About one in every hundred species of flowering plant is parasitic and obtain some or all of their carbon, nutrients and water from the sap of their hosts. They possess unique morphological and metabolic adaptations but are more than just botanical curiosities.
Gurney, A.L.; Press, M.C. Plant eats plant: sap-feeding witchweeds and other parasitic angiosperms. Biologist (2000) 47 (4) 189-193.