A possible comparison may be suggested also with Sigillaria Brardi as figured by Germar ([fig. 196], A) in which circular scars, which may be the scars of rootlets, occur below the leaf-base areas. It is not impossible that in the surface-features of Omphalophloios we have both leaf and rootlet scars represented.
General considerations.
The solid xylem core characteristic of the stele of some species of Palaeozoic Lycopodiales (e.g. Lepidodendron esnostense and L. rhodumnense) may probably, as Tansley and Chick[653] point out, be regarded as the lineal descendant of a primitive axial strand of water-conducting elements. In the course of evolution the centre of the tracheal column became partially converted into parenchymatous tissue, as in Lepidodendron vasculare. The arrangement of the short cells in regular vertical series is reminiscent of an early stage in the development of tracheae: instead of forming tubular conducting elements the central part of the stelar meristem acquired the short-celled form; some of the cells became lignified as isodiametric storage tracheae while others persisted as thin-walled parenchyma.
The production of secondary xylem and an increase in the girth of the whole stem led to reduction in the amount of centripetally developed conducting channels. Some of these assumed a new rôle and a shape in harmony with their functions. A later stage is represented by a further encroachment of the central parenchyma on the cylinder of centripetal xylem, as seen in Lepidodendron Harcourtii and other species. The next stage is afforded by ribless species of Sigillaria in which the primary xylem is broken up into separate conducting strands. As Kidston[654] reminds us, it is in the geologically more recent species of Sigillaria, such as S. Brardi, which persist into the Permian era, that this more extreme case of reduction occurs. The older genus Lepidodendron seems to have retained to the last the complete cylinder of primary xylem. In the stele of Stigmaria, the rhizome of Sigillaria and of Lepidodendron, reduction of the centripetal xylem has passed beyond the stage represented by the broken cylinder of the ribless Sigillarias. With the exception of the examples described by Renault[655] and by Weiss[656], Stigmaria is characterised by little or no centripetal primary xylem. It is, however, noteworthy that Renault’s Stigmaria, in which centripetal xylem forms a prominent feature, is attributed to Sigillaria Brardi, a species in which the vascular cylinder of the aerial stem illustrates a later and not an earlier phase in the replacement of centripetal by centrifugal wood.
It would seem, as Lady Isabel Browne[657] says, that most Stigmarian axes had reached a more advanced stage in specialisation than is shown in the stelar structure of the aerial shoots. The relatively greater and probably the more precocious development of secondary xylem in Stigmaria than in Lepidodendron or Sigillaria may have some significance in relation to the smaller amount of “old wood[658]” (in a phylogenetic sense) in their steles.
As is pointed out in a later chapter, recent researches into the anatomy of extinct members of the Osmundaceae by Kidston and Gwynne-Vaughan have brought to light a striking parallelism in evolutionary sequence between the Lepidodendreae and the ancestors of Osmunda and Todea, the two surviving genera of one of the most ancient families of ferns.
There can be little doubt as to a very close relationship between Sigillaria, Lepidodendron, and Bothrodendron. Sigillaria seems to have outlived Lepidodendron and Bothrodendron. The two latter genera are recorded from Upper Devonian rocks in several localities, Bothrodendron being particularly abundant in the pre-Carboniferous floras of Bear Island and other parts of the world. A remarkable stem described by Dr White[659] as Archaeosigillaria primaeva from Upper Devonian shales of New York is spoken of by him as “one of the most highly developed representatives of a fairly distinct archaic group foreshadowing the later genera Bothrodendron, Sigillaria, Lepidodendron and Lepidophloios.” The type-specimen, when first discovered, consisted of an apparently unbranched stem reaching a length of 5 metres. From the swollen basal part Stigmaria-like rootlets spread into the surrounding shale. At a higher level the fissured bark shows indistinctly defined leaf-cushions which pass gradually upwards into cushions and scars arranged in closer order on regular vertical ribs. The surface-features in this region are practically those of a ribbed Sigillaria. Traced farther upwards the vertical ribs die out and cushions of the Lepidodendroid form cover the surface of the bark. The leaf-scars, with a supraposed ligular pit and two vertically elongated parichnos-scars, are said to bear a closer resemblance to those of Sigillaria and Bothrodendron than to the leaf-areas of Lepidodendron. Nothing is known as to the anatomy of this stem, nor have fertile shoots been discovered. In the absence of more trustworthy evidence than is available conclusions of a phylogenetic nature must be accepted at their true value. It is however legitimate to describe Archaeosigillaria primaeva as one of the oldest examples of a lycopodiaceous plant which shows well-preserved external features, and these are of exceptional interest as indicating a combination of generic characters. This Devonian type lends support to the view that Lepidodendron and Sigillaria are offshoots, differing from one another in comparatively unimportant points, from a common ancestral type.
The generally accepted statement that arborescent Palaeozoic Lycopodiales bore their sporangia on specially modified leaves (sporophylls) grouped in cones which were usually produced at the tip of slender branches, has recently shared the fate of most rules. Prof. Bower in his Origin of a Land Flora mentions a Belgian specimen of Pinakodendron musivum Weiss from the Westphalian series (Middle Coal-Measures), to be described by Dr Kidston, which bore its sporangia “associated with the leaves of certain portions of the stem, without any cone-formation. The fertile and sterile portions are distinguished only by the presence or absence of sporangia[660].”
Lepidodendron and Sigillaria can hardly be claimed as the direct ancestors of any existing type of Lycopodiales, but while exhibiting points of contact with Lycopodium, Selaginella, and Psilotum they are perhaps more closely allied to Isoetes.
Lady Isabel Browne[661], who has recently published an excellent summary of the evidence on the relation of the Lepidodendreae to Isoetes, concludes her examination of the arguments by expressing the opinion that there is a strong probability of the correctness of the view that Isoetes may be derived “from the Lepidodendraceae in the widest sense of the word.” This decision seems to me to accord best with the facts.