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, x2, 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.
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, x2) 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, x2, 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, c1; 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, c2) 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, c3) 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.
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.
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.
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.
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.
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].
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].
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.
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.”
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.