SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 651 
protoxylem elements (px .) are sometimes recognisable in the enlarged ends of the 
rays. The phloem (ph.) occupies the bays of the stellate xylem, and forms a narrow 
zone outside the ends of the rays of xylem (fig. 72) ; it is often more or less 
completely decayed. 
Leaf-traces ( l.t .), consisting of a small centrarch strand of xylem surrounded 
by a zone of phloem, can be seen departing from the stele or on their way out 
to the leaf-bases. The much larger traces for the lateral branches (br.tr.) are 
occasionally met with, but, unlike the leaf-traces, pass out from the cortex of 
the stem into the branches. 
Attention may now be directed to some of the figures in which particular 
features mentioned in the above general account are shown especially clearly. 
The complete differentiation of a large stem is well shown in fig. 96, which will 
be referred to later in relation to the preparation for equal division exhibited 
by the stele. The corresponding tissues are labelled in the portions of other large 
stems in figs. 41 and 43. The epidermis is not evident, though the cuticle marks 
the limit of the stem. In the outer cortex ( o.c .), which is light coloured owing to 
the disappearance of the cell-contents, some leaf-traces (l.t.) are present. Within 
it. the outer zone of the inner cortex ( i.c.o .) is recognisable as a narrow and not 
- very well characterised layer. The broad middle zone of the inner cortex (i.c.m.) is 
prominent owing to the trabecular arrangement of the cells with wide intercellular 
spaces between the rows. The coherent tissue around the stele, dark in fig. 41, is 
the inner zone of the inner cortex (i.c.i.). The stellate xylem (x y.), with the 
widened ends of the rays divided and enlarged, is well preserved. Leaf-traces (l.t.) 
are present in relation to the rays. The phloem (ph.) between the rays of xylem 
has decayed and disappeared, while the narrow zone of phloem within the cortex 
and around the xylem is compressed, but recognisable. 
The somewhat smaller stem in fig. 42 does not show the zoning of the decayed 
inner cortex, but the dark remains of the cell-contents have persisted in the outer 
-cortex and render it prominent. The tissues of the leaf-bases and free portions of 
the leaves have a similar appearance. The stem in fig. 45 shows the zones of the 
cortex clearly. The stele has the massive four-rayed xylem well preserved, but the 
phloem has perished from between the rays. 
Fig. 44 is of a stem of fair size, but with a less massive, rayed xylem. This and 
the smaller stem in fig. 46 have a similar zoning of the cortex to the larger stems, 
and have the outline irregular from the projecting leaf-bases. The still smaller stem 
in fig. 47 shows the trabecular middle zone of the inner cortex clearly, and has the 
xylem of the stele three-rayed. The cortex of the three small twigs associated 
together (fig. 48) has so completely decayed, that their outlines are only dimly 
traceable. 
All the transverse sections of stems on PI. VI are magnified 14 diameters, and 
are thus strictly comparable. 
TRANS. ROY. SOC. EDIN., YOL. LII, PART III (NO. 26). 
101 
