ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES, 925 
by which the secondary tissues were formed, is well preserved. It is especially 
evident in the section from which fig. 42 on Plate 83 was drawn, in which the 
correspondence of the thin-walled tabular cambial-cells, with the radial series of 
secondary trachex, is perfectly clear. Other specimens in which the cambium is 
preserved, are illustrated in the photographs 19 and 20 on Plate 76. 
A relatively enormous amount of secondary cortical tissue was developed on the 
outer side of the cambium (Plate 75; Plate 83, figs. 40 and 43). It is often 
difficult to distinguish, in transverse sections, between the true phloém, formed 
directly from the cambium, and the internal periderms, which arose in abundance from 
deeply-seated layers of phellogen. The transverse section, shown in photograph 19 
and in fig. 43, is especially instructive. Here the actual cambium is only partially 
preserved ; immediately outside it the wide secondary cortex* begins. At the place 
shown in fig. 43, its maximum thickness was about 16 cells.t The radial series are 
continuous throughout, and a general continuity with those of the wood can also be 
traced. The inner zones of the secondary cortex consist of thin-walled cells ; each 
cell is occupied by a carbonaceous mass (omitted in the figure for the sake of clearness, 
but recognizable in the photograph). This carbonaceous matter may perhaps indicate 
the original presence of abundant cell-contents. The outer cortical layers are 
formed of cells with much thicker walls, and without any considerable carbonaceous 
contents. Between the two zones is a layer of somewhat flattened cells, with 
specially thin walls, which have sometimes broken down. 
The explanation which we propose for this structure is, that the inner zone is true 
phloém, formed directly by the cambium on its exterior surface, while the outer layers, 
with thicker cell-walls, constitute an internal periderm. The intermediate flattened 
layer would, in this case, be the phellogen, which must have arisen by the division of 
cells themselves belonging to the secondary phloém-parenchyma. This explanation also 
applies well to the other preparations in which the secondary cortical tissues 
are shown, though the carbonaceous contents of the inner layers are not always 
present. 
Longitudinal sections show that the periderm consisted of short cells, in very 
regular series, while the inner tissue, presumably phloém, was composed of much 
longer elements, possibly the sieve-tubes.{ In some specimens (see photograph 22) the 
phloém-elements are less regularly arranged than those of the periderm. 
The development of periderm seems never to have been external, but to have 
started, from the first, in deeply-seated tissues. A good example of its first forma- 
* Tt is convenient to use the general term “ secondary cortex ”’ for all secondary tissues external to the 
wood, (July 15, 1894.) . 
+The thickness of the secondary cortical tissues is very unequal at different parts of the 
circumference of the stem. It often, but not always, shows a maximum opposite each of the three 
protoxylem groups. 
t See WILLIAMSON, “Organization,” Part V., Plate 1, fig. 7. 
