Microphyll

Microphyll

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 are based. The traditional definition of microphylls and megaphylls will be discussed 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 small; those of Isoëtes can reach 20–100 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 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 are characterised by multiple venation. By this definition, the Psilotopsids, Lycopods and Equisetum have microphylls, as all extant individuals only bear a single vascular trace in each leaf.