654 DR H. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
the stem. When the free portions of the leaves are well preserved (fig. 37) the 
thick-walled epidermis encloses a uniform tissue, with dark contents similar to the 
cortex. As already mentioned, no vascular tissue is present in the free portion 
of the leaf. 
The wider inner cortex may be a uniform tissue (fig. 49), but it is usually 
differentiated into outer, middle, and inner zones (figs. 41 and 43). In the cases in 
which it is uniform the tissue has large intercellular spaces. When three zones are 
distinguishable, it is the middle zone that has the prominent intercellular spaces, and 
often shows a definitely trabecular arrangement of the rows of cells (fig. 67). The 
narrow collapsed cells between the intercellular spaces come out more clearly in 
fig. 71, though the trabecular arrangement is not shown in this specimen. Longi- 
tudinal sections show that the trabeculae were vertically extended plates, and not 
rows of cells. The outer zone beneath the outer cortex, and especially the wider 
inner zone around the stele, are composed of more compact tissue. The former is 
often ill-marked, while the inner zone is well characterised, and contrasts with the 
middle zone by reason of its closely associated small elements (figs. 67 and 68). 
While the cortical tissues show clearly the differences of the various zones, the cells 
have been so far altered in the process of decay as to make detailed description 
inadvisable. Longitudinal sections show that all the cells were somewhat longer 
than broad, and that those of the inner zone of the inner cortex were closely 
packed and more elongated. There is no evidence, however, to show that these 
latter elements were developed as fibres. 
As a rule there is no prominent layer marking off the cortex from the stele, but 
in one large stem * a well-marked layer in the position of an endodermis was so 
evident, that it cannot be passed over as without significance, although found in a 
single example only (fig. 69 ; cf. figs. 100 and 102). The soft tissues of the inner 
cortex and phloem of this stem were rather more than usually indistinct, though 
the zonation of the tissues was clear, and the layer of cells in question stood out 
between these two regions. Its cell-walls were prominent owing to their dark 
colour. The whole appearance suggests comparison with such an ill-defined endo- 
dermis as is met with in the stems of some species of Lycopodium. 
The soft tissues of the stele, which have been termed collectively the phloem, 
occupied the bays and formed a narrow zone outside the arms of the xylem. As has 
been seen, they are usually badly decayed or compressed, and* have often in great 
part disappeared, or been replaced by saprophytic fungi. In a few specimens of 
the leafy shoot they have been found well preserved. The more decayed examples 
must evidently be interpreted in the light of those better-preserved ones and of the 
sections of the transition region. It has been described above how, with the change 
from the circular to the stellate cross section of the xylem, the phloem of the stem 
can be clearly related to that of the rhizome. 
* This particular stem was undergoing branching of a peculiar endogenous type, as will be described below. 
