Everything about Microphyll totally explained
The terminology of fossil plants is in places a little confusing. In the discipline's 200+ year history, certain concepts have become entrenched, even though improved understanding has threatened the foundations upon which they're based. The traditional definition of microphylls and megaphylls will be employed in this article for simplicity; their merits will be discussed later.
Traditionally, a
microphyll is "an appendage supplied by a single, unbranched vein". Despite their name, microphylls are not always microscopic; those of
Isoetes (quillworts) reach centimetres in length, and the extinct
Lepidodendron bore microphylls over a metre long. In the classical concept of a microphyll, this vein emerges from the proto
stele, without leaving a gap.
Megaphylls, in contrast, leave a "leaf gap" when they depart the stele, with some
vascular strands leaving to supply the leaf, and the other strands closing up-stem of the divergence. Megaphylls are characterised by multiple venation. By this definition, the whisk ferns (
psilopsids), club mosses (
lycopods) and horsetails (
sphenopsids) have microphylls, as all extant individuals only bear a single vascular trace in each leaf.
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), 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's 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.][Further Information]
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