EtymologyFrom micro- (small) + "phyll" (leaf)
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), Even some conifer needles bear only a single vascular trace, but again, this evolved as a secondary simplification from a multi-veined leaf.
We have problems in the other direction, too. The lycopods are the only clade accepted to have evolved microphylls de novo, instead of by reduction from a megaphyll, but even they throw up exceptions: some Selaginella species have a complex venation instead of microphylls.
To make matters worse, there is also some debate about whether leaf gaps are unique and/or common to megaphyllous organisms.
This ambiguity leaves it difficult to distinguish between two competing hypotheses: that microphylls evolved via the reduction of megaphylls, and that they evolved independently, from enations. Taxonomically, the terms are perhaps better left undistinguished until more is known of their origins - perhaps the term "leaf" is more appropriate.