ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 931 
S. plurifohatum continuous rays do occur, more especially in the fascicular part of the 
wood,* so the distinction is by no means an absolute one. 
The pitting of the trachez is often beautifully preserved ; in tangential sections it 
is evident that the scalariform pits were bordered (Plate 84, fig. 50). On the surface 
of junction of tracheze and ray-cells the border is unilateral. 
Whether the tracheze were true vessels or tracheides can hardly be determined. 
If they were tracheides they must have been of great length, for they can often be 
traced all through a section without finding any terminal wall. In some cases, how- 
ever, we have observed, in tangential sections, what appear to be the very tapering 
ends of tracheee. We do not regard this observation as decisive, for such appearances 
might possibly be due to slight obliquity of the section. The question must therefore 
be left open. Now that the transverse lines seen in radial sections have been proved 
to represent the walls of cells external to the trachez, there is no longer any evidence 
for the existence of transverse septa in the latter. 
The cambium appears to have originated in exactly the same position in this species 
as in S. plurifoliatum. 
Appearances suggestive of annual rings are sometimes met with in S. insigne, but 
the same remarks apply here as in the case of the former species. 
In a longitudinal section of the largest stemt a protoxylem-canal is shown, which 
is partly blocked up by polygonal cells. As the canais are empty in the younger 
specimens, it is probable that these cells were of the nature of thyloses. 
It remains to consider the secondary tissues external to the wood. In the more 
advanced specimens (see Plate 84, fig. 48) the thick cortical zone{ is composed of 
radially arranged cells. Their radial series correspond with, but are usually more 
numerous than, those of the wood. The walls of the cortical cells become thicker 
towards the exterior. The limit between phloém and internal periderm is not always 
obvious. In favourable cases, however, where the delicate phloém happens to be 
particularly well preserved, or where, perhaps, it was developed in unusual amount, 
the whole structure is clear. This is the case, for example, in the largest specimen, 
of which corresponding transverse and radial sections are shown in Plate 84, figs. 51 
and 52. Adjoining the wood we see evident remains of the tabular cambial cells. 
Outside these we find a group of thin-walled tissue, in which the arrangement of the 
cells, though somewhat irregular, shows traces of a radial seriation. Some of the cells 
in this group are of large size, in fact not much smaller than the trachez of the 
wood. Beyond these, again, are flat, thin-walled cells, passing over on the outside 
into thicker-walled periderm, only a small part of which is represented in fig. 51. If 
we compare the radial section with this the corresponding parts are evident. The 
* This seems to be also the case in the Autun Sphenophyllum. See Renavtt, ‘Cours de Bot. Fossile,’ 
vol. 4, plate A, fig. 4, 7. 
+ O.N. 924. 
t Of. foot-note above, on p. 925. 
