Among vascular plants, parasites are limited to about twelve families of dicots. The bestknown examples are the Christmas mistletoes, Viscum album in Europe and Phoradendron serotinum in North America. While most mistletoes are tropical, the two mentioned above are common in the temperate regions. Unlike many parasitic flowering plants, mistletoes are green and photosynthetic and can thus produce their own food. Many species, including the Christmas mistletoes, produce fleshy, rigid mature leaves, but in other species the leaves are reduced to scales. Most species are epiphytic and grow in the branches of a host tree. Some tropical species are terrestrial, and at least one forms a 30-foot-tall tree. Even these tree species form a characteristic haustorium connecting the parasite to the host. Mistletoes cause severe economic losses in many areas. For instance, dwarf mistletoe (Anceuthobium) attacks many gymnosperms in the southwestern United States, particularly ponderosa pine.
Another wellknown flowering plant parasite is dodder (Cuscuta). Dodder is distributed worldwide. It is easily recognizable because its rapid growth can quickly cover a host plant with a network of yellowish stems and scalelike, yellow leaves. Although dodder seeds germinate on the ground, the root disintegrates as soon as haustoria make connections with a host, at which time the dodder becomes completely dependent on the host for nutrients. Dodder is designated as a noxious weed throughout the continental United States.
See also: Haustorium, Fungal Parasites
|
