haustorium, highly modified stem or root of a parasitic plant or a specialized branch or tube originating from a hairlike filament (hypha) of a fungus. The haustorium penetrates the tissues of a host and absorbs nutrients and water. In parasitic plants, such as dodder and mistletoe, the haustoria form a vascular union with the host plant to redirect the host’s nutrients.
The word haustorium also is used to indicate certain cell types in plant embryology. The African Mutelidae family of freshwater mussels have a parasitic larval stage that is called a haustorium.