Asteroxylon

Asteroxylon Temporal range: Early Devonian PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom:	Plantae
Division:	Lycopodiophyta
Class:	Lycopodiopsida
Order:	Drepanophycales
Family:	Asteroxylaceae
Genus:	Asteroxylon Kidston & Lang 1920
Species:	Asteroxylon mackiei Kidston & Lang (Type species)

Asteroxylon ("star-shaped xylem") is an extinct genus of plants of the Division Lycopodiophyta known from anatomically preserved specimens in an Early Devonian deposit of chert at Rhynie, Aberdeenshire in North-East Scotland that has been dated at 396 ± 8 million years ago. Asteroxylon is probably a stem group to the Drepanophycaceae.

Description

Asteroxylon is a genus of terrestrial vascular plant which flourished in the Early Devonian period. Dichotomously branching stems, which reached 12 mm in diameter and 40 cm in length, were erect, rising from a ground-running organ, from which also protruded underground "rhizoids" or "roots": these reached a depth of up to 20 cm below the surface. An actinostelic vascular bundle occupied the centre of the axes, with tracheids being of 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 externally similar genera of the same period, Drepanophycus and Baragwanathia, in that the vascular thread extends well into the leaf in these genera. The leaves of Drepanophycus and Baragwanathia are therefore by definition true microphylls, while those of Asteroxylon are not; see Drepanophycales for more details.

References

• The Rhynie Chert and Asteroxylon