Parasitism for plants is a relationship in which one organism uses the nutrients and water of another plant, the host, to the detriment of the host. A holoparasite is a relationship where parasitism is obligatory. A hemiparasite is exemplified by a plant that can live either as a parasite or on its own, hence this plant is a facultative parasite.

Parasitic plants belong to about 15 families of flowering plants. The many species of mistletoes, belonging to Viscaceae and Loranthaceae, comprise about three-quarters of all parasitic species. Several other families of parasitic flowering plants are also well known, especially the broomrapes (Orobanchaceae) and dodders (Cuscutaceae).

Parasitism has evolved many times in dicotyledons, i.e., there has been convergence toward several types.

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