Microphyll is a type of plant leaf with one single, unbranched vein. 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, in contrast, are characterised by multiple veins.By this definition, the whisk ferns, clubmosses and horsetails have microphylls, as in all extant species there is only a single vascular trace in each leaf. 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.[citation needed]