Parasitic plants

A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. All parasitic plants have modified roots, named haustoria (singular: haustorium), which penetrate the host plants, connecting them to the conductive system – either the xylem, the phloem, or both. This provides them with the ability to extract water and nutrients from the host. Some parasitic plants are able to locate their host plants by detecting chemicals in the air or soil given off by host shoots or roots, respectively. About 4,100 species of parasitic plant in approximately 19 families of flowering plants are known.

Parasitic plants are characterized as follows:

For hemiparasites, one from each of the three sets of terms can be applied to the same species, e.g.

Holoparasites are always obligate so only two terms are needed, e.g.

Plants usually considered holoparasites include broomrape, dodder, Rafflesia, and the Hydnoraceae. Plants usually considered hemiparasites include Castilleja, mistletoe, Western Australian Christmas tree, and yellow rattle.

Parasitic behavior evolved in angiosperms roughly 12-13 times independently, a classic example of convergent evolution. Roughly 1% of all angiosperm species are parasitic, with a large degree of host dependence. The taxonomic family Orobanchaceae (encompassing the genera Tryphysaria, Striga, and Orobanche) is the only family that contains both holoparasitic and hemiparasitic species, making it a model group for studying the evolutionary rise of parasitism. The remaining groups contain only hemiparasites or holoparasites.

The evolutionary event which gave rise to parasitism in plants was the development of haustoria. The first, most ancestral, haustoria are thought to be similar to that of the facultative hemiparasites within Tryphysaria, lateral haustoria develop along the surface of the roots in these species. Later evolution led to the development of terminal or primary haustoria at the tip of the juvenile radicle, seen in obligate hemiparasitic species within Striga. Lastly, obligate holoparasitic behavior originated with the loss of the photosynthetic process, seen in the Orobanche genus.

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