An intercellular space occurs at each angle of the three-rayed primary xylem strand, and spiral tracheae are abundant. The tracheae of the secondary wood have scalariform markings on the radial walls. Regular medullary rays extend through the secondary wood. The phloem contains large sieve-tubes.
This species occurs in the Calciferous sandstone rocks of Burntisland, and has lately been recorded from Germany. It characterises a lower horizon than S. plurifoliatum (Will. and Scott).
2. Sphenophyllum plurifoliatum (Williamson and Scott)[874]. Figs. 105, A, B, and D, and 106.
1891. Asterophyllites sphenophylloides. Will.[875]
The specific name plurifoliatum was proposed by Williamson and Scott for a type of stem originally described by Williamson[876] as an Asterophyllites, from the Coal-Measures of Oldham, Lancashire. This form of stem has not so far been connected with any of the older species founded on external characters, but it evidently bore foliage in which the leaves were deeply divided, as in Sphenophyllum trichomatosum (fig. 110, B).
Fig. 106. Sphenophyllum plurifoliatum, Will. and Scott.
From a photograph by Mr Highly from a section in the Williamson Collection (no. 899). × 27.
In this species there are no canals at the angles of the primary xylem, and there are fewer spiral tracheae than in S. insigne. The tracheae of the secondary wood have numerous small pits on the radial walls, and the medullary rays are chiefly composed of parenchymatous cells, which appear in transverse section as groups of cells between the truncated angles of the tracheae. The characters are fairly well seen in the xylem portion of a stele shown in fig. 106. The fascicular wood includes some rows of parenchymatous medullary-ray cells in addition to the characteristic groups, as seen in the figure. A slightly oblique transverse section of a stem is often convenient in the interpretation of histological features; one of the sections of S. plurifoliatum in the Williamson collection (no. 893), which has been cut somewhat obliquely, shows very clearly the differences in pitting exhibited by the different xylem elements.
b. Roots.
Our knowledge of the anatomy of Sphenophyllum roots is very limited. Renault has described a somewhat imperfect example of a silicified root from St. Étienne and Autun. The drawing in fig. 107, B, which is copied from one of Renault’s figures, shows a cylindrical mass of xylem with a small band of narrower elements occupying the centre, and surrounded by rows of larger secondary tracheae. The central bipolar band is described as the diarch primary xylem, around which the secondary pitted elements have been developed.