930 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
Lastly, we have the largest specimen, figured in transverse section by WILLIAMson,* 
the identity of which has been disputed. 
The primary xylem in this specimen has exactly the same structure as in the 
smaller stems; the canals at the angles, containing fragments of spiral tracheides, are 
perfectly evident. The secondary wood, which reaches a maximum thickness of 
36 elements, agrees in every respect with that of the less advanced specimens ; the 
medullary rays are alike, and so are the trache, with their scalariform radial walls.t 
In fact, if the more central portion of the largest specimen were seen alone, it could 
not be distinguished from the corresponding region of one of the younger stems, 
which still retain the typical cortex of a Sphenophyllum. 
In the most advanced specimens the primary cortex has, as we should expect, 
entirely disappeared ; it is replaced by an enormously thick bark, formed by succes- 
sive layers of internal periderm (see Plate 84, figs. 48, 51, and 52). 
We maintain, then, that there is no longer the slightest reason to doubt that the 
largest, as well as the smallest, specimens under consideration belong to one and the 
same species, namely, to that species of the genus Sphenophyllum to which we give 
the name of S. insigne. 
The sections in which no primary cortex is shown would, by themselves, suggest to 
any botanist the idea of a root. The root-like anatomy, however, is common to all 
specimens of the genus Sphenophyllum, and is no more remarkable in a large stem 
than in a small one. The comparison of specimens of various dimensions shows that 
the only change consists in the formation of additional secondary layers, and in the 
casting off of the primary cortex, owing to the development of internal periderm. 
The presence of true medullary rays in all parts of the wood is certainly exceptional 
in Sphenophyllum, so far as our present knowledge extends. This peculiarity, how- 
ever, is not confined to the larger specimens of S. insigne, but is already present in 
stems which still retain the unmistakeable cortex and node of a Sphenophyllum. 
Of the details of structure of the secondary wood little remains to be said. The 
medullary rays vary much in height. Many consist of a single series of cells, others 
of two series ; in extreme cases even as many as 15 rows may be superposed to form 
a single ray. The larger rays are usually two or more cells in breadth in some paris. 
The uniseriate rays, as seen in radial section, are perfectly similar to the radially- 
elongated cells of S. plurifoliatum, except that in S. insigne these cells lie in one and 
the same straight line (compare Plate 83, fip. 44, and Plate 84, fig. 49). The impor- 
tance of this difference is not to be underrated. It probably indicates a different 
mode of development from the cambium. We must, however, remember that even in 
* Loc. cit., Part V., Plate 4, fig. 21. 
+ Our figures 48, 49, 51, and 52 (all on Plate 84), are from this largest specimen. The figures in 
WiLitaMson, loc. cit., Part V., Plate 4, should also be compared. Our figures 47, 50, and Plate 85, fig. 53, 
are from smaller stems. 
