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 plant while visiting its native planet, Earth.

The Fifth Doctor has a habit of wearing a stalk of celery pinned to the lapel of his frock coat, beginning in the television story, "Castrovalva." In "Enlightenment" the Doctor replaces his old stalk with a fresh one. In "Caves of Androzani," he notes that the herb turns purple in the presence of gases in the Praxis range of the spectrum. Whether this allergy may extend to all Gallifreyans or only some of them, we do not know.

He also notes that 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. Interestingly enough, celery is sometimes consumed on Earth in whole or liquid form as a diuretic and cleansing tonic, and to relieve symptoms of rheumatoid arthritis.

External link
Wikipedia article on celery