Walnut

Walnuts were a type of tree found on Earth, as well as the edible nuts which grew from the tree.

The Doctor likened the size of the stegosaurus' brain to that of a walnut. (DW: Invasion of the Dinosaurs)

One Christmas eve, the Doctor and Romana snuck into Alistair Gordon Lethbridge-Stewart's house dressed as Santa Claus and left two oranges and a walnut in his Christmas stocking. (ST: Special Occasions: 3. Better Watch Out, Better Take Care)

When trapped in a subjective reality based on the works of Lewis Carroll, Peri ran from a pair of talking turnips who were trying to tell her a story called "the Walnut and the Cauliflower." (DWM: Salad Daze)

In 1840, the Eighth Doctor bought a walnut muffin whilst in London. It was knocked from his grasp when Penny Chapman ran into him. (DWM: The Curious Tale of Spring-Heeled Jack)

To battle the effects of cyanide, the Tenth Doctor needed, among other things, some sort of protein. Donna Noble found a jar of walnuts, which the Doctor wolfed down. (DW: The Unicorn and the Wasp)