New data about anatomy, branching, and inferred growth patterns in the Early Devonian plant Armoricaphyton chateaupannense, Montjean-sur-Loire, France

Abstract

This study provides detailed information, based on serial peels, on the anatomy of the primary and secondary xylem, lateral branch formation, and proposed growth pattern of Armoricaphyton Strullu-Derrien et al., an Early Devonian (Pragian) basal euphyllophyte. The centrarch primary xylem strand is circular to oval in cross section and includes P-type tracheids. The pattern of lateral branch departure is described in detail based on serial peels. The secondary xylem, illustrated in transverse, radial and tangential longitudinal sections, includes P-type tracheids with similar pitting in radial and tangential walls. The presence of one-walled spaces interpreted as rays is confirmed. This study also documents the earliest occurrence of secondary xylem at bases of next higher order of branches. A model of growth for this unusual Early Devonian plant is presented, with documented epidogenetic, menetogenetic and possible apoxogenetic phases. The difference in proportion of primary to secondary xylem is postulated to be a result of this mode of growth. The possible implications of xylem size and organization in the lateral organ in Armoricaphyton relative to the early evolution of the megaphyllous leaf are briefly discussed.