a. Stems.

i. Primary structure.

In a transverse section of a young Sphenophyllum stem such as that diagrammatically sketched in fig. 105, A, we find in the centre the xylem portion of a single stele with a characteristic triangular form. The primary xylem consists mainly of fairly large tracheae with numerous pits on their walls; towards the end of each arm the tracheids become scalariform, and at the apex there is a group of narrower spiral protoxylem elements. In the British species there is a single protoxylem group at the apex of each arm, but Renault has described some French stems in which the stele appears to be hexarch, having two protoxylem groups at the end of each of the three rays of the stele. The primary xylem strand of Sphenophyllum has therefore a root-like structure, the tracheids having been developed centripetally from the three initial protoxylem groups. This type of structure is typical of roots, but it also occurs in the stems of some recent Vascular Cryptogams.

Fig. 104. Diagrammatic longitudinal section of Sphenophyllum.
c, outer cortex; b, space next the stele, originally occupied by phloem etc.; a, xylem strand. (After Renault[871].) × 7.

As a rule the tissue next the xylem has not been petrified, but in exceptionally well-preserved examples it is seen to consist of a band of thin-walled elements, of which those in contact with the xylem may be spoken of as phloem, and those beyond as the pericycle. Succeeding this band of delicate tissue there is a broader band of thicker-walled and somewhat elongated elements, constituting the cortex. The specimen drawn in fig. 105, A, shows very prominent grooves in the cortex opposite the middle of each bay of the primary wood. It is these grooves that give to the ordinary casts of Sphenophyllum branches the appearance of longitudinal lines traversing each internode. In a longitudinal section of a stem, the cortical tissue (fig. 104, c) is found to be broader in the nodal regions, thus giving rise to the tumid nodes referred to in the diagnosis. The increased breadth at the nodes does not mean that the xylem is broader in these regions, as it is in Calamite stems. Small strands of vascular tissue are given off from the three edges of the triangular stele (fig. 105 A) at each node; these branch in passing through the cortex on their way to the verticils of leaves. The space b in the diagrammatic section of fig. 104 was originally occupied by the phloem and inner cortex. In some species of Sphenophyllum the apex of each arm of the xylem strand, as seen in transverse section, is occupied by a longitudinal canal surrounded by spiral tracheids, as in the primary xylem of the old stem shown in fig. 105, C.

Fig. 105. Sphenophyllum.

  1. Transverse section of young stem.
  2. Transverse section of the wood of a young stem; px, protoxylem; x, secondary xylem. (A and B. Sphenophyllum plurifoliatum.) × 20.
  3. Transverse section of an old stem; (S. insigne); a, phloem; b, periderm; c, fascicular secondary xylem; d, interfascicular secondary xylem. × 9. (No. 914 in the Williamson Collection.)
  4. Longitudinal section of the reticulate tracheae and medullary rays; r, r, r, of S. plurifoliatum. × 36.
  5. Similar section of S. insigne. × 75. (D and E after Williamson and Scott.)

ii. Secondary structure.