With the exception of very young twigs the petrified Sphenophyllum stems usually show a greater or less development of secondary wood. In the xylem-strand of fig. 105, B, the broad concave bays of the primary wood have been filled in by the development of two rows of large secondary tracheids, x, but opposite the protoxylem groups, px, there are no signs of cambial activity. In the unusually large stem represented by a rough sketch in fig. 105, C, the triangular primary xylem lies in the centre of a thick mass of secondary vascular tissue. The secondary and primary wood together have a diameter of about 5 mm.
After the bays between each protoxylem corner have been filled in, the formation of secondary wood proceeds uniformly along the stem radii, but the rows of tracheids and medullary rays which are developed opposite the corners of the primary strand, c, differ in certain characters from the broader masses of wood opposite the bays. For convenience, the secondary wood, c, opposite the protoxylem groups has been spoken of as fascicular wood, and the rest, d, as interfascicular wood.
The secondary xylem consists either of tracheae with numerous bordered pits on their radial walls (fig. 105, D), or of tracheae with broad and bordered scalariform pits (fig. 105, E). The suggestion of concentric rings of growth in the wood in fig. 105, C, is rather deceptive; there are no well-marked regular rings in Sphenophyllum stems, but irregular bands of smaller elements occasionally interrupt the uniformity of the secondary xylem. In some stems the medullary rays have the form of rows of parenchymatous cells, which in tangential longitudinal section are found to consist frequently of a single row of radially disposed elements; this type of medullary rays occurs in the species Sphenophyllum insigne, in which the tracheae are scalariform. Three medullary rays, r, are seen on the radial face of the scalariform tracheids in fig. 105, E, which represents a radial section of this species. In other species, e.g. S. plurifoliatum, the medullary rays have a peculiar and characteristic structure; in a transverse section of the stem they appear as groups of a few parenchymatous cells in the spaces between the truncated angles of the large tracheae (fig. 106). In longitudinal section these medullary-ray elements resemble thick bars stretching radially across the face of the tracheae (fig. 105, D, r); the apparent septa or bars are however thin-walled cells connecting the different groups of medullary-ray cells, as seen in a transverse section. These radial connecting cells are occasionally seen as short rays in transverse sections of stems.
The cambium and phloem elements are occasionally preserved in good specimens of older stems; the former consist of tabular flatted thin-walled cells, and the latter in some cases include large sieve-tubes and narrower parenchymatous elements.
The sections shown in fig. 107, E and F, illustrate the preservation of cambial and phloem tissue. In the transverse section of fig. 107, F, the secondary xylem with the medullary rays, r, is succeeded by a few tabular cambium cells, and external to these there are thin-walled elements of unequal size representing the phloem. In fig. 107, E, the scalariform tracheids are succeeded by narrow thin-walled cells, and the larger elements with transverse and oblique septa are no doubt sieve-tubes.
In the large stem of fig. 105, C, the xylem is succeeded by a band of tissue, a, which is no doubt phloem, and external to this there is a considerable development of periderm (b). The periderm in Sphenophyllum stems had a deep-seated origin, the phellogen or cork-cambium occasionally being formed in the secondary phloem-parenchyma, and in other cases in the pericycle, as in the stems of some living dicotyledons. Williamson and Scott[872] describe stems in which a succession of phellogens were formed at different levels, thus producing a scaly type of bark, such as we find in the Pine or the Plane tree.
SPHENOPHYLLUM PLURIFOLIATUM.
Before describing the structure of the strobili of Sphenophyllum, we may briefly point out the distinguishing features of two specific types of the genus recently described by Williamson and Scott. One of these species, S. insigne, was originally described by Williamson as an Asterophyllites; the numerous narrow linear leaves in each verticil led to the inclusion of the specimens in the latter genus. The material on which this species is founded is from the volcanic beds of Pettycur, Burntisland, on the coast of the Firth of Forth.
1. Sphenophyllum insigne (Will.). Figs. 105, C and E, and 107, E and F.
1891. Asterophyllites insignis. Williamson[873].