CHAPTER XVIII.
Bothrodendreae.
Bothrodendron. [Figs. 211–216].
Although in many respects the genus Bothrodendron agrees very closely in habit and in its anatomical features with Lepidodendron, there are reasons for referring it to a distinct family of Palaeozoic Lycopods. As the following description shows, the external features do not differ in any essential points from those of certain types of the genus Sigillaria, particularly such a species as S. rimosa, Gold.[608], which has recently been refigured and described by Nathorst[609] from Goldenberg’s type-specimen in the Stockholm Museum. The small size of the leaf-scars is, however, a characteristic feature of Bothrodendron ([fig. 212], F); but a more important point is the fact that in a recently described[610] English example of a cone of Bothrodendron ([fig. 216]), the sporangia are very like those of recent Lycopods, and differ from the radially elongated sporangia of Lepidostrobus. On the other hand, a French cone described by Zeiller[611] as Lepidostrobus Olryi, which is probably a strobilus of Bothrodendron, has the radially elongated type of sporangium ([fig. 212], E). The comparative abundance of Bothrodendron in Lower Carboniferous and Devonian rocks points to the greater antiquity of this member of the Lycopodiales as compared with Lepidodendron.
The name Bothrodendron was instituted by Lindley and Hutton[612] for impressions of stems from the English Coal-Measures, characterised by two opposite rows of large depressions like those shown in [fig. 211] and, in one of the specimens, by “a considerable number of minute dots, arranged in a quincuncial manner.” The minute dots were recognised as leaf-scars and the cup-like cavities were described as probably connected with the occurrence of large cones. On very slender evidence this Palaeozoic plant, which was named Bothrodendron punctatum, was considered by these authors as probably a member of the Coniferales. The large stem from the Coal-Measures in the neighbourhood of Mons, Belgium, shown in [fig. 211], affords a good illustration of Bothrodendron in a partially decorticated condition, exhibiting a row of depressions similar to those on the Ulodendron form of Lepidodendron Veltheimianum ([fig. 157]), but distinguished by the eccentric position of the scar at the bottom of each cup-shaped cavity: in the Belgian specimen, which is partially decorticated and shows the leaf-traces as small dots, the depressions have a diameter of 9 cm. It is believed by some authors that these Ulodendron shoots of Bothrodendron and Lepidodendron owe their characteristic appearance to the pressure of large cones, but, as I have already stated, there are reasons for preferring the view that these crater-like hollows are the scars of deciduous branches. Our knowledge of the strobili borne by Bothrodendron stems is still meagre, but we have no reason to assume the existence of any cones large enough to produce by the pressure of their bases such depressions as those shown in [fig. 211]. In one species at least the strobili were borne terminally on slender shoots ([fig. 213]). The Ulodendron condition has so far been recognised in one species only, B. punctatum.
In his catalogue of Palaeozoic plants, Kidston[613] included Bothrodendron punctatum as a synonym of Sigillaria discophora König, a mistake which he afterwards rectified[614]: the generic name Bothrodendron was generally ignored by authors in the belief that the specimens described by Lindley and Hutton were not generically distinct from the fossils originally figured by Rhode as Ulodendron. It was Prof. Zeiller who first demonstrated that the English authors were justified in their choice of a new designation for stems with large depressions in association with minute leaf-scars. In 1859 Haughton[615] proposed a new family name Cyclostigmaceae for some Upper Devonian plants from County Kilkenny, Ireland: he described three species of his new genus Cyclostigma, Cyclostigma kiltorkense, C. minutum, and C. Griffithsi; these are now generally recognised as a single species of Bothrodendron, though, as Nathorst suggests, the Irish plant should perhaps be separated as a sub-genus Bothrodendron (Cyclostigma) by reason of certain minor differences which distinguish it from other species of the genus.
Fig. 211. Bothrodendron punctatum. Part of a specimen from near Mons (Hainaut), in the Brussels Museum. (Reduced.)
Another generic name, Rhytidodendron, was instituted by Boulay in 1876 for stems characterised by a finely wrinkled bark and small spirally disposed leaf-scars. A short description of this type, which occurs in the Middle and Lower Coal-Measures, may serve to illustrate the external features of the commonest British example of the genus.