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Microphylls are photosynthetic flaps of plant tissue with a solitary, unbranched vein. They are most promiently found on the spore-bearing vascular plants, such as horsetails, though gymnosperms also possess them (as nonphotosynthetic sporophylls protecting their reproductive structures). They are considered evolutionary relics, and are theorized to have evolved from enations, flaps of veinless vascular tissue, whereas "true" leaves (megaphylls), are theorized to have evolved from flattened photosynthetic stems with flaps of webbing connecting them [1] (http://comenius.susqu.edu/bi/202/Journal/Vol8/number4/Geckospub.html). Microphylls are often small in size (though not always -- ancient lycophyte trees had very large microphylls), and generally don't contribute much in the way of photosynthesis. As a matter of fact, the microphylls on the fertile shoot of a horsetail (the reproductive structure of the plant) don't contain any chloroplasts at all.
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