A plant parasite is a plant that derives some or all of its nutritional requirements from another living plant. Parasitic plants have modified roots, called haustoria, which penetrate the host plant and connect directly to its vascular system. This allows them to siphon water and nutrients from the host plant. Many parasitic plants are small and inconspicuous, but some, like mistletoe, can be large and showy.
Most parasitic plants are found in tropical or subtropical regions, where they parasitize trees or shrubs. However, there are a few species that parasitize herbaceous plants, such as dodder (Cuscuta spp.). Some species of parasitic plant are specific to one host species, while others can parasitize a wide range of hosts.
Parasitic plants have evolved a variety of strategies for obtaining water and nutrients from their hosts. Some species tap into the xylem tissue, which carries water and dissolved minerals up from the roots to the leaves. Others tap into the phloem tissue, which moves sugars and other organic compounds around the plant. Still others form special structures called haustoria that penetrate directly into the host’s cells and steal water and nutrients directly from them.
Not all parasitic relationships between plants are negative; many are actually beneficial to both parties involved. For example, certain mycorrhizal fungi form symbiotic relationships with trees by colonizing their roots; in exchange for sugars produced by the tree’s photosynthesis process, these fungi help improve the tree’s uptake of water and minerals from soil – an interaction that benefits both partners involved.