Résumé
Thanks to its hydraulic and mechanic properties, the secondary xylem (wood) played a significant role in the evolution of the tallest trees from the Middle Devonian onwards. This tissue, produced by a vascular cambium, is characterized by radially aligned rows of vascular elements and transversally oriented rays consisting mainly of parenchymatous cells; anticlinal divisions of cambial cells contribute to the formation of new radial files of elements.Armoricaphyton chateaupannense, a basal euphyllophyte of late Pragian – early Emsian age, is the first known plant to have developed wood (Gerrienne et al., 2011; Strullu-Derrien et al., 2014). Within the euphyllophytes, the secondary xylem is a central element of the lignophytes (progymnosperms and seed plants) anatomy. On the other hand, the development of woodtype tissues within the fossil and extant plants affiliated to the ferns is less well understood, and their cambial origin has been challenged in some taxa (Rothwell and Karrfalt, 2008). The aim of this report is to characterize the wood of the Cladoxylopsida, the oldest known (Middle Devonian – Early Carboniferous) group of plants affiliated to the ferns sensu lato. Two species have been studied, Xenocladia medullosina, represented by Middle Devonian specimens from Kazakhstan, and Cladoxylon taeniatum, represented by early Carboniferous specimens from Germany. We characterized the wood of these species using the features defined by Tomescu and Groover (2019) in their publication hypothesizing the mosaicmodularity of the development of the secondary vascular tissues within the tracheophytesThe dissected vascular system of both species is typical of the Cladoxylopsida and consists of numerous vascular bundles immersed in a parenchymatous pith. In each bundle, the primary xylem is surrounded by radially aligned files of tracheids. No remain of a vascular cambium is observed. The radial diameter of the most external tracheids is reduced in some bundles. No evidence of phloem is noticed for Cladoxylon. For Xenocladia, a secondary phloem-like tissue is observed around some vascular bundles. In most cases, the secondary xylem layer produced is even, but some cases of cell files exceeding the bundle limits are observed. Anticlinal divisions are noticed for both species. In cross section, parenchymatous rays are visible for Cladoxylon. For Xenocladia, scarse ray-like structures are noticed.Despite the absence of an observable vascular cambium, these features suggest that the woodtype tissue of Xenocladia and Cladoxylon has a secondary origin and is determinate. Our observations support the modular development of the secondary vascular tissues in the euphyllophytes.