Microphyll

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Microphyll refers to the leaves of the Lycopodiophyta. Each has only a single, unbranched vascular trace ("vein"), and is typically but not always photosynthetic. They are usually small in size but can become reasonably large in Isoetes (quillworts) and up to a meter long in some extinct related taxa. They are a defining feature of the lycopodiophytes (clubmosses), in contrast to the megaphylls (leaves with multiple, branching vascular traces) of the other vascular plants. 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 strand of vascular tissue.

Some other vascular plants, including Psilotum (whisk ferns), Equisetum (horsetails), and Ephedra (ephedra) sometimes have greatly reduced megaphylls that resemble microphylls, but they have relatives (living or extinct) with megaphylls, and some have multiple vascular traces. This is supported both by phylogenies based on genetic data and by the fact that most of these plants have more complex types of stem vasculature (siphonosteles or eusteles) rather than the 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". 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.