haustorium (noun, pl. haustoria) – any growth that absorbs nutrients directly from another cell or organism, whether a single cell, a cell filament or several cells forming a specialized absorption organ. Haustoria are most frequently associated with forms of a parasitism acting as nutrient drain, depleting other organism or cells of valuable resources; e. g., the nutrient-absorbing cell of the embryo sac that depletes the endosperm. However, in some mutual symbioses haustoria may also facilitate the exchange of substances; e. g., haustoria of mycorrhizae deplete vascular plants from sugars but provide mineral salts in exchange. In fungi haustoria are typically formed by one or several hyphae that are closely appressed around cells (wall-to-wall apposition, cf. appressoria) or that invaginate another cell, sometimes penetrating only the cell wall (intraparietal), sometimes even penetrating through plasma membrane into the cytoplasm (intracellular).