<P> Complex animals are composed of organs and many of these organs evolved a very long time ago . For example, the liver evolved in the stem vertebrates more than 500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, and worms more than 600 million years ago . </P> <P> Given the ancient origin of most vertebrate organs, researchers have looked for model systems, where organs have evolved more recently, and ideally have evolved multiple times independently . An outstanding model for this kind of research is the placenta, which has evolved more than 100 times independently in vertebrates, has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa . Studies on the evolution of the placenta have identified a variety of genetic and physiological processes that contribute to the origin and evolution of organs, these include the re-purposing of existing animal tissues, the acquisition of new functional properties by these tissues, and novel interactions of distinct tissue types . </P> <P> The study of plant organs is referred to as plant morphology, rather than anatomy, as in animal systems . Organs of plants can be divided into vegetative and reproductive . Vegetative plant organs are roots, stems, and leaves . The reproductive organs are variable . In flowering plants, they are represented by the flower, seed and fruit . In conifers, the organ that bears the reproductive structures is called a cone . In other divisions (phyla) of plants, the reproductive organs are called strobili, in Lycopodiophyta, or simply gametophores in mosses . </P> <P> The vegetative organs are essential for maintaining the life of a plant . While there can be 11 organ systems in animals, there are far fewer in plants, where some perform the vital functions, such as photosynthesis, while the reproductive organs are essential in reproduction . However, if there is asexual vegetative reproduction, the vegetative organs are those that create the new generation of plants (see clonal colony). </P>

Is a leaf an organ or organ system
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