Leaf Evolution

True leaves appear to have evolved at least twice in plant evolution. Microphylls are believed to have evolved from enations (Chapter 8) and represent a synapomorphy for the Lycophyta. They are generally, but not always, small, have a single vascular bundle, and a leaf trace that leaves no gap when it departs the stele. The evolution of microphylls is discussed in more detail in Chapter 9. Megaphylls evolved from branching systems generally have more complex vasculature and produce a leaf gap when the leaf traces departs the stele. Contrary to the name, they are not all large in size. All groups of vascular plants except the lycopsids possess megaphylls, and the evolution of this leaf type will be discussed in Chapter 11.

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