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Microphyll
Microphyll is a type of plant leaf with one single, unbranched vein. In the classical concept of a microphyll, this vein emerges from the protostele without leaving a leaf gap. Leaf gaps are small areas above the node of some leaves where there is no vascular tissue, as it has all been diverted to the leaf. Megaphylls, in contrast, are characterised by multiple veins.
By this definition, the whisk ferns, clubmosses and horsetails have microphylls, as in all extant species there is only a single vascular trace in each leaf. Despite their name, microphylls are not always small: those of ''Isoëtes'' can reach 20–100 centimetres in length, and the extinct ''Lepidodendron'' bore microphylls over a metre long.
==Evolution of leaves: microphylls and megaphylls==
The "Enation theory" of microphyll development posits that small outgrowth, or enations, developed from the side of early axes (such as those found in the Zosterophylls).〔WN Stewart & GW Rothwell (1993) Palaeobotany and the evolution of plants. 2nd edition. Cambridge University Press.〕 Outgrowths of the protostele later emerged towards the enations (as in ''Asteroxylon''),〔 and eventually continued to grow fully into the leaf to form the mid-vein (such as in ''Baragwanathia''〔).〔 The fossil record appears to display these traits in this order,〔 but this may be a coincidence, as the record is very incomplete: microphylls may have originated by the reduction of a telome branch.〔
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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