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Asteroxylon
''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〔Kidston R & Lang WH (1920) On Old red sandstone plants showing structure from the Rhynie chert bed, Aberdeenshire. Part III. Asteroxylon mackiei, Kidston & Lang. Transactions of the Royal Society of Edinburgh 52, 643-680〕 that has been dated at . ''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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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