In plant anatomy and evolution a microphyll (or lycophyll) is a type of plant leaf with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the fossil record, and few such plants exist today. In the classical concept of a microphyll, the leaf 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, in contrast, have multiple veins within the leaf and leaf gaps above them in the stem.
Leaf vasculature
thumb|Microphylls contain a single vascular trace.
The clubmosses and horsetails have microphylls, as in all extant species there is only a single vascular trace in each leaf. 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. Despite their name, microphylls are not always small: those of Isoëtes can reach 25 centimetres in length, and the extinct Lepidodendron bore microphylls up to 78 cm long. Outgrowths of the protostele (the central vasculature) later emerged towards the enations (as in Asteroxylon), Some gymnosperms bear needles with only one vein, but these evolved later from plants with complex leaves.
It is not clear whether leaf gaps are a homologous trait of megaphyllous organisms or have evolved more than once.