Fig. 161. Halonia tortuosa L. and H. From a specimen in Dr Kidston’s Collection, from the Lower Coal-measures of Ayrshire (No. 1561); ⅔ nat. size.
In 1883 Williamson[308] described a specimen, now in the Leeds Museum, which convinced him that Halonia is merely a special form of Lepidodendron concerned with the production of fertile shoots or strobili. Feistmantel[309] also recognised that Halonia regularis is identical in the form of the cushions with the type known as Lepidophloios laricinus. It is worthy of note that under the name Halonia, Feistmantel[310] figured a piece of decorticated axis characterised by two rows instead of the usual spiral series of large cup-shaped scars. Recent researches have, however, tended to break down the distinction between Ulodendron and Halonia founded respectively on the biseriate and spiral arrangement of the scars or tubercles.
The interpretation of Halonial branches as cone-bearing members of Lepidodendroid plants has passed into a generally accepted statement of fact, but, so far as I know, only one specimen has been figured in which strobili are seen attached to an Halonia axis. This specimen, described by Grand’Eury[311] from the coal-field of Gard, is hardly sufficiently well-preserved to constitute a demonstration of the correctness of the generally received view, which, as is not unusual, has been repeated by one writer after another without due regard being paid to the nature of the evidence on which the statement is based. It may, indeed, be correct to describe Halonial branches as cone-bearing, but there are certain considerations which make one pause before unhesitatingly accepting this explanation. The vascular strand which passes from the central cylinder of the shoot to the tubercle or scar is composed of a solid rod of xylem distinguished from the main stele by the absence of a pith. In such petrified peduncles as have been discovered the stele is of the medullated type. The common occurrence of strobili terminating slender branches of lepidodendroid plants, though not a fatal objection to their attachment to Halonial shoots, shows that in many cases the cones were borne at the tip of leafy shoots. It may be that some of the Halonial scars are in origin like those of the Ulodendron axes of Bothrodendron and mark the position of deciduous vegetative branches.
The first account of the anatomy of Halonia we owe to Dawes[312]; this was followed by a fuller description by Binney[313]. The history of our knowledge of this type of branch has been given by Carruthers[314], who expressed the opinion that Halonia is merely a fertile condition of Lepidophloios and possibly of other lepidodendroid plants. He was also inclined to regard the Halonial tubercles as younger stages of the larger scars characteristic of the genus Ulodendron. Williamson’s contributions to our knowledge of Halonia are of primary importance; he supplied further proof of the Lepidodendroid nature of these branches and advanced our knowledge of their anatomy. In an early paper[315] he expressed the view that the differences on which Halonia and Ulodendron are separated are such as result from a difference in age and are not of generic importance. In the last memoir, of which he was sole author, published by the Royal Society[316], Williamson brought forward further evidence in support of this well-founded opinion.
That the fossils known as Halonia are branches of a lepidodendroid plant is at least certain, and it is probable that the lateral branches which they bore were fertile, though satisfactory proof of this is lacking. We know also that Halonia branches are characterised by the Lepidophloios form of leaf-cushion; there is, however, no sufficient reason to assume that such branches were never attached to stems with the cushions of the Lepidodendron form. The further question, namely whether Williamson was correct in his contention as to the absence of any essential distinction between Ulodendron and Halonia, does not admit of an unchallenged answer. In 1903 Weiss[317] described the anatomy of a specimen of a biseriate Halonia branch of Lepidophloios. The form of the leaf-cushions is unfortunately not very well preserved, but Weiss figures other specimens with two rows of tubercles on which the leaf-cushions are sufficiently distinct to justify a comparison with those of Lepidophloios. He believes with Williamson that it is the presence of tubercles in place of scars which distinguishes Halonia from Ulodendron, and that the arrangement of the tubercles or scars is a matter of little importance. He expresses the opinion justified by the evidence available that the absence or presence of tubercles is merely due to accidents of preservation or, one may add, to difference in age. Kidston[318] dissents from Weiss’s description of his specimen as a biseriate Halonia; he regards it as a Ulodendron branch of Sigillaria discophora (König). Until specimens with more clearly preserved external features are forthcoming it is impossible to settle the point in dispute, but on the facts before us there would seem to be a prima facie case in favour of Weiss’s contention.
The designation Halonia may be retained as a descriptive term for Lepidodendroid shoots characterised by spirally disposed scars or tubercles and bearing leaf-cushions of the Lepidophloios type. In the case of specimens showing prominent tubercles, the superficial tissues are usually absent and, as in the fossil represented in [fig. 161], the name Halonia does not necessarily imply the presence of leaf-cushions of a particular type.
vii. Anatomical characters of Vegetative Lepidodendron shoots (Lepidodendron and Lepidophloios).
The type already described under the name Lepidodendron vasculare differs from those dealt with in the following pages chiefly in the anatomy of the stele. The simplest and probably most primitive type of Lepidodendron stem is that in which the xylem forms a solid rod; the type of stele most frequently represented is that of L. Harcourtii, L. fuliginosum, and other species in which the diameter of the stele is greater and a cylinder of primary xylem encloses a comparatively large parenchymatous pith.