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. While the simple definitions (microphylls: one vein, macrophylls: more than one) can still be used in modern botany, the evolutionary history is harder to decipher. File:Vein sceleton hydrangea ies.jpg|Megaphylls have a complex network of veins. File:Psilotum.jpg|
Psilotum has secondarily lost leaves, and bears
enations resembling the microphylls of early land plants. ==See also==