Osmundites Kolbei Seward, [figs. 253–255].
This species was founded on a specimen obtained by Mr Kolbe from the Uitenhage series of Cape Colony[788]. The fossil flora and fauna of this series point to its correlation with the Wealden or Neocomian strata of Europe[789]. The type-specimen consists of several pieces of a stem ([fig. 253]) which reached a length of about 90 cm. On the weathered surface the remains of petiole-bases are clearly seen and on the reverse side of the smaller piece shown in the figure numerous sinuous roots are present in association with the leaf-stalks. The depression c in the larger specimen may mark the position of a branch: at a [fig. 253] (enlarged in [fig. 254], a) the vascular strand of a petiole is exposed as a broad U-shaped band and at b ([fig. 254], b) the form of the petiole-bases is clearly shown[790]. With the stem were found imperfectly preserved impressions of fronds referred to Cladophlebis denticulata, a common type of leaf which was found also in association with the slightly older New Zealand stem, Osmundites Dunlopi.
Fig. 255. Osmundites Kolbei Sew. Transverse section, from a photograph supplied by Dr Kidston and Mr Gwynne-Vaughan. (2½ nat. size.)
An examination of the internal structure of the South African stem by Dr Kidston and Mr Gwynne-Vaughan has revealed many interesting features, which will be fully described in Part IV. of their Monograph on fossil Osmundaceous stems. I am greatly indebted to these authors for allowing me to publish the following note contributed by Dr Kidston:—
“The section of Osmundites Kolbei Seward, shown in [fig. 255], presents the usual appearance of an Osmundaceous stock. The parts contained in this section are the stele, inner and outer cortex and a portion of the surrounding mantle of concrescent leaf-bases. The whole specimen has suffered much from pressure, but if restored to its original form the xylem ring must have been about 19 mm. in diameter. The number of xylem strands is about fifty-six and several of them are more or less joined as in the modern genus Todea. The tracheae are of the typical Osmundaceous type, that is to say, the pits are actual perforations and several series of them occur on each wall of the larger tracheae.
“The most interesting structural characteristic of Osmundites Kolbei is not well seen in the figure owing to the compression of the xylem ring. This consists in the occurrence of tracheae in the pith. In fact, we have here a mixed pith, composed of parenchyma and true tracheae, a condition which connects the Osmundaceae with a parenchymatous medulla with those possessing a solid xylem stele like Zalesskya and Thamnopteris and so completes the series of transitions extending from the older and solid-steled forms to the modern medullated members of the Osmundaceae.”
Osmundites skidegatensis, Penhallow.
This lower Cretaceous Canadian species, first described by Penhallow[791] and more recently by Kidston and Gwynne-Vaughan[792], is remarkable for the large size of the stem, the stele alone having a diameter of 2·4 cm. Penhallow figures a fragment of a leaf bearing a superficial resemblance to that of Osmunda Claytoniana, which may be the foliage borne by Osmundites skidegatensis. The xylem cylinder is broken by the exit of leaf-traces into 50 or more strands varying in size and shape, and it is noteworthy that the phloem is also interrupted as each leaf-trace is given off. In recent species the xylem cylinder is almost always interrupted, but the phloem retains its continuity. In the Canadian fossil an internal band of phloem occurs between the xylem and the pith, and this joins the external phloem at each leaf-gap. This internal phloem finds an interesting parallel in certain recent species[793], but in these the internal and external phloem do not meet at the foliar gaps as they do in the extinct type. In Osmunda cinnamomea the internal phloem occurs only at the regions of branching of the stem stele; in the fossil it is always present.
It is clear that Osmundites skidegatensis represents the most complex type of stem so far recognised in the Osmundaceae; it illustrates a stage in elaboration of the primitive protostele in advance of that reached by any existing species.