III. Notice of further researches on the fossil plants of the coal-measures

Abstract

My dear Dr. Sharpey,—Since I read my last communication to the Royal Society on the organization of the Fossil Plants of the Coal-measures I have done a large amount of work, having cut between two and three hundred new sections and with most satisfactory results. I have obtained a series of specimens almost completing the life-history of one plant from Burntisland, beginning with the tips of the smallest twigs and ending with the large stems. The former are mere aggregations of parenchyma with a central bundle of barred vessels mixed with a small amount of primitive cell-tissue. As the twig grew the leaves assumed definite form, and the central vascular bundle opened out at its central part, so as to form a cylinder, the interior of which was occupied by parenchyma. This cylinder grew rapidly, the number of its vessels steadily increasing; but they were all equally arranged as in, what I have termed, the medullary vascular cylinder, i. e. not in radiating series. We thus obtain the origin of that remarkable cylinder, and see that it is the expanded homologue of the central vascular bundles of the living Lycopods. Whilst these processes were in progress the cortical portion became differentiated into layers, and the parenchymatous cells of the pith continued to multiply, so as to occupy the expanding interior of the vascular cylinder. After attaining a certain size, through the above processes, a new element of growth appeared; an exogenous addition was made to the exterior of the cylinder, also consisting of barred vessels, but these are arranged in the radiating series described in my last memoir. This series continued to grow until it attained to considerable dimensions; but the entire vascular system always remains small, compared with the diameter of the stem, the chief bulk of which consists of an enormously thick bark. The structure just descibed is that of a true example of the genus Diploxylon of Corda. But I have got abundance of specimens with leaves on the exterior of the bark, demonstrating that the plant is a true Lomatophloios , thus indicating the correctness of my supposition, advanced in my last memoir, that sooner or later the genus Diploxylon would have to be abandoned.