Plant, Parasitic

a plant that completely or partially lives off the nutrients of other living organisms. Parasitic plants exist among both lower and higher plants, including flowering plants. Fungi, algae, and bacteria parasitize plants, animals, and man and are frequently the causative agents of infectious diseases. Parasitic flowering plants mostly infest and harm higher plants, including such cultivated ones as the sunflower, tomato, sorghum, and tobacco.

Facultative parasitic plants can exist by parasitism and other modes of feeding (for example, photoautotrophic). Obligate parasitic plants also use other sources of nutrition at times. Hemiparasitic plants are at the same time parasitic and photo-trophic organisms; for example, common toothwort is both a parasite and a carnivore.

Ectoparasitic plants penetrate the tissues of the host by means of haustoria, which extract nutrients. Such parasites include perisporaceous fungi (the causative agents of powdery mildew) and such flowering plants as broomrape and dodder. Endoparasitic plants, which include the Rafflesiaceae, develop completely or largely in the tissues of the host plant and emerge onto the surface of the host only to reproduce.

The evolution of plant parasites proceeded from accidental parasitism to facultative forms and, then, to obligate forms. It was accompanied by the disappearance of the capacity for photoautotrophic or saprophytic nutrition and by the acquisition of mechanisms for parasitic nutrition. The structures used for autotrophic nutrition gradually disappeared or were transformed into organs of parasitic nutrition. For example, the root tips of some flowering plants were transformed into haustoria. The developmental cycle of some parasitic plants changed as a result of parasitism. Highly developed parasitic flowering plants are characterized by metamorphic development, that is, the transformation of their organization in the process of ontogeny.

Complex forms of parasitism evolved in some plant groups. For example, flowering plants with endotrophic mycorrhiza are characterized by mutual parasitism of a flowering plant and a fungus (such as Orchidaceae and Pyrolaceae). There also are cases of epiparasitism, or hyperparasitism, in which a mycotrophic flowering plant obtains nutrients from a coniferous plant through a mycorrhizal fungus common to both (for example, yellow bird’s nest).