SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 653 
numerous leaf-traces cut across. The leaf-traces close to the periphery of the stele 
have well-developed xylem. The only other thickened elements are the poles of 
the stellate xylem, from which differentiation in the thin-walled tissue of the immature 
stele will proceed centripetally. This is more apparent in the next lower section of 
the same stele, which is more highly magnified in fig. 60. In this the poles of the 
imperfectly differentiated xylem, marked x., are readily distinguishable from the 
leaf-traces. Figs. 59 and 60 are apparently of sections just below the apex, where 
the lignified leaf-traces are crowded and pursuing a more vertical course, while the 
stem-xylem is only beginning to be lignified. 
The various tissues, the arrangement of which in transverse sections has been 
described, are of course found in longitudinal sections (fig. 63), but few good 
examples have been available for study. The longitudinal sections tend to be less 
satisfactory, since the preservation of the tissues in these shoots, though good up to 
a point, is by no means perfect. Owing, in part at least, to the effect of saprophytic 
fungi the cell-walls are often hardly to be traced, although the zones of tissue may 
be obvious. The difficulties arising from this imperfect preservation are especially 
felt in studying longitudinal sections of Asteroxylon (figs. 63 and 31). 
In proceeding to deal with the various tissues in detail, the information obtained 
from longitudinal or oblique sections will be combined with that from the transverse 
sections. 
The epidermis, as seen on the stem, leaf-bases, and leaves, is a single layer of 
cells, the outer walls of which are thickened, and covered with a distinct cuticle 
(figs. 61 and 62). The cuticle often persists when the rest of the cell-walls has 
disappeared. The outer walls of the cells may be flat or papillate (figs. 61 and 39). 
In surface view or tangential section the epidermal cells are somewhat longer than 
broad (fig. 65)» and in some well-preserved specimens their outer walls have shown 
a median line similar to that in Rhynia Gwynne-Vaughani (Part I, PI. VI, fig. 31). 
Stomata of ordinary form have been seen on the leaves, though even there they 
seem to be relatively few (fig. 40), and they occurred on the aerial stems also, as 
they did in the epidermis of the transition region (cf. fig. 73). A well-preserved 
stoma from the aerial shoot is represented in fig. 65, and another in vertical section 
in fig. 66. As the latter figure shows, the guard-cells were distinctly depressed 
below the general surface. The pore of the stoma on the aerial shoot was much 
smaller than on the transition region (cf figs. 65 and 73). 
The outer cortex is a narrow zone, some six cells deep. Its cells were 
tangentially extended, and are usually compressed (figs. 62 and 58). The contents of 
the cells may persist as a dark substance, making the region prominent (figs. 42, 62, 
and 58), or the contents may have disappeared and the zone appear clear (figs. 43 
and 45). This applies to the contents of the epidermal cells also (figs. 61, 62, 
and 65). 
The tissues of the leaves are continuous with the epidermis and outer cortex of 
