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.
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.