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). 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.
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