What is a Parasitic Plant?

The title of this database is a goal rather than an appropriate description of its present content because many plants which could be (and are, in fact) termed parasitic are not yet featured. Parasitic plants are here defined as vascular plants which have developed specialized organs for the penetration of the tissues of other vascular plants (hosts), and the establishment of connections to the vascular strands of the host to the end of absorption of nutrients by the parasite. These organs (albeit of different morphological nature in different parasitic plants) are termed haustoria. All known parasitic plants thus defined belong to the dicotyledons. Maybe, however this must be revised if the only known arhizous (lacking roots) gymnosperm Parasitaxus ustus (Dacrydium ustum, Podocarpus ustus, Podocarpaceae), an endemic of New Caledonia, can be shown to parasitize its "host" Dacrydium guillauminii (also Podocarpaceae) by haustoria. In the absence of suitable evidence, this decision has to be postponed.

Among parasitic plants commonly a distinction is made between holoparasites (lacking chlorophyll and photosynthesis, absorbing organic matter from the hosts) and hemiparasites (green, photosynthesizing, absorbing mainly anorganic nutrients from the hosts). The borders are, however, not entirely clear, and several representatives of holoparasitic groups have been demonstrated to contain at least traces of chlorophyll. Nevertheless, in this database only (mainly) holoparasitic plants are featured. The database thus comprises nearly 800 taxa (species or infraspecific subdivisions) in 56 genera and 8 families.

There may be various reasons why plants from several different evolutionary lines chose this striking way of heterotrophy. An obviuos one is presented by the epiphytic members of Santalales (Eremolepidaceae, Misodendraceae, some Santalaceae, Loranthaceae, and Viscaceae), where parasitism secures (at least) a steady supply of water for the parasite. Another evolutionary advantage is conveyed by the ability of parasitic plants to spend larger proportions of their life cycle on reproduction than their hosts.

Because of the enormous number of species (>2000), Santalales (only approaching holoparasitism in very few cases) have been omitted here. The same is true of most (hemiparasitic) Scrophulariaceae-Gerardieae, S.-Rhinantheae, and of the small hemiparasitic family Krameriaceae (only Krameria, with 17 species in the Americas). The parasitic groups as dealt with here are (with numbers of recognized taxa):

Balanophoraceae (Balanophora 26, Chlamydophytum 1, Corynaea 2, Cynomorium 2, Dactylanthus 1, Ditepalanthus 1, Exorhopala 1, Hachettea 1, Helosis 2, Langsdorffia 3, Lathrophytum 1, Lophophytum 4, Mystropetalon 1, Ombrophytum 4, Rhopalocnemis 1, Sarcophyte 1, Scybalium 4, Thonningia 1)

Hydnoraceae (Hydnora 4, Prosopanche 3)

Lauraceae p.p. (Cassytha 29)

Lennoaceae (Lennoa 2, Pholisma 3)

Rafflesiaceae (Apodanthes 7, Bdallophyton 4, Berlinianche 2, Botryocytinus 1, Cytinus 9, Hypolepis 3, Mitrastema 2, Pilostyles 17, Rafflesia 13, Rhizanthes 2, Sapria 2)

Convolvulaceae p.p./Cuscutaceae (Cuscuta 279)

Scrophulariaceae-Gerardieae p.p. (Alectra 48, Harveya 41, Hyobanche 7, Striga 50)

Scrophulariaceae-Rhinatheae p.p. (Lathraea 6, Tozzia 2)

Orobanchaceae (Aeginetia 10, Boschniakia 1, Christisonia 18, Cistanche 24, Conopholis 3, Epifagus 1, Gleadovia 3, Kopsiopsis 2, Mannagettaea 2, Necranthus 1, Orobanche 130, Phacellanthus 1, Phelypaea 2, Platypholis 1, Xylanche 1)

Jan Schlauer, March 25, 1996

Rick Walker
rick_walker AT omnisterra.com"