IV—On rhexoxylon, bancroft—a triassic genus of plants exhibiting a liane-type of vascular organisation

Abstract

In 1913 Dr. Bancroft instituted the generic nameRhexoxylonfor a portion of a petrified stem sent by Dr. Rogers, Director of the Geological Survey of South Africa, to Prof. Seward for investigation. The specimen was handed over to Dr. Bancroft, who, in a communication to the Linnean Society of London, gave a full account of its anatomy.Rhexoylon, as represented by the type-specimen, may be briefly described as follows:— The fragment of stem exhibits “parts of two more or less concentric series of vascular structures surrounding a large pith and embedded in a parenchymatous ground-mass,” which contains sclerotic nests and small vascular strands. The pith, in addition to sclerotic nests and small vascular strands, contains other structures, which are most probably of an anomalous nature. There is an inner ring or series of vascular structures, which were called, for convenience in description, “steles”; each consists of two portions of secondary wood, one centrifugal and the other centripetal, separated by a narrow zone of obliterated cells. The “steles” branch and rarely anastomose. Traces were described separating from the sides of the centripetal portions of the “steles” and taking a centrifugal course. Outside and opposite each member of this inner series is a mass of centrifugally-developed secondary wood, on the inner margin of which are numerous small protoxylem groups. The tracheids have bordered pits, arranged typically in two alternating series; the pits are flattened and in contact. The medullary rays are uniseriate. Tissue external to the xylem was described as phloem.