XVII. On Botrychioxylon paradoxum, sp. nov., a Palaeozoic Fern with Secondary Wood.

The stele of Botrychioxylon paradoxum has a simple, nearly cylindrical form. There is a “mixed pith” consisting of internal xylem and parenchyma; the outer zone of wood is entirely secondary, diminishing in thickness towards the upper part of the stem. The cortex is of very uniform parenchymatous structure, and is bounded externally by periderm. The branching of the stem is dichotomous. The leaf-trace is given off from the stele in a similar manner to that of Ankyropteris corruyata. The xylem of the trace appears, like that of the stele, to be largely of secondary origin. The petiolar bundle only shows primary tissues. It has short antennae and a long median band (“apolar”). There appear to be no peripheral loops, the sinus at each end of the xylem being open. Aphlebiæ are borne both on the stem and petiole; they are branched, spine-like organs. The adventitious roots are diarch and freely branched. They formed a periderm and sometimes secondary wood. The affinities of Botrychioxylon appear to be closest with the genus Metaclepsydropsis; in the stem of the latter, however, secondary wood is only formed in exeptional cases. Botrychioxylon shows close analogies with Botrychium, in which also internal xylem sometimes occurs. The affinity of the Zygopteridese generally with the Ophioglossacese is confirmed.