On the Organization of the Fossil Plants of the Coal-Measures. Part XIX

In the year 1832 the late Rev. Wm. Vernon Harcourt obtained an important fragment of a Lepidodendroid branch from a colliery at Hesley Heath, near Rothbury, in Northumberland. It was the first example seen of such a Lepidodendron in which much of the external organization was preserved, hence it naturally excited great interest. Probably no other fossil has ever obtained a wider notoriety. The specimen was first figured and described by Witham, of Lartington, then by the authors of the ‘Fossil Flora of Great Britain,’ and finally by the late Adolphe Brongniart. Witham gave to the specimen the name of Lepidodendron Harcourtii , after its discoverer, which name it has ever since borne; but his accompanying description was full of errors. Lindley and Hutton were not much more successful in their subsequent attempts to interpret the specimen. It was otherwise when Brongniart received a section of the fragment from Hutton, which he submitted to a very elaborate investigation. The specimen was not a perfect one. The outer cortex and leaves were wholly wanting; but the rest of its organization enabled Brongniart to base upon it the construction of a diagrammatic figure of the plant when living; this he believed would prove to be typical of that of the entire Lepidodendroid family. His pre-eminent position as a palaeobotanist caused his views to be universally accepted. Brongniart’s typical Lepidodendron contained in its interior a single vascular cylinder, enclosing a medulla, from the exterior of which cylinder the leaf-traces received their vascular elements. Later in life two fragments belonging to different plants were obtained by him, in each of which there existed the central cylinder seen in Lepidodendron Harcourtii ;but in both cases this cylinder, which he had designated etui medidlaire , or equivalent of the protoxylem or medullary sheaf of recent exogenous stems, was surrounded by a secondary zone of wood, developed exogenously through the instrumentality of a cambium layer. Brongniaiit was unfortunately biassed by the fact that no such secondary growth existed amongst living Cryptogams—to which group he correctly determined the Lepidodendra to belong. The existence of a secondary, exogenously-developed, vascular zone was a clear proof in his eyes that his new plants could not be Cryptogams; hence lie referred them to the family of Gynmosperms, and connected them with the well-known Carboniferous plants called Sigillariae . The conclusions thus arrived at by the great French palaeontologist were almost universally accepted at the Mine when I began my investigations into these subjects, some thirty years ago; I soon obtained clear proofs that many of the Carboniferous Cryptogams, unlike their living representatives, were provided with a secondary, exogenously-developed, vascular zone. ' That this is the case with most of Brongn Niart’s Lepidodendra is now an almost universally accepted fact. But these plants vary in a very remarkable manner as to the stage of growth at which this secondary development makes its appearance. In some, like the L. selaginoides , it exists in very young twigs. In others, like the Arran plant ( L. Wunshchianum , Will.), the branches have to attain to very large, even arborescent, dimensions, before the smallest trace of such a growth is discoverable. The L. Harcourtu is one of those in which no exogenous zone has yet been observed; but the history of the Arran plant teaches us to be cautious ere we conclude that it never develops such a zone of secondary xylem. The largest branch of L. Harcourtu yet met with is 31/2 inches in diameter, apart from its leaves. I have specimens of the Arran plant of yet larger size, equally devoid of all traces of secondary xylem; but when we come to stems of the latter plant 2 feet in diameter, we find in them the most magnificent examples of a secondary zone of wood that I have yet met with. This fact lias only become known to us through a rare and fortunate accident. We find in our Coal-Measures very many, even large, Lepidodendroid and Sigillarian Trees, but unfortunately these are all mere casts of the outer surface of the cortex, each interior being only a mass of inorganic clay or sand. Hence we know nothing of what the internal organization of these arborescent forms was. But the Arran stems have been preserved for us under very different conditions. They have been imbedded, not in sand and mud like those ordinarily met with, but in volcanic ash. Their central vascular axes, with their contained medullse, are as perfect as they were in the living plants. Not only so, but much of their cortex is equally well preserved. But for this happy and unique circumstance, we should have been entirely ignorant of the fact that a Lepidodendron might attain to a considerable magnitude without any trace of a secondary vascular zone making its appearance in its stem, and yet, ere it attained to its ultimate dimensions of a noble forest tree, it contained such a zone. Having, however, learnt this indisputable truth, we must admit that, though no branch of L. Harcourtu has yet been found possessing a secondary xylem, it would be presumptuous to deny the possibility that, if we could discover its matured stems, as we have done those of the Arran trees, corresponding results would follow. This conclusion becomes the more probable when we remember how very few there are, even of the smaller Lepidodendra , which have not developed a secondary xylem. The only exceptional cases that I am acquainted with are two or three of which we only know the young twigs.