Celery was an edible plant originally native to Earth.
The plant was transplanted to the planet Sphinx, where treecats developed a taste for it. Sphinxian celery had been genetically modified and in doing so produced a chemical related to one found in a native Sphinxian plant. The original genetic modifications were developed by Dr. Marjorie Harrington. (HH1, SK1)
With the introduction of treecat signing, the Sphinx variant of celery was revealed to act as a telepathy/empathy "vitamin", far more effective (and tastier) than the native Sphinxian plant which had been known and valued by treecats as far back as their memories went. Treecats adored celery in any form, even though they were effectively unable to digest it. It was the low level theft of celery that led to the discovery of the treecats by Stephanie Harrington.
According to Honor Harrington, after interviews with Nimitz and Samantha on how much treecats like celery: "Think of the most chocolate-addicted human being you've ever met, then cube her craving, and you'll start closing in on just how much they love it." (HH10)
- Detailed article about Celery on Wikipedia