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Aarabia
''Aarabia'' is a genus of extinct vascular plants found in central Morocco in outcrops of Early Devonian age (Emsian, around ). The leafless plant has a complex branching system with a main stem and at least three orders of side branches. In addition to these long branches, stems bore very short branches, which typically branched once into two curved sections. Spore-forming organs or sporangia were borne singly on reduced lateral branches in groups of at least three. The genus is thought to be related to the euphyllophytes – modern ferns and seed plants.〔
==Description==
Fossils were found in an outcrop believed to be of Emsian age (around ) northwest of the town of Azrou in central Morocco. They were 'adpressions' – a mixture of compressed and chemically altered remains of the original plant and impressions of its shape. Only central parts of the plant were found, the longest being 9 cm.〔
Stems (axes) were bare of leaves, spines or other protrusions, and were between 1.5 and 5 mm in diameter. Branching was 'pseudomonopodial', i.e. resulted in a 'main' stem and a lateral branch. There were up to four orders of branching; the branches were borne in two rows on opposite sides of the stem, producing a relatively two-dimensional structure. All orders of branches appear to have borne distinctive short branches which typically divided only once, the final sections having tips which curved away from each other and back towards the stem which bore them. Meyer-Berthaud and Gerrienne were unable to decide whether these short branches were generally capable of further growth or had ceased to grow and were adapted for some particular function.〔
Only poorly preserved specimens had sporangia. Up to three sporangia were found on a "reduced lateral branch system". Individual sporangia were 1.8 to 4 mm long and 1 to 1.4 mm wide and probably hung downwards. No spores were found inside the sporangia, but large spores with an average diameter of 270 µm were found close to sporangia and may have been released from them. If so, ''Aarabia'' had megaspores and could be the earliest known plant with 'heterospory', i.e. two distinct sizes of spore.〔
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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