But now that the immediate euphoria of recognizing a human partner in Hamish and bonding to him had passed, Samantha was back face-to-face with the realities under which her human friends lived. And because Samantha was extremely intelligent, and an empath, she knew just how badly her adoption choice had disturbed those realities, even if she was still working on fully assimilating all of the reasons why it had.

“If they can’t digest it, and if it, um, clogs their systems, then why do they all love it so much?” Emily asked.

“That was something that puzzled every human who ever studied ’cats,” Honor said. “So once they learned to sign, we asked them, of course.” She shrugged. “Part of their answer was exactly what you might have expected—they love the way it tastes. 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. But that’s only part of the reason. The other is that there’s a trace compound in Sphinxian celery that they need.”

“In Sphinxian celery?” Emily repeated.

“They love the taste of any celery from anywhere,” Honor told her. “But back when humans first came to the Manticore System, we had to make some minor adjustments in our Old Terran flora and fauna before we introduced them into their new environments. As,” she added in a dust-dry tone, gesturing briefly at herself, “we’ve done with human beings themselves, in a few other cases. We didn’t do anything really drastic in the case of Sphinx, but a few minor genetic changes were designed into most of the Old Terran food plants to prevent the fixing of elements we didn’t need in our diet and to discourage some particularly persistent local parasites and the plant diseases they carry. The basic idea was to get the genegineered plants to manufacture and store a Sphinxian organic compound that’s harmless to humans but serves as a natural insect repellant. It worked in all of them, but better in some than in others, and it was most effective of all in celery, of all things. The version in the descendants of the modified Old Terran plants is slightly different from that which occurs in the native flora, sort of a hybrid. But it appears to be either necessary or extremely beneficial to the maintenance of the ‘cats’ empathic and telepathic senses.”

“But where did they get it from before we came along with our celery?” Emily demanded.

“There’s a Sphinxian plant that produces the native plants’ version of the same compound. They call it ‘purple thorn,’ and they’ve known about it forever. But it’s scarce and hard to find, and, frankly, they say celery just tastes a whole lot better.” Honor shrugged again. “And that, it turns out, is the answer to the Great Celery Theft Mystery which first brought humans and treecats together.”

“That’s fascinating,” Emily said, gazing at Honor raptly, and then moved her gaze to Nimitz and Samantha. She watched them for a moment, and they looked back at her solemnly until she drew a deep breath and turned back to Honor.