928 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
formed by the rupture of the tissues, as so often happens in similar positions. The 
numerous, often uncoiled spiral trachez, at the angles of the primary xylem, are an 
indication that growth was still in active progress when the differentiation of the 
tissues began. Spirals are comparatively rare in S. plurifoliatum, a fact which 
harmonizes well with the absence of canals in that species. 
There can be no doubt that in S. znsigne, as in other species, the primary xylem 
was triarch and centripetal, the three peripheral groups of spiral tracheze marking the 
three points at which differentiation started. We cannot see any proof that there 
were two groups of protoxylem at each angle. It might indeed be possible to 
cousider the canal as separating the two arms of a crescentic group of primitive trachez, 
as is so conspicuously the case in the Autun species (see Plate 76, photograph 24), 
But the fact that we have repeatedly found spiral trachez immediately to the outside 
of the canal (see Plate 85, fig. 53) appears to us to negative such a view, and to 
prove that the protoxylem at each angle was a single group. 
As regards the other primary tissues there is little to detain us. The inner 
cortical layers, which cannot be distinguished from the pericycle and phloém, were 
thin-walled, and are very imperfectly preserved in the less advanced specimens. We 
can therefore give no information as to the primary phloém, though we shall find 
that the secondary phloém was well preserved in some of the older stems. 
The outer cortex consisted of somewhat thick-walled tissue, but less sclerotic than 
that of S. plurifoliatum. The cells which, in the internodes, are of considerable 
length, were generally parenchymatous in form (Plate 84, fig. 47). 
As regards the course of the leaf-trace bundles, it appears probable from the 
preparation shown in Plate 83, fig. 46, that two foliar bundles were given off from 
each angle of the triarch stele. 
For the mode of branching, we have only the evidence of one specimen,* the 
interpretation of which is doubtful. We shall return to the consideration of this 
specimen, after describing the secondary tissues. 
Secondary Changes. 
The development of secondary wood and phloém and of periderm proceeded in 
Sphenophyllum imsigne in the same general manner as in S. plurifolatum and other 
species of the genus. The structure of the secondary tissues, especially of the wood, 
differs in some respects from that in any other species at present investigated. 
As regards the largest specimens of 8. insigne, certain authors have expressed 
doubt or disbelief as to their belonging to Sphenophyllum at.all, and have supposed 
them to be roots, either of a Cycad, or of some unknown plant.t 
* C.N. 926; see Wittiamson, “ Organization,” Part V., Plate 5, fig, 27. 
+ L.g., Renavv, ‘ Cours de Botanique Fossile,’ vol. 4., p. 12 (1885) ; Scuunx, “ Die Fossilen Pflanzen- 
reste,” in ‘ Handbuch der Botanik,’ Bd. 4, p- 103; Sonms, ‘ Fossil Botany,’ p. 349, 
