ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 921 
teristic outline.* There are three sharp depressions, or furrows, which bear a 
definite relation to the internal structure, each depression lying midway between two 
of the prominent angles of the triquetrous strand of primary wood. ‘The cortical 
surfaces between the furrows are nearly flat. This is the form in the internode ; close 
to the nodes, however, a secondary elevation is found in the middle of each furrow.t 
The characteristic outline of the cortex agrees with that in several other species of 
Sphenophyllum.{ It is only to be observed in the younger stems, for, as we shall see, 
the whole primary cortex soon becomes replaced by periderm, and cast off. 
We will now describe the anatomical structure. Many points can be passed over 
rapidly, as they have already been sufficiently dealt with in the previous memoirs 
above cited. 
Primary Structure. 
The stem is traversed by a vascular cylinder or stele, the primary structure of 
which is simple, though, for a stem, highly peculiar. 
The wood, as seen in transverse section, is triangular ; the sides of the triangle are 
somewhat concave, the angles are slightly truncated. The xylem is a solid mass of 
tracheides; there is no trace either of a medulla or of xylem-parenchyma. The 
tracheides near the middle of the stele are pitted, and of large size ; as we approach 
the three prominent angles we find that the size of the tracheides rapidly diminishes, 
and their walls have here scalariform thickening. At the actual angles we find 
reticulated and spiral elements of very small diameter (see Plate 83, fis. 40 and 41), 
There can be no doubt that the tracheides at the angles are the primitive elements, or 
protoxylem, and that the primary wood constitutes a centripetal triarch strand—a 
structure which is very unusual in stems, though so familiar in the case of roots. 
The angles of the triquetrous xylem are often blunt, though this is not always the 
case. We could not convince ourselves that more than a single group of protoxylem 
is present at each angle, though, in other species, there are undoubtedly two such 
groups, and the stele is then hexarch (see Plate 76, photograph 24, from the Autun 
Sphenophyllum ; also many figures in M. ReNAULT’s works above cited). 
The wood is surrounded by a thin-walled tissue of considerable width, but in the 
youngest specimens, such as we are now describing, this zone 1s seldom well pre- 
served. The cells near the wood are smaller than those more towards the exterior. 
We can hardly be wrong in regarding the tissue immediately surrounding the wood, 
as phloém ; the outer thin-walled region is, perhaps, best interpreted as a pericycle, 
+n which case we should take the commencement of the more peripheral thick-walled 
zone as the inner limit of the cortex. It must, however, be remembered that, in the 
* See WiitiaMson, loc. cit., Part V., Plate 1, fig. 1, &e. 
+ Wiuiamsoy, loc. cit., Part V., Plate 1, fig. 4. 
t See Renavty, loc. cit. 
6 BQ 
