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PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION

section. Utterly unable to interpret tbe relations of these several structures, I adoptedthe plan which proved so successful in the case of Ileterangium Grievii (Phil. Trans.1873, pi. xxx. figs. 37-44), and had about an inch of the specimen cut up into the seriesof closely consecutive, transverse sections represented in figs. 2-12. By this process apartial interpretation of these sections, at least, has become sufficiently easy.

Vascular cylinder. —The central vascular cylinder (figs. 2-12,5) consists of from fiveto seven clusters of vessels, which clusters are intimately blended together at theirperipheral borders. This arrangement is more clearly shown on the enlarged scale of18-^ diameters in fig. 13.

Medulla. —The medullary cells constitute a central mass (a), which sends off severalirregular, narrow, diverging radii (a 1 , a 1 ). These diverging portions, which occasionallydichotomize, 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 thetransverse section, are of the ordinary parenchymatous type, varying in size from '003to '00125 *. Some of these cells either contain isolated secondary cells within theircavities, or distinct primordial utricles, detached from their proper cell wall. Scatteredamongst them also are some small vessels. In the longitudinal section (fig. 14) the cellsof the medulla are seen to be of irregular shapes and sizes, but often narrow, elongatedlongitudinally, and square-ended. The vessels of the surrounding cylinder are generally oflarge 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, notexceeding '002, resembling, in this respect, some vessels which are scattered throughthe medullary parenchyma. On examining the longitudinal section (fig. 14) of thisaxial cylinder, we find that the vessels are of the usual barred type, and that the largestof them occupy the central parts of each cluster—a fact also shown by fig. 13. Butthe most remarkable feature of these vessels is further illustrated by figs. 15 & 16. Infig. 15 the dark, strongly marked walls of the vessels enclose densely packed masses ofcells («'). The longitudinal section, made nearly through the centre of a single vessel(fig. 16), shows it to be similarly crowded with iylose throughout its entire length.The existence of this singular tissue in the vessels of recent plants has long beenknown to botanists, and Professor W. T. Thisleton Dyee has described a similar tissueexisting 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 fossilCryptogam, and especially in one from these ancient Carboniferous deposits. The factof the existence of tylose at this remote period affords a striking example of thepersistence of types of elementary tissue, additional to those which we already possess.

Bundle-sheath. —The axial vascular cylinder is enclosed in a somewhat irregularsheath of cellular tissue seen at figs. 13, ^ & 14, <7. This consists of three or four rows

* As in the preceding Memoirs, all these measurements are recorded in the decimal parts of an inch.

f “ On some Fossil Wood from the Lower Eocene,” by "W. T. Thisleton Dveb, B. A., B.Sc., F.L.S., ‘ GeologicalMagazine,’ vol. ix. no. 6, June 1872.