Temporal range: Early Devonian
Kidston & Lang 1920
Kidston & Lang (Type species)
Asteroxylon ("star-shaped xylem") is an extinct genus of vascular plants of the Division Lycopodiophyta known from anatomically preserved specimens described from the famous Early Devonian Rhynie chert and Windyfield chert in Aberdeenshire, Scotland. Asteroxylon is considered the most basal member of the Lycopsida.
Asteroxylon is a genus of terrestrial vascular plant which flourished in the Early Devonian period. This plant consisted of aerial, isotomously and anisotomously branching stems that reached 12 mm in diameter and 40 cm in length.  The possibly procumbent aerial stems arose from a leaf-less rhizome which bore smaller-diameter, positively geotropic root-like branches.  The rhizomes reached a depth of up to 20 cm below the surface. The xylem or conducting tissue at the center of the aerial stems is distinctly star-shaped in cross-section and has been considered an early actinostele or an "Asteroxylon-type" protostele.  The tracheids are of the primitive annular or helical type (so-called G-type). "Leaves" – not true leaves, but protrusions – were of the form of unbranched strap-shaped enations up to 5 mm long; a single vascular trace branched from the main bundle in the centre of the stem to terminate at the base of each enation. Enations and axes bore stomata, indicating that their tissues were capable of photosynthesis.
Asteroxylon differs from other similar Early Devonian lycopsids such as Drepanophycus and Baragwanathia in that the singular vascular leaf trace in these latter plants extends into the leaf.  The leaves of Drepanophycus and Baragwanathia are therefore considered to be true microphylls or, alternatively, small leaves. 
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