706 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
impressions. These must have belonged to stems of enormously greater size, but we 
cannot absolutely prove their identity with Lyginodendron (see below, p. 741). 
Full descriptions of the characteristic structure of the stem have been given in 
previous memoirs.* It will, however, be necessary to recapitulate what is already 
known, for our re-examination of the material, with the aid of additional specimens, 
has thrown new light on various points. Photograph 1, Plate 18, represents the 
transverse section of a stem of medium size which we may take as a typical example. 
This specimen, however (of which we have three sections), though unrivalled for 
the preservation of the most important details, is not so perfect, as a whole, as are 
some of the others. In fig. 1, Plate 21, a transverse section of another stem from an 
entirely new specimen is shown, the preservation of which is remarkably complete. 
The middle of the central cylinder is occupied by a solid parenchymatous pith, 
imbedded in which are groups of dark sclerotic cells. At the periphery of the pith 
there are several distinct strands of primary xylem. Two of these, from the same 
specimen as photograph 1, are shown on a larger scale in Plate 21, figs. 2 and 3. 
Beyond the primary strands of xylem, we come to a broad zone of secondary wood, the 
elements of which are arranged with great regularity in radial rows. This secondary 
wood is divided up by numerous medullary rays, both primary and secondary. 
{f any doubt could be entertained as to the mode of development of the woody zone, 
it is removed by the fact that at its outer limit the cambium itself is often preserved 
in great perfection (see Plate 22, fig. 7). 
On its outer side the cambiumn is continuous with a zone of thin-walled tissue, 
which is made up of phloém-groups, separated from one another by the phloém-rays. 
The greater part of this layer is secondary, as is shown by the radial seriation of its 
elements. On its external border, however, the primary phloém-groups can still some- 
times be recognized (see Plate 21, fig. 1, and Plate 22, fig. 7, ph.). The phloém-zone is 
again surrounded by a ring of thin-walled tissue, which is best regarded as a pericycle 
(see Plate 22, figs. 5, 6, and 7). Imbedded in this we find groups of the same some- 
what sclerotic cells, which are so conspicuous in the pith. At the exterior of the 
pericycle there is usually a layer of periderm. 
Beyond the periderm we come to the cortex proper. Its inner zone is usually the 
worst preserved part of the specimen. Only fragments of it are present in the 
specimen shown in Plate 18, photograph 1; in the stem represented in the drawing, 
Plate 21, fig. 1, it is better preserved. The inner cortex consists of large-celled 
parenchyma, among which numerous sacs (probably secretory) are scattered. 
The outer cortex is made up of the well-known alternate radial bands of scleren- 
chymatous fibres and parenchyma, forming what is termed by Count Sonms-LAauBAcH 
the Dictyoxylon cortex.t As has often been described, the sclerenchymatous strands 
anastomose in the tangential direction, forming long meshes, which are occupied by 
* Wituiamson, “ Organization,” Parts IV. and XVII. 
+ “Fossil Botany,” p. 7. 
