About 380 million years ago, plants with vascular tissue first evolved a special type of leaf, referred to as a microphyll. A microphyll typically has a single midvein, and arises from a stem which does not have leaf gaps, in regions of parenchyma (i.e, unspecialized) tissue where the vascular strand leads into the leaf base. The microphyll may have originated as an outgrowth of a vascularized stem, or by evolutionary simplification of a complex branch system. The leaves of certain modern plants in the Lycopodophyta (Lycopods) and Sphenophyta (Horsetails) are classified as microphylls. Although the microphylls of these modern plants are quite small, some of their fossilized relatives had very large microphylls.
About 350 million years ago, plants first evolved megaphylls, the leaf type of modern seed plants and ferns. A megaphyll typically has a complex venation pattern, and arises from a stem which has leaf gaps, or regions of parenchyma tissue where the vascular strand leads into the leaf base. One theory proposes that megaphylls, as well as other plant organs, evolved by modification of branch systems. In other words, megaphylls may have evolved by the flattening of a three-dimensional branch system, and connection of the flattened branches with a cellular webbing.
Many botanists believe that the four different whorls of a flower (sepals, petals, stamens, and carpels) originated by evolutionary modification of the megaphylls of a free-sporing plant. Shortly after the evolutionary origin of the Angiosperms (flowering plants), there was a major division between the monocots (plants whose embryos have one cotyledon) and dicots (plants whose embryos have two cotyledons). The leaves of modern monocots, such as grasses and lilies, tend to be narrow and have parallel venation; the leaves of dicots tend to be wide, and have reticulated venation.
Many modern plants have evolved complex and highly specialized leaves. For example, the insect-eating organs of carnivorous plants, such as Venus Flytrap, Sundew, Pitcher Plant, and Bladderwort, are all highly specialized leaves. Dischidia rafflesiana, a tropical epiphyte, has among the most specialized leaves of any plant. Its leaves are tubular in shape and they collect forest debris and rain water, providing a habitat for the colonies of ants which live inside. As the ants die, their bodies dissolve and special roots of the plant absorb the nutrients that are released, providing nourishment for the plant.
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Wilkins, M. Plant Watching. Facts on File Inc., 1988.
Peter A. Ensiminger