._
The designation Knorria, after a certain G. W. Knorr of Nürnberg, was
proposed by Sternberg in 1826[273] for casts of Palaeozoic stems of a
type figured more than a century earlier by Volkmann[274]. Goeppert, in
his earlier works, published drawings of fossil stems which he referred
to Sternberg’s genus: one species he at first called _Didymophyllum
Schollini_. He afterwards[275] described some specimens which showed
that the features characteristic of _Knorria_ may occur on partially
decorticated stems with leaf-cushions of the true Lepidodendron type.
His specimens, preserved in the Breslau Museum, demonstrate the
accuracy of his drawings and conclusions. Goeppert, and after him
Balfour[276], drew attention to the different appearances presented
by branches of _Araucaria imbricata_ when preserved with the surface
intact and after partial decortication, as illustrating possible
sources of error in the determination of fossil stems.
Although it is now a well-established fact that fossils bearing the
name _Knorria_ are imperfect lepidodendroid stems, the use of the term
may be conveniently retained for descriptive purposes. The specimen
from the Commentry coal-field of France, shown in fig. 156, affords
some excuse for the institution of several generic names for different
states of preservation or decortication of one species. The cortical
level exposed at _e_ is characterised by spirally disposed peg-like
ridges with truncated apices: it is this form of cast which is usually
designated _Knorria_. The ridges vary in size and shape in different
types of stem; they may be narrow as shown at _e_, fig. 156, or short
and broad with rounded distal ends. In some cases they are forked at
the apex, as in the partially decorticated specimen of _Lepidodendron
Veltheimianum_ represented in fig. 185, A.
[Illustration: FIG. 156. A dichotomously branched Lepidodendroid stem
(_Knorria mirabilis_ Ren. and Zeill.). (After Renault and Zeiller.)
(¼ nat. size.) The original specimen is in the Natural History
Museum, Paris.
_a–g_, surface features exposed as the result of different degrees
of decortication. (See vol. I. p. 102, fig. 23).]
The _Knorria_ state represents the impression or cast of the outer
cortical region too deep below the leaf-cushion region to retain
any indications of the cushion-form; the ridges are the casts of the
spaces produced in the cortex by the decay of the sheath of delicate
cells surrounding each leaf-trace and by the decay of the thin-walled
cells of the parichnos. The occasional forked apex of a ridge is the
expression of the fact that the cast was made at the region where the
parichnos divides into two arms (cf. p. 100). In certain specimens it
is possible to connect the Knorria casts with associated lepidodendroid
stems which may be determined specifically; but when we have no
evidence as to surface-features the fossils may be designated casts
of lepidodendroid stems in the Knorria condition. Such casts are
illustrated by numerous drawings in palaeobotanical literature[277].
_b._ _Bergeria._
This is another name first used by Sternberg in his classic work,
_Die Flora der Vorwelt_, for casts of lepidodendroid plants such as
Steinhauer[278] had previously figured as _Phytolithus cancellatus_.
Brongniart[279] recognised that the application of the generic title
_Lepidodendron_ should be extended to include specimens referred by
Sternberg to _Bergeria_, and a few years later Goldenberg[280] realised
that this name does not stand for well-defined generic characters.
The correctness of these views was, however, first satisfactorily
demonstrated by Carruthers[281] and by Feistmantel[282].
If a Lepidodendron stem loses its superficial layers of outer
cortex and in this condition is embedded in sand or mud, the cast
is distinguished from that of a perfect stem by the absence of
the leaf-scars and by other features. It may, however, still show
spirally disposed areas, corresponding approximately to the original
leaf-cushions, which are characterised by a small depression or pit
either at the apex or near the centre of each oval area: the pit marks
the position of the leaf-trace and its parichnos strand. In some
cases the exposed surface may be smooth without any indication of
leaf-cushions, while narrow spirally arranged grooves represent the
obliquely ascending vascular bundles passing through the cortex to the
leaves.
Fig. 185, B, shows the Bergeria state of _Lepidodendron Veltheimianum_,
which differs from the Knorria condition in the fact that decortication
had not extended below the level at which the form of the leaf-cushions
could be recognised. It is clear that no sharp line can be drawn in all
cases between the different degrees of decortication as expressed by
the terms _Knorria_ and _Bergeria_.
A list of synonyms of _Knorria_, _Bergeria_, and _Aspidiaria_ forms of
stem and a detailed treatment of their characteristic features may be
found in a recent work by Potonié[283].
_c._ _Aspidiaria._
In one of the earliest English books on fossil plants, the
_Antediluvian Phytology_ by Artis[284], a specimen from the
Carboniferous sandstone of Yorkshire is figured as _Aphyllum
cristatum_, and a similar fossil is described as _A. asperum_. These
are impressions of Lepidodendron stems in which the characteristic
leaf-cushions are replaced by smooth and slightly convex areas with
a narrow central ridge. To this type of specimen Presl gave the
name _Aspidiaria_[285], under the impression, shared by subsequent
writers, that the supposed external features were entitled to generic
recognition.
It is to Stur[286] that we owe the first satisfactory interpretation
of fossils included under the name _Aspidiaria_: he showed that on the
removal of the projecting convex areas from some of his specimens a
typical Lepidodendron leaf-cushion was exposed (fig. 144, A, _a_). The
Aspidiaria condition (fig. 144, A, _b_) represents the inner face of
the detached shell of outer bark of a Lepidodendron stem, while in the
Bergeria casts we have a view of the external face of a stem deprived
of its superficial tissues.
In a Lepidodendron stem embedded in sediment the more delicate portions
of the leaf-cushions would tend to shrink away from the internal and
more resistant tissues of the outer cortex, thus producing spaces
between each cushion; further decay would cause rupture of the
leaf-traces and the superficial tissues would thus be separated from
the rest of the stem. The tendency of Lepidodendron stems to split
along the line of phellogen in the outer cortex is seen in fig. 148,
A, _g_. The deposition of sediment on the exposed inner face of this
cortical shell would result in the production of a specimen of the
Aspidiaria type: the reticulum enclosing the spirally disposed convex
areas is formed by the impression of the firmer tissue between the
leaf-cushions.
vi. _Lepidodendroid axes known as_ Ulodendron _and_ Halonia.
_a_. _Ulodendron._
This generic name was suggested by Lindley and Hutton[287] for two
specimens from the English Coal-measures characterised by leaf-cushions
like those of a _Lepidodendron_, but distinguished by the presence of
two vertical rows of large and more or less circular cup-shaped scars.
These authors, while recognising the possibility that the fossils
might be identical with _Lepidodendron_, regarded them as generically
distinct. The generic title _Ulodendron_, though no longer denoting
generic rank, is still applied to certain shoots of lycopodiaceous
plants which may belong to the genera _Lepidodendron_, _Bothrodendron_,
and according to some authors[288], also to _Sigillaria_.
The large specimen from the Belgian coal-measures, represented in fig.
211, affords a good example of the Ulodendron form of shoot of the
genus _Bothrodendron_, which is described on page 249. The specimen
shown in fig. 157 shows the _Ulodendron_ shoot of _Lepidodendron
Veltheimianum_.
Casts of large Ulodendron scars are occasionally met with as separate
fossils bearing a resemblance to an oval shell.
In Steinhauer’s paper on _Fossil Reliquiae_[289] a drawing is given of
a Ulodendron stem under the name _Phytolithus parmatus_ and a similar
stem specifically identical with that shown in fig. 157 was figured by
Rhode[290], one of the earliest writers on fossil plants, under the
comprehensive designation “Schuppenpflanze.”
[Illustration: FIG. 157. _Lepidodendron Veltheimianum._ Ulodendron
condition. (From a photograph by Dr Kidston of a specimen from the
Calciferous Sandstone series, Midlothian; ⅖ nat. size.) [Kidston
(02) Pl. LVII.]
There has been no lack of ingenuity on the part of authors in offering
suggestions as to the meaning of these large cup-like depressions,
and there is still difference of opinion as to their significance.
Lindley and Hutton[291] described them as the scars of branches or
masses of inflorescence. Sir Joseph Hooker[292] speaks of a specimen
of _Ulodendron_, shown to him by Mr Dawes, on which a large organ,
supposed to be a cone, was inserted in one of the depressions, but
he was unable to arrive at any conclusion as to the real nature of
the fossil. While most authors have seen in the scars pressure-areas
formed by the pressure of sessile cones against the surface of a
growing branch, others, as for example Geinitz[293], have described
the depressions as branch-scars. Carruthers[294] regarded the scars
as those of adventitious roots and Williamson referred to them as the
scars of reproductive shoots. The depressions vary considerably in
size. The Belgian example shown in fig. 211 possesses scars 9 cm. in
diameter. A specimen of _Bothrodendron_ in the Manchester Museum from
the Lancashire Coal-Measures, to which Williamson[295] has referred,
bears two rows of scars 11–12 cm. in diameter on a stem 112 cm. in
girth and 233 cm. long. The scars occur in two alternate series, on
opposite faces of the axis, the distance between the successive scars
in the same row being 29 cm. The surface-features of this large stem
are not preserved.
Before considering the nature and origin of the scars it is important
to remember the considerable size to which they may attain; other
points of importance are the occurrence, either in the centre of each
depression or in an excentric position, of an umbilicus or slightly
projecting boss, in the centre of which is a pit formed by the decay of
an outgoing vascular strand. The sloping sides of the scars sometimes
bear elevations resembling leaf-cushions like those on the rest of
the stem surface. In the specimen shown in fig. 157 the lower margin
of each cup shows indistinctly the outlines of what appear to be
leaf-cushions, while the rest of the sloping face is characterised by
radial ridges, which may be due to bracts or leaves.
It is obvious that in these cups we have the scars of some lateral
organ, but the evidence afforded by specimens of which the depressions
contain the remains of such organs is by no means conclusive. A
Ulodendron has been figured by D’Arcy Thompson[296], in which the lower
part of a lateral organ is attached by a narrow base to one of the
scars, but the preservation is not sufficiently good to enable us to
decide whether the organ is a cone or a vegetative shoot. Kidston[297]
has described other examples showing portions of organs in connexion
with the scars, but an examination of the specimens in his collection
failed to convince me that his interpretation of them as strobili is
correct.
The phenomenon known as cladoptosis, as shown on a stem of the Conifer
_Agathis_[298] and certain Dicotyledonous trees such as _Castilloa_,
suggests a possible explanation of the Ulodendron scars. This
comparison was made by Shattock[299] in 1888, but he did not accept the
resemblance as a real one. An objection may be urged to the cladoptosis
hypothesis that in _Ulodendron_ the branch, whether vegetative or
reproductive, was not attached to the whole of the depressed area.
On the other hand, a lateral branch originally attached by a narrow
base may have continued to increase in diameter until its base became
slightly sunk in the bark of the stem, thus producing a cup-like
depression which, on the fall of the branch, would retain traces of the
original surface-features of the stem.
Mr Watson[300] of Manchester recently published a paper on Ulodendron
scars, in which he adduces fresh and, as it seems to me, satisfactory
arguments in favour of the branch-scar hypothesis. Fig. 158, from
one of Mr Watson’s blocks, illustrates the nature of his evidence.
He points out that in the obverse half of a large specimen of
_Bothrodendron_ in the Manchester Museum, the umbilicus consists of a
cylindrical hole, 18 mm. deep and 8 mm. in diameter, surrounded by a
projecting ring of mineral material which doubtless represents some
portion of the original plant: on the reverse half of the specimen
the continuation of the ring is seen as a prominent cone fitting into
the cup-like depression in the obverse half: the conical cast shows
that numerous small vascular strands were given off from this ring of
tissue, and these strands have the same arrangement and size as the
dots which are found on typical Ulodendron scars. He interprets the
ring surrounding the umbilicus as the remains of the primary wood and
the small strands as leaf-traces supplying the branch.
[Illustration: FIG. 158. Diagrammatic section through the base of a
branch to illustrate the Branch theory of the Ulodendroid scar.
(After Watson.)]
In the diagrammatic section shown in fig. 158 the outer cortex of the
main stem is represented by _oc_ 1; this consists of secondary tissue.
The corresponding tissue in the branch is seen at _oc_ 2. The stele of
the stem is shown at Tr. St. and that of the branch at Br. St.; _lt_,
_lt_, mark the position of the leaf-traces. If we assume the branch
to be detached along the line LS, the depression would show numerous
spirally arranged dots representing the points of exit of leaf-traces
and the vascular axis would be exposed in the umbilicus. This
explanation appears to me to be in harmony with the surface-features
of Ulodendron scars on both Bothrodendron and Lepidodendron stems. The
occasional occurrence of leaf-cushions on a portion of a Ulodendron
scar is a difficulty on the cladoptosis hypothesis. Assuming that true
leaf-cushions occur, their presence may, as Watson suggests, be due to
the folding back of a piece of the outer cortex of the branch which has
been “crushed down on to the area of the scar[301].”
Since this account was written a note has been published by M.
Renier[302] in which he describes a specimen of _Bothrodendron_ from
Liège, one face of which shows a projecting Ulodendroid scar with an
excentric umbilicus. On the other face is a dichotomously branched
shoot with surface-features corresponding to those on the scar; the
evidence that the scar represents the base of the branch is described
as indisputable.
Stur[303] held the view that the depressions on Ulodendron stems
represent the places of attachment of special shoots comparable with
the bulbils of _Lycopodium Selago_, or, it may be added, with the short
branches occasionally produced on _Cycas_ stems. If the depressions
were formed by the pressure of the bases of cones, it is clear that
the size of the cavity must be an index of the diameter of the cone.
The larger Ulodendron scars exceed in diameter the base of any known
lepidodendroid strobilus. Another obvious difficulty, which has not
been overlooked by Kidston who holds that the scars were produced
by sessile cones, is that in _Lepidodendron Veltheimianum_ strobili
were borne at the tips of slender branches; the same difficulty is
presented by _Bothrodendron_ (Fig. 213). It is unlikely that two types
of strobili were produced on the same plant, particularly as the cone
of _L. Veltheimianum_ was heterosporous.
The cones of certain species of _Pinus_ remain attached to the tree
for many years and their bases become embedded in the stem; this is
particularly well shown in the drawing of a cone of _Pinus clausa_
(fig. 159), for which I am indebted to Mr Sudworth, Dendrologist in
the United States Forest Service. Mr Sudworth has drawn my attention
to _P. attenuata_ and _P. muricata_ in illustration of the same
phenomenon[304]. The example shown in fig. 159 cannot, however, be
matched by any known specimen of _Ulodendron_; in the case of the
depressions on the stem of a Pine the cone-base fits the circular scar,
but in the fossil stems it is practically certain that this was not the
case.
[Illustration: FIG. 159. _Pinus clausa._ ½ nat. size.]
There can be little doubt that certain Palaeozoic Lycopods shed their
branches by a method similar to that employed by the Kauri Pine of New
Zealand and by some species of Dicotyledons. The evidence adduced in
the case of _Bothrodendron punctatum_ is a strong argument in favour of
extending the same explanation to other Ulodendron shoots.
[Illustration: FIG. 160.
A. _Lepidophloios scoticus_ Kidst. From a specimen from the
Calciferous Sandstone, Midlothian, in Dr Kidston’s Collection;
rather less than ⅓ nat. size.
B. _L. scoticus_ cone. From a specimen from the Calciferous
Sandstone of Midlothian in Dr Kidston’s Collection; slightly
reduced.]
_b._ _Halonia._
The branched axis with _Lepidophloios_ leaf-cushions, represented
in fig. 160, A, illustrates a special form of shoot described by
Lindley and Hutton[305] under the generic name _Halonia_. The original
specimens referred to this genus are decorticated axes showing remains
of Lepidodendroid leaf-cushions. The spirally disposed circular scars
in the specimen of _Halonia_ (_Lepidophloios scoticus_[306]) shown in
fig. 160 constitute the characteristic feature of the genus; they may
have the form, as in fig. 160, A, of circular discs with a central
umbilicus marking the position of a vascular strand, or, as in the
sandstone cast of _Halonia tortuosa_ shown in fig. 161[307], they may
appear as prominent tubercles. The latter example illustrates the
condition characteristic of partially decorticated stems.
[Illustration: FIG. 161. _Halonia tortuosa_ L. and H. From a specimen
in Dr Kidston’s Collection, from the Lower Coal-measures of
Ayrshire (No. 1561); ⅔ nat. size.]
In 1883 Williamson[308] described a specimen, now in the Leeds
Museum, which convinced him that _Halonia_ is merely a special form
of _Lepidodendron_ concerned with the production of fertile shoots or
strobili. Feistmantel[309] also recognised that _Halonia_ _regularis_
is identical in the form of the cushions with the type known as
_Lepidophloios laricinus_. It is worthy of note that under the name
_Halonia_, Feistmantel[310] figured a piece of decorticated axis
characterised by two rows instead of the usual spiral series of large
cup-shaped scars. Recent researches have, however, tended to break down
the distinction between _Ulodendron_ and _Halonia_ founded respectively
on the biseriate and spiral arrangement of the scars or tubercles.
The interpretation of Halonial branches as cone-bearing members of
Lepidodendroid plants has passed into a generally accepted statement
of fact, but, so far as I know, only one specimen has been figured in
which strobili are seen attached to an Halonia axis. This specimen,
described by Grand’Eury[311] from the coal-field of Gard, is hardly
sufficiently well-preserved to constitute a demonstration of the
correctness of the generally received view, which, as is not unusual,
has been repeated by one writer after another without due regard being
paid to the nature of the evidence on which the statement is based. It
may, indeed, be correct to describe Halonial branches as cone-bearing,
but there are certain considerations which make one pause before
unhesitatingly accepting this explanation. The vascular strand which
passes from the central cylinder of the shoot to the tubercle or scar
is composed of a solid rod of xylem distinguished from the main stele
by the absence of a pith. In such petrified peduncles as have been
discovered the stele is of the medullated type. The common occurrence
of strobili terminating slender branches of lepidodendroid plants,
though not a fatal objection to their attachment to Halonial shoots,
shows that in many cases the cones were borne at the tip of leafy
shoots. It may be that some of the Halonial scars are in origin like
those of the Ulodendron axes of _Bothrodendron_ and mark the position
of deciduous vegetative branches.
The first account of the anatomy of _Halonia_ we owe to Dawes[312];
this was followed by a fuller description by Binney[313]. The
history of our knowledge of this type of branch has been given
by Carruthers[314], who expressed the opinion that Halonia is
merely a fertile condition of _Lepidophloios_ and possibly of other
lepidodendroid plants. He was also inclined to regard the Halonial
tubercles as younger stages of the larger scars characteristic of the
genus _Ulodendron_. Williamson’s contributions to our knowledge of
_Halonia_ are of primary importance; he supplied further proof of the
Lepidodendroid nature of these branches and advanced our knowledge of
their anatomy. In an early paper[315] he expressed the view that the
differences on which _Halonia_ and _Ulodendron_ are separated are such
as result from a difference in age and are not of generic importance.
In the last memoir, of which he was sole author, published by the Royal
Society[316], Williamson brought forward further evidence in support of
this well-founded opinion.
That the fossils known as _Halonia_ are branches of a lepidodendroid
plant is at least certain, and it is probable that the lateral
branches which they bore were fertile, though satisfactory proof of
this is lacking. We know also that Halonia branches are characterised
by the Lepidophloios form of leaf-cushion; there is, however, no
sufficient reason to assume that such branches were never attached
to stems with the cushions of the Lepidodendron form. The further
question, namely whether Williamson was correct in his contention as
to the absence of any essential distinction between _Ulodendron_ and
_Halonia_, does not admit of an unchallenged answer. In 1903 Weiss[317]
described the anatomy of a specimen of a biseriate _Halonia_ branch of
_Lepidophloios_. The form of the leaf-cushions is unfortunately not
very well preserved, but Weiss figures other specimens with two rows
of tubercles on which the leaf-cushions are sufficiently distinct to
justify a comparison with those of _Lepidophloios_. He believes with
Williamson that it is the presence of tubercles in place of scars which
distinguishes _Halonia_ from _Ulodendron_, and that the arrangement of
the tubercles or scars is a matter of little importance. He expresses
the opinion justified by the evidence available that the absence or
presence of tubercles is merely due to accidents of preservation or,
one may add, to difference in age. Kidston[318] dissents from Weiss’s
description of his specimen as a biseriate _Halonia_; he regards it as
a Ulodendron branch of _Sigillaria discophora_ (König). Until specimens
with more clearly preserved external features are forthcoming it is
impossible to settle the point in dispute, but on the facts before
us there would seem to be a _prima facie_ case in favour of Weiss’s
contention.
The designation _Halonia_ may be retained as a descriptive term for
Lepidodendroid shoots characterised by spirally disposed scars or
tubercles and bearing leaf-cushions of the Lepidophloios type. In the
case of specimens showing prominent tubercles, the superficial tissues
are usually absent and, as in the fossil represented in fig. 161, the
name _Halonia_ does not necessarily imply the presence of leaf-cushions
of a particular type.
vii. _Anatomical characters of Vegetative Lepidodendron shoots_
(_Lepidodendron and Lepidophloios_).
The type already described under the name _Lepidodendron vasculare_
differs from those dealt with in the following pages chiefly in the
anatomy of the stele. The simplest and probably most primitive type of
Lepidodendron stem is that in which the xylem forms a solid rod; the
type of stele most frequently represented is that of _L. Harcourtii_,
_L. fuliginosum_, and other species in which the diameter of the stele
is greater and a cylinder of primary xylem encloses a comparatively
large parenchymatous pith.
1. _Lepidodendron esnostense_, Renault[319].
This species was founded by Renault on petrified specimens from the
Culm beds of Esnost in France. The surface of a young twig bears
prominent leaf-cushions of elongated rhomboidal form similar to those
of _Lepidodendron obovatum_ (fig. 173) and other species. In older
branches the primary cortex is replaced by a considerable thickness
of radially disposed secondary cortical tissue which, as shown in
tangential section, consists of a reticulum of elongated pointed
elements with comparatively thick walls enclosing meshes filled with
large-celled parenchyma. It is worthy of note that if such a branch
were exposed to decay, the earlier destruction of the more delicate
tissue in the meshes of the secondary cortex would produce a series of
oval depressions, corresponding to the parenchymatous areas, separated
by a projecting reticulum of the more resistant elements: a cast of
this partially decayed surface would be indistinguishable from that
of some types of _Sigillaria_ or of a _Lyginodendron_. The inner
regions of the cortex of the type-specimens have not been preserved.
The xylem, which is the only part of the stele represented, has the
form of a protostele or solid cylinder of scalariform tracheids with
peripheral groups of narrower protoxylem elements which mark the
points of exit of the leaf-traces: in a branch 1–2 cm. wide the xylem
column has a diameter of 3 mm. The small leaves (fig. 143, B, C),
similar to those of a _Sigillaria_, are sub-rhomboidal in section
near the base and approximately circular near the apex[320]. The
mesophyll consists of palisade cells having the appearance of typical
chlorophyll-tissue. The heterosporous strobili attributed to this
species bore microsporangia on the upper and megasporangia on the lower
sporophylls; the megaspores, of which a considerable number occur in
each megasporangium, are identical in size with those of another Culm
form, _Lepidodendron rhodumnense_. Some of these have retained traces
of prothallus tissue, and in one spore Renault figures what he regards
as an archegonium: the drawing is by no means convincing.
2. _Lepidodendron rhodumnense_, Renault[321].
The species from the Culm of Combres (Loire) agrees in its solid xylem
cylinder and in the differentiation of the secondary cortex, as also in
the association of two kinds of spore, with _Lepidodendron esnostense_.
A comparison of the leaves of the two types reveals certain differences
which may be of specific rank, but, apart from minor differences, these
Culm species may be classed under one anatomical type.
3. _Lepidodendron saalfeldense_, Solms-Laubach[322].