Celery

Celery was a plant largely composed of water. The Fourth Doctor once claimed that he hated celery. (AUDIO: The Beautiful People) During his fifth incarnation, the Doctor took to wearing a stalk of celery on the left breast of his frock coat. This began after his regeneration when he visited the city of Castrovalva. He considered it as a sign of civilisation. (TV: Castrovalva) He replaced his celery at the party of the Eternal, Captain Wrack. (TV: Enlightenment) Despite carrying one with him at all times, the Fifth Doctor positively hated celery. (AUDIO: The Gathering)

Although a later incarnation of the Doctor later referred to the celery as a "decorative vegetable", (TV: Time Crash) the celery also served at least two practical purposes.

The Fifth Doctor once used the leafy part to test airflow in the Tractator-made tunnels of Frontios. Because the leaf was especially light, it would have indicated which way the air was flowing — had there actually been any circulation in those caverns. (TV: Frontios)

However, the main use was medical. The Fifth Doctor claimed to be allergic to certain gases in the praxis range of the spectrum. Celery, he said, would turn purple in the presence of such gases. In that case, he would eat the celery; if nothing else, he said, at least it would be good for his teeth. The Doctor apparently also believed the celery to possess some restorative ability, as he attempted to revive a dying Peri by having her smell the celery. Her human olfactory sense did not suffice for the plant to help her. (TV: The Caves of Androzani) Centuries later, the Eleventh Doctor asked for celery while being physically tortured by Silurians, seemingly confirming its beneficial effects on his anatomy. (TV: Cold Blood)

Peri Brown once found herself in a dimension modelled upon a combination of the Alice in Wonderland book she had been reading and the salad she had been eating. Among the creatures she met in this world was a talking stalk of celery. (COMIC: Salad Daze) After the Sixth Doctor fainted because of a time-slip with the Second Doctor, she offered him celery to feel better. (TV: The Two Doctors)

In Belgium in 800, the Tenth Doctor, who had been mistaken for Charlemagne's food taster, detected the smell of celery in some soup — much to his disgust, as he claimed he had never liked celery. (PROSE: The Lonely Computer)

When the Eleventh Doctor had been captured by the Silurians, Malohkeh attempted to decontaminate him of the "human bacteria", the side effect of which process actually harmed the Doctor. After pleading with the Silurian doctor that he was "not ape", Malokeh stopped the decontamination process. After taking a couple of breaths, the Doctor said "That's much better, thanks. Not got any celery, have you?" (TV: Cold Blood)

The Twelfth Doctor was wary of celery. After smelling it throughout North Lees Hall, he told Clara Oswald that celery was "always a sign that something's wrong". (COMIC: Unearthly Things)

The Father of Time, an incarnation of the Doctor from far in the First Doctor's future, wore a stick of celery on his lapel along with a cat brooch. (COMIC: The Test of Time)

Following the Thirteenth Doctor's forced regeneration, the new incarnation worse a celery stick on his lapel. (TV: The Power of the Doctor)

Behind the scenes

 * While the Fifth Doctor was sometimes portrayed as loving celery, Peter Davison hated it. During a scene in Castrovalva in which he had to eat celery, he spat it out after the first take. (DCOM: Castrovalva)
 * The celery worn by Davison was artificial. Davison has described the one he wore during his television run as looking "like a piece of embroidery" up close, and found the replica celery he was given to wear when reprising the role for Time Crash much better-looking. Upon learning that Davison's celery hadn't been real, David Tennant said he was "devastated."
 * The first piece of celery the Doctor pinned to his lapel was from the artificial reality of Castrovalva. When he replaced it in Enlightenment, it was from a similarly artificial environment.
 * The notion of an individual wearing a celery stalk on his lapel did not originate with the Fifth Doctor. In a 1965 episode of The Avengers (a series, coincidentally, created by Doctor Who co-creator Sydney Newman) entitled "Two's a Crowd", an individual impersonating John Steed is criticised for a particularly leafy choice of flower to wear on his lapel. A character remarks that he might as well have worn a piece of celery instead.
 * The Fourth Doctor had occasionally been seen to wear a brooch on his lapel, most notably one shaped like an artist's palette in City of Death, but the celery was the first lapel augmentation worn consistently by the Doctor. The theme of having the Doctor wearing something unusual on his lapel was continued with the Sixth Doctor wearing a variety of cat-shaped pins. The Seventh Doctor was also seen on occasion to wear items on his lapel, but not as regularly as his predecessors. By the arrival of the Eighth Doctor, the Doctor's lapels were accessory-free and,, remain so.