It is probable that the specimen described by Renault is a root of Sphenophyllum, but my impression gained from an examination of the section was that the diarch primary strand is not quite so clear as in the published figures. Until we possess better material we cannot attempt any very satisfactory description of the anatomical features of the roots of this genus.
A section of a Sphenophyllum stem has been figured by Felix[877], in which a lateral member is being given off; this may possibly represent the origin of an adventitious root, but the preservation is not sufficiently distinct to render this certain.
c. Leaves.
Renault[878] has described some silicified leaves of Sphenophyllum from Autun in which the laminae consist of thin-walled loose parenchyma, traversed by small groups of tracheids constituting the simple or forked veins. The epidermis is made up of a single layer of cells, with here and there indistinct indications of stomata. A more perfect stoma has, however, been described by Solms-Laubach from the epidermis of a bract in a strobilus (fig. 107, A).
Fig. 107. A. Stoma in a bract of Sphenophyllostachys. B. Root of Sphenophyllum. C. Sphenophyllostachys Römeri, Solms. s, sporangiophore, b, bract. D. Sporangium. E and F. Sections through the cambium, phloem and secondary xylem of Sphenophyllum insigne (Will.). s, sieve-tube. G. Sporangium and pedicel. A, C, D. After Solms-Laubach. B. After Renault. E–G. After Williamson and Scott. E. F. × 100. G. × 115.
d. Cones.
The history of the recognition of the cones of Sphenophyllum has already been briefly alluded to in chapter V., p. 100. The main points in the structure of the cones of this genus were known for several years, before the fact was established that they belonged to Sphenophyllum stems. In 1871 Williamson[879] published an account of an imperfect fossil strobilus from the Lower Coal-Measures of Oldham, Lancashire, under the name of Volkmannia Dawsoni. The generic term Volkmannia has been used by different writers for cones varying considerably in structural features; in the case of Williamson’s fossil, Weiss[880] substituted the name Bowmanites, a genus instituted by Binney[881] for a strobilus apparently of the same type as Volkmannia Dawsoni. In 1891 Williamson[882] described some additional specimens of Bowmanites Dawsoni, and, as in his earlier paper, he compared the strobilus with Asterophyllites and Sphenophyllum, but it was still a matter of speculation as to what was the form of the vegetative branches. Soon after the more complete account of the English cones was published, Zeiller[883] recognised a close agreement between some French and Belgian specimens of Sphenophyllum strobili and the strobilus described by Williamson. A closer comparison thoroughly established the connection between Bowmanites Dawsoni and Sphenophyllum; and there is little doubt that this strobilus belongs to the stem known as Sphenophyllum cuneifolium (Sternb.)—a well-known species of the genus.
STROBILUS.
The most important morphological features of the strobilus of Sphenophyllum may best be illustrated by a detailed account of one specific type, and by a brief reference to other forms which are characterised by certain differences in the number and attachment of the sporangia. When we know that a given strobilus must have grown on a Sphenophyllum stem, the obvious name to assign to it would seem to be that of the plant which bore it; but there are advantages in making use of special generic terms for detached cones, which cannot be referred with certainty to a particular species of stem. The genus Calamostachys affords an example of a name which is intended to denote that a cone so called belongs to a Calamarian plant; similarly such a name as Sphenophyllostachys may be used for Sphenophylloid cones which cannot be connected with certainty to particular species of Sphenophyllum. It has been suggested that the genus Bowmanites, first used for a cone which was afterwards recognised as belonging to a Sphenophyllum, should be employed instead of the sesquipedalian term Sphenophyllostachys. The latter is used here as being in accordance with a generally accepted and convenient system of nomenclature, and as a name which at once denotes the fact that the fossil is not only a cone but that it belongs to a Sphenophyllum.