OF THE FOSSIL PLANTS OF THE COAL-MEASURES.—PART XIII. 
291 
masses, of unequal size and shape, of sclerous cells. Examples of these are seen at 
t, t, in Plate 21, fig. 1, Plate 22, fig. 2, Plate 21, fig. G, and Plate 22, fig. 8. The true 
relations of these masses of sclerenchyma will be seen on examining the vertical 
sections of the cortex. 
Plate 3, fig. 9, represents a vertical section made radially, and passing through the 
medullary axis at A, the exogenous xylem zone at B, the phloem zone at C, and the 
innermost border of the inner cortex at D. This section has passed from above 
downwards in a slightly oblique manner, so as successively to intersect five of the 
vascular laminae, d, and the intervening secondary medullary rays, h, of the fibro- 
vascular bundle through which the section passes. Each of these alternating layers 
comes successively into view as we pass from the upper to the lower margins of the 
Plate. As is so often the case with the fossil Cryptogams, we find the cubical paren¬ 
chymatous cells of the medullary axis, Plate 23, fig. 9, a, a, exhibiting a tendency to 
arrange themselves in vertical lines and with approximately rectangular horizontal septa. 
The vessels or tracheids, b, of this axis vary much in diameter. As already pointed 
out, those at its periphery and in contact with the inner surface of the xylem cylinder 
are very small; but the largest of those seen in the isolated bundles (Plate 21, fig. 3, b) 
have a mean diameter of at least = ’001 of an inch. More externally we have 
the vessels of the exogenously developed xylem, the laminae of which are arranged in 
parallel radiating series. Their innermost vessels (Plate 23, fig. 9, e ) are much smaller 
than the more external ones, e !, of the same lamina. The latter have a mean diameter 
of from 120 0 = ’0008 to y- 6 ^o = *00062. 
Alternating with these laminae of vertically disposed vessels, we have, at Plate 23, 
fig. 9, h, h, the secondary medullary rays composed, in these radial sections, of horizontal 
lines of cells arranged in a mural manner. In tangential sections of this exogenous 
xylem we find each of these secondary rays composed of one or two vertical series of 
cells, much compressed laterally. At C we obtain a radial section through the phloem 
zone of the stem. Here, again, we find numerous long, narrow, vertically arranged 
tubes (fig. 9, l) alternating with murally disposed masses of cells, n, constituting the 
primary or secondary phloem rays. The tubes have a mean diameter 3-y^o = *00031 
of an inch. Of course it is to be presumed that these tubes may be regarded as 
representatives of the sieve-tubes of the higher Phanerogams, but I can detect in 
them no traces of transverse septa, or of any structural peculiarities justifying my 
affirming that they are actually sieve-tubes. It must be remembered that we are 
equally unable to verify the identity of the similar tubes in the phloem of the 
Selaginellse with the true sieve-tubes of Ferns and of the Phanerogams.* 
Plate 21, fig. 10, represents a tangential section of the phloem of our plant. We 
here see that the tubes, /, just described, display a tendency towards an undulating 
arrangement, the spaces between the curves of which enclose horizontally disposed 
lines of cells. I have already called attention to the fact that transverse sections 
* See De Bary, ‘ Comp. Anatomy of Phanerogams and Ferns’ (Engl. Transl.), p. 182. 
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