Langoxylon asterochlaenoideum Stockmans: Anatomy and relationships of a fern-like plant from the Middle Devonian of Belgium

Restudy of Stockmans' [Stockmans, F., 1968. Végétaux Mésodévoniens récoltés aux confins du Massif du Brabant (Belgique). Mém. Mus. Roy. Hist. Nat. Belg. 159, 1-49.] Middle Devonian Langoxylon asterochlaenoideum clarifies many details of its anatomy. The main axis has a large multi-ribbed actinostele from which traces of two size classes depart in an irregular helix to decurrent lateral appendages. Most laterals are branch-like and supplied by major traces that are initially terete, expand distally, become bilaterally ridged, and then radially fluted just before appendage separation. Other minor traces supplied ultimate appendage- or leaf-like organs, which have smaller terete traces that fork and contract distally. Inner metaxylem of the main axis and branches consists mostly of apparent xylem parenchyma. Primary xylem is mesarch with two types of protoxylem strands. Protoxylem of the main axis appears to consist only of tracheids and occurs as several strands in each of the deeply cleft actinostele ridges (i.e., of the 'radiating' type). Protoxylem of lateral appendages occurs only at the tips of the xylem ridges (i.e., of the 'permanent' type) and has a core of parenchyma cells surrounded by protoxylem tracheids. This latter type has been called a peripheral loop (boucle périphérique) and shows minimal cell elongation. It differs from protoxylem lacunae of similar-appearing iridopterid taxa where parenchyma is absent and stretching of central tracheids results in their destruction to produce a vertical cavity. These multiple forms of protoxylem architecture might be useful for systematics of euphyllophytes. Where preserved, primary phloem is concentric. The cortex has inner parenchymatous and outer fibrous zones. Langoxylon bears only a superficial resemblance to Asteropteris and Asterochlaena, to which Stockmans compared his genus, and seems to be unrelated to them or to iridopterids like Ibyka, Arachnoxylon, Iridopteris, and Compsocradus, or the lignophyte Stenokoleos. Langoxylon compares with Middle Devonian archaeopterid progymnosperms like Actinopodium and Actinoxylon, but has a mixture of anatomical characters that were previously used to subdivide Euphyllophytina into Moniliformopses and Radiatopses types. Langoxylon is thus a 'fern-like' Middle Devonian euphyllophyte of uncertain phylogenetic relationship. Its most important contribution is to stimulate further investigations of the characters used to reconstruct early plant phylogeny.