Celery

Celery is one of any varieties of a plant of the species Apium graveolens. The vegetable is known on Gallifrey, possibly being introduced there by Time Lords who had discovered the vegetable while visiting its native planet, Earth. Gallifreyans early on discovered the aromatherapeutic properties of celery, which have restorative benefits. Unfortunately, humans, such as the Doctor's companion  Peri, have comparatively underdeveloped olfactory senses and thus the vegetable's healing scent has little or no benefit for them ("The Caves of Androzani").

The Fifth Doctor has a habit of wearing a stalk of celery pinned to the lapel of his frock coat, beginning shortly after his regeneration while visiting the city of Castrovalva in Andromeda ("Castrovalva"). While attending dinner as a guest (or captive) of the Eternal Captain Striker, the Doctor replaces his old stalk with a fresh one ("Enlightenment"). The Fifth Doctor claims to be allergic to certain gases in the Praxis range of the spectrum, in whose presence the herb turns purple. Whether this allergy may extend to all Gallifreyans or only some of them, or to all the Doctor's incarnations or only his fifth, is not known (The Caves of Androzani).

External link
Wikipedia article on celery