214 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
section. Utterly unable to interpret the relations of these several structures, I adopted 
the plan which proved so successful in the case of Heterangium Grievii (Phil. Trans. 
1873, pi. xxx. figs. 37-44), and had about an inch of the specimen cut up into the series 
of closely consecutive, transverse sections represented in figs. 2-12. By this process a 
partial interpretation of these sections, at least, has become sufficiently easy. 
Vascular cylinder. — The central vascular cylinder (figs. 2-12, b) consists of from five 
to seven clusters of vessels, which clusters are intimately blended together at their 
peripheral borders. This arrangement is more clearly shown on the enlarged scale of 
18^ diameters in fig. 13. 
Medulla. — The medullary cells constitute a central mass (a), which sends off several 
irregular, narrow, diverging radii (a', a'). These diverging portions, which occasionally 
dichotomize, as is the case with the one descending to the lower portion of fig. 13, 
partially separate the clusters of vessels from each other. The cells, as seen in the 
transverse section, are of the ordinary parenchymatous type, varying in size from ‘003 
to ‘00125 *. Some of these cells either contain isolated secondary cells within their 
cavities, or distinct primordial utricles, detached from their proper cell wall. Scattered 
amongst them also are some small vessels. In the longitudinal section (fig. 14) the cells 
of the medulla are seen to be of irregular shapes and sizes, but often narrow, elongated 
longitudinally, and square-ended. The vessels of the surrounding cylinder are generally of 
large size, ranging from ‘007 to ‘005 in diameter, and with strong, sharply defined walls; 
but at the inner and outer boundaries of the cylinder we have many small ones, not 
exceeding ‘002, resembling, in this respect, some vessels which are scattered through 
the medullary parenchyma. On examining the longitudinal section (fig. 14) of this 
axial cylinder, we find that the vessels are of the usual barred type, and that the largest 
of them occupy the central parts of each cluster — a fact also shown by fig. 13. But 
the most remarkable feature of these vessels is further illustrated by figs. 15 & 16. In 
fig. 15 the dark, strongly marked walls of the vessels enclose densely packed masses of 
cells (i). The longitudinal section, made nearly through the centre of a single vessel 
(fig. 16), shows it to be similarly crowded with tylose throughout its entire length. 
The existence of this singular tissue in the vessels of recent plants has long been 
known to botanists, and Professor W. T. Thisleton Dyee has described a similar tissue 
existing in a fossil Tertiary exogenous wood from Herne Bay and the Isle of Thanetf ; 
but so far as I am aware this is the first recorded example of its appearance in a fossil 
Cryptogam, and especially in one from these ancient Carboniferous deposits. The fact 
of the existence of tylose at this remote period affords a striking example of the 
persistence of types of elementary tissue, .additional to those which we already possess. 
Bundle-sheath. — The axial vascular cylinder is enclosed in a somewhat irregular 
sheath of cellular tissue seen at figs. 13 ,g Sc 14, g. This consists of three or four rows 
* As in the preceding Memoirs, all these measurements are recorded in the decimal parts of an inch. 
t “ On some Fossil Wood from the Lower Eocene,” by W . T. Thisietok Dyee, B.A., B.Sc., F.L.S., ‘ Geological 
Magazine,’ yol. ix. no. 6, June 1872. 
