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THE INTERNAL STRUCTURE OF SIGILLARIA ELEGANS. 541 
The complete zone of cortex bore twenty-eight ribs, of which nine are not seen in the 
section, but the figure allows one to estimate the proportional size of the vascular 
cylinder to that of the circumference of the stem. 
The stele, which measures slightly under 13 mm. in its longest compressed diameter, 
consists of a perfectly continuous ring of primary wood, «’, which is surrounded 
by a zone of secondary wood, 2”, 
The pith and the tissue which originally composed the inner portion of the cortex 
and all remains of the phloem elements, have been entirely destroyed. The only other 
remaining portion of the stem in which the structure is preserved is the outer layer of 
the cortex on which the leaf cushions are situated. The structure of these parts may now 
be considered in detail. 
The Primary or Centripetal Xylem.—At fig. 3, Pl. L, is shown a transverse section 
of the vascular cylinder, enlarged about 43 times. ‘The ring of primary xylem «.’ is 
quite continuous, and is about 0°70 mm. thick. As seen in fig. 4, Pl. L, its outer margin 
is deeply and regularly undulate or crenate, so as to form a number of blunt ridges 
alternating with as many intervening furrows. The inner margin of the xylem ring is 
very uneven, sending irregular toothed projections into the now empty pith-cavity (PI. I. 
me. 4, w.’ and p.c.). 
The main mass of the primary xylem consists of large tracheides, more or less 
hexagonal in transverse section, and without any intervening parenchymatous cells. 
They diminish slightly and gradually in size towards without, but just underneath 
the ridges a decrease in size takes place somewhat abruptly, and the ridges themselves 
are composed of much narrower elements, that are to be regarded as constituting the 
protoxylem (Pl. I. fig. 4, prx. Text fig. 1, B, pra.). 
At the inner margin of the primary xylem a certain amount of thin-walled parenchy- 
matous cells are to be found between the tracheides, and a few of the latter may even be 
surrounded by parenchyma, and separated from the rest of the mass. Some especially 
narrow tracheides also occur here and there along the inner margin. 
Radial longitudinal sections, Pl. II. figs. 6 and 8, show that all the, elements of the 
primary xylem, both protoxylem (Pl. II. fig. 7, prx.) and metaxylem (Pl. II. fig. 9, x.’), 
are elongated scalariform tracheides with pointed ends (fig. 7, pre., fig. 9, .’), except a 
few of the innermost tracheides bordering on the pith, which are quite short, blunt-ended, 
‘and irregular in shape, but still scalariform. So far as observed, no spiral or annular 
elements at all were found at any point in the primary xylem, not even in the 
protoxylem. 
The protoxylem elements are seen in radial section at prz., figs. 6 and 8, Pl. II. 
They are long and narrow, and terminate in an elongated conical point (PI. II. fig. 7, pra.). 
Secondary or Centrifugal Xylem.—This forms a zone of varying thickness surround- 
ing the primary xylem, and attaining at its widest part a breadth of about 0°75 mm. (Pl. 
I. figs. 3, 4, and 14, x.”). Its inner margin follows the crenulate outline of the primary 
xylem. The outer margin exhibits the same crenulations, but to a slightly less degree. 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART III. (NO. 23), 81 
