In 1831 Mr Witham[340] published an anatomical description of a fragment of a Lepidodendron which he named Lepidodendron Harcourtii after Mr C. G. V. Vernon Harcourt from whom the specimen was originally obtained. The fossil was found in rocks belonging to the Calciferous series in Northumberland. Witham reproduced the account of this species in his classic work on Fossil Vegetables[341], and Lindley and Hutton[342], who examined Mr Harcourt’s material, published a description of it in their Fossil Flora in which they expressed the view that Lepidodendron is intermediate between Conifers and Lycopods. Adolphe Brongniart[343] included in his memoir on Sigillaria elegans an account of Witham’s species based on material presented to the Paris Museum by Mr Hutton and Robert Brown. Dr Kidston[344] has shown that the actual transverse section figured by Witham is now in the York Museum; a piece of stem in the same Museum, which is not the specimen from which Witham’s section was cut, supplied the transverse section figured by Brongniart. The figures given by Lindley and Hutton do not appear to have been made from the York specimens. In 1887 Williamson[345] published a note in which he pointed out that some of the specimens described by him as L. Harcourtii should be transferred to a distinct species, which he named L. fuliginosum. Subsequently in 1893 he gave a fuller account of Witham’s species; it has, however, been shown by Dr Kidston and by Mr Watson[346] that certain specimens identified by Williamson as L. Harcourtii differ sufficiently from that type to be placed in another species, for which Watson proposes the name L. Hickii.

A paper on L. Harcourtii published by Bertrand[347] in 1891 extends our knowledge of this type in regard to several anatomical details. It was recognised by Williamson that the absence of secondary wood in shoots possessing the anatomical characters of L. Harcourtii is a feature to which no great importance should be attached. It is possible that the large stems from the Isle of Arran described by Williamson[348] as Lepidodendron Wünschianum, in which the secondary wood is well developed, may be specifically identical with the smaller specimens from Northumberland and elsewhere which are recognised as examples of Witham’s type.

The diagrammatic sketch shown in [fig. 179], A, was made from a section figured by Williamson in 1893[349]; it has a diameter of 9 × 8·5 cm. The stele is of the medullated type like that of L. Wünschianum, and the outer edge of the primary xylem is characterised by sharp and prominent projecting ridges similar to those of L. fuliginosum but rather more prominent. Parenchymatous cells succeed the xylem, as in other species, but in this case there is no indication of meristematic activity; beyond this region occur occasional patches of a partially destroyed secretory zone. Remains of a lacunar tissue are seen in the middle cortical region; also numerous leaf-traces, lt, consisting of a tangentially elongated xylem strand accompanied by a strand of secretory zone tissue enclosed in a sheath of delicate parenchyma. In the inner part of the outer cortex, c3, the leaf-traces lie in a space originally occupied by the parichnos; in the outer portion of the same region a band of secondary cortex, pd, has been formed; immediately internal to this occur numerous patches of secretory tissue, represented by small dots in the drawing close to pd; one is shown on a larger scale in fig. B.

The position of the phellogen is seen at a; external to this are radial rows of rather large cells with dark contents.

Fig. 179.

Fig. 179, C, x, shows the characteristic form of the primary xylem edge, beyond which are seen oval or circular leaf-traces with a mesarch protoxylem, lt, px. It is possible that this specimen may not be specifically identical with Witham’s species, but it represents a very similar if not identical type; it may on the other hand be referable to L. fuliginosum. The importance of the specimen, apart from its precise specific position, is that it serves to illustrate the general appearance of the xylem surface met with in both species, L. Harcourtii and L. fuliginosum. A tangential longitudinal section, taken through the line ab in fig. C, is represented in [fig. 179], D. The xylem of the leaf-traces lt, consisting chiefly of scalariform tracheae, alternates with patches of crushed and delicate parenchyma which immediately abut on the primary xylem; at p, p, the section passes through some of the projecting arms of the xylem cylinder; at m is seen a patch of meristematic zone tissue. This section together with the similar section of Lepidodendron vasculare described on a previous page demonstrates that the projecting ridges of the primary xylem form apparently vertical bands: they are not characterised by a lattice-work arrangement as described by Bertrand and by other authors who have accepted his conclusions. If a reticulum of intersecting ridges were present on the face of the xylem cylinder its existence would be revealed by such a section as that represented in [fig. 179], D.

Fig. 180. Lepidodendron Wünschianum. From Arran. (⅕ nat. size.) (Sedgwick Museum, Cambridge.)