ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 929 
These doubts, as we shall proceed to show, are groundless. We have a series of 
specimens with secondary wood of all thicknesses up to 36 elements in each radial 
series. The smaller of these specimens (such as that shown in the photograph 23, on 
Plate 76) still retain the characteristic primary cortex, and no doubts have been, or can 
be, entertained as to their nature. Their structure, however, already presents the 
same peculiarities which have given rise to suspicions in the case of the larger speci- 
mens. ‘The latter only differ from the former in the greater bulk of their secondary 
tissues and in the loss of their primary cortex, which is cast off in S. insigne exactly 
as in other species of the genus. 
If we first consider one of the moderately thickened specimens (such as that shown 
in photograph 28), we find that the primary triarch wood has the structure already 
described (cf. fig. 45 on Plate 83). Upon this primary strand several layers (7-10 in 
the specimen photographed) of secondary xylem have been deposited. This tissue is 
thickest opposite the middle of each of the three concave sides of the primary wood, 
and. thins out considerably towards its angles. The xylem elements are arranged with 
characteristic regularity in radial series. The distinction between fascicular and 
inter-fascicular wood is already well marked. 
There are, however, two distinctive characters presented by this wood as compared 
with that of the other species. 
(1.) The tracheze have scalariform markings, at least on their radial walls. This is 
shown very plainly, both in oblique sections* and in the longitudinal section already 
referred to (C.N. 1420, see Plate 84, fig. 47). The tangential walls do not generally 
show any pits; when present on these walls the pits are small and rounded. 
(2.) The more striking difference consists in the presence, in all parts of the wood, 
of regular medullary rays, one or two cells in breadth, extending through the whole 
thickness of the secondary zone. Occasionally isolated cells occur at the corners of 
the trachez, but, as a rule, the parenchyma of the wood takes the form of continuous 
rays. Comparison with a longitudinal section (see Plate 83, fig. 47B) shows that 
these rays may be one cell in height or more. Their elements have a lenticular form, 
as seen in tangential sections of the stem. 
The primary cortex is well preserved in these younger specimens (see photograph 23). 
A zone of secondary tissue, no doubt including both phloém and periderm, surrounds 
the wood ; its structure is more cvident in the more advanced specimens. 
Other examples, with secondary wood from 11 to 22 elements in radial thickness, 
show precisely the same structure. 
Here, however, the secondary cortical tissues have reached a considerable thickness, 
and their cells are in regular radial series. Outside this zone the remains of the 
primary cortex can still be traced.{ 
* Asin O.N. 910. 
+ For the details of the wood, see Plate 84, figs. 49-52, and compare with the structure of the 
younger stem shown in fig. 47. 
t¢ C.N. 922 and 926. 
6c 2 
