Microphylls and megaphylls

[2] These leaves are narrow because the width of the blade is limited by the distance water can efficiently diffuse cell-to-cell from the central vascular strand to the margin of the leaf.

[4] Outgrowths of the protostele (the central vasculature) later emerged towards the enations (as in Asteroxylon),[4] and eventually continued to grow fully into the leaf to form the mid-vein (such as in Baragwanathia[4]).

[1] Horsetails (Equisetum) bear only a single vein, and appear to be microphyllous; however, the fossil record suggests that their forebears had leaves with complex venation, and their current state is a result of secondary simplification.

[2] Consequently, Psilotum was long thought to be a "living fossil" closely related to early land plants (rhyniophytes).

[1] While the simple definitions (microphylls: one vein, macrophylls: more than one) can still be used in modern botany, the evolutionary history is harder to decipher.