Microphyll

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Microphylls are poorly-developed leaves with only a single, unbranched vascular trace ("vein"), typically but not always photosynthetic. They are usually small in size but can become reasonably large in Isoetes (quillworts) and some extinct related taxa. They are a defining feature of the most primitive vascular plant lineage, the lycopodiophytes (clubmosses), which never have well-developed megaphylls (leaves with multiple, branching vascular traces). In addition to having a single unbranching vascular trace, microphylls also occur only with the simplest type of stem vasculature, a protostele in which stems contain a single solid strand of vascular tissue.

More advanced plants, including Psilotum (whisk ferns), Equisetum (horsetails), and Ephedra (ephedra) sometimes have greatly reduced megaphylls that resemble microphylls, but their leaves are currently thought to have evolved through loss of leaf function from an ancestor with well-developed megaphylls rather than representing true microphylls. This is supported both by phylogenies based on genetic data and by the fact that most of these plants have more advanced types of stem vasculature (siphonosteles or eusteles) rather than the primitive protostele of true microphyllous plants. Psilotum is particularly interesting in that it has a protostele and its leaves are reduced to flaps of photosynthetic tissue completely lacking any vascular traces, called "enations", which resemble fossils of the earliest evolutionary steps in the development of leaves. As a result, Psilotum was long thought to be a "living fossil", descending unchanged from the first land plants. More recent phylogenies of the vascular plants based on genetic data, however, suggest that Psilotum actually represents a loss of leaves from an ancestor that had megaphylls.