Chapter XV: Arborescent Lycopodiales (1)

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.

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.

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.

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).

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.

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.]

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.

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.

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.

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.

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.

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.

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.

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.

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.

(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.

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.

_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].