Celery stalks (actually the petiole of Apium graveolens) are the fleshy stalks of an ancient vegetable known to the ancient Greeks, Romans, and Egyptians. They have a crunchy texture and slightly bitter, astringent flavor that makes them integral to many world cuisines, notably Classical French's mirepoix and Cajun's "Holy Trinity".
The characteristic aroma of celery comes from three chemicals: apiole, sedanolide, and 3-butylphthalide. Celery can also cause anaphylactic shock in sensitive people. It is most common in Central Europe.
Celery stalks are usually used as a garnish for the Bloody Mary.