CHAPTER XI.
II. SPHENOPHYLLALES.
I. SPHENOPHYLLUM.
The genus Sphenophyllum is placed in a special class, as representing a type which cannot be legitimately included in any of the existing groups of Vascular Cryptogams. Although this Palaeozoic genus possesses points of contact with various living plants, it is generally admitted by palaeobotanists that it constitutes a somewhat isolated type among the Pteridophytes of the Coal-Measures. Our knowledge of the anatomy of both vegetative shoots and strobili is now fairly complete, and the facts that we possess are in favour of excluding the genus from any of the three main divisions of the Pteridophyta.
In Scheuchzer’s Herbarium Diluvianum there is a careful drawing of some fragments of slender twigs, from an English locality, bearing verticils of cuneiform leaves, which the author compares with the common Galium[860]. As regards superficial external resemblance, the Galium of our hedgerows agrees very closely with what must have been the appearance of fresh green shoots of Sphenophyllum.
A twig of the same species of Sphenophyllum is figured by Schlotheim[861] in the first part of his work on fossil plants; he regards it as probably a fragment of some species of Palm. Sternberg[862] was the first to institute a generic name for this genus of plants, and specimens were described by him in 1825 as a species of the genus Rotularia. The name Sphenophyllites was proposed by Brongniart[863] in 1822 as a substitute for Schlotheim’s genus, and in a later work[864] the French author instituted the genus Sphenophyllum. Dawson[865] was the first to make any reference to the anatomy of this genus; but it is from the examination of the much more perfect material from St. Étienne, Autun, and other continental localities, the North of England and Pettycur in Scotland, by Renault, Williamson, Zeiller and Scott, that our more complete knowledge has been acquired.
The affinity of Sphenophyllum has always been a matter of speculation; it has been compared with Dicotyledons, Palms, Conifers (Ginkgo and Phyllocladus), and various Pteridophytes, such as Ophioglossum, Tmesipteris, Marsilia, Salvinia, Equisetum and the Lycopodiaceae[866].
DEFINITION.
We may define the genus Sphenophyllum as follows:—
Stem comparatively slender (1·5–15 mm.?), articulated, usually somewhat tumid at the nodes; the surface of the internodes is marked by more or less distinct ribs and grooves which do not alternate at the nodes, but follow a straight course from one internode to the next. A single branch is occasionally given off from a node. Adventitious roots are very rarely seen, their surface does not show the ridges and grooves of the foliage-shoots.