Appendicula, commonly known as stream orchids or ? (niu chi lan shu), is a genus of flowering plants in the family Orchidaceae. Orchids in this genus are epiphytic, lithophytic or rarely terrestrial plants herbs with many flat, often twisted leaves and small resupinate, white or greenish flowers. The sepals are free from each other but the lateral sepals and labellum are fused to the base of the column.
The genus Appendicula was first formally described in 1825 by Carl Ludwig Blume who published the description in Bijdragen tot de flora van Nederlandsch Indië. The name Appendicula is the diminutive form of the Latin word appendix meaning "appendage" or "addition", hence "little appendage", referring to the "inward-facing appendages on the labellum".