= Parasitic plant
A <parasitic plant> obtains resources directly from another living <plant>, its host, through a specialized connection called a <haustorium>. This differs from an <epiphyte> using another <plant> merely as a physical support. Some <parasitic plants> retain <photosynthesis>, so green color alone does not establish nutritional independence.<Biology 2e by Mary Ann Clark and others>[Section31.3, Nutritional Adaptations of Plants: parasitic plants and haustorial connections]{ref}
Chemical cues can be important before the connection forms. In 1966, investigators isolated the crystalline germination stimulant <strigol> from <cotton> root exudates and tested its activity on a root-parasitic <Striga>. This connected a soluble substance released by roots with a developmental response in another <plant>.<Germination of Witchweed (Striga lutea Lour.): Isolation and Properties of a Potent Stimulant by C. E. Cook and others>[Primary abstract: isolation, chemical characterization and germination activity]{ref}
A positive <germination> assay is not itself proof of successful parasitism. A seed can respond to a chemical without subsequently reaching a suitable host, forming a <haustorium> or acquiring enough resources to complete its life cycle. These are successive biological questions, requiring different observations.
For an original comparison, place a green plant on a branch and ask separately whether it photosynthesizes, where it gets water, and whether its tissues connect directly to a living host. Neither the place it grows nor a single metabolic ability is an adequate substitute for observing the resource-acquisition relationship.
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